Learn about the science behind the current exploration of the solar system in this free class. Use principles from physics, chemistry, biology, and geology to understand the latest from Mars, comprehend the outer solar system, ponder planets outside our solar system, and search for habitability in our neighborhood and beyond. This course is generally taught at an advanced level assuming a prior knowledge of undergraduate math and physics, but the majority of the concepts and lectures can be understood without these prerequisites. The quizzes and final exam are designed to make you think critically about the material you have learned rather than to simply make you memorize facts. The class is expected to be challenging but rewarding.
Lecture 1.20 Origin of low latitude ice
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From the course by Caltech
The Science of the Solar System
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From the lesson
Unit 1: Water on Mars (week 2)
Meet the Instructors
Mike BrownProfessor
Planetary Astronomy
So we've that ice on Mars is not merely a polar phenomenon.
Ice extends in the subsurface to very low latitudes.
Glaciers appear in many places around the planet.
We talked a little bit how high obliquities
could change where the polar caps go on Mars.
Let's look at that in a little more detail.
The last time I was a little bit hand waving.
Let me show you some actual scientific studies that have been done to try to
understand where the ice would go under different circumstances.
The first question to ask, of course, is how would you figure that out?
And an answer, an answer that's been used quite effectively, is to
use something that we use on the Earth to understand the climate on the Earth and
transfer that to Mars.
What we use on the Earth to understand climate, to understand long-term
implications of changes are things called Global Circulation Models, GCM's.
Global Circulation Model is in principle relatively simple, you, you solve for
things like heat coming in at different locations, heat being re-radiated out.
The effects of the atmosphere, the motions of the particles in the atmosphere.
And you predict what's going to happen.
This is how you get things like weather forecasts.
Weather forecasts are not necessarily made from global models of what the weather is,
but they can do smaller scale models that can incorporate more details.
And weather forecasts,
if you've been paying attention, have gotten better over time.
Even though we all like to complain about the weather forecasts,
you can now look further ahead in time because the people who do the modeling
understand better how to use those models to do the weather.
We don't care so much about weather, we care about climate.
Climate is the long-term behavior.
And to understand climate you have to take one of these global models and run it for
a very long time with different conditions and see what happens.
On, on Earth people do this to try to understand things like the implications of
dumping all this extra CO2 in the atmosphere,
how the heating of the Earth is going to affect the climate of the Earth.
On Mars we can do the same thing where we take Mars as it is now.
Tilted to side with a new obliquity and see how that affects the climate in
detail by putting it into these large scale computer simulations.
And when you do, you get, you get a lot of things.
But one of the things you can look at is the distribution of ice,
surface ice on Mars over the course of years.
Here's, here's a set of simulations run over 30 year time period.
And what's plotted is the depth of the seasonal water ice cap.
And now notice, it's only plotted up to about 75 degrees south,
75 degrees north, because up here, there's a lot of water, and
we know that there's a lot of water up there.
What we're really interested in is how much water can
possibly come down to these lower latitudes.
And what you see in this simulation these, these triangles are, these are,
are once a year.
Here's the south polar cap growing, retreating.
The north polar cap in, in turn grows, retreats, south, north.
So, the sawtooth continues, throughout the seasons.
This is done for a 25 degree obliquity of Mars, which is close enough to what Mars
is today, that it's a, we can think of this as current Mars.
Interestingly, it starts out like this, and it's more or
less the same throughout the 35 years of this run.
Which means that, if you start Mars out the way it is now and
you keep the obliquity the way it is now, things don't change very much,
which is not very surprising.
But now, wait.
We can take the same simulations, we can start out the exact same way.
Start out as Mars is today, but
suddenly tilt Mars more on its side, make it have a higher obliquity.
Let's see what happens.
Now we suddenly have Mars at 35 degree obliquity, and again,
let's remind ourselves what obliquity here means.
Here we have a zero degree obliquity means that as Mars goes around the sun there
would be no seasons at all because the north pole of Mars points
directly in the same direction as the orbit of Mars pole points.
20 degree obliquity, 25 degree obliquity is something like that.
35 degree, you're tilting more and more on the side.
If you have a 90 degree obliquity, you'd be tilted like this and
your north pole would completely get fried one part of the year and
then completely freeze up the other part of the year.
At 35 degrees though, things have already changed pretty dramatically
from that 20 degree case that we looked at just a minute ago.
Yes, the south pole looks more or less the same in color, green.
Green is, oops, I cut this off down here.
But green is somewhere in the range of 0.2 centimeters.
That's what this is over here.
Notice what's going on in the north.
You start to have these regions of quite big build up.
These are two plus centimeters.
This is a two.
Two centimeters of build up over here, up at 75 degrees.
Much more up here at the pole, we're not worrying about that now.
And the reasons the Northern Hemisphere and
the Southern Hemisphere are not symmetric is for a lot of different reasons.
You know that the Northern Hemisphere is much lower than the Southern Hemisphere,
so you might think, oh, it should be hotte.
Therefore, there should be less ice.
