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1:18

Empathy is our best policy. Hello,

1:22

everyone, and welcome to the Mindscape Podcast. I'm your

1:25

host, Sean Carroll. As I will

1:27

mention very briefly in today's podcast, when

1:29

I was a kid growing up, I

1:32

knew that I wanted to do science. In fact,

1:34

theoretical physics is what I knew I wanted to

1:36

do from a very early age. But

1:39

theoretical physics, you know, Einstein's equation,

1:41

quarks, things like that, this is

1:43

not something anyone in my family

1:46

or circle understood. They

1:49

did understand that it was somehow

1:51

related to space and astronomy and

1:53

things like that, so I would be frequently given, you

1:56

know, telescopes or books about

1:58

astronomy as gifts, Which is great,

2:00

I love that stuff too, it just wasn't what I

2:02

actually wanted to do for a living. But

2:05

as a result of this, circa, I

2:07

don't know, 1980 was kind of

2:09

my peak knowledge about modern astronomy.

2:11

Even though I went on as

2:14

an undergraduate and a graduate student to

2:16

be an astronomy major, get

2:18

a bachelor's degree as well as

2:20

a PhD in astronomy, by then

2:22

I was actually focusing more on

2:24

learning fundamental physics. So I had

2:27

to sit through my required courses

2:29

in astronomy, but I wasn't like in

2:31

my spare time catching up on

2:33

the most recent discoveries about planets

2:35

and stars and galaxies and stuff

2:37

like that. And even today,

2:39

you know, I just as much

2:42

as anyone else follow the news items. I

2:44

get to talk to my colleagues, it's true,

2:46

so I probably get more inside scoop than

2:48

the average person. But I'm

2:50

absolutely not at the cutting edge

2:52

of what's going on broadly in

2:54

astronomy, more than many

2:57

people are. So since I

2:59

did have that knowledge back in the 70s

3:01

and 80s, it's always fun to catch up

3:03

on what's been going on since then.

3:06

And when it comes to something like

3:08

planets, we've just learned so much more

3:10

about the planets, both in our solar

3:12

system as well as exoplanets of course,

3:14

than we did back then. I

3:17

mean, not only was Pluto a planet

3:19

back when I was still learning this

3:21

stuff, but we had just started in

3:23

the 1970s sending spacecraft to

3:25

other planets. We had learned to

3:27

our surprise that the atmosphere of

3:30

Venus was kind of inhospitable. Probably

3:32

we knew that even before we sent the spacecraft there,

3:34

but it was a surprise when we learned it. We

3:37

were still hoping to find life

3:39

on Mars of some sort, not

3:41

just little microbial life, but maybe

3:43

something more exotic. The

3:46

very first landers, the Viking landers, were

3:48

sent to Mars, but also they were

3:50

mission sent to Venus that just plunged

3:52

right in. Even mission

3:54

sent to Mercury, as

3:56

well as of course, famously, the pioneer

3:58

in Voyager missions. to the outer planets.

4:01

So it was a very exciting time back

4:03

then, but so much more

4:05

has happened now landing on all

4:07

sorts of planets, investigating them, measuring

4:09

their properties with much greater precision.

4:12

So today we have to catch up on

4:14

some of this knowledge with Sabina

4:16

Stanley, who is actually an astronomer

4:18

at Johns Hopkins and has

4:20

recently come out with a book

4:23

published by Johns Hopkins University Press

4:25

called What's Hidden Inside Planets? And

4:28

the idea is, of course, that there's the atmosphere

4:30

and the outer layers of planets, but there's

4:32

also the very fun interiors of planets, which

4:35

is kind of a place where

4:37

we know a lot, but much less

4:39

than we would like to. Even the

4:41

Earth, as we will learn in the

4:43

podcast, we know things indirectly,

4:45

not directly. We have not journeyed to

4:47

the center of the Earth in reality

4:49

as much as we like to imagine

4:51

doing so in fiction. So

4:53

it's always fun to learn how

4:55

clever scientists have been to figure

4:57

out what's going on in places

4:59

we can't see, including the ground

5:02

right beneath our feet and extending

5:04

that knowledge then to other planets,

5:06

making predictions for what their gravitational field

5:08

should be, what their magnetic field should

5:10

be, watching those predictions go

5:12

wildly wrong, updating our models and going,

5:14

oh yes, we forgot about sulfur. That's

5:17

kind of important or something like that.

5:19

So there's a whole mess of things we're

5:21

going to learn, and we're going to learn

5:24

about the diamond iceberg floating on liquid oceans

5:26

in cold planets far

5:28

away and how all that

5:30

stuff happens and how much more we have

5:32

yet to learn. So let's go. Veena

5:51

Stanley, welcome to the Winescape podcast. Thanks so much

5:53

for having me. So we're going to talk

5:55

about what's inside planets. I wanted to

5:57

set the stage just by remembering when

6:00

I was a kid, we had terrestrial

6:02

planets and we had gas giants,

6:04

right? And of course, these

6:06

days, we kicked Pluto out and we've discovered

6:08

exoplanets and things like that. Is it still

6:11

though basically true that we have those

6:14

two categories or it has our space

6:16

of possible planets to think about grown

6:18

bigger? I definitely

6:20

think the space of possible planets has grown bigger,

6:22

right? Even in our own solar system, we even

6:24

now with the giant planets, we know that there's

6:27

Jupiter and Saturn, which are these gassy giant

6:30

planets. Then you've got the ice rich planets,

6:32

Uranus and Neptune, so they can be quite

6:34

different. We have little worlds in our solar

6:36

system with 16 Psyche, this

6:38

asteroid that the Psyche mission is going to go

6:40

to as well. So there's a

6:42

lot of variety even here in our solar system and everything

6:45

we've found outside of our solar

6:47

system just shows us how many more possibilities

6:49

there are. Well, my not

6:51

quite expert impression

6:53

from the exoplanet research is that

6:56

we have been surprised by

6:58

various properties of planets. Have we actually

7:01

discovered new kinds of planets? Yeah,

7:05

absolutely. It's really interesting to think about, I

7:07

remember when I was learning this stuff in

7:10

undergrad, that we had this sort of real

7:12

belief that we understood how planets formed and

7:15

that wherever we would

7:17

look, it should be that you'd have the rocky planets

7:19

and kind of the closer to the star system and

7:21

then you'd have the gas giant planet further

7:23

out and then boom, the first exoplanet

7:25

we see, suddenly you've got something

7:27

bigger than Jupiter orbiting closer than

7:29

Mercury does in our own solar

7:32

system. So this early

7:34

exoplanet discovery has just really showed us

7:36

that we needed to kind of rethink

7:38

how planet formation occurs

7:40

and what the possibilities are out

7:42

there. And I guess it made sense

7:45

to think that back in the day, right?

7:47

Because we thought that in

7:49

the early stages of the formation of

7:51

the planetary system, the atmosphere would get

7:53

blown off of the planets. I mean,

7:55

that's my, again, I'm a very theoretical

7:57

physicist here, but I have this feeling.

8:00

that the inner planets are rocky

8:02

because that's all that was left and the outer planets are

8:04

gaseous because they could keep their atmosphere. It's

8:07

a little bit, I would say it's a little

8:09

bit different. It's more that the inner planets are

8:11

rocky because there was no gas. They couldn't

8:14

grow fast enough to collect the gas whereas the

8:16

outer planets could grow faster because they had more

8:18

building blocks. But I think

8:20

the key thing that we learnt from looking

8:22

at these exoplanet systems is there was a

8:24

process that we had kind of

8:26

thought wasn't that important for our

8:28

own solar system but turns out to be

8:30

important in other solar systems and that's planetary

8:32

migration, the fact that planets can move their

8:36

orbits over time. Well, I

8:38

will, we can jump around. We don't need to be

8:41

logical order here from what we're talking about.

8:43

So I'll confess when I was looking into

8:45

your book and thinking about this podcast, I

8:48

never knew that people thought that Jupiter

8:50

might have started its life much, much

8:53

closer to the sun than it is

8:55

now. Yeah, it's possible

8:57

that it actually moved a lot. It started further

8:59

out then came in kind of closer to kind

9:01

of where Mars currently is

9:04

and then switched again and started

9:06

moving out again. And Uranus and Neptune

9:08

might have actually switched places. It used

9:10

to be that Neptune was closer than Uranus. So these

9:12

are all reasonable possibilities

9:14

based on what we see

9:17

in terms of the orbits of a lot of the

9:19

asteroids and Kuiper Belt objects out there in the outer

9:21

solar system right now. Is there some

9:23

just human scale difficulty because when we see

9:25

the solar system, it seems like more

9:27

or less the same from when we're born to when

9:30

we die and extrapolating it back a billion years is

9:32

kind of hard. Yeah, I

9:34

think that's a natural issue that we

9:36

always have with things that are on

9:38

such long timescales are really far away

9:40

is putting it on the scale. But

9:42

I was even, I know this is kind of

9:44

in a separate vein, but I think

9:46

it's interesting to think about the fact that

9:48

it's possible that Saturn didn't have rings when

9:50

there were dinosaurs on Earth. So things do change, right?

9:54

And even in sort of

9:56

the timescales, we're used to trying to comprehend

9:58

even if it's not on a human level. lifestyle

10:00

a life lifespan So

10:03

let me just quickly get your opinion. Did

10:05

you do you think Saturn did have rings

10:07

back when the dinosaurs were around? Oh gosh,

10:09

I think It's not something

10:11

to have an opinion about I think it's really

10:13

interesting to think about how the rings are Populated

10:16

and and how they change. I think we

10:18

just need more data to study that and

10:21

what is your okay? Here is an

10:23

opinion question Pluto planet or not Okay,

10:27

here's the here's my answer. It's not a

10:29

planet and that's okay if Pluto

10:33

is just really cool. It was such an

10:35

interesting Object that it

10:37

started its own class of planetary object

10:39

the dwarf planet So why would you want

10:42

to be a planet when you can just

10:44

start your own class of planetary objects?

