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 4.08: Mars -- Methane?
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From the course by Caltech
The Science of the Solar System
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From the lesson
Unit 4: Life in the solar system (week 1)
Meet the Instructors
Mike BrownProfessor
Planetary Astronomy
Even at the time of the, the Viking
biology results, and the, the inconclusive Viking biology results.
People suggested that perhaps a better approach, rather than picking up some
soil and looking for microbial activity in a small amount of soil.
Perhaps a better approach would be to look in the entire Martian
atmosphere and see if you could see the effects of biological processes.
As we've learned you can certainly see that in the case of the earth.
And in the early earth you would have seen the same thing.
It was in many ways quite exciting then when it 2003 three
separate groups announced that they had
discovered methane in the atmosphere of Mars.
Methane in abundances which were hard to explain by just normal processes.
At least in the first paper that came up there
was a suggestion that biology was the most likely explanation.
Here's that first paper.
And while it doesn't quite say evidence for
life period, it certainly says evidence for life?
With the implication being that perhaps they think the answer is yes.
And then, they come down pretty clearly on the side
that maybe this is a good explanation for what's going on.
This is a pretty dramatic conclusion and
in a pretty traumatic conclusion like this.
It's important to look carefully at the data and come
to your own conclusions about what you think is going on.
So I remember when this paper came out very quickly flipping through
the actual printed copy, which is mostly what I looked at back then.
Flipping through the printed copy and coming to the first figure
which showed their data in which there was supposed to be methane.
Let me walk you through how this works.
First, the idea is exactly the same idea that
we used back when we were looking at Mars before.
And trying to determine whether or not Mars had water in the atmosphere.
Remember what happened?
We put the Sun here and the Earth here like usual.
And if Mars was here or Mars was here, which meant
that the Earth was moving towards Mars or away from Mars.
It was enough of a Doppler shift in the absorption lines of water
that, that lines could be Doppler shifted
out of the terrestrial water absorption lines.
And we could potentially see them.
Remember how we saw those from the early paper [UNKNOWN].
To detect methane you have to do the same thing.
There's sufficient methane in the Earth's atmosphere that you're going to
have a hard time seeing it in the atmosphere of Mars.
Unless you can shift those lines.
Okay, so these observations were made either here or here,
we'll be able to tell by looking where it is.
And I'm going to show you first the data which
show you most clearly the water in the Martian atmosphere.
Okay, so this is units of wave number, usually we
saw things in wave length, which I usually call lambda.
Wave number is simply 1 over the wave length.
And for calibration purposes this regions is right around 3.3 microns.
So this is an infrared.
Remember those early observations from high-spin rad we're at around 0.8 microns.
This is what you could do with photographic plates back in the 60s.
In the, by the 2000s, we had nice infrared
detectors, and could take these, these beautiful infrared spectra.
And what do you see?
Well, here is terrestrial water absorption.
Terrestrial, terrestrial, terrestrial.
And every time you see a terrestrial water absorption, you
see a little blip to the side that looks like that.
You don't see much there, but you see it there, you see it there, you see it there.
That is Martian water a very clear detection of water on Mars.
In this region spectrum, you also have terrestrial methane.
And what you should expect is that there be
a little blip shifted by precisely the same amount.
And well, a little blip would be hard to see here, how about here?
Yeah, a little hard to see here.
But in this region of the spectrum, this is a region where
there is so much absorption from
terrestrial methane that there is no light.
This is this scale actually literally goes down to 0.
No light gets through here because methane is absorbing so much.
Right here at the very edge, though, there is a little bit of
a line and there should be similar line due to Martian methane right there.
The strongest methane line in this region, as you can see
because it's so deep here, compared to these very weak lines here.
So this would be the strongest Martian line in this region too.
Let's go look really carefully at this region and see what we see.
Okay, here we have it.
We actually just have this region cut off right
about here and going up to right about here.
Blowing it up in gory detail and this
is the exact spot where Mars methane should be.
This very thin line that you see from here going
up to here is what terrestrial methane should look like.
It should be very flat in through here.
And so if you subtract this expected signal, the flat signal
from this kind of wavy signal, you get something that looks like this.
And multiplied by a factor of 2 just so you can see it better.
And you see, yes indeed, right there, as you
can even already see in the raw data, right there.
Precisely where Mars methane is expected, you see the deepest dip of any.
That is the detection of methane on Mars.
Now, I remember looking at this and I look at
it again and I think that is an easily disputable claim.
Look at this, this is, yes this
is the deepest absorption right there negative point.
But these wavy patterns go up here.