But the, the detailed circulation that occurs takes the water vapor,
water vapors is what you need to form the ice, not just the cold temperatures.
And the water vapor condenses up here in the north
faster than it does down here in the south.
But, you even get a little bit of ice all the way down to the equator.
So every spot on Mars,
even at just 35 degrees, every spot on Mars gets ice on it.
But not for long.
We get a little bit of ice at the equator, it melts away.
What happens if we take this whole thing and tilt it to 45 degrees?
Remember, 45 degrees is the maximum obliquity we think that Mars had
in its past based on those detailed computer simulations.
So what's going to happen?
Well, remember we said before that as we tilt more and
more to the side, the poles will start to melt more.
So what happens when the poles melt?
Water vapor goes into the atmosphere.
When water vapor goes into the atmosphere,
you can have huge buildup in different locations.
But of course,
the southern pole is going to melt when Mars is over here on the other side.
And what's going to end up happening is that there will be a band
in the middle here.
The equator, the tropics, which we like to think of the tropics as being warm but
that's because we have a relatively low ob, obliquity.
If you have an obliquity as high as 45 degrees,
the tropics keep on getting water pumped into them throughout the year.
And it, they can become cold enough to get nice and frozen there as you will see.
The effect is quite dramatic here at 45 degrees.
In fact, you suddenly no longer mostly have this polar stuff going on.
You have a permanent band here.
Not only is it a permanent band, remember,
all these simulations started out with Mars' current condition.
If you start out with Mars and its current condition where you have the poles
getting warmer and colder, warmer and colder year after year.
And you suddenly switch to 45 degrees, how long does it take for
those poles to melt and form that equatorial band?
Five years, not very long at all.
And notice that it's still growing.
This, this ice is now 10 centimeters thick.
There's nowhere yet that's 20 centimeters thick, but you can ge, kind of guess
that as time goes on, you're going to get big thick ice bands in through here.
Very little happening there at the poles, where it continues to melt.
If we go all the way up to the far north, and the far south,
you still would see those polar caps are forming in the winter on both sides.
Imagine what happens now.
You have these, these tens of centimeters of ice that form in through here,
freezing out from the atmosphere.
And then, Mars does what it does a lot of, which is has dust storms.
These dust storms can cover this ice, and if you cover this ice with dust,
you cover this ice with, with the ground, it is stable suddenly.
Before we couldn't have ice at the equator.
Now, if there's enough insulation of dust, ice can be stable underneath that surface.
Ice can also not just be stable as ice, but the ice can sort of seep into
the pores of the rock and be, become part of that subsurface.
This appears to be the process by which the entire
surface of Mars has some amounts of water in it,
as seen by that gamma ray spectrometer and as seen by that neutron spectrometer.
So in looking at that, that extra ice, you're looking at the fossil record
of these high obliquity times that occurred in Mars' history.
Gotta tell you though, that 20 centimeters of ice,
ten centimeters of ice, that does not make a glacier.
A glacier requires enough ice that it's compressed, that it will flow.
That it can develop these flow features that make us recognize it.
So where do the glaciers come from?
There's a very nice study that came out at about the same time which took the same
idea, used a terrestrial climate model, terrestrial weather model and,
apply it to Mars and see what might happen in different conditions.
But now instead of doing the global modeling,
the GCM, this is using a more local model.
Like you would use to predict the weather in a specific spot on the earth,
rather than the, the global environment.
And the specific spot they chose on Mars, was the region around Tharsis,
and Olympus Mons, where large scale flow features were seen.
Here's the region around Tharsis that they looked at.
Here are our favorite three volcanoes again, and Olympus Mons.
And these yellow regions that you can see here are the regions that have been mapped
out that look like some sort of ice flow features in them.
Interestingly all on the same sides of these,
these giant volcanos and many of them up here in these,
these high regions where you might think glaciers would indeed form.
So the way one of these local weather computer models work is you put
in all the terrains, you put in all the elevations and you force it from a,
a larger global simulation, but
then you let the weather develop on the faces of these mountains.
And, and in the course of doing this, here's, here's what they found.
They found regions where you have net ice accumulation.
And where are they?
Well, they're on these sides of the mountains,
these side of the mountains, and okay, on the southern side of Olympus Mons.
There is not a 100% match between these two, but I think it's a pretty good match.
The fact that you're finding out that one, you do have ice accumulation.
How much centimeters per year?
This is already now five centimeters of accumulation in these regions per year.
This is not a five centimeter layer, this is how much continues to be accumulated.
And remember, that's how a glacier works.
Accumulation happens at the top of it, it drives the flow down the front of it.
And you finally get evaporation down at the tail of it.
So you have this accumulation happening here, here and here, and
you end up having these flows going off downhill on each of these.
And I should have said, but I didn't, that this is for a 45 degree obliquity.
I find these two pairs of computer simulations in, incredibly convincing.
We know that Mars went through these periods of 45 degree obliquity.
We see that it would drive ice down to the equators, and
we see from this it would also lead to accumulation.
The ubiquitous ice and
glaciers in the present day seems like a mystery that we actually have solved.
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