10:46

So, you know, I will I will confess

10:48

I said this before but in

10:50

the early days I was against kicking Pluto out

10:52

of the Planet Club on the basis of you

10:54

know The idea of a planet

10:57

is something we human beings made up we can grandfather

10:59

in Pluto But I had Mike Brown who was a

11:01

good friend of mine from Caltech on the podcast and

11:03

I read his book and I did Change my mind

11:05

like a good scientist should so I think

11:07

that the scientists are right about this one I

11:10

agree. Okay. So let's get to what

11:12

you actually do for living as I understand

11:14

it the interiors of planets Which

11:16

is a little bit harder than

11:18

the exteriors, right? We can't actually see them

11:20

I presume we start by thinking about the

11:22

earth and what we know about its interior

11:26

Yeah, absolutely you know It's frustrating because when

11:28

you try to think about what's going on

11:30

inside a planet your first instinct is you

11:32

know Let's let's dig in there

11:34

and get some samples and try and figure that

11:36

out But it's just it's impossible for earth right

11:38

the furthest we've ever drilled into

11:40

the planet is under 10 kilometers,

11:42

right? It's basically another under

11:45

10 miles. Sorry. It's basically nothing for

11:47

a planet with a say

11:51

6,000 kilometer radius, right? So we're just kind

11:53

of getting at the skin here So we

11:55

have to be really sneaky and clever in

11:57

how we figure out properties of the

11:59

entire planet interior of the planet do

12:01

a lot of things that like doctors do to figure

12:03

out what's wrong with you when you go to the

12:06

Doctor right. Hopefully they don't drill first and kind of

12:08

figure out things later, right? So

12:10

it's a lot of Honing techniques

12:13

to give us the information we're looking for for the

12:16

interiors of planet It's kind of sad

12:18

that we've only gone down ten less than ten miles

12:21

is that Ambition

12:24

the planetary scientists have like, you know Like

12:26

particle physicists want to build a bigger bigger

12:28

collider to planetary physicists want to dig deeper

12:30

and deeper. I Don't

12:33

think it's so much nowadays that there's

12:35

this goal of digging We want to learn

12:38

more and more about the deep interior But

12:40

we're open to the fact that there are

12:42

better sometimes more efficient other ways to do

12:44

this, right? so nowadays, I

12:46

think we rely a lot on the

12:48

combination of Sort

12:52

of Non digging type

12:54

technology, right? Relying

12:57

on sensors and and waves to study the interior

12:59

but also the fact that we can get samples

13:01

can come up from depth Whenever

13:04

diamonds come to the surface some of

13:06

them actually preserve bits

13:09

of the mantle in their Inside of them

13:11

and they're like as inclusion so we can learn about

13:13

things that way Meteorites tell us

13:15

about the interiors of asteroids and other bodies

13:17

out there. So that we're willing to get

13:19

the information However possible. I'm not sure

13:21

that we're all kind of Hell

13:24

bent on digging anymore Well,

13:27

I'm sorry just to follow up on this crazy

13:29

scenario But if I did want

13:31

to build a little robot that had a

13:33

drill and could just dig down deeper and

13:35

deeper What is the obstacles is it the

13:37

heat of the density or the power source?

13:40

So the combination of the pressure and the heat right?

13:43

You every time you're gone a little

13:45

bit deeper inside the planet temperatures are

13:48

rising equipment doesn't like hot temperatures Doesn't

13:50

like high pressures doesn't humans

13:52

don't like them So it's harder to get down there to

13:54

fix things as well just as it is if you're going

13:56

out into space So it's the combination

13:58

right if you think about about sort of the deepest

14:01

minds we have that humans can function

14:03

in, right? You're talking about things that are in

14:05

the two-mile depth range, one

14:07

to two-mile depth range, right? So

14:10

it's a combination of those two issues.

14:12

Okay. So we're stuck with being the

14:14

doctor who cannot perform surgery. What

14:17

do we actually do? How do we know about,

14:19

I guess maybe what is the Earth's interior like

14:21

and then how do we know? Yeah.

14:24

So the Earth, which is kind of a good

14:28

prototype or typical example of a rocky

14:30

planet, the outer part of it

14:32

is made of sort of magnesium silicates, what we

14:35

would normally consider as rocky materials.

14:37

And then the inner part of the planet is

14:40

iron. So we have an iron core.

14:43

In Earth, the innermost part of that, about the

14:45

innermost 1300 kilometers is solid. And then you've got

14:47

a liquid iron core for another 2000 or so

14:49

kilometers. And so you got

14:53

this separation, right? The heaviest stuff, the densest stuff

14:56

is at the center and

14:58

then the outer layers are rock. And that's true for the

15:00

other rocky planets as well. So

15:02

how do we figure this all out? Combinations

15:05

of methods. One

15:07

of the coolest methods to me to talk about, the

15:10

one that gives us a lot of information is seismology.

15:12

So every time there's an earthquake somewhere, waves,

15:15

sound waves essentially travel through the Earth

15:18

and we can record when they arrive

15:20

at different locations on the surface and

15:22

those waves, whatever region they travel through,

15:25

the speed of the wave is completely

15:27

related to the material

15:29

properties that they're traveling through.

15:31

So we can use that information at

15:33

the surface and kind of backtrack all the

15:35

waves that go through the Earth and learn about the

15:37

material they pass through. That's how we learned that the

15:39

Earth has an iron core, but the outer part

15:41

of it is liquid. We

15:43

can learn about phase transitions in the

15:46

Earth's mantle so when minerals change their

15:48

structural properties

15:50

into other physical

15:52

arrangements, all sorts of stuff. So that's

15:54

kind of been the sort

15:57

of workhorse of planetary interior studies.

16:00

Obviously, I've heard that story before and it

16:03

does make sense, right? You have a sound

16:05

wave traveling through and it's kind of, it

16:07

is kind of like what doctors do, whether it's a

16:09

CAT scan or an MRI or whatever. But

16:13

it still seems a little crude, you

16:15

know? I mean, hearing these sounds from

16:18

earthquakes thousands of miles away and

16:20

saying, okay, I have now inferred

16:22

the internal structure of the Earth.

16:24

I mean, what's our confidence level

16:26

here? So here's the

16:28

amazing thing. We have lots of

16:30

earthquakes. They travel through different parts

16:32

of the Earth. They travel in different directions, through

16:34

different materials. So with sort of

16:36

modern day analysis techniques and computational

16:38

methods, we can actually get a

16:40

lot of really great data. We

16:42

can see things like volcanic

16:45

plumes coming up from the core mental boundary

16:47

all the way to the surface

16:49

of the Earth. We can see subducting

16:52

slabs, so places on the planet

16:54

where one tectonic plate

16:56

is descending back into the Earth under another

16:58

one. We can see that colder material descending

17:00

into the Earth almost all the way down

17:03

to the core mental boundary. So we're really

17:05

at the point where we're getting like lateral

17:07

structure. It's not just a density

17:10

as a function of depth. It's

17:13

really like imaging now of the

17:15

interior. So we're getting something like a 3D

17:17

picture of what the Earth's interior

17:19

looks like. Absolutely.

17:22

And okay, so we have the core, the

17:24

inner core and outer core and mantle are the three

17:27

that I remember from high school. Yeah.

17:29

That's still true. Like everything else I learned in my

17:31

high school science classes is not true anymore. It's

17:34

still true. It's just it gets more, the

17:36

more we learn, the more we can break things

17:38

up, right? Now the mantle's got the upper mantle,

17:41

the lower mantle. You can get to talk about

17:43

transition zones. You can talk about all fun sorts

17:45

of phase transitions, stuff like that. But to a

17:47

basic level, that's still accurate. And which

17:49

parts are liquid and which parts are solid? So

17:52

the only liquid part in the interior of the

17:54

Earth is the liquid iron outer core. So there's

17:56

about 2,000, 2,500 kilometers or so. near

18:00

the center of the Earth that's liquid. So

18:04

the very core is also iron but solid.

18:07

Yeah, that is correct. And

18:09

there's an interesting property in

18:11

the deep interiors is that so pressure is

18:13

increasing as you're going deeper and temperature is

18:16

increasing as you're going deeper. So the

18:18

very center of the Earth, even though the temperature

18:20

is much hotter than the outer parts

18:23

of the layers, it's solid because it's pressure

18:25

frozen. It's basically squeezed so much that it

18:28

has to be solid. So just

18:30

a fun thing when you're thinking about how things are

18:32

different inside the Earth than they are, say, at the

18:34

surface. And the mantle, I

18:36

guess, this is part of my inner picture,

18:39

which is probably faulty, but I think of

18:41

it as what's coming

18:43

up in lava and volcanoes and things like

18:45

that, which look liquid to me, but it's

18:47

actually solid. So yeah,

18:50

this is such a common understanding

18:53

that needs to be corrected, right?

