Welcome to the third annual upon your first lecture in Planetary Geology.
The name of the lecture series is in recognition of the generous support the
Department has received from one of its alumni on the Keith OLBERMANN costume.
So as the lap on offer a staff ski associate professor of planetary geology, it falls on me to introduce our speaker, Professor Raymond here.
Burke was recently arrived here in Oxford last July to become the Hailey Professor of Physics.
So he is of course, familiar to many of you in the room now.
His research involves developing conceptual and analytical models, addressing big picture questions in climate,
incorporating physical as well as chemical and biological processes.
And he's grown from the presence climate to such earlier or exotic environments as the snowball earth, the early Earth and early Mars.
So he has been involved also in assessing the socio political aspects of climate change, and he was one of the lead authors in the third IPCC report.
So his background includes an undergraduate degree in physics from Harvard,
a fellowship at applied maths and theoretical physics at Cambridge, a Ph.D. in aeronautics and Astronautics from MIT.
Faculty positions in meteorology at MIT, in Geosciences at Princeton, in the University of Chicago,
and most recently, a sabbatical as the Carl the King Carl the 16th Gustaf professor at Stockholm University.
He is a fellow of the AGU, is a fellow of the American Academy of Arts and Sciences, and he is a chevalier of the academic.
My apologies to French speaker and an avid accordionist.
His interest presently take him beyond the present earth system to the early Earth, to other planets, and to exoplanets.
And while he professes to have so far limited himself to spiritual objects, he plans on exploring other shaped bodies in the near future.
And we look forward to hearing what that means.
So for the moment, he is going to tell us about the origin and evolution of exoplanet atmospheres and oceans.
Okay. Thank you. Thank you very much. It's very nice to be here. The reference to non spherical objects is basically asteroids.
I didn't think that there could be anything so interesting about objects without an atmosphere,
but there's as since I've come here, the asteroids have started to look more and more interesting.
But in this talk, it will be all or all spherical objects.
I provided my talk with a kind of subtitle or re title, The Geology of Planetary Atmospheres.
This this is a lecture series on planetary geology.
And as I've gotten more and more involved in exoplanets, I realised that that the the boundaries between geology,
between processes that we think of as involving rocks and processes that we think of as involving ices or clouds or atmospheric substances,
those those boundaries have gotten more and more blurred.
And various questions that we're beginning to treat in atmospheric sciences and planetary atmospheres start to look more and more
geological and need to draw more and more on bodies of knowledge that are traditionally held in in geology and earth science departments.
I want to I want to acknowledge my collaborators funding whose might me get that cursor out of the way?
My collaborator funding, who is one of, is my graduate student,
funded by the NASA's Astrobiology Institute and my recent post-doc, Robin Wordsworth, who's now on the faculty of Harvard.
But who both of them collaborate,
contributed a great deal to many of the papers and results that I'm going to be talking about today and the funding for a lot of this work earlier.
Part of this work came from the Virtual Planetary Laboratory, one of NASA's Astrobiology Institutes.
So this is the canonical scatterplot.
Just to give you an idea of the greatly expanding territory we have for planetary geology, we used to have just the solar system to think about,
but now we have a vast number of new objects about which we're learning more and more,
which knowing the existence of these objects is quite interesting.
Even this graph is a little bit outdated. Is this goes back to 2011, there quite a lot more dots in here.
But just to situate you generally in terms of the range of different temperatures and compositions, we may be dealing with this this axis.
This axis is the is the amount of energy received from the planets star normalised to how much energy the earth receives from the sun at one.
So anything along this line one would have an earth like temperature if it had an earth like atmosphere.
And this is on a long plus that we're going out two factors of 10,000 here and 1/1000 and 1/1000 over here then.
So this is think of this is roughly temperature. This is this axis here is the radius of the planet in units of Earth Radio, either five Earth radii.
And that provides that provides a window into that provides a window into the composition because things that are big things
that are sufficiently big are or are certain to be hydrogen dominated because hydrogen is the only thing abundant hydrogen,
helium things that are abundant enough to make something that big.
As you get into these smaller things, you have the possibility for a for more rocky compositions.
And so now we found that some of these smallish things actually have low densities density similar to what water would be if you squeezed it.
So there are a lot of surprises all along, all along the way.
But I want to focus a lot of the possibilities for planetary geology come from the fact that we now know a lot of planets, atmospheric geology,
that we now know a lot of planets in the range where we need to think about the
composition of atmospheres that are that are not necessarily hydrogen dominated.
But the other other cases where we have to think about planetary geology or atmospheric geology is how do these very hot things,
even gas giants, these planets are hydrogen dominated,
but they're so hot that the cloud forming materials are things like institute that we that we
would think of or even iron vapour that we would normally think of as as minerals on on earth.
And each of these dots is one, one planet candidate from the from the Kepler planetary detection detection mission.
And so you can see we're actually edging into seeing more and more of these earth sized things.
And so and in some cases we can even characterise some things about the atmosphere we can characterise
in many cases the density of these objects which gives us another window on the composition.
And so, so the, the, the things get especially interesting for, think for planets that are, are more that are smaller and potentially rocky.
