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 4.18: Detecting exo-life
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Del curso dictado por Caltech
The Science of the Solar System
286 calificaciones
De la lección
Unit 4: Life in the solar system and beyond (week 2)
Conoce a los instructores
Mike BrownProfessor
Planetary Astronomy
Imagine that you have just discovered a new planet.
It's in something like a habitable zone around its star and, and this
is critical, you have determined that it's a super earth or maybe Earth.
As opposed to a, a Sub Neptune or water world.
Sub Neptune, water world, well, they might have habitable moons or something on them.
But we are not thinking about those as
places where things could live on the surface.
So you've found all the right things.
It's got a temperature that's suitable for liquid water.
It's got a solid surface.
What do you do next?
You can probably guess the answer by now because
we've talked about this a lot in this class.
The next thing you want to do is get a spectrum of that object.
How are you going to get a spectrum of that object?
Well, there are two ways to do it.
It's very difficult if you have the star here and
the planet here, and you just want to isolate the
light from that planet away from the star, and get
the reflectant spectrum, the sunlight
reflecting off here, reflecting towards you.
Very hard thing to do because that's star is so close, and so
bright, particularly if we're talking about something
in a habitable zone, an earth-like distance.
Really, really, really hard.
Also hard, but significantly less hard is to use that technique we've talked about.
A transit wait for or probably discover the planet by the
fact that it transits over the cross of its host star,
and then do a spectrum of the entire event, the entire
star plus planet or star minus planet light in this case.
And look at the transmission spectrum.
This is what you saw demonstrated by Professor Knudsen in her lectures.
And you can do the same thing
for an earth-sized object around a sun-sized star.
Well, not today.
You couldn't do it today.
But you could do it with the next giant space telescope that's going into space.
The James Webb space telescope that's supposed to launch somewhere around 2018.
The James Webb space telescope will be able to get a spectrum of this event.
You compare the spectrum when the planet is in the middle of the star, to aspectrum
on the planet is not, and you get how much light is being removed by that planet.
You get a spectrum of that light, you get
the light that shines through the planet coming towards you.
This is not only perfectly feasible, this will be done with
the James Webb space telescope when it goes off into space.
This is something that is, indeed, coming up in, in, less than half a decade away.
So it's pretty exciting times to see what might happen.
You get a spectrum, and then what?
Well, you look at it and you try to figure out what's going on.
Let's look at some examples of what you might see and what might be going on.
Here are six models of what the earth would have looked like
if you could've looked at it at six separate times in its history.
And, I'm sorry you can't read this very well, this is a
spectrum from five microns right here, up to 20 microns right here.
These numbers should start to mean something to you at this point.
This is the region where the bulk of the transmission, the
thermal emission of the, of the Earth comes from in this region.
This is where the greenhouse gases are causing, my biggest problem is,
is the same spectral region five up to 20 on this side.
And this is what we see.
Epoch zero is the, the earliest stage of the Earth's history.
The very beginning when the atmosphere would have been dominated by CO2.
You see a little bit of water, barely, little bit of water
here, barely, but mostly you see this big broad absorption due to CO2.
The CO2 would have been, from things like out
gassing of volcanoes it's very similar to what the atmosphere
of Mars looks like, except for it's a little thicker,
a lot thicker, than what the atmosphere is on Mars.
What happens over time?
Well, we're in the Archean period.
In the Archean period, there are these
archaea, these microbes that are, they're not
evolved in, in oxygen photosynthesis, they're involved
in other reactions that lead to their methanogenesis.
Other things that and, and you, as
the atmosphere builds up from those microbes, over
here into stage one, you start to see signatures of methane coming up in here.
And still the CO2 dominates.
Up to stage two the methane is continuing to come in.
And you can really see its signature right here.
Very clear that you have methane in this atmosphere.
Just like that thought that a, that methane
must have been coming from microbes on Mars.
Certainly microbes on the Earth early on would have made this very clear signature.
The transition from epoch two to epoch three is an epoc transition.
And that's because this is now the rise of
the oxygen producing bacteria, and do you see the oxygen?
No.
Oxygen actually doesn't have any spectral features
in this region here, but what does, ozone.
I dunno if you can read that right there.
That is, ozone.
We have a lot of ozone in the Earth's atmosphere.
For the most part it's confined up in the top of the stratosphere.
We have some around here in the form of, sort of, smog which
we don't like, but up in the stratosphere there's a lot of ozone.
That ozone comes from oxygen molecules, which are very abundant as you know.
Photon comes in and destroys an oxygen molecule leaving two oxygen atoms.
And that oxygen atom quickly combines with another oxygen molecule and makes O3.
The fact that we have O3 in the atmosphere and the fact that you can
see it is one of the strongest indicators that O2 is present in the atmosphere.
And as you watch, as the oxygen rises, starting from
epoch three, going to epoch four, going to the present day.
You see the change from, CO2 dominated, with a lot of methane, to
oxygen coming in, CO2 starting to go away, methane still there very strong.