18:56

And we see it at the surface. Yes,

18:58

lava is liquid, but that's because you took

19:00

something that was under really high pressure and

19:02

you quickly depressurized it, right? So that

19:04

material that's coming up at volcanoes, it wasn't

19:06

liquid inside the Earth. It was solid. It

19:08

just got depressurized so that then that expanded

19:11

volume made it into a liquid. Okay,

19:13

but the

19:16

Earth is four point something billion

19:18

years old. Should we be

19:20

surprised that it's as active as it is, that

19:22

it's still sort of turning around and plate tectonics

19:24

and all that stuff? Why hasn't it settled down

19:26

yet? That's a great question. So

19:29

yes, the Earth is very old. All

19:32

the planets are, but all the planets

19:35

have some hints of some sort of activity on the

19:38

inside. We're the only planet with plate tectonics,

19:40

but you got Mercury generating a dynamo on

19:42

its core, which means that its core is

19:45

still convecting and very active. You have tectonic

19:47

processes happening on Mars. So the crust in

19:49

the outer parts of the planets are

19:51

shifting around in response to like they're flexing

19:53

in response to say thermal gradients or

19:56

other tidal forces and so on. The

19:59

key thing with planets is all

20:02

planets start out really hot, the centers of planets

20:04

are much hotter than the space, and

20:06

so they're all cooling and most of the motions, most

20:08

of the processes we see happening are a result of

20:11

that cooling and so

20:13

that activity is the cooling

20:15

and it takes a really long time to cool

20:17

down a planet. So that's why we're still

20:19

seeing activity everywhere. And part of that is

20:22

that these interiors are not only iron,

20:24

they have heavier radioactive elements that are

20:26

still providing some heat. That's

20:28

exactly right. So you've got the initial

20:30

heat of formation when these planets form,

20:32

they've stored a lot of heat inside,

20:34

but planets also have uranium, thorium, these

20:36

long-lived radionuclides that can actually generate heat

20:38

today. About half of the heat coming

20:40

out of the earth today is coming

20:42

from radioactive elements in Earth's mantle. That

20:45

always, you know, again, my intuition

20:47

fails me here, right? Because there's

20:49

not that much uranium and thorium

20:51

in there, but I guess

20:53

there's a lot of volume in the earth so it's enough

20:55

to keep it hot. Exactly. And

20:59

how do we know how much uranium is

21:01

in the middle of the earth? Is it reverse

21:03

engineering from how hot it is? No,

21:06

it's actually based on, so if we

21:08

look at samples that we have of

21:10

Earth, so it's mostly based on estimates

21:12

we have from the crust or maybe the upper mantle,

21:14

you take samples from there, you actually

21:19

measure how much uranium and thorium or their

21:21

daughter products that you have there, and from that

21:23

you come up with estimates of what you think is

21:26

in the earth. It's a combination of just direct measuring

21:28

and then also understanding, okay, so

21:30

I've got a rock and when

21:32

it melts, does uranium and thorium prefer to

21:35

be with this part of the melt or that

21:37

part of the melt? So it's a lot of geology and

21:39

geochemistry involved that can tell you where you should expect

21:41

to find the uranium. Yeah, it's

21:43

always reminded me because as a

21:46

physicist I will sometimes teach

21:48

general relativity and it's this

21:50

beautiful pristine logical edifice, right? And I

21:52

love teaching it and sometimes

21:55

I'll teach cosmology and it's

21:57

a mess, like every week you have to do something else, like there

21:59

might be a lot of thermodynamics and E&M or whatever

22:01

particle physics, I imagine that your job

22:03

is even more of a mess than

22:05

cosmology is in terms of all the

22:07

different kinds of knowledge that come in.

22:10

Yeah, absolutely. But honestly, that's what I love about it.

22:12

I love the fact that in order to make, to

22:15

have progress in understanding the interior of the

22:18

Earth and planets, you need to combine the

22:20

sort of fundamental physics knowledge, the chemistry knowledge,

22:23

the methods and like

22:25

sensors and observational

22:27

methods knowledge, right? It's

22:29

a big puzzle and you got to bring in all

22:31

these different types of knowledge to get an answer. Speaking

22:34

of which, okay, we talked about the seismic

22:37

information. I guess I should ask, is

22:39

there, is that more active or

22:41

passive? Like do we have detectors

22:44

that were set up specifically

22:46

to understand the interior of the Earth or do

22:48

we sort of piggyback off of the fact that

22:50

we want to know where earthquakes are happening anyway?

22:53

So over time, there's been more

22:56

and more interest

22:58

in having seismic

23:00

sensors, basically all over

23:03

the surface of the Earth. And there are these

23:05

great sort of dense arrays of sensors, for example,

23:07

all over the US, there's this moving US

23:09

center network that goes around and other countries and

23:11

regions of the Earth are doing this as well.

23:14

So we're very actively looking for

23:17

putting up sensors so that we can measure when earthquakes

23:19

happen where they are. We

23:22

are also kind of moving out into the

23:25

solar system, right? We have had seismometers on

23:27

the moon since the Apollo missions. They were

23:29

turned off in the, I guess it was

23:31

the early 80s. But we've

23:33

very recently put a seismometer on Mars and

23:36

been able to measure Mars quakes there and

23:38

from that, from those Mars quakes be able to

23:40

learn about the interior of Mars as well. So

23:43

I think there's a major move

23:45

to using seismology on other planetary bodies because

23:47

of the wealth of information it provides. Cool.

23:50

And I guess probably there wasn't a lot on the

23:52

moon or are there moonquakes all the time? There

23:54

are moonquakes all the time. So this is amazing.

23:57

So and

23:59

a lot of the Artemis mission, there

24:01

are plans to put new seismometers on

24:03

the Moon in different locations so that we can

24:05

start studying these again. Moonquakes

24:07

actually happen for a variety of

24:09

reasons. Sometimes you have impact, so

24:12

the Moon is hit with meters

24:14

as well as Earth is and all other

24:16

planets over time. So we can measure Moonquakes

24:18

from that. But there are also these very

24:20

deep Moonquakes. They happen much deeper in the

24:22

Moon. And they're actually caused by tidal flexing

24:24

of the Moon. So the Moon experiences tidal

24:26

forces just like the Earth does, why we

24:28

have tides, the Moon has tides. And

24:31

so we can actually measure rumblings

24:33

in the inside of the Moon from those tidal

24:36

forces. All right. And then what

24:38

else besides seismic information do we use to learn

24:40

about the interior of the Earth? Yeah.

24:43

So then take a combination of

24:45

fields. So gravity fields, magnetic fields,

24:47

those are probably the biggest ones

24:49

there. So gravity fields, the fact

24:51

that when we teach our

24:53

intro physics courses, we tell everyone, you

24:56

know, G is 9.8 meters per

24:58

second squared on the surface of the Earth. That's not

25:00

true. As you walk around on the surface of

25:02

the Earth, the value of G actually varies and

25:04

it depends on how much mass is directly below

25:07

you. And so we can

25:09

measure those variations in gravity and

25:11

use that to actually learn about

25:13

variations in density below our feet.

25:15

And so we can do this for other planets as well.

25:17

There have been gravity missions sent to, well,

25:20

basically any planet that we've sent a mission

25:22

to, we have gravity data from. And

25:24

from that we can learn about the

25:26

interior mass distributions inside planets. You

25:29

know, in cosmology right now, there's a famous Hubble

25:31

tension. We measure the Hubble cost in two different

25:33

ways to get two different answers. I

25:36

could imagine Earth's

25:39

core tension, if you measured

25:42

its properties seismically one way and

25:44

then magnetic fields or gravitational fields in other

25:46

ways, is there any such thing

25:48

on the horizon or is everything completely compatible? That's

25:52

an interesting question. First of all, I'm

25:54

frustrated with the, even though I'm not in

25:56

the field, I hate that it's called attention because I'm like,

25:59

it's not attention. It's a complete

26:02

like the disagreement. Hey French,

26:04

but the clearly different numbers. That's not attention But

26:06

anyway going back to the earth. I think

26:08

it's much more that the methods are very complimentary. So

26:12

gravity tells you something about bulk So

26:15

right the gravity field can't really tell you about

26:17

what the density is a particular location But you

26:19

combine that with the seismology and the seismology tells

26:21

you tell hey You have an iron core at

26:23

the center then that you combine that with the

26:26

gravity and you can use that to

26:28

really infer More details about stuff, right?

26:30

So all the methods are

26:32

really complimentary. There isn't any tension that I

26:34

can think about hand there are actually There's

26:38

the latest tension that's interesting

26:40

is with

26:42

what seismology and gravity are telling us

26:44

for example about mark what Mars is

26:46

core is made of and what

26:50

we think is true about About

26:54

the Material that was

26:56

around in the solar system all planets were

26:58

forming. So in Mars

27:01

insight mission right measures the radius of the core for

27:03

the first time very near the end of the mission

27:05

We were waiting for like the big one on Mars

27:07

and it finally came Like a couple

27:09

months before we were showing down the mission and from

27:11

that we were able to figure out the radius of

27:13

Mars's core And it's a

27:15

little bit bigger when we thought we

27:17

knew from gravity But gravity tells

27:20

you a bulk measurement So essentially

27:22

if the core is bigger it may

27:25

means that it has to be a little less dense

27:27

a little lighter Than what we

27:29

thought but if it's a little lighter,

27:31

that means that it combined with the iron in the

27:33

core there's some lighter elements and

27:35

it's really hard to kind of Figure

27:38

out how these light elements got to the

27:40

center of Mars Based

27:42

on what we thought the building blocks of planets were

27:44

so that's kind of a little bit of attention right

27:46

now Although I think there are ways around

27:48

it. We just need to understand the geochemistry of planet

27:50

formation a little more I think the better thing for

27:53

you to do is to label it the Mars core

27:55

crisis and Grand

27:57

money and the publicity will start rolling in

28:01

I will take that advice that's amazing and

28:03

you've mentioned something a little provocative before about

28:06

mercury and Convection

28:08

and magnetic fields so magnetic fields

28:10

are obviously the other way as

28:12

you mentioned gravity and magnetism What

28:15

does the earth's magnetic field let

28:17

us infer about its interior? Yeah,

28:20

great question. So magnetic fields happen to be my favorite thing to

28:22

talk about and You can learn

28:24

a lot from magnetic fields for any planet. So let's start with

28:26

her The key thing about

28:28

magnetic fields if a planet has a magnetic field Then

28:31

first of all, you know, it has to have a good

28:33

electrical conductor somewhere on the inside and that's great iron at

28:35

the center of the earth does that for you? You

28:38

know it has to have motions in it And

28:40

so that tells you that first of all, you have to

28:43

have a liquid to have the motions be fast enough for

28:45

this to occur and there needs to be a

28:47

power source for those motions and So

28:49

this is how we know for example that Earth's

28:52

core there's convection going on in Earth's core. It's

28:54

trying to remove heat Through

28:56

that convection and so that tells us a

28:58

lot about how much energy and power is stored

29:00

inside the earth So tell you you learn

29:02

a lot about the thermal evolution Of

29:05

a planet by knowing that it has an

29:08

active magnetic field generates today It gets generated

29:10

by this dynamo action right similar sort of

29:12

process that runs your generators or

29:14

your bike lights But lots

29:17

of information by seeing a magnetic field. So

29:19

the convection is presumably in that

29:21

liquid outer core Absolutely,

29:24

and it really is just sort

29:26

of a constant turning because of

29:28

thermal disequilibrium somehow So

29:30

yeah basically it's like when you put a pot

29:32

on your stove bottom hotter than the top if

29:35

You try to get heat through there

29:37

faster than can be conducted through the material you're gonna

29:39

get convection so it's the same sort of thing inside

29:41

the core of the planet and That

29:45

so if it in the absence of that if you

29:47

didn't have that you would not have the magnetic field. There's

29:49

no other way That's right. Yep, that

29:51

is correct So for

29:53

example Mars today doesn't have an actively

29:55

generating magnetic field today It doesn't have

29:58

a dynamo, but it does have rocks on

30:00

the surface that are magnetized, which tells us that

30:02

it did have a dynamo in its path. So

30:05

we've learned something about the thermal

30:07

evolution of Mars four billion years ago

30:10

by looking at these rocks on the surface that are

30:12

magnetized. But there's no convection going

30:14

on in my refrigerator magnets. So

30:17

that's a different kind of magnet. So when we'll

30:19

have permanent magnets, so the inside of the planets

30:21

aren't permanent magnets. These are what are called induction

30:23

processes creating magnetic fields. So it's the moving

30:26

around of currents that are creating new

30:28

magnetic fields. It's not like permanent magnets

30:31

like your fridge magnet. And

30:33

so that does sound like a pretty consistent

30:35

story overall. Like if we didn't know about

30:37

the magnetic field, would the

30:39

seismic observations have led us to conclude

30:42

that part of the core was liquid?