So these are some of.
And there's a particular interest in planets that are in the so-called habitable zone, which should be called the potentially habitable zone.
These are planets that are that are that are at a distance from their star, where given the right kind of atmosphere,
they could have temperatures that would support liquid water at the surface of the planet.
And the habitable planets, the possibly habitable planets,
really are generally thought of as being the smaller ones which could happen, which have the potential to have a rocky surface.
Jupiter, a Jupiter, a gas giant could have habitable temperatures.
But that but biologists tend to think that you need some kind of substrate for life to evolve on at least initially.
And so one doesn't think of gas giants, even with the right temperature as having as having a habitable layer.
So these are all the small planets in what is generally called the habitable zone that have been found just by the Kepler mission alone.
They're quite a lot of them there. These are the ones around stars.
These are those are main sequence stars that have a surface temperature like the sun, a mass similar to the sun.
There's only one of those. And then K stars are slightly smaller, cooler stars.
And then M stars are the most common form of star in the universe.
And they provide terrific, terrific real estate for potentially habitable, habitable planets.
There are relatively few m star and star planetary detections in Kepler receptor wasn't really configured to to look at M stars.
It was worth configured to look at G stars, which are more which are like our sun.
But these are very interesting planets. But so but although we call these, does that potentially have the potentially habitable zone?
None of these planets would have a climate anything like Earth if they didn't have have the right kind of atmosphere.
That doesn't have to be in it has the same kind of atmosphere as Earth.
But there are some kinds of atmospheres that could give you conventional liquid water, habitable conditions and others which which which don't.
And right now, we have very little information about whether any of these planets, except for Earth, has an atmosphere.
And and that's the big breakthrough that's going to come in the near future.
But even though we have very little data yet about how much about what kind of atmosphere,
if any, these these habitable zone, these small habitable zone planets have,
we can start thinking about we can start thinking about what kind of atmosphere they might have
based on what we know about how atmospheres actually are made and in some ways in atmosphere.
So why I call this one of the reasons I call this subject atmosphere.
The geology of planetary atmospheres is that atmospheres are not something that a planet is endowed with at the beginning.
It doesn't just come in and sit there. And monsters are actually an outgrowth of of what's happening in the interior of a planet.
Atmospheres are very dynamic entities that that involve which sit between to search to sources and sinks.
And so at the and so at the at the ground or and the interior, we have we have things that are outcast from the interior of the planet.
So everything is so the amount of what's out guest is connected with subjects like volcanism, mantle dynamics and so forth.
The chemistry of, of the, of the mantle and also the early planetary formation system,
which determines which elements are segregated into a core and which ones are in a dynamic layer like a convecting mantle,
which can in principle exchange or out gas into the interior.
And you get things like water vapour, nitrogen, hydrogen, methane, carbon dioxide, outgassing from from the interior of a planet.
You some of those things and the right conditions will condense into an ocean or dissolve in an ocean on the earth and perhaps other planets.
You have thin continental crust, which can be subducted. And so the chemical reactions,
notably the formation of carbonates that happens through silicate rock reactions and the continental
crust can be subducted where the where some of those some of those volatiles are recycled.
So you have all these interchanges with the surface, many of which are are affected strongly by life.
If the planet has life. Then you've got atmospheric dynamics which determines things like how much
water or other contemptible substances makes it up into the upper atmosphere.
The vertical structure of temperature in the atmosphere, providing cold trap determines.
And we'll see more of this later in the talk. Determines to a large extent whether any water vapour in our case makes it into the upper
parts of the atmosphere where ultraviolet from the star can break it up into hydrogen.
And an oxygen where after the hydrogen escapes to space, obviously can get oxygen,
gets left behind and reacts with iron and sulphur and the crust can get bound up or then later eventually
accumulate in the atmosphere and then later help by photosynthesis build up an oxygen atmosphere.
So you've got all sorts of exchanges at this end where the atmosphere and the ocean interact
with the crust in the interior and then you've got loss of substances to space on.
For earth like conditions, it's really only hydrogen that can be lost very rapidly and with a good flux.
Other things do trickle out.
But on hotter planets or planets that are bombarded with more ultraviolet or more stellar winds, even carbon can be lost from that.
From the top. The planet's evolving by losing volatiles from the top, it's exchanging with the crust.
What you sequester in the crust determines a lot about what is shielded,
what can be shielded from loss up here, what can be generated if you lose atmosphere.
But you also have volatiles coming in from space, comets, whatever, bring in volatiles.
So it's a to a change up there as well. And and and so in between where we live is, is, is this atmosphere.
But to some extent, if you know something about what goes out, what can go out here,
if you know enough about what the planet is made out of, you should be able to reconstruct what kinds of things are in the atmosphere.
And so in the game with exoplanets right now,
we're largely we're largely at the stage not having enough data to directly observe exoplanet atmospheres for small planets.
In most cases, with the exception of one that I will show you,
we are playing the game of of knowing something about what the composition of the system is
by looking at the spectrum of the star and trying to figure out what range of atmospheres
a planet could build given its temperature because we know its orbit and from that we can
infer its temperature and try to find a self-consistent atmosphere and temperature model.
