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 4.04: Alternative energy sources
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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
Well, I started talking about photosynthesis and the ways that
photosynthesis was this amazing ability to take common things, like
carbon dioxide and water, and create a storage system for
energy that can be used, and moved around, and exploited.
And I'd said that this is the way that most of the
living organisms on the surface of the earth get and use their energy.
It's arguably not true, sort of depends on how you count most there is an entirely
different energy source that a set of, of
organisms uses, and entirely different set of energy sources.
That a set of organisms uses and, in fact, looks like these are
the organisms that were the very earliest things to evolve on the Earth.
These are called Archaea, and these are entirely separate domain of life.
A domain of life if you remember your
biology, the other two are Eukaryotes and Bacteria.
Eukaryotes are multi-celled, anything that you see multi-celled,
animals people, plants, fish, house cats, all eukaryotes.
Bacteria are single-celled things that we know and love, or sometimes don't love.
Archaea are entirely separate from those two.
They are, used to be classified as bacteria because they are single-celled
things, but they are distinct in a couple of very important ways.
One is that they, they have very different cell structures.
They are, I like to think of them as just sort of loosely held together.
It, we'll see one of the reasons why
that they might have very different cell structures.
It's also because presumably because they were some of the various, very earliest
things to evolve, and eventually led to these other things, or maybe not.
That part is still being argued.
For our purposes, the interesting things
about the Archaea are their energy sources.
Let's look at some of the very different
ways that that the Archaea get their energies.
There are a lot of different versions of these.
One are called methanogens.
You might guess, very quickly, methanogens are methane generators.
Let's see how that works.
The key reaction that they typically do is take a hydrogen molecule and a carbon
dioxide molecule and change it to methane and water.
And to get this to balance right, we better make that a two, and
we better make that a four, five, six, seven, eight, better make that a four.
This looks pretty innocuous.
Remember our previous reaction was, that we
had a CO2 plus water yielding carbohydrates.
And of course, this reaction only occurred when
there was a photon that gave it energy.
This reaction occurs spontaneously, in fact, this reaction gives off energy.
This reaction is more akin to the carbohydrate plus oxygen, which gives
energy for animals, bacteria, and eukaryotes to survive.
So, methanogens are doing something really interesting, we as a people,
eat carbohydrates that were created initially by plants, and we breathe O2.
We combine those two chemically, and we get energy out.
Methanogens are breathing CO2, which is in the
environment just like the O2 is in the environment.
They are eating or harvesting or something,
H2 hydrogen gas, combining those and getting energy.
Now, the obvious objection is, well, if, if you
can create, combine H2 and CO2 and get energy
then that's a reaction that will just occur, so
there's no way this methanogen can do it itself.
So the meth, methanogen has to build an apparatus that carefully harvests
the H2 and harvests the CO2, combines them together, makes the energy.
And then it gets the energy directly from there.
Now what it doesn't get, the nice thing about carbohydrates is that it may
not just a storage system for the energy, we can move the energy around.
Carbohydrates also allow you to build big structures.
starches, celluloses, here, we're just getting energy directly,
and then we get methane as a waste product.
Hydrogen as a waste product.
You can do some more complex chemistry with
the, with the methane and eventually build up
some, some sort of structures and some more
complex things that can carry some energy around.
But in general, you don't get nearly the,
the nice structures that you get with the carbohydrates.
This is sort of one of the reasons why these
methanogens, the archaea in general, which are not making carbohydrates.
These, these archaea are, are significantly more
primitive structures than bacteria, and certainly than eukaryotes.
Where would you find these sorts of methanogens?
Well, they are living in places where there's an
availability of H2, and where there is no oxygen.
Oxygen is incredibly toxic to these methanogens.
So they live in anoxic environments.
Anoxic environments, well, what is that?
That's like a marsh, or the inside of your stomach.
Or in sediments at the bottom of the sea.
In these sorts of environments, they harvest the H2, combine
it with CO2, which they can find, and produce methane.
What happens to that methane?
Well, in humans, from our stomachs, methane comes out
as waste products when we eliminate wastes from our body.
In cows, we know that cows produce methane when they burp.
Methanogens are one of the key archaea that live in these anoxic environments.
And so the presence of abundant methane in bogs, from swamps, in cows, in our
stomachs is a, is a clear signature
of metabolism taking place in an anoxic environment.
This general idea of taking chemicals and combining them to get
energy, as opposed to using the sunlight, is, is called chemosynthesis.
And this is just one of the many pathways that you can
create energy directly out of the chemicals in the environment around you.