There should be no reason for 'em to go up so you can just
use that up and down to sort of mentally calibrate how accurate this technique is.
If it goes up, we know that that's an inaccuracy
so it goes down might also be part of the inaccuracy.
Here it goes down not quite as much but almost as much.
Again, helping your brain calibrate the inaccuracies there.
This should be 0.
Look at these, these go up and it goes down below 0.
You can't really go below 0.
All of this stuff is the error, the error level of what's going on,
and the error level, I'm telling you, looks awfully similar to the actual data.
They don't show you the data extending through here,
but it should be a straight line through here.
And you would see these downs and ups and
downs and ups are not dissimilar to this one.
I look at this and I think, I am not
even convinced you have detected methane, much less evidence for life.
But, as I said before this was not the only detection.
There were two other groups that detected methane independently.
And you would think that this would be an argument that maybe it must be real.
I think this is actually an argument of jumping on the wagon.
I think that people heard that the other groups were going to
announce the existence of methane and they sort of had these sort of dubious spectra.
This specter that looked like this and wanted
to jump in and say the same thing first.
I don't know who said it first and who didn't say it first.
But I think this is the case of, of
really is the bandwagon effect rather than three independent verification.
And I really think that there is not very compelling
evidence that there is any detection of methane from this data.
It took a couple of years, but eventually there were data that I
have to say, looked more compelling, at least from a pure data sense.
And let me show you those.
Okay, and this is complicated looking so let me, let me walk you through it.
This is, again, a spectrum.
The telescope on the earth looking at
Mars, looking for absorption in the martian atmosphere.
And now instead of looking at the whole of
Mars, like that original data that I showed you did.
They are taking a small slit, putting it across the face of
Mars like this, and independently looking at every location along that slit.
That's what you're seeing here.
Each of these little bitty lines that you see here is
an independent spectrum at every location along the surface of Mars.
You see, again, wave number down here, inverse centimeters so 1 over lambda.
Again, this is around 3,000 wave numbers, so that's about 3.3 microns.
It's not exactly the same wavelength region
as the previous one, but more or less.
And you see here a model of what the terrestrial atmosphere should be doing.
Here's a big absorption due to methane in the Earth's atmosphere.
Water in the Earth's atmosphere.
Water again.
Water, water, and methane again.
And so what we're looking for are absorption features
shifted by a little bit compared to the terrestrial.
This time because Mars is on the opposite side of
the Earth and the shifts are going in this direction.
And let's look at the water first, because we know that there's
water there and we should be, clearly be able to see it.
And sure enough, here is the terrestrial line, here is the Martian line.
Now, these are not the raw data.
If I blew up the raw data right here, it would look like, something like this.
And here's the terrestrial line and here's the Martian line.
And they took their model of the terrestrial
line and subtracted it from the raw data.
[SOUND] And have the residuals right here.
But anyway, you see the Mars water line.
And you see it's stronger at the North Pole
than it is down here at the South Pole.
This is a consequence of the season.
Something that's been known for a long time.
Here's water again.
Little bitty line here, because there's a little bitty line here.
Big strong line here, big strong line in through here.
Good.
We know things are working.
Let's take a look at the methane.
Here is the terrestrial methane.
Exactly the same shift should be, right there.
Look at that, and that, and that, and that.
And all these stronger here than down here, just like the water line.
Let's look at a few more.
The other strongest methane line is this one right here.
Should there be something?
Yes.
Strong there all the way down to weak there.
That is, one the face of it, I have to say, a pretty convincing detection.
It's not just a single detection that you could argue
one way or the other that something funny was going on.
It's a detection where you see it more in one location than another so.
If there were any systematic problems in the
whole data, they should be equally systematic across here.
It's, it's, it's convincing except for one really important
thing, the lifetime of methane in the Martian atmosphere.
If you put methane in the Martian atmosphere, the sync, the
reason that methane eventually goes away is destruction by photons, by sunlight.
Sunlight will come in.
It will eventually hit the molecule and turn into a CH3
radical, and an H and the CH3 will then do something else.
No more methane.
The lifetime to this photo destruction is something like 300 years.
If you put a methane molecule in the Martian
atmosphere, it will last for, on average 300 years.
In 300 years, you would mix that methane very uniformly through out Mars.
In fact, in just a few years you
would mix that methane very uniformly throughout the Earth.
It is very difficult to imagine how you would have more
up here at the north, then down here at the south.
There's an even more troublesome thing that happened, is, is that I'm
showing you the single best slice of data that they have right here.
That they observed many times.
And many times, they saw no methane at all.
The conclusion that's drawn, is that not only is the methane spatially variable,
more in the north this season than in the south in this season.