So this is the universe, as it appeared to astrophysicists,
as is generally thought of by astrophysicists, the primarily the universe is hydrogen and helium.
The Big Bang did make a little bit of lithium and beryllium, which gets tense.
It tends to be rather quickly consumed in stars. But you can, not far wrong in just taking the universe is originally just hydrogen and helium.
And then there are all these other things. Everything else is called metals.
And so that's that's astrophysical geology as it was generally conceived in the term
metallicity in a system which characterises various planetary systems or stars,
refers to the ratio of these not of these things heavier than helium two to hydrogen, to hydrogen and helium.
It's not very geological, but it's the beginning of a kind of kind of geology.
But now that we have exoplanets in the astrophysical literature, you start to see this other substance, which is called rock.
And, and so then we get a little bit closer to, to to geology.
And so so there's there's increasing interest in what this substance is and in various
planetary systems and how it interacts with what kinds of atmospheres you might have.
Now in terms of of where this where these metals come from, the the metals,
as geologists think of them and and rock as geologists would think of them,
that that's all a matter of nucleosynthesis, which I'll get to in the next slide.
But, but just as an example of the sort of things that, you know,
elements that you use that you see or elements of substances that you assemble a planet with,
you've got things that we conventionally think of as metals,
like the like sodium and iron and nickel, etc., which tend to segregate into into a very dense core.
There are some other things that actually make that are metals like magnesium or calcium or iron that
make that make compounds with silicate and make up the bulk of what we normally consider rock silicate.
The earth is mostly a silicate silicate envelope around a metallic iron nickel core with various contaminants.
And these kinds of things here, like calcium carbonate and and silica and various silicates, make up what we can eventually consider to be rock.
But actually there are these other things that we that on earth we think of as atmospheric constituents into CO2, carbon monoxide, water, methane.
But if you're on if you're on Triton, which is a satellite of Pluto, that that has a temperature of maybe 80 Kelvin at the surface, nitrogen isn't.
Well, if you are Triton. What? There is no more volatile on Triton than than quartz is on earth.
So water is a rock on Triton and water is basically a rock on, on on Titan.
The satellite of Saturn is as as well. And so.
And on the other hand, if you get to some of these really hot exoplanets,
the hot Jupiters and the hot rocky planets, I'm going to show you many of these things.
For vapours that can form quite substantial rock vapour atmospheres and condense and form clouds which are actually observable,
observable even from from the earth by various spectroscopic techniques.
And so, so really as we expand our range of temperatures, we have to generalise your thinking about what we actually mean by a rock or mineral,
because in essence, the things that we call rock or rocks or minerals on earth there are nothing different from a form of ice.
And ice is a rock at a at happens to be at a certain temperature range that we tend to think make sense,
make us call it ice and and so so so the boundary between these things which
may be in the atmosphere or may be condensed in other cases and form oceans,
there's a lot of interest in carbon dioxide oceans. The boundary between these these and these things is not the basis of physical properties.
It's just a matter of what what temperature range you're thinking about now in terms of where these things come from.
It's all a matter of nucleosynthesis in stars.
In the beginning, you basically have hydrogen mainly and and the first generation stars don't have any of this stuff that makes planets interesting.
And it can can possibly get can make earth like planets or make us make any form of life.
You just have you basically have hydrogen being fused into helium and first generation stars sufficiently massive stars make everything,
loosely speaking, make everything up until iron all these fun things that we know and love.
Carbon, oxygen, nitrogen, silicon up to, up to, up to iron.
You don't actually need supernovas to make those things.
Some of those so heavy elements of those metals, as they're called in rock forming substances,
can just can just be blown off by gas, by a by a gas, just the wind, stellar winds, basically outgassing from stars, as it were.
So some gets out into the rest of the universe.
But a lot of it a lot of a lot of the this these heavier materials only easily get dispersed through through the rest of the galaxy,
through supernovas. A supernova is not only help to disperse all the other stuff that is made,
but they also create heavier elements like uranium and thorium, which exist in the earth in very small quantities,
but are part of the engine that makes plate tectonics go, that makes mantle convection go, drive volcanism and so forth, and then all those things.
So all those things get into some nebula that condenses into planets.
But but the galaxy is not well-mixed. And so the particular recipe, the particular combination of elements we have on Earth is not is not universal.
And so, so, so we have to actually learn how to think about how rocky planets might behave if you had different elements in the mix.
But this is just an example. This is an estimate. It's hard to actually the estimate what what solar composition really is.
But if you assume that what's in the outer part of the sun is typical of what's what the it originally in the in the in the solar
and the nebula this this is this is abundance relative abundance with silicon scale to ten to the sixth on a low on a long scale.
And so this is this is what's called solar composition or at least an estimate of solar composition.
You can see it, of course, you have hydrogen and helium are way up there.
But but carbon, nitrogen, oxygen, magnesium, silica, sulphur, they're all very abundant.
They're all abundant. And so so all these things that we used to make.
So there's a lot of oxygen around. We make silicates out of combinations of things of of calcium, magnesium, silicon and oxygen.