30:45

So, yes. So the seismic observations, luckily,

30:47

can give us that information in a

30:49

completely different way. It's because a certain

30:51

type of wave doesn't travel through liquids. So

30:54

these shear waves that are called S waves inside

30:56

planets, they don't travel through liquids. So when we

30:58

see them disappear in our seismic records,

31:01

we say, oh, they must have gone through a

31:03

liquid. But what magnetic fields can add

31:05

to it is, first of all, the motion. We can't

31:07

tell that there are motions in the core without

31:10

magnetic fields. And the other thing

31:12

that magnetic fields can really do for you is tell

31:14

you about the history of a planet. So because the

31:16

rocks on the surface record magnetic fields at the time

31:18

they form, that's why we learned about

31:20

plate-tech chronics on the surface of the Earth and where the land

31:23

masses were in the past and so forth. And

31:25

the exact same sort of thing on Mars. We

31:27

could learn that the core of Mars was liquid from seismology,

31:30

but we never would have been able to learn that

31:32

it had a dynamo in its early history

31:35

if it weren't for magnetic fields being recorded

31:37

in the rocks. And

31:39

the magnetic field of the Earth does all

31:41

these weird things. Like it wanders around, occasionally

31:43

it just reverses its polarity, right? And as

31:46

far as I know, we

31:48

can't predict when and we're not exactly sure why.

31:51

Yeah, that's a great way of looking at

31:53

it. So it's an interesting comparison

31:56

when you think about the Sun. So our Sun

31:58

also is a magnetic field and that's magnetic

32:00

field also reverses, but it does

32:02

so like clockwork every 11 years.

32:04

Poof, reversal, right? In

32:06

the Earth, we're aware of reversals because we

32:08

have rock record that tells us that there

32:11

are reversals in the past, but it's

32:13

not periodic. But it's happened, if

32:15

you were to take all the ones we know

32:18

about and divide by the amount of time they've

32:20

happened over on average, every half million years or

32:22

so the Earth's magnetic field reversals. The

32:25

last reversal was about 750,000 years

32:27

ago. So in some metric, we're

32:29

a little bit overdue for a reversal, but

32:31

it's all thrown one kind of non-periodic

32:35

process. So it could just be normal right

32:37

now. It could be another quarter million years before

32:40

it happens. Exactly. Would it be bad if it

32:42

happened tomorrow? Would it break the internet? It's

32:45

an interesting question. As far

32:47

as we know, again, having not lived through a

32:49

reversal ourselves and be able to measure it, what

32:51

we can see in the rock record tells us

32:53

first of all that reversals

32:55

probably take a bit of time. They might take somewhere on

32:57

the order of a thousand years or so to actually fully

33:00

complete. So I like to hope

33:02

that as humans, any

33:04

of the complications associated

33:06

with a reversal, we

33:08

could actually adjust for, right?

33:10

So the main issues we would have if

33:12

a reversal occurs is actually due to our

33:14

technology, right? So we rely

33:17

very heavily right now on satellites

33:19

orbiting the Earth. They do everything sort of from

33:21

GPS to navigation to all that

33:23

stuff, right? Our magnetic field

33:26

actually very much shields all of

33:28

those satellites from the high energy particles

33:30

that come from the sun, the solar wind and cosmic

33:32

rays. So during a reversal, the Earth's

33:35

magnetic field actually decreases somewhat,

33:37

gets more chaotic. And

33:39

so satellites in orbit would actually be more

33:42

susceptible to being hit by these high

33:44

energy cosmic rays and solar wind. And

33:46

so they could get knocked

33:48

out for example. But

33:50

if that happens on say a human

33:52

life time scale, hopefully we could change our technology

33:54

in time to deal with that. I do

33:56

remember reading that the over the last 150

33:58

years, the magnetic field has been diminishing

34:01

slightly in magnitude. Yeah,

34:03

slightly. That's true. But it's

34:05

interesting. If you look at a longer time record,

34:07

it was actually pretty high recently. So the diminishing

34:09

that's happening now is still putting us above the

34:11

average of, let's say, the past 10,000 years. So

34:15

I think we

34:17

have to look at a longer time record before we

34:19

can decide if there's some weird anomaly.

34:21

Are we in the beginning or reversal or

34:23

not? For the young people out there who

34:25

are deciding on their future research careers, is

34:28

understanding the Earth's magnetic field, something that is

34:30

still very much an ongoing project? Absolutely.

34:34

And there's different ways you can tackle this, right?

34:36

So for people who really like studying

34:39

fluid dynamics and nonlinear dynamics, chaos,

34:41

that kind of stuff, there's understanding

34:43

the fundamental processes

34:45

involved. For people who really

34:47

like observational studies, trying to get

34:49

data now from satellites

34:51

in orbit, lots of cool

34:54

data analysis projects. We're really trying to understand

34:56

the magnetosphere, the region surrounding Earth, because that's

34:58

important for our understanding of space weather and

35:00

that helps us in keeping our

35:02

technology going. And then of course, for

35:04

other planets, we're trying to learn about them from their magnetic field

35:07

as well. So yeah, there's lots of work to be done here.

35:09

It's a very data-driven field, lots

35:11

of use nowadays of

35:15

data science, machine learning, computational

35:18

models, lots of cool stuff going on. And

35:21

something you're implying is that both the

35:23

plate tectonics on Earth and the

35:25

magnetic field are kind of temporary.

35:27

I mean, eventually those radioactive

35:29

materials will decay away and the Earth

35:32

will just cool off. Yes,

35:35

that is accurate. All right, so we should

35:37

enjoy the magnetic field while we have it. Absolutely.

35:40

Maybe we'll find new ways to generate it or something.

35:42

Maybe. Yeah, the other thing that

35:44

makes the Earth special here in the solar system is

35:47

the moon, right? I mean, the moon is much

35:49

bigger compared to the Earth than any other planet

35:51

satellite is. Do we learn

35:54

about the Earth by studying the moon

35:56

or vice versa? Or is there still

35:59

a lot of uncertainty? about how the whole thing came together

36:01

in the first place. Absolutely. So

36:03

it's really interesting to me

36:06

that especially if you're thinking about the early

36:08

history of Earth, right? You had mentioned Earth

36:10

4.67, so billion years old. And

36:15

the surface of the Earth is very young

36:17

because we have plate tectonics. The surface gets

36:19

recycled back into the interior. There's very little

36:22

old rock on the surface. Fortunately

36:24

for us, there's lots of old rock on the moon.

36:28

And the Earth and the moon formed

36:30

from the same sort of material. There

36:32

was a giant impact very early in

36:34

Earth's history. And so there's a lot

36:36

of similarities between the Earth's material

36:38

and the moon's material. And

36:40

being able to look at the rock record on the

36:42

moon actually tells us a lot about,

36:44

first of all, the early solar system in general,

36:46

but also about the earlier. Is

36:48

that impact theory more or less the consensus these days?

36:51

Yeah, it's the only one that can explain all the

36:54

observables at the moment. I

36:56

read that there was recently a story,

36:58

a claim, that we could... So if there

37:00

was an impact, then there was the proto-Earth and some

37:02

other planet... The planet has a name that I forget

37:04

now, came and smashed into us. And

37:07

we could actually identify chunks of

37:09

the planet inside the Earth right

37:11

now. Is that credible? So

37:14

my understanding of that research is

37:17

that we do computer simulations of that

37:19

impact nowadays. So you take a body

37:21

that was a Mars-sized body and believed to be... It's

37:23

usually given the name Fea. Fea. And

37:26

when it crashed into the Earth, you can ask the question, where

37:28

did the material go? And

37:30

you do find that some of the material

37:33

from the impactor gets

37:35

put inside the Earth. And so the question is,

37:37

does it mix in all the way or so

37:39

forth? And there have been some computer

37:42

simulations of these processes that suggest

37:44

that some of it ends up at

37:46

the bottom of the mantle, kind of right above the core

37:48

mantle boundary. And it turns out that we

37:50

have these weird features in the mantle that we've learned

37:52

from seismology that are down there. And

37:55

so I would put it at

37:57

the moment as a hypothesis with

37:59

some simulation. suggesting it's feasible,

38:01

but there are potential other explanations

38:03

for the materials that we see

38:05

down at the core mantle boundaries. So it's not

38:08

definitive, I would say at the moment. But should

38:10

we get the impression that the

38:12

simple cartoon that we

38:14

see of the cutaway earth with the

38:17

inner core, outer core mantle, the reality

38:19

is not quite so pretty and symmetric

38:21

as that? Yeah, that's definitely

38:23

true, right? It's never as pretty as the

38:25

simple models. But

38:27

also the movie, The Core, was probably not realistic. The

38:31

movie, The Core, is my favorite movie in the whole

38:33

universe. But

38:36

it is accurate to say that there are some things

38:38

in it that are not realistic, but still a

38:40

great sum of the stuff in there was that bad.

