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Lecture 1.15: Was early Mars warmer and wetter?
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En provenance du cours de Caltech
The Science of the Solar System
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À partir de la leçon
Unit 1: Water on Mars (week 2)
Rencontrer les enseignants
Mike BrownProfessor
Planetary Astronomy
We've seen enough geological evidence to be convinced, I think,
that liquid water indeed flowed on the surface Mars in the past.
Now, maybe that liquid water came out of aquifers and, and
seeped from the underground in some places.
Maybe it was even covered by ice or something in some places.
But there are certain places that you look at, and it's very hard to
convince yourself that it's anything other than precipitation filling the area.
Here's one of the best examples.
And it's region called Warrego Vallis.
I have no idea where that name came from.
And you can see, looking at it,
that it's not just the sort of typical river channels that you see other places.
But it's secondary channels,
tertiary channels fourth order channels, tenth order channels.
And in fact, it looks like there is not a single spot anywhere around here
where you don't have a drainage from the high points down to the low points
everywhere through this entire region.
That is exactly the signature of, not just water flow, but precipitation.
Of precipitation inundating this entire region and
all of the water needing to flow downward.
It's images like this that make us need to explore the possibility that it indeed was
raining on the past in Mars.
And indeed, liquid water was stable on the surface.
So how do you make liquid water stable on the surface of Mars at least in the past?
To understand that, you need to look at the phase diagram of water.
Phase diagram is the diagram that tells you at different conditions of temperature
and pressure.
What phase that material is in, is it in a gas phase, liquid phase, or a solid phase?
And the phase diagram for water looks something like this.
In, here's pressure on one side, temperature on the other side.
As the temperature increases, not surprisingly,
the gas phase becomes more stable.
As the pressure increases, again, not surprisingly, solid phase,
liquid phase, anything but the gas phase becomes more stable.
At low temperature, it's solid in ices.
And the specific behavior here matters.
This is the spot where the phase diagram meets.
This is called the triple point of water.
It's the spot where you can have liquid, solid, and
gas all at the same temperature and pressure.
And that triple point of water is 273 degrees Kelvin,
which is equal to zero degrees Celsius.
And it's at about a 100th of an atmosphere, an earth atmosphere.
I'm going to use these non-standard earth atmosphere units for today.
And in fact, we also know that at one atmosphere where the earth is currently.
Here's one atmosphere, the temperature, the transition from liquid to gas,
we call that the boiling temperature, that's 373 or 100 degrees Celsius.
And this is still about zero, this line is nearly straight.
And so that's why we have this range where liquid water
is stable between 0 and 100 C.
It's steam above, and it's solid below.
What's going on on Mars?
Well, the average surface temperature of Mars is 220 degrees.
That's way down here.
And the pressure on Mars is a hundredth of an atmosphere.
So Mars, Mars is something like here.
Okay, I have to back up for
a minute because people often get confused by this point.
I've been talking about water vapor and the temperature and the pressure and
where you're above or below the triple point.
And we talked about the pressure on the surface of Mars.
What really matters though is the partial pressure of water
vapor above the surface of Mars.
The water doesn't care how much of anything else there is.
Because the way that water goes from being a liquid into a gas form or
into a solid is it's exchanging constantly between the atmosphere and the surface.
And what's it exchanging?
Well, it's exchanging gaseous water or liquid water.
So it doesn't, can't exchange any of those other things.
So even if you had many, many bars of CO2 atmosphere, if you didn't have very
much water, you still would have no liquid water be stable on the surface of Mars.
What would happen?
Well, you would have that liquid water on the surface.
And it would just evaporate.
Until, either it all evaporated out, or it would evaporate until there
was enough in the air that those two things could be stable together.
Just how little of partial pressure of water vapor is there on Mars?
Well, if you remember the pressure of Mars is something like seven millibar.
That's, that's seven 1,000ths of the earth's atmosphere.
The partial pressure of water vapor, that is,
the pressure of the water vapor if everything else went away,
is only something like 0.0018 millibar.