There's plenty of that around.
But then then we have these things that form many of the key atmospheric gases for Earth, like temperatures, carbon, nitrogen and oxygen.
And there's a competition when you're building a planet between where the oxygen goes,
most of our oxygen winds up and various silicates and you're that's points up in the cell bulk silicate bulk silicate earth.
But then there's then these things go through the hydrogen mostly winds up in the star and then the giant
planets and then whatever's left behind is what you make your what you can make a rocky planets out of,
plus some little contamination, some debris has of giant hydrogen dominated.
We could have a lot of silicates in there in their atmosphere,
but we have to learn how to think about how what different ways planets could be assembled if if we had systems which had more wear carbon.
So I'm going to add more carbon, for example, relative to oxygen.
Okay. Now I'll come back later to the difference to the difference between solar composition and
what we think made up the planetesimals that the rocky planets were put together out of.
So I'm going to tell three stories. I'm going to tell three, three different stories that in some sense can be considered atmospheric geology.
The first is a story where the atmosphere is geological,
where the atmosphere itself is is dominantly made of various kinds of rock vapours or things that we conventionally consider rocks on earth.
And largely based on one fascinating object 55 can create.
And then I'm going to talk spent quite a bit of time talking about nitrogen,
which in the Earth's atmosphere is represented dominantly by, by and too, but which is a but,
which undergoes a lot of which which has a lot of other chemical forms largely created by both forms of life on earth,
but which play a big role in, in the nitrogen cycling on the Earth and also in the in in the way life works all together.
And nitrogen is a very mysterious substance,
but it turns out to be way more important to habitability and making a planet that is conceivably earth like than than is often, often considered.
It's a very difficult substance to think about. And then I'm going to tell a very rich story of carbon.
Carbon is the most familiar story in the last couple of months.
So many interesting things came out about these two stories that I decided to truncate,
what I was going to say about carbon and just tell you basically one important thing where the one of the many ways that the carbon cycle,
that the deep carbon cycle comes into fundamentally into notions of planetary habitability.
So there's going to be a very a very abridged story.
But this is already a very, you know, a lot of carbon cycle stuff is will already be fairly familiar to many of you here.
And I just and so I will talk about the less familiar the less familiar story.
And of course, the carbon cycle story is one that's still being written by us as we massively perturb the carbon cycle on Earth.
We're really a force of more than geological proportions in terms of the way that the normal carbon cycle,
deep carbon cycle would work on a on a cell, a rocky planet with silicate carbonate.
Okay. So now it's going to. Magma oceans and rock vapour atmospheres.
So the planet that simulates that, that's kind of the poster child for for this subject of geology of atmospheres,
rock paper atmospheres is is 55 cancri.
Most of the new planets we work on don't have names, and there isn't even a real convention for naming, for actually giving these planets real name.
So they're named according to they're named according to what star they're around.
So 55 Cancri is a star in the constellation of cancer.
It's not quite visible to the naked eye, I don't think. And but it's right here.
You can find if you find Gemini and you find constellation of cancer, the crab right up here by this point is is the star.
And there are there are five planets known in this system,
which the one I'm going to talk about is is E and this is not anybody's idea of a habitable world,
but it's nonetheless a very something that illuminates our picture of planets quite well.
It's actually and to let you know a little bit more about this, the system 55 Cancri is actually a double star system.
It turns out that planets are very common around double stars.
And so 55 Cancri actually is a is a slightly cooler g type main sequence star,
a little bit cooler than the sun has a photosphere temperature of 50 about 5400 kelvin and it has a companion quite a long way away,
which is an m dwarf, an end sequence low mass star 55 B This is not actually close enough to significantly
perturb the orbits or or provide any illumination for for for these planets in here.
But so here's a star that is basically basic as a luminosity almost as much as the sun.
So it's a bright star and 55 Cancri e is orbiting so close that its orbital period is three quarters of an Earth Day.
So mercury is a hot place. It has an orbit of 80, 88 Earth Day.
So here's the thing that is is so is is sitting whizzing around once every three quarters or three quarters of an Earth Day.
And then so it's in a very it's in a very close orbit.
And in fact, the the the at the distance that's implied by its orbital period, we get the distance from Newton's law and the orbital period.
The, the, the, it's so close that it's the illumination, the amount, this flux of stellar energy going into going towards the planet is,
is more than 4000 times the more than 4000 times the watts per square metre that the earth gets from
the sun and and using the Steffen Boltzmann law sigma cheat of the fourth to solve for temperature.
Even if you had no interesting atmosphere, a greenhouse effect or anything like this.
The temperature in equilibrium with the sub stellar point it would be about 3000 kelvin so it's it's similar.
It's above the liquid. It's above the melting point of most kinds of rocks.
What's more, planets that are this close to their star have such strong tidal stresses that
unless we are completely wrong about all sorts of notions of tidal stress,
that they're almost certainly tied locked to the to the to the star, just like the moon is tied like to the earth.
So and it has a fairly circular orbit. So so there's one point on this planet, one geographical point where where it's noon all the time,
and there's a nightside which never gets any stellar energy.
