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 1.08: Water on earth
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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 1)
Meet the Instructors
Mike BrownProfessor
Planetary Astronomy
I'm going to step back and digress for a lecture, and ask a question that maybe we
should have asked before, which is, okay, we're, we're worrying about water on Mars,
and whether there is, or should be, or could have been water on Mars.
We know the Earth is full of water.
Where did all the water from the earth come from?
Should Mars have had water from the same source that the terrestrial water has come
from, or not?
It's really hard to answer that question unless we know what the source is.
So, let's think a little bit about what the source could have been.
Traditionally, when people thought about the formation of the planets and
the sun, they think about a starting, a forming star, you know,
everything collapses out of a big cloud of dust and gas in the interstellar medium.
There's a little bit of rotation to it.
As the collapse occurs, that rotation goes faster and faster and, and
a disc is formed, generally called the protoplanetary disc,
disc of materials, sort of like this.
And the planets form from the coagulation of materials in that protoplanetary disc.
Over the course of the class we'll talk much more about this protoplanetary disc
and, and where things are inside of it.
One important point, though, is that the disk,
of course, is hottest closest to the sun, then cools down as it moves further out.
Generally, close to the sun, it is too hot for water to condense.
Water will be in a gas phase, and only when you get outside of
some distance will the disk be cold enough that that gas,
that water gas, can freeze out and become ice cubes, basically.
Ice cubes, once they freeze out, are, are essentially like rocks.
So, in, inside of here, you only have rocks freezing out.
It's kind of funny, you think of rocks freezing out, isn't it?
But you have gases of these silicates that the rocks are made out of,
gases of least the materials that are in the silicates.
And those things are able to condense out, and become the rocks in here.
Out there, the silicates condense out, too.
But also, ice cubes, this is one of the explanations, early explanations, for
why Jupiter is suddenly so much larger than the terrestrial planets.
That Jupiter got to incorporate all these ices as materials into it, and
therefore, it led to a quicker formation, and eventually a bigger formation, too.
So, this is the important point.
This is called the snow line in the Nebula.
And these days, there are many more complicated ideas.
And so, the snow line is a pretty simplistic way of looking at things, but
it's a good way to start, at least.
So, inside the snow line, no water except in gas form.
Outside the snow line, solid water.
In the earliest ideas about the snow line, the Earth formed well inside the snow
line, even Mars formed well inside the snow line.
And only things like Jupiter formed outside the snow line.
Maybe, if we were lucky, some of the asteroids in the asteroid belt between
Mars and Jupiter, some of them might have formed out here with a little bit of ice.
Most of them just would have been rocky inside of here.
So, where did the Earth's water come from?
Well, one idea would be that it came from not around in here,
but it was transported from outside the snow line into the earth.
And one way to do that is from comets.
Comets, we'll talk a lot about the formation of comets, and
where they've been for the past four and a half billion years.
But comets, presumably formed out here, as conglomerations of ice and
rock, they spend most of their time out here where it's very cold, and
then every once in a while a gravitational perturbation will bring them in.
Again, we'll talk about this in detail, but imagine that these things come in,
they impact the Earth, and they deliver their water onto the surface of the Earth.
It's an entirely plausible theory, and in fact, if you think about that,
that means that the same thing should be happening to Mars.
Same thing should be happening even to the moon.
The moon is too small to hold onto its water, but
Mars is big enough that, if it had, all that water had been delivered by comets,
there should still be water there today.
That's one idea.
Another idea is that perhaps this snow line
is actually not quite as far out as you thought.
Maybe the snow line is only really, maybe right here, and a lot of these asteroids
that formed, actually formed with a lot of water on them.
That's one of the things I'm actually currently investigating, and
one of the things that I have recently been to the telescope to look for.
Look at these asteroids that are here that formed around here,
and see if I can find that they actually had water ice on them or not.
Answer, I think they do.
If they had water ice on them, and as the solar system was forming,
they get jostled around, they impact Mars.
They impact the Earth.
In fact, more of them might impact Mars, because Mars is a little bit closer.
So, again, Mars should probably have water.