It is tiny.
There's very little water in Mars' atmosphere.
And unless you can get that number much higher,
liquid water is not going to be stable on the surface.
How are we going to get liquid water stable on Mars?
Well, there is only one solution,
there is no liquid stability zone for this very low temperature.
We can play with the pressure all we want in this direction.
But we're still going to have ice on the surface of Mars.
Water is in the form of ice.
How do we get it in the form of liquid?
Not surprising answer, heat it up.
You need to heat it up from this value,
about 220 degrees up to more like 273 degrees.
But that's not all.
As you can see, simply making it warmer means that the ice will easily move from
solid to the gas phase.
It will sublime.
But unless there is higher pressure, then there still won't be liquid phase.
So we to not only need increase the temperature on Mars,
we have to increase the pressure on Mars.
How are we going to do that?
Well, first,
increasing the temperature by something like 50 degrees is not very easy.
As you remember,
the temperature of Mars is basically set by a pretty simple balance of sunlight,
the energy of sunlight coming in, and the black body radiation of Mars going out.
When those two are equal, you've reached an equilibrium,
that's the surface temperature of Mars.
If those two weren't equal, if, if the temperature of Mars were higher,
then it would radiate more than it gets in.
And if it radiates more than it gets in,
it cools until it hits that equilibrium point again.
We could easily, once again, calculate that e,
equilibrium point by remembering that the thermal emission from Mars
is sigma T to the fourth, where sigma was the Stefan–Boltzmann constant.
And that has to be equal to the amount of sunlight absorbed.
That's going to be the solar constant times 1 minus the albedo, the reflectance.
Anything that's not reflected is absorbed.
And the solar constant is usually considered for the location.
The earth, Mars is 50% further away.
And as you know, the energy does, goes down as the distance squared.
So we need to square that.
Solar constant is 1,362 watts per meter squared.
And Stefan–Boltzmann constant is one of the easiest numbers in the world to
remember because it is 5.67 times 10 to the minus 8.
And the albedo is somewhere around 0.3.
But one other thing that we have to consider is if we're thinking
about something on the surface of Mars,
it doesn't get the full sunlight all day long, and yet, it radiates all day long.
So how much sunlight does it get?
Well, you can think that it maybe gets half of this total amount,
because half the time, it's nighttime.
But it really, it only gets this full amount at noon.
And the total amount it gets decreases from sunrise or
increases from sunrise to noon, decreases back to sunset, and goes to zero.
So, a, a real typical number is more like, I think it's a quarter on average.
A quarter of the total sunlight.
Throw in all those numbers, and
you get the temperature of Mars is something like 220 degrees Kelvin,
which is a pretty good estimate of the average surface temperature of Mars.
220 degrees Kelvin.
Again, we have a long ways to go and not much we can do to change things over here.
We could change the albedo if we have the albedo of zero.
Mars absorbs everything it gets.
That's a very trivial change.
Th, there's a temperature to the fourth power,
means that small changes over here don't really do very much.
And I have worse news for you.
The worse news for you is that this solar constant, we call it the solar constant,
is not constant.
Astronomers are fairly, fairly sure about, at the time, that the early,
early solar system, at the time of say the Noachian, that this solar constant was,
perhaps, only 70% of what it is now.
That's the faint young sun it's called.
Stars, in general, get, slowly get brighter as they're on the main sequence
where they spend most of their lives.
And it's, the sun has been doing that this entire time.
So, not only is it really hard to, to get up from 220 degrees,
if we use the faint young sun, the temperature at the time really should have
been something like 200 degrees Kelvin.
How do we get something warmer than that?
The answer is all in the greenhouse effect.
We talked about that a little bit earlier when we talked about the spectrum of
the earth.
And where light can get out of the earth's atmosphere and
come in to the earth's atmosphere.
And in general, the light that comes into the earth's atmosphere or the Martian
atmosphere, in this case, comes in in the visible portion of the spectrum.