And so that actually makes the sub stellar point especially hot because you're not losing if it were if you were in our atmosphere.
We'll talk about that issue soon. If there were no atmosphere, you don't lose any energy by bye bye.
Heat flowing away to the night side of the planet or cooler parts of the planet.
And and so so the picture of this planet is that you would have you would have a permanent magma ocean.
You have a permanent magma ocean on the on the day side.
And who knows what's on the nightside. But we now have some idea of what the temperature of the nightside is from very recent observations.
Just it just came out in the last two months. This in terms of the size and the likely composition, it's two earth radii.
So it's in a range of of radio sizes that are generally called super-Earths and it's masses eight Earth masses.
Actually, it's been upgraded somewhat. The density with with this mass and radius is a little bit light for a rocky planet.
It couldn't really be compatible with much of an iron core some of the initial thoughts on this planet or that it was actually.
The two, the density was too low to even be pure silicate and that it either had either was mostly silicate with some kind of a massive
get a gaseous envelope or that it was to bring down the mean density or it might have been pure carbon or carbon dominated,
which was cute because if you make a planet mean the end of carbon, the interior is one big diamond.
And so reporters like all of that recently, the masses,
the density has been upgraded a bit to where maybe this thing is not carbon dominated, but could be a silicate dominated thing.
But it's still a it's a fairly low density, a fairly low density density object.
And so so the question is, okay, if this has a magma ocean, permanent magma ocean on the dayside,
is the magma ocean just some isolated thing sitting there radiating to space?
Or is or will it generate an atmosphere of its own?
And this is where you have to go to someone who knows experimental petrology knows how
how to how to deal with equilibrium of vapours with with melts of geological substances.
Because in temperatures that we in moderate temperatures that we're used to in terms of oceans, like if you heat up water,
if you have water at water at 300 kelvin or so, the vapour that's an equilibrium with water at 300 Kelvin is is water vapour.
You get water vapour out if you have CO2 at at temperatures of around 300 Kelvin, the the we're actually a little bit less than 300 Kelvin.
Let's say the what you have out here is the CO2 atmosphere.
Of course, if you get if you put CO2 in a 2000 Kelvin atmosphere,
it starts to thermally dissociate and you'll get mostly carbon monoxide and oxygen and even water can thermally dissociate.
But this is even more so this business of of dissociation of the components,
the molecular components of the melt is even more significant when you're talking about these fairly complex silicate melts.
And so if and so if you were to take something that was representative of the
bulk silicate earth and you have various compounds like this in the melt,
you put it at 3000 Kelvin even though you have say silica in here,
what actually out gases is not CO2, but most of the vapour that you get out is a silicon oxide and you make some oxygen,
O2 and things like that and that even if you have two or three, you would get quite a lot of iron vapour out and things like that.
And the, the, this is the standard stuff of, of a lot of, of Cosmo chemistry and, and experimental petrology and so forth.
And, and there aren't that many experiments on the directly looking at the composition of rock vapours in equilibrium with melts.
In fact,
I was just talking to the experimental Petrology group today about the possibility that you might be able to directly look at some of these things.
But the inference of what composition you get in the rock vapour atmosphere over a
magma ocean have been mostly done in terms of have been mostly done by indirectly
by measuring gives free energies in various ways and then and then minimising
Gibbs Gibbs energy to figure out what vapours you have in equilibrium with a melt.
And so this is the so this is the generalisation, as it were, of the causes factor in relationship.
We used to figure out how much humidity you have in the in the Earth's atmosphere.
And so this is temperature here. And if I just pick out a couple lines here so that this this red line is is is cellulose.
So this is a this is more or less bulk silicate earth composition at temperatures of of to of around 2500 Kelvin the SIO which is
this red line way dominates over silica CO2 is a logarithmic scale which is about an order of magnitude lower and vapour pressure.
So you actually get mostly soil. That's quite important from the standpoint of observations and also the climate because CO2 is like CO2,
it's a tri atomic molecule, it's a good greenhouse gas, it's a hot greenhouse gas, it's a good greenhouse gas.
SIO being diatomic has very well separated spectral lines and so like something like H.f. or SEO, it has very distinct spectral characteristics,
character, but doesn't, doesn't actually doesn't actually block much trapped much infrared because the lines are too well separated.
But down here actually sodium with something like a box on earth, the dominant atmosphere is sodium.
At 2500 Kelvin, you generate you generate something like a 200 millibars sodium vapour atmosphere.
When you get up to 3500 Kelvin, it's the SIO is the silicon oxide that that dominates the atmosphere.
And at 3500 Kelvin you sow on 55 cancri you'd expect something like a one bar,
something like a one bar Soho atmosphere with a little bit of sodium mixed in.
And the picture is schematic picture of 55 cancri with something like this,
all the volatiles, carbon and so forth at these temperatures should have escaped.
And and then you you are cooking out SIO and sodium and then smaller amounts of these other things which then
stream away because there's high pressure here at the saturation vapour pressures on that previous graph.
They go up, they cool off by adiabatic cooling, you make clouds, but the clouds are made of SIO and sodium and so forth.
