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Lecture 4.03: Water
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The Science of the Solar System
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Unit 4: Life in the solar system (week 1)
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Mike BrownProfessor
Planetary Astronomy
In the last lecture, we talked all
about energy and energy creation and energy transport.
We touched on one other thing, that might be critical for life.
And that was, of course, water.
Why might water be critical for life?
In fact, why is so many other
things we think about habitability associated with water.
There are a couple important points to consider.
One is that it's very difficult to conceive of
life that is not intimately associated with a liquid.
Forget about water for a minute, let's just say a liquid.
Why is that?
Well because, this life is having these chemical reactions,
extracting energy somehow, storing energy somehow, releasing it later somehow.
All these processes in, in life as we know it and life as we
can conceive it require chemical reactions that
really, mostly only take place in liquids.
You're not going to have gas life.
I mean, imagine trying to have, have these chemical reactions
taking place in a gas somehow in balloons or something.
I don't know.
It's very hard to imagine a gas life.
That doesn't just immediately dissipate solid forms, of course you can have
imagine having solid H2O and solid CO2 and energy input and nothing will happen.
They don't react if they're in the solid form.
They don't have a chance to interact in this solid form.
Liquids allow them to be dissolved inside this liquid and
allow them to actually have the chemical reactions that are needed.
In fact, I, I, I think it's fair to say to think of life on the earth including
us as basically we are these big bags, made
out of carbohydrates, that are designed to carry water around.
And we need to carry water around, so that we
can have the chemical reactions, that allow us to be alive.
Think about that, next time, you see somebody
you don't really like, walking down the street.
Think, you're just a big bag, designed to carry water around.
Although.
So are you.
So we need some liquid.
I, I think it's fair to say that that is a necessity.
I can't imagine any way to make general life that
doesn't involve some sort of liquid for storing, for transporting chemicals.
But is it water?
Do we need water?
Is water something special?
Well, there are couple of reasons why water is special.
First, astrophysically.
Water is an abundant molecule in the solar system.
In addition to being common, it's one of a very small number of molecules that
we know to be a liquid in a liquid phase anywhere in the solar system.
And it's by far the most common one and
by far the most common liquid in our solar system.
So, those are good reasons to be intimately concerned with water.
Of course photosynthesis the, the version we showed involves water.
You don't have to have water for photosynthesis.
That's, that's a, we showed that because that's the one is most common.
So, that's not a good reason think there has to be water.
There's one other good reason that water is, is a critical thing here.
And that is the water is a polar molecule.
you, you might have heard water being described as the, the universal solvent.
Things dissolve in water better than in almost anything else, and that
the inherent reason for that is because water is a polar molecule.
Let me show you what I mean by polar molecule.
Here's a sketch of what water molecule looks like.
Here's, here's the oxygen atom here in the middle,
hydrogen on the end, hydrogen on the other end.
And it's bent, which is just a function of
the physical chemistry of these particular atoms coming together.
And what I'm showing you in the, in the shaded
part here, is the residual electrostatic charge that this molecule has.
Okay, let's think of it this way.
The hydrogen atom is really just a proton which is a positive charge.
And another proton with its positive charge.
No neutrons involved.
And, of course the, the oxygen nucleus has
a bunch of positive charges in through here.
But the oxygen also has a lot of electrons around.
And, and this covalent bond that water has
they are hydrogen and the oxygen are sharing
electrons back and forth between them in orbitals
that look, you know, sort of like this.
The fact that they're sharing the electrons
is what causes this bond between them.
But it also means that in, in very simplest
terms, the electron that's associated with this hydrogen atom tends
to spend more time between the two of them
than it does on [SOUND] this side of the hydrogen.
The electron tends to spend over here, more time than it does over here.
As a net result, you could almost think of it as this bare
proton over here sticking out on this end, sticking out on this end.
And because it's overall neutrally charged, you can tell from
that that there is a net negative charge on this end.
This is what's meant by a polar molecule.
It has a polarity just like a battery has a polarity.
And this shading that you see in the
background shows you the overall regions of the charge.
Over here it's generally positively charged.
Over here it's generally negatively charged.
Now what happens?
Well, if we have this thing that's generally
positively charged over here and generally negatively charged
over here, and we have water, which is
a fluid which can move around, what's going to happen?