Methane is now nearly gone, CO2 is much more relaxed, and this is the signature.
If we saw this, we would have a signature of an oxygen dominated atmosphere.
What would that tell us?
I think it would make people very excited.
I think people would immediately announce that they had discovered a world
that was not only habitable, but inhabited by some sort of oxygen-creating microbes.
And then the arguments would start because there are a lot of complexities of these.
Are there geochemical ways of making these
molecules, are there some sorts of global reactions?
Possibly.
If we saw something as clear as this I think
maybe we really would be allowed to jump up and down.
I suspect the first sorts of things we
see will not be clear cut signatures of oxygen,
but maybe it'll be these signatures of a
CO2 dominated atmosphere, with perhaps some methane in it.
Methane could well be a strong signature of a microbial source,
or we know that methane is produced in, in certain geochemical cycles.
In fact, methane is one of those things that the microbes at the,
the vents in the bottom of the ocean, like to live off of.
Methane can produce, can be produced by the breakdown of rocks
in a, in a process called serpentinization, and methane is released.
So, there could be a lot of reasons that there's methane in through here.
You know if we got, I got us, a spectrum that looked
like this, like a very rich methane atmosphere in a CO2 atmosphere.
Again, people jump up and down, but again, people would probably, try to
be very skeptical and see if there were other explanations that were possible.
And there may well be.
It's, it's not a very easy thing to look at a spectrum of the
atmosphere and, definitively discern that the spectrum,
the gases in the atmosphere must be biological.
What else might you do?
Well, people have talked about, rather than
looking in the infrared, where most of
the thermal emission is coming, look instead
in the visible part of the spectrum.
The visible part of the spectrum would just
be sunlight reflected off the surface of the planet.
And there is one very distinct feature that you
see in reflected sunlight off the surface of the Earth.
And that distinct feature is caused by plants.
It's caused by chlorophyll.
What do we know about plants?
Plants are green.
Why are they green?
If you looked at the reflectant spectrum of a leaf of a plant,
what you would see is that there is around half of a micron,
which is a good spot, which is where your eye is very sensitive,
where there's a lot of sunlight, the leaf reflects a moderate amount of light.
And then there's this big spot where no light is reflected from the leaf.
And then, in the infrared, our eyes can't see it but our cameras can easily see it.
Suddenly the reflectance shoots up.
This reflectance shoots up because leaves reflect all of the
infrared photons, nearly all of these infrared photons that hit it.
Now what's going on?
This region in through here where no light is reflected from leaves is the
spot where the chlorophyll is collecting the photons and using it to make energy.
Leaves are very efficient at collecting all the
photons in this region, but right here at
around 0.7 microns, just beyond where our eyes
are sensitive the, the, chlorophyll can't use those photons.
The wavelengths of those photons are too big and so they can't use
them effectively, and so they completely reject
them, they reflect them off of them.
Nobody knows exactly why.
Maybe a, letting them enter the leaf would cause too much heat.
Who knows?
But in any case, it's a very dramatic signature of a leaf,
of a plant, of a forest, of a planet covered in vegetation.
It's a potentially very interesting way to
look for something like vegetation on a planet.
Because there are a couple things about it.
One is, it's a very dramatic spectral signature.
There are really no minerals that could mimic something like
that, that really have this dramatic red edge here like this.
What's more, you might expect it to change seasonally.
Certainly, if you look at the earth,
there's a stronger vegetative signature in northern summer.
Northern summer because the north has more continents than the south does.
And you can see that change with season.
There's one really good reason why it's not
a very good thing to go actually looking for.
It's because we have no idea if microbes on other planets would be photosynthetic,
much less if they would use the same chlorophyl pigments that plants here use.
There's no specific physical reason why plants have to
use this one very specific region of the spectrum.
Now, you might say, okay if there's some sort of
photosynthesis there, you'll see that same sort of thing somewhere.
Maybe, if not right here.
And the answer is, maybe, you might.
I wouldn't want to count on it.
But I certainly would want to take a look.
And, in fact, I think this is really the key point of
looking for actual life, and the effects of actual life on other planets.
Assuming we're going to see one of these things, much like the history
of the Earth, or something like this, it's something like the Earth.
Those assumptions are clearly very bad assumptions.
Even being a little bit more generous and saying okay, we're not going to assume
the spectrum looks exactly the same, but
maybe we'll assume similar sorts of atmospheres.
We can then make computer models of what those atmospheres would like like if they
were around hotter stars or cooler stars because
that would certainly change a lot of things.
Even then, I think that's just not a very good way to go about it.
I think the right way to go about it is our general feeling of exploration.
We do not know what the atmospheres
of terrestrial planets' inhabitable zones will be like.
Let's go find out.
When we find out, I suspect that we will end up throwing away most of these
models that we've created, even though I think it's a great thing to have done it.
Oh, and I should have mentioned where this model came from.
And I think these models are great.
And making these models is great for generally guiding our thinking.
But in the end, our exploration will simply require finding out
what's there and seeing if we can make sense of it.
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