And they're basically expanding into a vacuum. And if the pressure gradient is big enough, you get a supersonic flow from from the magma ocean.
And most of this stuff then condenses out before it gets very far to the Nightside in the Nightside is is really cold,
maybe as low as 100 kelvin or something like that, depending on the internal, internal heat flux.
And so so there are mathematical models of this kind of rock vapour atmosphere which actually are quite analogous to a much colder,
much colder situation in the solar system, SO2, sulphur dioxide at the temperatures of of EOE, the satellite IO actually generates local atmosphere.
Is that snow out on to the surface and have supersonic flow from the source to their sinking condensation,
which are mathematically very analogous to these things, except there more more components involved.
There's a lot of interesting stuff to think about in terms of in terms of what this magma ocean actually does,
how hard is it melt down in how to connect up to the interior flow and so forth?
But you would have a lot of interesting patterns deposited on to the surface as you segregate,
you fracture these various minerals and snow them out in different places,
and then the oxygen make make a lot of oxygen when you do this, which actually doesn't condense out anywhere.
And we tend to build up into an oxygen atmosphere if it doesn't escape to space, which is one of the calculations I'm gearing up to do.
But now the reality on this planet is this is interesting enough.
This is a very interesting sort of climate system, but the reality is even weirder.
This this this was a result that just came out this year by the group at University College London.
Now, this is a typical way that one tries to determine what's in the atmosphere of planets.
The U. Of 55 cancri crosses in front of its star, and so it blocks some of the starlight.
But you can also you can observe that transit. You can observe how much starlight is blocked at different frequencies.
And so the planet looks bigger at some frequencies than others. And that's a measure of where the atmosphere absorbs or scatters.
Well, and so this is this is a measure this is from from Hubble, I think.
And the and they and the data are they are the vertical grey grey lines.
And so this is the this is the the how much light is blocked in parts per million relative to the total starlight.
And it ramps up, it ramps up this, this, this red line is what you would expect for a pure hydrogen atmosphere.
This green line is what you expect with it for a pure hydrogen atmosphere that that that carries
some additional solar composition contaminants up out altitudes where you can absorb it,
where you can observe it by this kind of by this kind of technique.
And it's very the data is not great.
The error bars are pretty big,
but it's still more compatible with having a low molecular weight atmosphere like hydrogen than any other thing anyone's been able to to think about.
That's really very, very hard to swallow because at these temperatures and this close to a star, hydrogen should have escaped long ago.
So so there are physical problems with the hydrogen atmosphere.
And that was bad enough as that was perplexing enough. How can you actually keep a hydrogen atmosphere on something like this?
And then then they've got even more puzzling.
Paper by Bryce Demaree came out just about a month ago in which by looking at the emission, the thermal emission from the planet as it's going around,
as it's going around in its orbit, you see different you see the planet from different angles at different at different times in its orbit.
So you get to see the different proportions of dayside versus nightside.
There's this dot here is the sub stellar point, which should be.
Hottest point on the planet. This and then these are the temperatures, the north south, average temperatures on the planet.
The people who make these kinds of phase curve measurements like to plot them on a map.
There's no actual resolution in this direction.
It's just a way of trying to make you think more about what you know, what you're looking at on the planet here.
And and so the the hot spot is phase shifted from relative to this sub stellar point, which means that there has to be heat transport,
there has to be systematic heat transport over in this direction, which is also also demands a fairly massive atmosphere to move that heat around.
And it has to that atmosphere has to be for transporting heat quite effectively because
at temperatures of around 3000 Kelvin segment to the fourth is radiating so fast.
You've got to move heat fast to make up for just how quickly the atmosphere is cooling off.
What's more, the nightside is 1300 Kelvin and the Nightside gets no stellar radiation at all, has no energy source.
The energy source by diffusion of heat from the interior is trivial.
And so somehow you have to move energy to the nightside also to keep the temperature to to 13 to 1300 Kelvin.
And so so that doesn't this doesn't demand a hydrogen atmosphere.
We can start to ask questions now. We can start to do modelling and say, can we, can we get this kind of pattern with the hydrogen atmosphere?
And that's that's a job for three dimensional climate modelling, which is one of the things I do.
And so and so my student finally was able to adapt our, our planet, our exoplanet model to do pure hydrogen and pure CO2 atmospheres.
There are a lot of a lot of these all going on in these simulations which, which I don't have time to talk about and a lot of subtleties.
But the upshot is that is that, hey, this is there's this sub stellar point right here in both of these cases,
right at the centre, hydrogen has very high heat capacity.
If you if you were to I just arbitrarily show ten bars of hydrogen here the if we put in a pure hydrogen atmosphere
with no extra stellar absorbers no extra solar absorbers then so that you have a deep convective troposphere,
then actually the hydrogen is is so good at transporting heat that you get a
pretty isothermal planet around 2000 Kelvin and we're doing the sweep know
we may be able to get something that looks like the data with a lower with a
lower hydrogen content in the atmosphere like a one bar hydrogen atmosphere.
But actually we have, we tried things with a with a what with lower surface pressure of hydrogen.