38:44

You got to take what you can get for

38:46

Hollywood Entertainment, that's fine. Okay, so

38:48

with The Moon, what do we know about

38:51

its interior? You said there are moonquakes. Does

38:53

it also have a hot little core? So

38:56

the moon does have a core, but the core is

38:58

much smaller than, for example, Earth's core is relative to

39:00

the size of Earth. So the moon's core

39:03

is only about 400 kilometers in

39:05

radius, right? The moon's radius is about 1800, or crap,

39:07

what is it? The

39:10

moon's radius, yeah, it's about 1800 kilometers,

39:12

1700 kilometers or so. They can look that up,

39:14

don't worry. Yeah, yeah, so we'll Google that later.

39:16

So it's a smaller core, and that actually

39:18

makes sense when we think about how the

39:20

moon forms because the collision that would have

39:23

created the moon, when

39:25

you do a glancing impact, probably the

39:27

core of the moon, that

39:29

thea body ended up more inside the

39:31

Earth, and more rocky material

39:33

from Earth's mantle, and from thea

39:35

ended up in orbit around Earth,

39:38

and that then created the moon. So

39:40

it makes sense that there's less

39:42

iron in the moon, if

39:45

it formed from that impact. And

39:47

it also, sometimes I worry when things

39:49

make sense, that I think I understand

39:51

them, but I really don't. So it

39:53

sounds like it makes sense that the

39:55

moon is

39:58

cooler on the inside and doesn't have

40:00

a magnetic field and doesn't have plate tectonics just

40:02

because it's smaller in addition to the formation history.

40:04

I mean, it should cool off quicker, right? So

40:08

this is interesting. Yes, it makes sense that way.

40:10

However, we have to be very careful with reasoning

40:12

like that. And there's a great story about the

40:14

planet Mercury when we do reasoning like that. So

40:17

the first mission that went

40:19

to Mercury to study the planet

40:21

in detail was Mariner 10 in

40:23

the mid 1970s. And there's this

40:26

great paper that came out a couple years

40:28

before spacecraft got to

40:30

Mercury. And it said, Mercury is a

40:32

very small planet, which is true. And

40:35

so it should cool down fast, which is true.

40:37

And so it shouldn't have an active

40:40

dynamo generating magnetic field today because the

40:42

core should be completely solidified. And

40:44

then boom, Mariner 10 gets to Mercury and measures

40:46

an active magnetic field. And so

40:49

luckily, right, and that's okay. Predictions are,

40:51

you know, meant to be there as

40:53

based on what our understanding of the

40:55

series at the time. But

40:57

after we actually saw the magnetic field, then

40:59

scientists went back to the drawing board, they're

41:01

like, okay, maybe the core is not

41:04

pure iron mixed in a little bit of sulfur

41:06

in that iron, and you change the melting temperature

41:08

so much that you could actually

41:10

keep the core liquid much longer. And

41:12

so it was just it was this actual data, I

41:14

guess I kind of say data is really important

41:16

before we use

41:19

sort of just very

41:21

basic principles to try and understand what's going on

41:23

inside a planet. The details tend to matter a

41:25

lot. Are we lucky that

41:27

Mariner was equipped with a magnetometer? Yes,

41:30

because there was no other way to know

41:32

that from

41:34

that. It was not until much

41:36

later, just before the messenger

41:38

mission that to Mercury in the early 2000s,

41:42

that we actually had another way to determine that

41:44

there was a liquid core inside Mercury. And that was

41:46

from a really interesting study of

41:48

radar observations from Earth, looking

41:51

at Mercury and seeing how the planet wobbles

41:53

while it spins. Okay. And

41:55

so because Mercury has this very elliptical

41:57

orbit around the Sun, it's

42:00

length of day essentially changes a little bit depending

42:02

on how far it is from the Sun and

42:05

we could actually measure that wobble and from that

42:07

get the moment of inertia and from

42:09

that realize that the Amount

42:11

that the planet was wobbling meant there had

42:13

to be a liquid layer Decoupling the outer

42:15

part of the planet from the interior part

42:18

and so we didn't get that information into

42:20

the early 2000s But it again

42:22

confirmed what the magnetic field was already telling

42:24

us that there must be a liquid iron

42:26

core inside mercury Here's how much

42:28

astronomy I've forgotten Is mercury totally

42:31

locked is it the same? So

42:33

mercury is in this three

42:35

to two. Yeah in orbit

42:38

locking so it's not purely tied a lot So it's not that

42:40

one Face spaces the Sun

42:42

all the time. So one year doesn't equal

42:44

one day instead. It's the three to two ratio

42:47

Okay, so that but that's a very nice thing

42:49

for the observations of wiggles and so forth so

42:52

we can be back They should be what they

42:54

are. Okay. Good. All right. Well, so then

42:57

Should we be a little chagrined that

42:59

our? Theorists

43:02

didn't predict something like that ahead of time

43:04

Like how good is the state of the

43:06

art of we astrophysicists being able to say

43:08

here is what planet formation was like?

43:10

And therefore what planet should be like? I?

43:14

Like to think of it as I think what

43:16

we're learning is that the details really matter

43:18

and so you need to understand very specific

43:20

details of a planet or a situation in

43:22

order to understand what to expect and That

43:25

also means that more and more data actually really

43:27

helps us every time we send a mission to

43:29

a planet We basically rewrite the textbooks about

43:32

what we know about that planet, right? We

43:35

aren't just refining sort of a number

43:37

or a very specific theory We're actually

43:39

having to be very creative in coming

43:41

up with new explanations for phenomena. We see

43:43

whenever we go to a planet now So

43:46

let's just like run through the menagerie.

43:48

I guess we have the four

43:51

inner planets They're all terrestrial, but they're all

43:53

also kind of different which is weird and

43:55

fun And how well do we how are

43:57

they different and how well do we understand?

43:59

Why? Yeah, great

44:01

question. So I think this is something that

44:03

actually I think we need to be very

44:05

careful about when we're especially thinking about exoplanets

44:08

nowadays is that I would

44:10

argue that the reason the four innermost

44:12

planets are so different is because of

44:14

really tiny circumstances

44:17

essentially. Mercury, why

44:19

is it so tiny? It has such a huge

44:21

iron core probably because it got hit by a

44:23

giant impact or very early on its history. So that

44:25

one giant impact completely changed the history of

44:27

that body. Venus

44:30

and Earth, so similar in terms

44:32

of fundamental properties, mass and radius, so

44:35

different in terms of living environment on the

44:37

surface and that's likely because

44:39

Venus is just a little bit closer

44:41

to the Sun so it's a little

44:43

bit hotter and went through this runaway

44:45

greenhouse process. Again, a tiny detail, a

44:47

few degrees in temperature difference. Mars, lots

44:51

of planetary formation models when they try to create

44:54

the inner solar system, they cannot make a small

44:56

Mars. Mars is supposed to be big. The

44:59

problem is though, what we think is the

45:01

reason for that is because if you include

45:03

the outer planets, Jupiter ends up disrupting a

45:05

lot of planet formation in the Mars region

45:08

and so it's hard to build a big

45:10

planet where Mars is. So

45:12

again, depends on what was near you. What did

45:14

you have a Jupiter planet just outside of you?

45:16

So lots of like individual circumstances

45:18

with each planetary body that ultimately determines

45:20

its evolution. So I think that's really

45:23

important to think about when for example

45:25

we're looking at exoplanets and thinking about, is

45:27

there life out there? What makes a habitable

45:29

planet? Well maybe it's not just about the

45:32

distance from the Sun or star

45:34

and the radiation environment. They're

45:36

going to be very specific details that

45:39

determine whether a planet is actually habitable.

45:41

A lot of history and a lot of probabilistic

45:43

events. Exactly, exactly. And

45:45

so Venus and Mars are not

45:48

that different in size from the Earth. They're

45:50

slightly different distances from the Sun like you

45:52

said but are their interiors comparable in some

45:55

way? So both, so Earth,

45:57

Venus and Mars are all roughly the

45:59

same in terms of them having half their radius

46:01

be about iron and the other half be rock.

46:03

So in that sense, their interiors are very similar

46:05

on a basic level. Yeah.

46:08

Mercury is the outlier there and that is mostly

46:11

iron, very little rock. But we think we understand

46:13

that from a giant impact. How

46:15

do we know about the interior of

46:18

Venus? We cannot go down to the

46:20

surface and do seismology. So

46:22

I go on about this in my book a

46:24

little bit. Venus

46:26

is very frustrating. It's the

46:28

worst planet out there. It's

46:30

right there. It's the closest planet. It's

46:33

right there. But as planetary

46:35

scientists, we've developed all these methods to study

46:37

the interiors of planets. And

46:39

almost every single one of them fails when it comes

46:42

to Venus because of some reason or another, right? Venus

46:44

doesn't have an active dynamo today generating magnetic

46:46

fields. So we can't use magnetic information to

46:49

learn about its interior. It

46:51

rotates so slowly that

46:54

it's really hard to use information

46:56

we can get from the shape of a

46:58

planet. So for example, the Earth is a

47:00

little bit bulgy, right? Its equator is wider

47:02

than its pole-to-pole region. All the

47:04

planets are bulgy because they're rotating. Venus

47:07

rotates so slowly that you can't really tell about

47:10

how bulgy. The bulge doesn't tell you a lot

47:12

of information about the interior. Whereas because of the

47:14

gravitational effects of it, when we look at the

47:16

bulge of another planet, the shape of another planet,

47:18

we can actually say something about the density in

47:20

its interior. Venus,

47:23

you know, you want to put a seismometer on

47:26

Venus? Sure, except it has to live in

47:28

a completely hostile environment and it would basically

47:30

melt right away and no

47:33

one can go down there because of the toxic atmosphere. So

47:35

we can't use seismology to study Venus

47:37

either, right? So all these wonderful ways

47:39

we've found to discover what's going on

47:41

inside a planet just fail when you

47:43

get to Venus. So it's very frustrating.

