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How Do We Know it When We See it?
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From the course by Princeton University
Imagining Other Earths
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From the lesson
What is Life?
This lecture introduces different approaches to defining life, a necessary step if we are going to search for life. We then introduce the basic building blocks of biochemistry. One potential way of detecting life is through its byproducts like Methane. We discuss efforts to discuss life on Mars.
Meet the Instructors
David SpergelCharles Young Professor of Astronomy on the Class of 1897 Foundation and Chair
Department of Astrophysics
In this segment I want to address the question, how
will we know life when we see it on another planet?
If we, or when we, and as we explore Mars, for example.
How do we know when we encounter life?
And one of the answers to this question
is one posed by the great jurist Potter Stewart.
Potter Stewart was a member of the US Supreme Court.
Wrote many opinions.
But the one that most of us know him for, is
when he was asked to define pornography, or discussing what pornography is.
He said, I can't define it. But I know it when I see it.
And sadly for Justice Stewart this is, of
all his writing it's what most of us remember.
But it's I think a useful thing.
When we think about life, that
and many of us have a sense for if we were to encounter life on Mars.
Encounter some of the signs of life elsewhere.
We'll know it when we see it. But, how might we see it on Mars?
How might we find signs of life?
And I think a useful way to think about that problem, is turn it around.
Ask, if you were an alien observing the Earth from afar.
How might you detect signs of life?
Well one way of detecting the signs of our advanced civilization.
Is that we're making movies, we making television.
We're broadcasting television and radio signal out into space.
And since the 1950s, we've been sending out I Love Lucy and other of those
early 1950s shows. And those TV shows can now be watched
by aliens, you know, 60 light years away. And there's a lot of planets.
We don't know if they have life, but there's
certainly a lot of planets within 60 light years.
They also might detect life by some of the chemical processes life produces.
Plants on Earth produce oxygen. In fact, if there were no
plants, no photosynthesis in the oceans.
Our atmosphere would not contain oxygen.
We'll come back to this later in course, when
we look at the geological history of the Earth.
And we'll see that a major, and
many ways, catastrophic event in the Earth's history.
Was the relatively rapid rise in the abundance of
oxygen, which actually poisoned many of the life forms.
Oxygen is very reactive.
If it wasn't for life, it wouldn't be there.
So just detecting oxygen would be a useful sign.
Other things produced by life include methane and carbon dioxide.
Now there are planets
where methane or carbon dioxide could persist
for a long time without any biological source.
So in some cases there would not be signs of life.
But as we'll see when we come to Mars,
that methane is a potential signature of Martian life.
And let's talk a little bit more about methane.
And talk about the creatures that create methane.
These are called methanogens. They're members of archaea.
An archaea, remember, is the oldest form of life.
It was the most recently discovered.
We've known about you know, bacteria and eukaryotic life for a long time.
But archaea actually,
well the prokaryotes have some of the properties of the eukaryotes.
They're an interesting and comediate case.
And we think they're the oldest case.
And one of the interesting things about these
methanogens is we find them in really extreme environments.
You find them actually inside solid rocks.
And the case of methanogens that do things like
convert carbon dioxide and hydrogen into methane and water.
They're in your guts.
And they are among the things responsible for your flatulence.
So you can thank methanogens for that.
Now that's not why I'm talking about methanogens.
But the fact that they're found in these really extreme environments on Earth.
And if you look at the environments, you find them on Earth.
And later on we're going to be be talking
with Tullis Onstott, a geologist here at Princeton.
Who looks at these methanogens on Earth, mostly because they're
the kind of life form that could survive on Mars.
They're pretty intriguing things to think about.
And people recognized that one of the ways we might detect life on Mars, is if
there were methanogens below the surface. Those would produce methane.
And methane doesn't survive very long in the Martian atmosphere.
About 300 years, it's completely destroyed.
So if we had no source of methane, nothing producing it.
In the, underneath the Martian surface, we wouldn't expect to see any.
There's a lot of excitement over the last decade as
observations first from Earth, and then from a European orbiter suggested
evidence of methane.
And even seasonal variations in the methane level.
So when people observe Mars at different parts of it's orbit.
They would find the levels of methane would fluctuate up and down.
Now this observation was controversial.
It's very difficult to absorb, observe the methane from the Earth.
And in doing those observations, you have to observe it through
the Earth's atmosphere.
When you study another planet, you need to
be careful about what astronomers call telluric lines.
There are gases in the Earth's atmosphere that are emitting
or absorbing light, at the same frequencies as the methane on Mars.
And this methane result has been quite controversial for the past
ten years, as some scientists seem to think
they see evidence from methane in the data.
While others don't really find much evidence.
And that's why there was so much excitement when
the Curiosity Rover landed and started to search for methane.
And the Curiosity Rover was designed to
have something called a Tunable Laser Spectrometer.
A very sensitive device that could detect methane
at the level of a part in a billion.
Relative to the other components that make up the Martian atmosphere.
And with this enormous sensitivity, if those previous results were right,
the Mars Curiosity Rover would see them. And one of the results
that many of us look forward to, Was a definitive detection of perhaps a
byproduct of life by the Curiosity Rover. So far we've been disappointed.
In fact, the results so far, don't seem to find any methane.
And they appear to contradict the previous studies.
And perhaps
those results were contaminated.
This is something where we should all look
forward to more data over the next few years.
Perhaps up to now, Curiosity has been looking
in the wrong places at the wrong time.
For this flatulence produced by life underneath the Martian surface.
Or perhaps those life forms just aren't there.
Time will tell.
[BLANK_AUDIO].
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