We and it's really a very,
very narrow range of pressures for which you get the right kind of phase shift of the hot spot relative to the relative to the sub stellar point.
This is with a pure CO2 atmosphere, which is much more plausible in terms of being able to hold on to a high molecular weight atmosphere,
although real in reality of these temperatures, it would be part CO2 and part C0.
There's the sub stellar point. You get these characteristic wave patterns on the nightside and cold spot there.
The temperatures are a little bit too cold, but around around a thousand Kelvin and the dayside a little bit,
not quite hot enough up to about 2600 Kelvin.
But but with with carbon dioxide which gives you different wave speeds in the atmosphere and different and and different heat capacity,
we actually get the phase shift about right. But so this is just to give you the flavour that we're at the stage now with the data
that we can get and we can start to test hypotheses about what's in the atmosphere.
So, so there's a lot more to do, to do with this.
But one place we now have to think more of now that now that there's evidence that
there is some kind of atmosphere other than a local thin rock vapour atmosphere,
we have to start thinking about what what you out?
Gas from a magma ocean. If you've got some carrier gas above, what if you have an H2, CO2 or water vapour atmosphere above the magma ocean?
What how does that affect the chemistry of what's in the of what's in the atmosphere?
And in particular the simulations I showed you did not have the rock vapour
clouds in and they did not have the latent heat released from from rock vapour.
And that we're we're quite a long way off from being able to handle that kind of dynamics.
And so and so the next stage in trying to do this is to understand how the condenses will rock vapour mixed into a
conventional background gas effects the the radiating temperature of the planet by affecting the temperature directly.
But also by. By affecting the outgoing radiation by.
Clouds. Okay. So now let's go on to nitrogen for somewhat briefly.
In some ways, nitrogen is a rather boring gas. It's like the air.
It's it's there. But you don't think about it very much.
In fact, in Earth, it is the air. It's about 80% of the air. N2 is not very reactive.
It's practically a noble gas doesn't form minerals easily.
And therefore, unlike, say, carbon, there's not much cycling between the atmosphere and the mantle.
And it's also not a greenhouse gas. It's a little bit of a greenhouse gas.
But but but it's a very weak, very weak greenhouse effect, at least at Earth like temperatures.
And it only condenses at very low temperatures. So it just sort of sits there not doing very much.
But it turns out to be one of the key gases in the evolution of planetary atmosphere in the termination of climate.
There are multiple reasons for this, many of which I've been working on over the last few years.
One is that it affects this cold trap that I mentioned that protects water vapour from getting up into the upper
parts of the atmosphere where it can break up under ultraviolet ultraviolet illumination and then escape to space.
If you have a lot of nitrogen in your atmosphere,
the temperature goes down very rapidly with height because the heat release by condensing water is shared over a bigger mass.
So you you get something close to a dry 80 about for nitrogen, you get very cold minimum temperature.
And so by clause, yes, Klapper and all the water vapour is trapped is trapped down low.
If you take away a lot of the nitrogen,
then then the temperature contrast in the vertical and you can just show this from straightforward thermodynamics is very weak,
which means that you have very moist upper atmosphere.
This water vapour is then exposed to energetic ultraviolet which breaks it up and the hydrogen escapes
oxygen is left behind which which on a rocky planet reacts with with various minerals in the crust.
And you essentially have permanent water loss. This is this is believed to be what happened to Venus and how Venus got to be the way it is now.
There's another way that nitrogen affects the atmosphere,
although nitrogen isn't much of a great greenhouse gas itself or another way the nitrogen affects the climate,
the nitrogen isn't much of greenhouse gas itself. Spectral lines that do absorb infrared would be very, very thin if it weren't for collisions.
And so the collisions of nitrogen with being has gases like CO2 or water vapour
have a very strong effect on the on the strength of the greenhouse effect.
So, so so if you have this is the infrared cooling to space, the generalisation of signatory to the forth as a function of the surface temperature,
four atmospheres containing 1% CO2 and water vapour,
50% saturated and a vertical structure on the 80 of that, the standard sort of climate calculation.
So this is with an earth like atmosphere with roughly one bar of carbon dioxide,
you you balance the outgoing infrared, the cooling, the space against the amount of sunlight received.
You get the surface temperature. And let's say that we have a surface temperature of 280 with a conventional one bar nitrogen atmosphere.
If you give the you are two bars of nitrogen at the surface.
Just the increase in the greenhouse gas from collision or broadening gets your surface temperature up to 292.
You lose some of that because in reality with two bars you reflect a little more sunlight by really scattering.
But but the greenhouse effect wins.
But if you go down to a quarter bar of nitrogen, your temperature goes down to 270 Kelvin and the Earth freezes over.
And so a quarter of our nitrogen doesn't look very good for the early earth, especially when when the sun was fainter than it is now.
But well, the world will see a surprise in the next couple of slides with regard to that.
So there's another way that the nitrogen affects the climate, which could partly offset that pressure effect that I showed in the previous slide.
If this these are from some simulations that are coming out in proper succession.
This is the altitude and this is the latitude equator, North Pole Circle.