47:45

But we are making progress. I don't

47:47

want to make it sound impossible. There

47:49

have been very recent papers where people

47:51

have been measuring kind of the

47:54

slow rotation and a little bit about the

47:56

precession rate of Venus to learn

47:58

about what's going on. going on inside

48:01

the planet. And hopefully the

48:03

new missions that will hopefully go to

48:05

Venus will learn even more. I

48:08

mean, if an advanced alien civilization wanted to hide

48:10

out in the solar system from us, the surface of

48:12

Venus would be a great place to do it,

48:14

right? Yeah, if they can survive there, absolutely.

48:16

Well they're an advanced alien civilization. I'm going to give

48:18

them credit for that. But nevertheless

48:22

we do think that the interior is

48:24

similar. Is there, I mean, or maybe

48:27

there's no liquid part to the core

48:29

because there's no magnetic field. So

48:31

that's interesting. So we

48:33

don't know for sure, but we do think

48:36

that the core of Venus is probably liquid.

48:38

It's probably just not experiencing the motions, the

48:40

convective motions that we have inside the Earth

48:42

to create a magnetic field. And

48:44

that might be because of the fact that

48:47

it's not cooling fast enough to get convection

48:49

to happen. Now, you start

48:51

asking, well, why? Why wouldn't Venus convect?

48:54

And it turns out a better

48:56

question is why on Earth is Earth's core

48:58

convecting? Because when you start doing the math,

49:00

when you start looking at how much heat

49:02

you would need to have escaping the Earth

49:04

to generate a magnetic field and you look

49:06

at how much heat could actually have just

49:08

been carried by conduction, the numbers are really

49:10

close. And so we're just like barely

49:13

able to convect on Earth.

49:16

And so Venus might be

49:18

more the norm. Venus might be the planet that's

49:20

kind of cooling at a just

49:22

below the rate that would result in convection.

49:25

The fact that Earth also has

49:27

plate tectonics tends

49:29

to be really important as a cooling mechanism for

49:31

the planet. So imagine you're trying to cool a

49:34

cake or let's say

49:36

you have a baked potato. This is my

49:38

favorite. If you have baked potato, you could

49:40

just let it sit there and cool through

49:42

conduction or you can try to cut it

49:44

up so that the interior gets exposed and

49:46

cools down immediately. And plate tectonics is kind

49:48

of like the cutting up of the potato

49:50

because you're constantly exposing new material to the

49:52

surface and descending cold material on the inside.

49:55

So the fact that we have plate tectonics on

49:57

Earth might be ultimately responsible for why

49:59

we have a dynamo generated magnetic field today

50:01

because it's a very efficient way to

50:03

remove heat from a planet. Venus

50:06

doesn't have plate tectonics. And it's

50:08

closer to the sun, does that matter? The

50:12

reason we think that matters is because

50:14

what happened in the atmosphere. So the

50:16

runaway greenhouse ultimately removed all the water

50:18

from Venus. Now on Earth,

50:20

yeah, we have water on the surface in our

50:23

oceans and in our atmosphere, but we also have

50:25

water inside the planet and water

50:27

can actually be used to lubricate the

50:30

plates as they move around. So we

50:32

think plate tectonics actually relies on having

50:34

these volatile materials like water inside the

50:36

planet. So it's possible that Venus doesn't

50:38

have plate tectonics because it got rid of

50:40

its water so quickly through the runaway greenhouse

50:43

effect. So this convection-conduction

50:45

distinction is interesting. I want to make sure

50:47

the audience gets it. So you're saying that

50:49

if I have a hot

50:52

end of an object and a cold end, but

50:54

it's a very smooth gradient,

50:56

it's not that much hotter on one

50:58

end, that much cooler, then you can

51:01

just transfer that heat by conduction. But

51:03

if it's a dramatic thing, there's going

51:05

to be swirls and convection. Absolutely.

51:07

Yeah, that's a great way to think about it. And

51:10

I always like to go back to the pot on the stove,

51:12

right? You got your porridge on the stove or something like that.

51:15

If your burner's not on high enough, the

51:18

temperature difference is not so big.

51:21

So you don't have to transfer a bunch of

51:23

heat through it. You can manage it through conduction. But

51:25

as soon as you make that temperature high enough at

51:27

the bottom, then the heat transfers much higher and you

51:29

get the bubbling and the moving around the stuff. So

51:33

I'm going to guess that since Mars

51:35

is smaller and further away from the

51:37

sun and has less atmosphere, it

51:40

does not have a liquid

51:42

core. Help me out. Tell me

51:44

I'm right. So seismology actually

51:46

told us that Mars does have a liquid

51:48

core, but again, it's not, it's

51:51

again the issue of the motions, right? So

51:53

again, it's the lack of plate tectonics on

51:55

Mars that doesn't

51:57

allow it to transfer whatever heat it has coming

51:59

out. out, but it's also very

52:01

true that it's cooler nowadays. So even

52:03

if it had plate tectonics, it's unclear

52:06

if there would still be enough heat transfer to allow

52:08

convective motions in the core. Okay,

52:10

so liquid but no convection as far as we

52:12

know. Exactly. All right, good.

52:14

And then there's like this radical change. I

52:17

was, when I was a kid, I always

52:19

liked theoretical physics, but in my family's

52:21

world, that was just astronomy. So they

52:24

would give me these astronomy books. And

52:26

so I was a huge believer

52:28

that there used to be a planet in

52:30

between Mars and Jupiter that got destroyed or

52:32

something, but that's not right. We

52:35

just go out there and we have the

52:37

gas giants. And what you told me earlier

52:39

was that there's more heterogeneity

52:41

there than we originally thought. Yeah,

52:44

absolutely. So it's interesting to think about

52:46

sort of our textbook model

52:49

of what happens inside say Jupiter, right? Picture

52:51

Jupiter as being this mostly

52:54

hydrogen gas ball and

52:56

probably has some sort of rocky core at the center.

52:59

If you think about how planets form, people usually

53:01

talk about that rocky core being about 10 earth

53:03

masses in size. That's when it

53:06

got big enough that it could attract all the gas in

53:08

the early solar system to become this giant gas planet. But

53:11

then the Juno mission got to

53:13

Jupiter and through very careful gravity

53:16

measurements inside the planet was

53:19

able to determine that it's not just this

53:21

like center of rock and then

53:23

this fluffy atmosphere around it.

53:26

Instead a lot of it is much more

53:28

mixed inside. So there's almost like this gradient,

53:30

this decreasing amount

53:33

of rock as you go further and further out

53:35

of the planet. So we talk about this now

53:37

as Jupiter having this fuzzy core. It's not just

53:39

this like sharp boundary between the rock layer and the

53:41

gas layer. Instead it's much more mixed

53:43

and we're trying to understand how that's possible and what

53:45

it means for the formation. Do

53:48

you know how to quite visualize that? Are

53:50

there like little pebbles floating in the thick

53:52

atmosphere? So this is one

53:54

of the hardest things to think about because

53:57

we have to take materials that we're used to how they

53:59

behave on. surface of

54:01

Earth and think about what happens to them

54:03

when there are millions of degrees in temperature and

54:05

millions of atmospheric pressures under that

54:07

type of pressure, right? And it's just completely different.

54:10

These things are usually, so the hydrogen and the

54:12

rock is probably mixed. It's probably like a solution

54:14

of some sort. It's

54:16

just completely different way that materials behave

54:18

under really high pressure and temperature. Well,

54:21

when you say rock, do you mean

54:23

solid or are you talking about the

54:25

constituents? Yeah, I think I'm pretty much

54:27

talking about the constituents more. Are you talking about higher

54:30

density elements, higher mass elements like

54:32

magnesium, silicate, probably some iron too.

54:34

Basically anything that's not gas, not

54:37

hydrogen and helium. Let me put

54:39

it that way. Anything that's

54:41

not hydrogen and helium for the center

54:43

of giant planets, we probably talked about as

54:45

rock. Okay. So the fuzzy

54:47

core, what kind of phase is it in? Well,

54:51

that's an interesting question. We're used

54:53

on the surface of the Earth just thinking about liquid

54:55

solids and gases, but when you go deeper inside planets,

54:57

it's probably accurate to call it a fluid. It's

55:00

not really a liquid. It's not a

55:02

gas. It's not a plasma. It's

55:04

in that weird phase space where the properties

55:06

of the material can behave very differently. Have

55:09

we sent probes just diving into Jupiter to

55:11

see how long they last? We

55:14

have. So we sent one probe into

55:16

Jupiter with the Galileo mission. I

55:20

can't remember how far deep it went, but very

55:22

much just the outer part of the atmosphere, right?

55:25

It's like it's hard to dig inside the Earth. It's

55:27

hard to go under high pressures inside

55:29

gas giant planets as well. But we

55:31

actually, it was a really interesting probe

55:34

because the goal, one of the main goals of it

55:36

was to measure the amount of water in the atmosphere of

55:38

Jupiter because water on Earth, for example, is

55:41

so important to determine what happens in our

55:43

atmosphere in terms of storms and things like

55:45

that. And it just

55:47

so happened to descend in Jupiter in like

55:50

the driest spot in Jupiter's atmosphere. So we

55:52

measured like no water whatsoever. Too bad.

55:55

So, you know, things happen. But

55:57

yeah, so there was also, this is a That's

56:00

why probes can be so important though because they can give us kind

56:03

of like real in

56:05

situ data from a particular region.

56:07

But you generally want a lot of them or

56:10

more than just one spot so that you can get

56:12

some sort of more general understanding of the planet. Is

56:14

there any prospect for a probe that will sort

56:16

of dive in but then come back out? Not

56:20

for the giant planets, no. Maybe the

56:23

closest analogy to that, it's not a probe,

56:26

but it also happens to be my favorite mission to think about

56:28

in the future is the Dragonfly

56:30

mission that's planned to go to the

56:32

moon Titan. So Saturn has this moon Titan, very

56:35

cool moon. And

56:37

one of the amazing things about the moon is

56:39

it has an atmosphere very similar to Earth's in

56:42

the sense that it's mostly nitrogen and

56:44

the pressure at the surface is about 1.5 bars. So

56:47

1.5 the pressure of Earth's atmosphere. Okay.