The visible portion of the spectrum, as you can tell by looking around,
is pretty clear.
We see most of the sunlight, and the sun is peaking in that region.
The sun, the, the thermal radiation that's emitted by Mars or
by the earth is at a much lower temperature.
And it peaks at somewhere around ten microns.
Ten microns has things like absorption from water, has CO2.
And on the earth, these two gasses account for most of the greenhouse warming.
On the earth, we get a greenhouse warming of something like 33 degrees Kelvin,
Celsius, which most of it's water, a little bit of it is CO2.
By most of it's water, I mean that the, that the outgoing radiation
that's coming from the surface of the earth goes up into the atmosphere and
is absorbed in the atmosphere by the water vapor.
When you're absorbed in the atmosphere by the water vapor, you heat the atmosphere.
And that hot atmosphere then goes back and heats the surface.
Again, that is the greenhouse effect.
So if water is in effect a greenhouse gas on the earth,
then let's see if we can make it work on Mars.
We have water ice.
We have these huge polar caps that if we could melt and
put into the atmosphere would cause significant greenhouse warming.
And if we could cause just a little bit of greenhouse warming,
then more of them would melt, and we would cause more.
And then more would melt, and we would cause more.
And this is called, of course, the runaway greenhouse effect.
And if we get the runaway greenhouse effect working on Mars,
then perhaps we could have had a warmer, wetter early Mars.
The problem is that a,
a water vapor-based runaway greenhouse effect cannot work on Mars.
It's still too cold.
There's still too little sunlight coming into Mars.
That even if you try to do that, if you put all that water vapor into the Martian
atmosphere and heat things up for some amount of time, eventually,
it rains back down and cools back down and freezes up again.
You can't sustain it.
One of the ways that we know that is that we can look at
the difference between Venus and Earth and Mars.
As they go from closest to the sun, middle, furthest from the sun.
And on Venus, sure enough, the the runaway greenhouse effect have,
has completed dominated the atmosphere there.
And the surface is baking.
On the earth, we have plenty of water with the runaway greenhouse effect.
Water vapor-based were effective at these amounts of radiation.
Then we should have it, and we don't.
Mars is kind of hopeless.
Okay, if it's not water, what's the next best thing?
The next best thing is CO2.
It's the second, most powerful greenhouse gas that we have on the earth.
And it is the most abundant species in the Martian atmosphere.
So it's a good place to, to go looking.
First, we should draw the phase diagram for CO2.
The triple point is something like minus 80 degrees C, so
it's way over in this direction.
And the pressure of the triple point is five atmosphere.
So the triple point is more like right here.
And then the rest of the phase diagram has some similar-looking behavior to water.
The details don't matter.
So we have solid CO2 here and vapor CO2 here, gas phase of CO2 here.
Where's Mars? Well, Mars is sitting right here.
And Mars is clearly in the vapor phase of CO2.
You'd have to add a ton more pressure to ever make it get into the liquid phase.
That's why we don't ever really see liquid CO2,
it needs five atmospheres of pressure for it to happen.
But CO2 is easily in the gas phase at these temperatures.
On the poles of, of Mars, we now know, of course,
that CO2 condenses out in the winter, but there's a pretty thin film of CO2.
The nice thing about this is, is that at these colder temperatures, Mars is,
Mars is, we're in the gas phase.
And so we can get that CO2 up into the atmosphere.
That CO2 in the atmosphere can then cause a greenhouse effect.
Can it do enough? Well, there's, there's debate.
Some people believe that they can get something like with one atmosphere of CO2,
that you can get enough warming to, to heat up Mars up into this region in here.
And then, if the temperature gets high enough,
water is suddenly stable in its gas form, instead of its ice form.
It's not in a liquid form yet because there's not enough partial pressure of
water in the atmosphere, still.
But let's see what happens if there's, if there's say, ice caps, or
ice in other places around the planet.
That ice starts to sublime, it starts to go into the atmosphere.