But it's also temporally variable, that there
are releases of methane at certain seasons.
That the methane spreads and then is quickly disappears.
This is totally contrary to this idea, not just idea, this very well
understood fact that photo destruction takes 300 years on Mars.
Now is there, is there a possibility
that something else could be destroying methane?
Sure, nobody has ever been able to demonstrate
any possible idea of what it could be.
Is there some way that methane is sucked into the surface?
Maybe.
But, again, no convincing case was made.
The conclusion that I came to.
And I think, a pretty large majority of the the skeptical
scientists who both understand how difficult these sorts of observations are.
And understand the difficulty of getting
away from this 300 year photo-chemical lifetime.
The conclusion was no, there is not variable methane on Mars.
In fact, there's probably non, none of these
things are actually detections of methane on Mars.
But there is still some systematic problem that's going on.
What comes to the rescue?
Well, the Curiosity Rover.
One of the instruments the the Curiosity Rover has on it
is a spectrometer to measure the abundance of methane on Mars.
Rather than having to look through
a telescope they actually take the atmosphere.
And put inside of a chamber and pass a light through that chamber.
And in fact, they take a light at, at several different wave lengths.
Pass it through the chamber and look to see if it's
absorbed right at those wavelengths where methane is supposed to absorb.
They actually do a really cool trick and put it through
something called a Herriott cell, where they take two mirrors like this.
They inject the light right here.
Here's the atmosphere.
The light goes in, hits a mirror, bounces,
a very specific pattern that I can't really draw.
But it bounces some ridiculously large numbers of times.
Before it finally passes out a hole right here.
What that means is even though we have
a small amount of atmosphere in through here.
The light passes through that atmosphere many many many times.
And so the probability of getting absorbed is much higher, and
it lets you measure these very very small abundances of methane.
And what was the results?
No methane, no methane and really low level at a part
per billion, is the upper limit to what could possibly be there.
That's a factor of something like
ten lower than what these measurements suggested.
That's so small that it's, it's something like the amount of
methane that would be produced from the burps of 100 cows.
If 100 cows live there, and they had methanogens living inside of their guts.
And they burped.
You would see this much methane in Mars.
Not very much.
What's going on?
Well, the people who really would like to believe
in the telescopic observations say, oh, no, no, it's okay.
We knew it was variable.
You just happened to look when there's not any there.
It's a hard argument to dispute because well, maybe
they just happened to look when there's nothing there.
The Curiosity mission will continue to monitor for methane.
In fact, they've looked over many months now.
And have still continued to not see more.
And what's even better is that they can take the atmospheric gas
and suck in even more and concentrate it to very high pressures.
And do it again.
And they should have a much more robust measurement of just how
little methane there is there, if there is methane there at all.
Maybe don't to the level of, of one cow.
So pay attention to this methane discussion in, in the years to come.
The the lack of the detection of methane by Mars Curiosity was,
was a big result that made the news in a lot of ways.
And sometimes the news said, scientists are surprised that there's no methane.
Well no, most scientists had already concluded
that the previous observations were probably not true.
If it comes down to less than one cow worth
of methane on there, you know who will be surprised?
Me, as I said earlier it is inevitable that there's life on Mars.
Mars was contaminated by Earth early on.
It seems to me that there's no way you
could prevent there being methanogens there somewhere living deep underground.
Pumping methane into the atmosphere.
If they were there, if they were pumping the methane
into the atmosphere, 100 cows worth is not very much.
The fact that there is less than one part
per billion in the global atmosphere, I'm already surprised.
I'm already a little bit worried.
Maybe my, my assertion that it is inevitable, maybe my
idea that there's no way you couldn't have life on there.
It could just be wrong.
Eventually there has to be some amount of methane in the atmosphere.
Because eventually you'll, you'll get to the level where
just normal chemistry where, where decays of, of rocks.
And, and radioactive decay in materials eventually making methane.
And that level might be down to that one cow level.
But as it is, the small, small amount of methane
that we have there is perhaps the strongest argument that exists.
That there are no methanogenic bacteria living anywhere on Mars.
Because regardless of where they lived, they would get into the atmosphere.
And would last for long enough that Mars Curiosity would eventually see it.
Is that lights out for Martian Biosphere?
Maybe not, you know, maybe there are not methanogens.
Maybe there are things that are producing other waste products, instead of methane.
But I have to tell you, for, those of us who, kind of were pretty sure
that there would be, large amounts of these
microbes, if you looked in the right places.
This, this low, methane number came as quite a blow.
And, un, an even lower, is going to be, if not the
death, of Martian biology something that smells, a lot like it.
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