56:50

In that sense Titan's atmosphere is very much

56:52

like Earth. It's much colder planet. But

56:57

the other cool thing about the fact that it's

56:59

a moon, it's small, its gravity is really

57:01

low. Yeah. So dense

57:03

atmosphere and low gravity means it's really easy to

57:06

fly. So we

57:08

are sending an octocopter, something

57:10

with basically eight helicopter blades

57:13

that is going to land on the surface

57:15

of Titan, do a bunch of science at

57:17

a particular location, then fly

57:19

up again, look for somewhere new to

57:21

land, go travel to that spot,

57:24

land again, do a bunch of more science

57:26

and do a bunch of the kind of

57:28

traverses across the surface of Titan.

57:30

So it's the first mission where we'll have

57:34

more than, we'll have in situ

57:36

information at more than one location

57:39

over a large distance, right? We aren't talking

57:41

about rover, small rover distances like on Mars.

57:43

We're talking about hundreds of kilometers of travel

57:45

on the surface. Because the atmosphere

57:47

is thicker than Mars, so it's easier to fly.

57:50

Exactly. Yeah. When is

57:52

it going to actually? Basically, you could put cardboard on your

57:54

arms and flap them and you could fly on Titan. There's

57:57

probably other obstacles to doing that, but yes, that sounds like

58:00

a good idea. That's very evocative. So when

58:02

is this schedule to occur? So

58:04

good question. So the mission is in

58:06

development right now probably launching

58:09

sometime in the next decade That

58:12

it takes let me patient some amount

58:14

of years to get there. Yes, okay So,

58:16

you know, I would be thinking late 2030s

58:18

by the time we're this will kind of set

58:21

us back data That will be really cool And

58:23

even though we've had a pretty good track record

58:25

of late with things like this It's always possible

58:27

that thing just fails, right? I

58:30

mean did it I'm scared to say yes,

58:32

of course, it's always possible that something could

58:35

fail But the scientists who are working on

58:37

this it's always

58:39

amazing to me how many of the missions that we

58:41

send out the planet succeed the way they do because

58:43

there's So much that could go

58:45

wrong, but there's so much work done to Really

58:48

ensure that nothing goes wrong, right? So it's

58:50

so it's it's quite amazing to

58:53

me the feet of the engineering and science that goes

58:55

Into every single mission we send up. So

58:57

back to back to Jupiter I know

58:59

that there's metallic hydrogen

59:02

taking up a lot of Jupiter

59:04

and liquid metallic hydrogen And so

59:07

is that like a little fun part of

59:09

the inner structure is that most of Jupiter?

59:12

That is most of Jupiter So again,

59:14

this is great this kind of a great

59:16

example of hydrogen what we think of as

59:18

hydrogen on earth This is gas you might

59:20

expect put in under enough pressure.

59:22

It becomes a metal So it's actually a

59:25

really good electrical conductor and this happens at

59:27

about let's say About six

59:29

or seven thousand kilometers deep. So about ten percent you

59:31

go ten percent into the planet and

59:33

pooch you get this phase transition You're

59:36

in metallic hydrogen now metallic

59:38

hydrogen is great electrical conductor. That's

59:40

what's generating Jupiter's magnetic field So

59:42

rather than a liquid iron core in Jupiter

59:44

giant man, so you've got this giant metallic

59:47

Region inside Jupiter generating its immense magnetic field

59:49

that we see So

59:52

good. That's a success story for the

59:54

theory and experiment Combining with

59:56

it all fits together and the

59:59

other thing that Maybe this is not fair because

1:00:01

you're an interior of the

1:00:03

planet person, but I'm always amazed

1:00:05

at how colorful and stripey Jupiter

1:00:08

is. Why hasn't it all just

1:00:10

mixed together by now? Yeah, this

1:00:12

is a great question. First of all, I

1:00:14

love the fact that the color ... Hydrogen's

1:00:16

a clear gas, so if Jupiter

1:00:18

were pure hydrogen, we wouldn't see

1:00:20

any color at all. Yes, exactly. All

1:00:23

the colors we see are from tiny

1:00:25

bits of pollution, I would say, in

1:00:27

the atmosphere of these things,

1:00:29

things like ammonia and sulfur and stuff like this that

1:00:31

are floating around that we can see. The

1:00:35

stripey-ness is really great because

1:00:37

it shows us an important concept that's hard

1:00:40

for us to kind of put

1:00:42

our minds around. That's the fact

1:00:45

that rotation is really good at

1:00:47

separating regions inside a fluid. If

1:00:51

you spin a fluid, it's

1:00:53

really hard to get it to mix on the

1:00:55

inside. This is a result of the forces that

1:00:57

occur, the Coriolis forces, and how they affect fluids.

1:01:00

The fact that we have these bands, these

1:01:02

stripey bands, is almost a direct

1:01:05

result of the fact that we have spinning

1:01:07

fluids and they don't mix when they're spinning that

1:01:09

fast. Just so

1:01:11

people know, Jupiter's spinning really fast. Jupiter's

1:01:14

spinning really fast. Day on Jupiter is what? It's like

1:01:16

10 hours or something like that? It's much bigger

1:01:18

than the Earth, so that's very fast

1:01:20

indeed. Yeah. Yeah,

1:01:22

Jupiter was always my favorite planet. I would like to

1:01:24

go visit Jupiter someday. Then there's

1:01:27

Saturn, which is comfortable

1:01:29

in some ways, but very different in others. It

1:01:31

doesn't have quite the colorful stripey bands that Jupiter

1:01:33

does. Yeah. Saturn's

1:01:36

interesting because although it doesn't have as

1:01:38

many observable bands, it does have these

1:01:40

... We saw with the Cassini mission,

1:01:42

it has these amazing storms at the

1:01:44

poles, so this hexagonal feature. I

1:01:47

don't know if you've seen this as hexagonal storm

1:01:49

at the poles. Right? There's great

1:01:52

fluid waves that are occurring to cause that

1:01:54

pattern. The winds on Jupiter are actually

1:01:56

very fast. It's just that they're not as stripey.

1:01:58

There's not as many of them. bands that go

1:02:00

around the planet. You said

1:02:03

Jupiter, but Saturn again, giant

1:02:06

planet, a little bit smaller than Jupiter still

1:02:08

has metallic hydrogen on the inside generating a

1:02:10

dynamo and a magnetic field. Um,

1:02:13

the rings on the outside, the amazing thing about Saturn

1:02:15

to me is that you've got these gorgeous

1:02:17

rings and we can, you know, you can see them in a telescope,

1:02:20

but there are waves in those rings that

1:02:22

are actually caused by motions

1:02:25

inside Saturn itself. So we can

1:02:27

use the rings as a probe

1:02:29

of motions going on inside Saturn. And what

1:02:31

do we learn from those waves?

1:02:33

So yeah, what we've learned from that

1:02:36

is that the innermost part of Saturn

1:02:38

is actually what we call stably stratified.

1:02:40

It doesn't have this convective motioning happening

1:02:43

in the deepest part of Saturn

1:02:45

because there are these gravity waves, this kind of like

1:02:47

what you would like when you see like the surface of

1:02:50

like the ocean or whatever, kind of move

1:02:52

up and down there are these gravity waves, but

1:02:54

it's not circulating like convecting motions are. So

1:02:57

that's one thing we learned from the rings. I know that

1:02:59

there's an attempt in some circles to

1:03:02

police the language of gravity waves

1:03:04

versus gravitational waves. Gravitational waves we

1:03:06

detect from black holes in spiraling,

1:03:09

but gravity waves are motions in

1:03:11

planetary interiors. Yeah, I can tell

1:03:13

you that as someone who kind of grew

1:03:16

up kind of doing physics and astronomy and

1:03:18

planetary science, that was very confusing. Very,

1:03:22

very different things. But otherwise Saturn

1:03:24

and Jupiter kind of related to

1:03:26

each other qualitatively. Yeah, qualitatively Saturn

1:03:28

is a bit smaller. So the pressures are a

1:03:30

bit lower inside the temperature a bit lower, but

1:03:33

along the same the physics is the same. But

1:03:35

then Uranus and Neptune are actually kind of different.

1:03:39

Yeah, Uranus and Neptune seem to be completely different

1:03:41

beasts. So they seem to be what would

1:03:44

have happened if you had while you were building

1:03:46

Jupiter and Saturn, but you didn't get big enough

1:03:48

fast enough to attract a bunch of gas. So

1:03:50

instead, you've got these rocky icy

1:03:52

balls left with a little bit of gas on

1:03:55

top. We think they're

1:03:57

mostly stuff like water. Although

1:04:00

it's really hard to actually figure out

1:04:02

what goes on deep inside these these

1:04:04

planetary bodies Their magnetic fields are completely

1:04:06

different than any of the other planets in the

1:04:08

solar system So rather than having this nice dipolar

1:04:10

field like we have on earth with like a

1:04:12

north pole and a south pole They're

1:04:15

multipolar. They have a bunch of north and south

1:04:17

poles all over the place Um,

1:04:19

so we spend a lot of time trying to understand why that

1:04:21

is but also it's

1:04:23

a great kind of Um Test

1:04:26

bed for what happens to water when it's under really

1:04:28

high pressure and temperature and you get

1:04:31

really cool New phases of water

1:04:33

you get something called Succorionic water where the

1:04:35

oxygen atoms become a lattice and the hydrogen

1:04:37

atoms just flow through The oxygen

1:04:39

just really weird stuff happens at high pressure

1:04:42

and temperature when you have water And

1:04:44

is this from data or from theory?

1:04:47

So it started to be from theory but

1:04:49

very recently in the past say five years.

1:04:51

We actually now have experiments Uh

1:04:54

that can take materials at some of our

1:04:56

biggest Uh particle

1:04:59

colliders and you basically hit a material

1:05:01

with a big shock wave And

1:05:03

boom you put it under really high pressure temperature

1:05:06

and we've actually created suprionic

1:05:08

water in labs here on

1:05:10

earth now and I know

1:05:12

that I forget whether it's purely

1:05:14

hypothetical for axial planets or even for

1:05:18

Our outer planets but people love

1:05:20

the idea of diamonds in the planets,

1:05:22

right? Either, you know diamond

1:05:24

rain or icebergs or something like that.