Well, you're going to have things sort of forming like this.
Positive and negative charges like to move together.
You're going to have hydrogen bonding, it's called.
Hydrogen bonding is not the same as the bonds
that, that form the water molecule, the, the, oxygen
and the hydrogen that the, that, are very tightly
bounded in this covalent bond, where they're sharing electrons.
It's a weak bond, it's just that there's a slight force between the two of
them and so they tend to preferentially line up like this in the hydrogen bond.
Why does this matter?
Well, because, not just do they do it in the water molecules themselves, but
if I put some other substance in there, let's say NaCl, also known as salt.
What happens to NaCl?
Well, NaCl, when you put it in water, it it dissolves, and when it
dissolves, it turns into a positively charged
sodium atom, and a negatively charged chlorine atom.
It turns into that because this polar molecule wants to grab onto
those things it'll attach hydrogen bond to this water molecule this way.
There's the positive, positive-negative.
The chlorine will be over here.
There'll be the oxygen would grab onto the sodium and
that, that attraction will pull those, will break this ionic bond.
Pull those things apart, the object will dissolve.
Other liquids that don't have this polar nature, they're not
very good solvents, they will not dissolve these ionic compounds.
Why do you need things to be good solvents?
Again if your going to transport molecules, if your going to transport
energy, if your going to transport things that you're going to build things,
and you're going to transport them in a liquid, you need to
be able to dissolve in that liquid, if i wanted to transport.
Sodium chloride, and, I, put it
into something where sodium chloride didn't dissolve.
Well, you can imagine trying to push those salt grains along in some sort of river.
but, it's a lot easier if you can dissolve the salt grain and just, move, tiny bits
of fluid around a cell, instead of trying
to, push salt grains, which would never actually happen.
There are other reasons that this hydrogen bond is important.
One other good one is that one of the
reasons that water is so ubiquitous, liquid water is
so ubiquitous in the solar system, is because it
actually has a fairly wide range of liquid stability.
At atmosphere pressure it's from, you know,
0 degrees celsius, up to 100 degree celsius.
And that's a wider range than most other liquids.
And one of the reasons for that wide
range is because it takes extra energy to vaporize
water because of this extra hydrogen bonding that has
to be broken to get it into vapor form.
But really, the key point, I think, is this dipolar nature.
Without the dipole, we would probably not have
the ability to form life here on Earth.
If you need a polar liquid and you're on Earth, what are you going to find for it?
Well, you're going to find water.
It's the most common liquid on Earth, and it's polar, perfect for life on Earth.
What about life elsewhere?
Everybody's other favorite polar liquid is ammonia.
Why is ammonia polar?
Well, you can't, maybe, see it as well from here.
This is the nitrogen.
Here's the hydrogen.
Hydrogen, hydrogen but, it too is a bent molecule, the nitrogen is here like
this and the hydrogen make sort of a pyramid structure coming down like this.
So, you have a nitrogen on top, all the hydrogen is on bottom.
It's very similar to the H2O where you have
the two hydrogens down here and the oxygen up here.
So, you have a polar molecule with the positive charges down here
on the bottom and a negative charge up here on the top.
If I drew it from the side, I would be able to draw, like this.
Here's nitrogen and the three hydrogens.
This one sticking backwards.
And the positive charges all down here.
Negative charges here.
You get this same sort of hydrogen bonds in ammonia.
And ammonia is also a very nice solvent.
Where might you have liquid ammonia?
We'll talk about that.
Ammonia is a moderately abundant substance in the solar system.
It's one of the main places you find nitrogen in the solar system.
If you remember your commentary compositions, ammonia was
one of the more abundant of the species.
Of course, relative to water, everything was relative to water on
the comments, it was only a few percent relative to water.
But still, there's plenty of ammonia out there
in the solar system and, presumably, in the universe.
For most of the rest of these lectures,
we're going to fixate on water, itself, much the
same way NASA does when it flies to Mars and, with the mantra, follow the water.
There's a reason for that; we need some sort
of liquid to be able to have the chemical
reactions for life, we need some sort of polar
liquid in order to dissolve things, to carry them around.
What is the most abundant polar liquid in the entire solar system?
I would even be willing to bet, in the entire universe, water.
Let's go follow the water, see what's there.
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