And there's one other way to do it, which is to say that just that as the earth and
Mars were forming, and there were all these little rocks, little dust grains,
that were cong, conglomerating to form planets,
there was water vapor in through here at the same time.
But gas vapor could be absorbed onto these grains, and you could incorporate water,
not as solid water itself, but as water on grains.
Then, as you make the planet, and heat up those now rocks,
those planets, that water, and in fact, all those gasses, without gas, and
you would then end up with an atmosphere that included water in it.
All of these are pretty plausible ideas.
How would you figure out what the answer is?
One of the favorite ways that people have had,
is to look at the ratio of hydrogen to deuterium in water.
I'll remind you that water is H2O,
which means it's an oxygen atom, two hydrogen atoms.
On occasion, though, it's not.
It's an oxygen atom, hydrogen atom, and a deuterium atom.
Deuterium with twice the mass of hydrogen,
because it has a neutron that hydrogen doesn't have.
So, this is, this is deuterated water, heavy water.
You could also have two deuteriums, but that's pretty uncommon.
The interesting thing about hydrogen and deuterium is that that ratio of hydrogen
to deuterium in a closed system is conserved.
So, if you can figure out the ratio of hydrogen to deuterium on the earth
right now, and
you could figure out the ratio of hydrogen to deuterium in all these different source
regions, you might have a way of getting at what the actual ratio is.
There's some problems, because if you listen carefully,
I said the ratio of hydrogen to deuterium in a closed system is conserved, and
the Earth is definitely not a closed system.
Hydrogen, in particular, and even water, escape from the top of the Earth's
atmosphere, and as you might imagine, it's just a little bit easier for
something with a tiny bit less mass to escape than for
something with slightly more mass to escape.
It's not a huge effect, but over time,
that means that deuterium sticks around more than hydrogen was.
So, one of the ways to look at that is to,
is to forget about the atmosphere, is to look at the ocean,
and measure the deuterium to hydrogen abundance in the ocean.
This is called SMOW, Standard Mean Ocean Water,
where the ratio of deuterium to hydrogen is 156 parts per million.
Deuterium is not particularly abundant, but
it's easily measurable and and it's there.
And so, the question is, we take this standard mean ocean water, and
we look at the sources.
We can look at the asteroids,
because sometimes we get meteorites that hit the Earth, and
they have a little bit of water content, and then absorbed into the minerals.
Comets are significantly harder, but we can, as a comet gets in close,
we can look at the gaseous emission in the tails of the comet,
in the coma of the comet, and try to measure d to h ratios.
And when we do that, we find that both asteroids and
comets seem to give approximately the right number.
Now, I have to tell you, I find this a pretty unconvincing argument,
particularly because the commentary measurements are so rare.
We've measured the d to h ratio in a small handful of comets.
They are kind of all over the place, but
you'll still see these headlines every once in a while.
Somebody will measure the d to h ratio in a comet,
and they'll come up with the number that is close to 156 parts per million.
And the headline in the newspaper will say, our oceans have come from comets!
And, maybe a year later, somebody else will measure another comet, and
it'll be a very different number.
It'll be 300 parts per million, and the headline in the newspaper will be,
astronomers have determined that our oceans cannot have come from comets.
And newspapers, of course, don't have any sense of what they just said a year ago.
So, they will ignore the fact that just a year ago,
they said that they came from comets.
What that really means is that these measurements are hard.
I think there's a lot of variability in comets.
Even if there's not a lot of variability in the comets themselves,
there's a lot of variability in the gaseous emissions from comets.
Just like I talked about how the small differences in weight of
water can make big differences, in a comet it can do really be the same thing, too.
Asteroids are part of the same story.
In the end, I'm going to tell you that we don't know the answer.
I, I think it's likely that comets and
asteroids are the source of water, but I think we really just don't know, and
it's a, it's a very difficult question to answer.
The interesting part of that question, though is, so far,
for any of these ideas that I've told you, it doesn't matter.
Mars should have had the same process, if not more of it.
For example the asteroids are more likely to, to hit Mars, and
it should have gotten that same delivery of water.
Maybe it was an external delivery of water, but
it still should have had that same delivery of water.
Mars should have started out with that water on it.
And so, if we are seeing no water on it today,
that water has all had to go somewhere.
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