1:05:27

Is that a urinous and neptune phenomenon?

1:05:29

So that could be a urinous and neptune phenomena.

1:05:32

So here's the thing in addition to water. There

1:05:34

are things like methane ch4

1:05:36

right so carbon-based elements out there

1:05:38

and so you start asking what

1:05:40

happens to The h4 when

1:05:42

you put it under high pressure and we

1:05:44

know about the diamond um

1:05:46

phase of carbon even here on earth you

1:05:48

put carbon under enough pressure you're going to get diamond

1:05:51

Uh, and so that's likely to happen inside

1:05:53

neptune and urinous as well The

1:05:56

question is does anything weird kind of happen

1:05:59

and it turns out that But if you go, if

1:06:01

we understand the exact mixture

1:06:03

of like, say, water, ammonia,

1:06:06

methane inside the giant planets like

1:06:08

Uranus and Neptune, the carbon

1:06:10

could actually separate out from the other materials

1:06:13

and it's heavier so it could sink. And

1:06:15

so there's a theory out there that you would actually

1:06:18

have a diamond sea kind of

1:06:20

in the deep interior of Uranus and

1:06:22

Neptune. And an interesting thing about diamond

1:06:25

is at the melting point, it has the same

1:06:27

property that water does at the surface of the

1:06:29

Earth where the solid phase is a little bit

1:06:31

less dense than the liquid

1:06:33

phase. And so you could have diamond

1:06:36

icebergs on the diamond sea that float

1:06:38

on it just like water icebergs or

1:06:40

float on our oceans.

1:06:42

So some interesting things to think about what

1:06:44

might be happening in Uranus and Neptune and also

1:06:47

on exoplanet. Is that right

1:06:49

below a sort of gaseous layer? So

1:06:52

that might be below a gap layer, but

1:06:54

also below like a water layer, a mixture

1:06:56

of things like water, ammonia, methane, but just

1:06:58

certain pressures and temperatures where suddenly stuff separates

1:07:00

out. So you're talking about, I mean, we

1:07:02

don't know exactly the depth of this,

1:07:04

but let's say roughly think about roughly halfway through

1:07:07

the planet or so. This is

1:07:09

going to wreak havoc with the world's diamond

1:07:11

markets once we actually start excavating these icebergs.

1:07:14

You'd think so, but let's be honest here,

1:07:16

we can actually make diamonds in labs nowadays.

1:07:18

The only reason diamonds are expensive is because

1:07:21

people are trying to stop

1:07:23

them from being made in labs and to go make

1:07:25

them something that's hard to get. That's

1:07:28

right. No reason to worry Uranus and Neptune to get

1:07:30

diamonds. We could just make them in lab and sell

1:07:32

them for a buck each day. Again, I don't think

1:07:34

you're maximizing your grant money potential

1:07:36

here by telling the truth

1:07:38

about diamonds. So well,

1:07:41

I mean, then I guess

1:07:43

we should give a shout out to

1:07:45

all the little tiny things in the

1:07:48

solar system, whether it's dwarf planets like

1:07:50

Pluto or asteroids or hyperbell objects, et

1:07:52

cetera, et cetera. There's an enormous array

1:07:54

of different kinds of ways that matter

1:07:56

comes together in our solar system. And

1:08:00

I love the small bodies in the solar system.

1:08:02

I absolutely love because they're basically the

1:08:04

ingredients that created the planet. And

1:08:06

so imagine, you know, imagine you make a bunch

1:08:09

of cookies, for example, and you show up at someone or

1:08:11

someone else did and you show up at their house and

1:08:13

they're like, here are a bunch of cookies,

1:08:15

eat some and you'd like to know what they're made

1:08:17

of because maybe you have an allergy. But

1:08:20

they don't for some reason, they don't tell you right. One way

1:08:22

you could figure out what they're made of is by looking at

1:08:24

kind of the remnants of stuff left on the counter where they

1:08:26

were just made. So you might see some sugar floating

1:08:28

around somewhere, some some water stuck

1:08:30

on the counter or whatever. Right. And that's

1:08:32

exactly what the asteroids and comets and Kuiper Belt

1:08:35

objects are. There are leftover ingredients of planet

1:08:37

formation. And so we can really learn a lot

1:08:39

about how Earth and the other planets formed

1:08:42

by studying these leftovers. And

1:08:44

comets, I presume also, which have a lot of

1:08:46

volatiles and is

1:08:48

it true that comet collisions contributed to

1:08:50

a lot of the atmospheres, the interplanets?

1:08:53

So that's a good question. We don't, the short answer is

1:08:55

we don't know. We do know that

1:08:57

comets have a lot of water and that comets

1:09:00

end up on these weird orbits sometimes that could bring

1:09:02

them to the inner solar system so they can deliver

1:09:05

water to planets. But when we

1:09:07

look at something called the

1:09:09

D to H ratio of comets, so how

1:09:11

much of the two isotopes

1:09:13

of hydrogen and H2O in water, the

1:09:17

deuterium isotope, the heavier isotope, there's

1:09:21

like a particular signature that our oceans have

1:09:23

of this D to H ratio that

1:09:25

tells you something about where our water is

1:09:27

from. And it

1:09:29

doesn't really match exactly what comet, the ones

1:09:31

with comets we've gone to when we look

1:09:33

at their ratios weren't the same. So

1:09:35

it might be a mixture. It might be a

1:09:38

little bit of comets. It might be a little

1:09:40

bit of some asteroids that we know also have

1:09:42

some water. And it might be that a lot

1:09:44

of our oceans and stuff just actually came from

1:09:47

water that was able to be stored inside of

1:09:49

Earth that essentially got outgassed from volcanoes, for example.

1:09:52

So there's still lots of learning to be done when it

1:09:54

comes to the solar system. And I

1:09:56

guess we haven't even had a chance to

1:09:58

talk about the other hundred

1:10:00

billion planetary systems or whatever in

1:10:03

our galaxy. But what

1:10:05

is the current vibe

1:10:08

amongst people who think about

1:10:11

exoplanets? On the one hand, very exciting. We've

1:10:13

found all these planets. We're going to find

1:10:15

a whole bunch more. On the other hand,

1:10:17

we've kind of been humbled at how different

1:10:21

our predictions were than what we've actually seen.

1:10:23

So where are we kind of landing right

1:10:25

now? Yeah, I 100% agree

1:10:27

with you. And I think what this is is

1:10:29

a major opportunity. Because now it used to be

1:10:31

the case that if you wanted to come up

1:10:33

with a theory for something on Earth, you'd

1:10:36

say, okay, how can I test this theory? Well, we

1:10:38

can't build another Earth and test it, see if it

1:10:40

happened there. So we'd have to look at the other

1:10:42

planets in our solar system. And if we had a

1:10:44

good explanation about, you know, if

1:10:46

Earth has X, then Y must happen. It better

1:10:48

also explain why some other planet that

1:10:50

has X also had Y happen. But

1:10:53

you only have eight other planets and some

1:10:55

or seven other planets and some other small

1:10:58

bodies and stuff. But now we've got these

1:11:00

thousands of exoplanets out there. And it's just

1:11:02

an incredible test bed for all the theories

1:11:04

that we use to develop for our

1:11:07

own solar system in our own Earth form. So I

1:11:09

think it's an immense opportunity. And it means that we

1:11:11

have a lot of learning to do about

1:11:13

what's possible. Well, your research career

1:11:15

spans this era where we found all

1:11:17

these planets. I mean, what is the single most surprising thing

1:11:19

to you that we've learned so far? Oh,

1:11:22

gosh. Single

1:11:25

most surprising thing. I don't

1:11:28

know if anything is, I think the hot,

1:11:30

find that first exoplanet

1:11:33

orbiting sort of a

1:11:36

live star, this giant

1:11:38

hot Jupiter, so close to

1:11:41

the paradise. That was a major surprise, right? I

1:11:43

think for everyone, I'm not just me, right? But

1:11:47

and the fact that it showed us how much planetary

1:11:49

migration really matters, the planets can move around. I

1:11:51

think that's still the most surprising. And

1:11:53

we always get down to the end of the

1:11:55

podcast where we let our hair down and have

1:11:57

fun. So life these

1:12:00

other planets, what are your prospects? So

1:12:04

someone the other day asked me if I had to bet what

1:12:06

planet or object will we find life on

1:12:08

next and I went with Titan. Titan. So

1:12:10

this moon of Saturn as the place we're

1:12:12

most likely to find life if it's out

1:12:15

there. So the key thing here is you

1:12:17

think about as far as what we know

1:12:19

about how life formed on Earth, where

1:12:21

are the ingredients and the conditions right for it

1:12:23

to happen? So you're looking for a

1:12:25

place, turns out that liquid water seems to be important,

1:12:28

having an energy source, but the life seems

1:12:30

to be important, having complex

1:12:32

molecules around that can use that

1:12:34

energy source along with the catalyst

1:12:37

environment of liquid water to build

1:12:40

larger and larger molecules. And

1:12:42

the place where that all seems to be there

1:12:45

is on Titan. So you've got water,

1:12:47

this water ocean underneath the surface of

1:12:50

Titan, you've got a

1:12:52

surface that's basically formed out of hydrocarbon.

1:12:54

So it's a bunch of organics on the

1:12:56

surface. And you have

1:12:58

energy sources from tidal interactions and so

1:13:01

forth on the interior. So I'm

1:13:03

guessing we find it at Titan. Any

1:13:06

chance for life on the diamond ocean

1:13:08

of Neptune? If

1:13:10

so, we're talking about life that can live at much

1:13:14

higher temperatures and pressures than anything we have found

1:13:16

on the Earth. So it's probably something that we wouldn't

1:13:18

fully understand, but it would be cool if it

1:13:20

was there. That's what makes it exciting. Looking forward

1:13:22

to what happens next. So Sabina Stanley, thanks so

1:13:24

much for being on the Winescape podcast. Thanks

1:13:27

for having me here.