What it requires though, is that you're in an
environment where there are chemicals that are out of equilibrium.
Where would you find such an environment besides your stomach?
One classic location is at the spreading
centers between two plates as they move apart.
The two plates spread apart.
Hot magma is drawn up closer to the surface and as the sea floor spreads,
new sea floor is made, made here with, as the magma cools, and there are cracks.
In these cracks, sea water flows down into the
subsurface and that seawater is mixed with hot fluids here.
Get very got water, it dissolves the rock that's at the bottom here.
And you get all sorts of important metals and chemicals that then comes, coming out
as the water gets heated and comes streaming
out through here into these very hot regions.
These are called smokers, black smokers or
white smokers depending on how they actually
look when you see them, I'll show you some pictures here in a minute.
Where you have super heated water coming out, and super heated water is
saturated with chemicals that you might be able to use to form life.
Here's a whole list of some of these
chemicals that you might see in through here.
H2, a very important one you also
have another thing's good for chemosynthesis, H2S.
It can easily combine with CO2 also.
Its waste product rather than methane is
sulfur, solid sulfur which you can sometimes
see in the cells of of the animals that do this sort of chemosynthesis.
We haven't talked about all these other elements.
But these other elements are, are rich nutrients that are important.
We've only talked about energy.
Nutrients are things that you also use to help
build your body and, and transport energy within your body.
You can't live on energy alone.
You actually need to be able to harness that energy effectively.
If you go look at one of these, these deep
sea vents in the, in the center of the Pacific
Ocean where the two plates are spreading apart and creating
new ocean floor all the time, they look something like this.
You have some interesting things going on here.
You can see right here, this is a a little mound that's building
up of minerals that are precipitating out as they come out of this.
Hot water is coming out of here.
This was a white smoker.
You can see this is the smoke coming out, super heated fluid.
More stuff precipitating out through here.
These regions are rich with life.
This is the bottom of the ocean floor,
which is generally a pretty dead place to be.
But around these deep sea vents they are, they are rich with life.
These microbes exist, they're archaea in particular and things eat the microbes.
There are big tube worms, there are shrimp.
There are all these things at the very base of the
ocean living off of the chemistry that's coming out the earth.
We don't know very much about this.
Because it's exceedingly difficult to get to the
bottom of the ocean to see these things.
the, there are a couple of submarines that people can take to the bottom.
The Alvin submarines where you have two people,
a pilot and one scientist and they can get
down to the bottom, spend a little bit of
time down there, collect things, take pictures, come back.
And we're just really now starting to explore what
the environments are like in these deep, deep sea regions.
These, these microbes live in almost every environment that you can imagine.
Things that we think of as, as crazily
inhospitable very saline or very hot or very cold.
Almost anywhere you look, you can find some sort of
microbe that has eked out a living in that spot.
To me, perhaps the single most amazing one are
these microbes that have been found kilometers deep underground.
Some of them have been found kilometers deep
under the ocean floor in big drilling experiments.
And others have been found kilometers deep inside of
mines in South Africa where you can get down.
As, as low as you can go, and you look in the subsurface waters there, and there
are archaea, and they're generally using that same reaction,
that that H2 plus CO2 and and making methane.
Getting energy out of that plus all the other stuff that they do.
Getting the energy from that.
And, what is the source of the H2 in that location?
Very slow radioactive decay of things like uranium,
which eventually leads to a few protons being emitted.
Which, which turn into these molecules.
And then are eaten by the archaea.
How's that for crazy?
Something that lives off of the
products of radioactive decay, kilometers deep underground.
Suffice it to say that microbes have exploited, I would
say almost every environment you could possibly think that they would
be physically capable of exploiting and even some you wouldn't think
they would be physically capable of exploiting here on the earth.
There are limits.
If you get too hot, then your, your cell walls can't
hold together, or whatever walls that you have can't hold together.
If you get too cold, you freeze, although some of them make
their own anti-freeze so they can be colder than zero degrees celsius.
But, in general, they live almost everywhere.
I tell you this story because, when we think of looking
for life these are the things that were here first, these archaea.
These are the things that have been here the longest.
If you were just even coming to the Earth at a random point
in time and you were looking to see if it had life on it.
And you were looking specifically for something like photosynthesis.
You'd be looking in the wrong place.
What you really should be looking for are things like these
archaea, in fact, maybe you should be looking for things like methane.
Maybe you should be looking for those sorts of products, rather
than these oxygen-mediated photo-synthetic products that
we think are so important today.
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