And so and in an atmosphere that has a small amount of water vapour mixed in with a large amount of non danceable nitrogen,
the relative humidity, the percentage of saturation can be quite low because there are various ways of creating some saturated air dynamically.
That's one of the things that we talk about in this paper. So this is with earth like nitrogen.
But as you reduce the amount of nitrogen for a given temperature, then the atmosphere becomes more and more saturated.
And and so the greenhouse effect of water vapour gets stronger and stronger.
And it turns out that if it weren't for this, if it weren't for this sub saturation effect,
the earth would already be in a or in a range where it would turn.
To Venus, where we would go into a runaway greenhouse, where all the oceans would evaporate into the atmosphere.
And so this sub saturation effect, which is made possible by a non pronounceable background gas,
especially one that doesn't heat up the surface too much, actually is very important.
And of course, nitrogen is the stuff of life. Nitrogen, nitrogen.
The end to bond is really hard to break. But this wonderful biologic you can break it with with lightning.
But other than that, biology mostly does it the this wonderful enzyme nitrogenous, which is in bacteria and root nodules of legumes.
It's in and it's in various kinds of even if it's in blue green algae, it's in green sulphur bacteria, very early metabolism, it can make ammonium.
And then once you have oxygen, the the ammonium can oxidise to nitrate.
And both of these things are biologically available.
Ultimately, they work their way into amino acids or bases of all proteins and also the base pairs that are in rDNA.
So nitrogen biologists have no idea how you could get or get biology, at least anything like what we know working without nitrogen.
So it's quite important. But nitrogen is almost not there on earth at all.
There's very little of it. And so remember in the and in solar and in solar abundance, nitrogen out to carbon, we're all about the same.
This is the abundance in and see when comrades which are often taken as in as as is similar to what the earth, what rocky planets are made of.
And so carbon is quite depleted relative to the oxygen and nitrogen is extremely depleted.
But notice that this is .001 here. The Earth is even more depleted in nitrogen than conjugate.
So contracts are depleted in nitrogen relative to solar abundance.
And then the earth is depleted in nitrogen, insofar as we know at all relative to current to contract,
because you have something like a part 4000 of nitrogen and contracts,
but three parts per million of nitrogen based on rather crude estimates of how much nitrogen might be in the mantle.
And so, so, so the difference between how much we have in having nothing is really quite important.
And but this raises a lot of a lot of questions.
If if you if you were to lose an ocean or lose a lot of CO2 by either a young stage of a star when there
is a lot of ultraviolet or when or or if you had the moon forming collision or something like that,
you could be generated by outgassing. But nitrogen has no place to hide.
If you lose it, it's gone for good. You may be able to hide some of it in the mantle.
If you make a lot of ammonium nitrate that can substitute can can be incorporated into various mineral crystals to some extent,
but you mostly need life to do that to any great extent. But so there's a question, you know, what is the evidence?
Can nitrogen really exchange with the rocky planets interior?
And so this is the other big surprise that came out this year, is that by looking at the size of I,
looking at the size of vesicles in ancient lava, 3 to 3 and a half billion year old lavas,
and and using some fairly clever techniques since the pressure of these law is which there's geological argument that these formed
at sea level the pressure at the near the surface of the lavas when they solidify determines the pressure the size of the bubbles,
the vesicles and these and these lavas. And they have a good argument that these vesicles were their size was preserved.
And so this is the paper that just came out in nature geosciences.
They they they find an indication that the total surface pressure on the earth in the archaean may have been as low as a quarter of a bar,
and which would imply that there was a whole lot of nitrogen cycling. And what does that tell us about nitrogen cycling on other planets?
And do you need to know what was the role of life in allowing that amount of nitrogen cycling?
And is it real? Of course, you know, when it comes to single study syndrome.
Okay. So there's no time. I mean, you just have to skip the story of carbon altogether.
And then the because we're I knew I knew this would probably happen.
But let me just point out that that we are really poised to be able to characterise it to test some of
these hypotheses much better and characterise what is in the atmospheres of these small rocky planets,
because they have all these missions that will be able to cover more spectral space both from the ground and from space and and for
the first time characterise what's in the atmosphere of small rocky planets and give us more data to test some of our theories.
But to make sense of all of it, we really need to.
Think more broadly about how things that are traditionally thought of as geology interact
with things that are traditionally thought of as astrophysics or or or atmospheric science.
And so it it's very much like the story of the old style of thinking about planets was
very much like the story of the blind in the story of the blind man and the elephant.
You know, this one this this one thinks that, you know, it's like a snake, it's like a spear and so forth.
And I've been through this once where we had to learn how to see the whole elephant when I was a graduate student at MIT.
You know, there were these various disciplines in atmospheric and oceanic science, like geophysical fluid dynamics, radiative transfer and so forth.
And and the we sort of talk to each other, but these were sort of considered separate things with separate specialities.
But we had to learn how to see the whole that whole elephant with regard to
atmospheres and oceans and where we are right now with this new generation,
this new, vastly expanded territory of planetary and atmosphere,
geology is learning how to see the whole elephant with regard to the integrated picture of atmospheres and rock.
Thank you.