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.