As it goes into the atmosphere,
the partial pressure of water continues to increase.
And if there's enough ice on the planet, anywhere,
that can sublime, then the atmosphere can finally reach a high enough
partial pressure of water that it is above that triple point of water.
When it sits above the triple point of water,
then liquid water can be stable on the surface.
The temperature is high because of the CO2.
The high temperature melts the ice.
It doesn't melt the ice, it sublimes the ice into the atmosphere.
Finally, there's enough water in the atmosphere that liquid water is stable
on the surface.
Other people are not convinced.
They don't think that even with this much CO2, you don't get that much heating.
And some of the reasons for
the debates are similar to some of the reasons for the,
the, the difficulties in understanding greenhouse warming on the earth.
And that's because understanding the effects of things like
feedback from clouds.
When you have more of an atmosphere, you have more clouds, which reflect can,
can cause big enough differences that it's unclear whether this going to be enough.
But let's just go with the case that this is enough.
If you had one atmosphere of CO2, Mars would suddenly be up in this region.
And you would easily have liquid water flowing on the surface.
This is about 100 times the amount of CO2 that's currently in there and, and
how do you make it happen?
Well, you can imagine that you have these massive polar caps of CO2.
And that when these climate cycles happening,
when you have liquid that starts tilt, tilts towards the sun, those melt and
redistribute around the planet, you distribute that CO2 into the atmosphere.
You get massive warming, melts everything un, until all those the,
those caps are gone.
And you have this nice, warm, wet, early Mars like we see some evidence for.
There's only one big question big problem with this idea which is,
that is a lot of CO2, where did it go?
I'll give you three ideas I don't know the answer, but I'll give you three ideas.
Idea number one is that it could be in the polar caps.
Right now, those polar caps are pretty big as you'll see in the topography data when
we, that we'll talk about in a subsequent lecture,
there's this big mound at the poles.
And that mound, if it were all CO2, that might be enough ice.
Why doesn't it sublimate?
Well, maybe it's covered with dust, covered with something, so
it doesn't disappear.
And it's just sitting there, storing all that ancient CO2.
As again, we'll talk about later on,
there have now been radars that have flown across the surface of those ice caps.
And the radars can penetrate through and see what's down inside.
And it looks like it's mostly water ice, as we expected.
It does not look like massive mounds of CO2 ice.
So polar caps, probably not.
Next option, carbonates.
Carbonates are what happens when you have CO2 in the atmosphere.
You get carbonic acid rain.
That carbonic acid rain hits the rocks, weathers the rocks.
And you get calcium carbonates.
Calcium carbonates, CAC03.
And as you can see, you store a lot of carbon dioxide in that CAC03.
Calcium carbonates are things like limestone.
They're these beautiful locations on the earth where they're these things called
cap carbonates,
big caps of massive layers of carbonate where some similar process happened.
The earth had a lot of CO2 in it.
There was a lot of rain.
There was a lot of weathering. And these limestone layers,
limestone-like layers formed.
If this happened on Mars, if you took all this,
this 100 times current atmosphere Mars and turned it into carbonates, you
would get a carbonate, global carbonate layer something like 10 meters thick.
Now maybe it's covered by other things.
But we know that we're seeing some of these terrains that are,
that are Noachian terrains, that are Hesperian terrains.
And so these have not been covered up.
So where are the carbonates?
We don't know the answer to that one.
We'll discuss these sorts of questions a lot in subsequent lectures.
I'll give you one more possibility, and
that's that the CO2 existed in the Martian atmosphere, but it was lost.
How could it be lost?
Well, it turns out there are a lot of ways that you can lose an atmosphere.
And in the next lecture, we'll actually go into detail on atmospheric loss.
And how you possibly could have gone and
gotten rid of something like an atmospheric CO2.
Is it true?
Did it happen?
Can, like many of these things, we still don't know the answers.
But at least, we are getting to some very specific questions
that we'd like to be able to answer.
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