The search for life on Venus with Clara Sousa-Silva | WIRED Live

Clara Sousa-Silva is a molecular astrophysicist

at the Harvard Smithsonian Center for Astrophysics.

Clara spends most of her time studying molecules

that life can produce so that one day,

she can detect an alien biosphere.

Her favorite molecular bio signature is phosphine,

a terrifying gas associated with most the unpleasant life.

Please welcome Clara Sousa-Silva.

Hi there, yes, I am Clara, and oh,

I can see everything, yes.

Thank you so much for the introduction.

It's a absolute pleasure to be here.

What I do is exactly as it was described,

I try to understand how to detect life beyond Earth,

and today I'm gonna be talking to you

about how we're getting that done.

Now, we know it's possible to look for life beyond Earth

because we have been telling the galaxy

that we have life on Earth for a long, long time.

Sorry, without even meaning to,

humans have been sending out radio signals

for over 100 years

and audio visual signals since our first strong TV broadcast

in the late 1920s.

So we know if there are any intelligence species out there

in the galaxy, some would intentionally

or otherwise be sending out signals like these, radio.

And all we have to do is wait for the message, listen in.

But so far we've heard absolutely nothing, just silence.

And the silence would be very disappointing

for someone like me whose entire career

is focused on finding life,

but there's one important point to make here,

and that is that in our planet,

there have been billions of species, but only one of them

has been sending out radio signals into space

and only extremely recently.

But Earth itself has been teeming with life

for billions of years.

And although of course I would love

to find some advanced benevolent alien civilization,

I would happily settle for finding alien zebras

or alien forests, alien algae.

So how can we detect life on a planet

when it's not actually broadcasting its presence

through radio or TV?

Well, to illustrate that, let's reverse the problem.

Imagine an alien astronomer

was on the other side of the galaxy

looking for a habitable planet in her night sky,

about 200 years ago

before humans started fidgeting with radio.

If this alien astronomer

pointed their powerful spectroscope,

their telescopes or our sun,

they would be able to split our sun's light

into its spectrum

and this is exactly what they would see.

They would see that our sun's light has been broken up

into a rainbow, a spectrum,

but they would also see that some of the light

would've been absorbed by atoms and molecules in the sun

and leave behind these tiny shadows

that we call absorption lines.

And that's because molecules behave in a really unique way.

Molecules have to follow the laws of physics,

but they're so small that they have to follow the rules

of the quantum world.

And one of those very strict rules

is that they have to absorb quantized amounts of energy

and these correspond to specific wavelengths of light

and consequently, unique absorption lines on a spectrum,

different stars of different compositions

and consequently a different spectrum and this is our suns.

So that an alien astronomer would look at this spectrum

and using their universal knowledge of how atoms

and molecules behave

know exactly what the composition of the sun was.

But if they were patient and fortuitously aligned,

they would notice that every year,

a few additional absorption lines would appear

and then disappear, and then a year later appear again

and these new lines would be due to the molecules

and the atmosphere of the Earth

as it passes in front of the sun.

And an alien civilization

with only slightly better technology than we have now

would be able to study these absorption lines

and learn that we are a thriving complex living planet.

You can think of our atmosphere

and its subsequent spectrum as a planet size message,

communicating to the galaxy

that we have oceans and forests and rich life cycles,

and we can do the same to them.

Look for the signs that alien life didn't mean to make,

all the molecules that life can produce

and release into an atmosphere and form a biosphere

that we can decipher.

Molecules, for example, like these,

which life that is familiar to us

produces in often in large quantities.

The problem is none of these molecules

would on their own necessarily indicate life

as they all have false positives,

by which I mean non-biological sources

that can also emit these molecules into an atmosphere,

things like vulcanism and photochemistry.

These molecules are wonderful,

but thermodynamically speaking, they're easy to make

and so detection of any of these molecules on their own

in an atmosphere wouldn't necessarily indicate life.

Now, there are two solutions to this false foster problem.

The first is to think of context.

For example, oxygen on its own

is not a particularly good bio signature,

but in the context of our planet, our sun,

and the other components of our atmosphere,

oxygen is a wonderful bio signature.

Sadly, context is not always easy to determine,

mainly because atmospheres could be made up

of a lot of molecules and so this context is very complex.

My old group at MIT tried to come up with a list

of all the possible atmospheric gases

that could form the context of a biosphere

and they found that that list contained 16,367 molecules.

So you can imagine quite how hard it would be

to establish atmospheric context,

which in turn makes it extremely difficult

to resolve the false positive scenarios

for those popular biosignatures,

which brings me to the second solution

to this false positive problem.

And that is look for molecules

that although may be less popular

are biosignatures with very low false positives

and so need less context to signify life

and my favorite example of these molecules is phosphine.

Now, when I first met phosphine,

it was very much considered a bad bio signature.

In fact, phosphine was only known for two things.

One is a marker for violent storms on Jupiter and Saturn.

Phosphine is detected in the upper layers of these planets,

but it's a little surprising

because phosphine is not supposed

to be able to be formed there.

Phosphine needs much higher temperatures

and much higher hydrogen pressures than we find

in the observable layers of these planets.

What happens is phosphine,

after being happily formed in the hellish steps

of these planets, is aggressively dragged up

by strong currents, surviving to the top

with very large concentrations

before ultimately being destroyed by the sun

and other radicals in the atmosphere.

But rocky planets like the Earth

don't have those extreme environments

like you find in the depth of Jupiter.

So phosphine is never made spontaneously,

which brings me to the second thing phosphine is known for.

On Earth, phosphine is notorious for being a lethal

and foul smelling molecule.

Phosphine interacts fatally with oxygen metabolism,

so it's a very effective killer.

For this reason, we often use phosphine as pesticides.

And sadly for also for this reason,

humans have used it as a chemical warfare agent

in the First World War and most recently by ISIS.

So phosphine is deadly, but it is only deadly

because of this interaction with oxygen metabolism.

And so it's deadly with a very important exception,

that is life that doesn't rely on oxygen

can happily produce phosphine.

And on Earth,

we have such life forms in places such as sewage,

marshlands, rice fields, lake sediments,

the intestinal tract of fish,

the intestinal tract of babies,

the feces of penguins, the thoughts of badgers,

and actually the intestines and increments of most animals.

And what all of these ecosystems have in common

is that they host anoxic life

that produces phosphine quite happily

since phosphine is not toxic to them.

And for the majority of time the life exists on Earth

a long time ago, it also didn't rely on oxygen.

So other planets with lifeless oxygen

loving than that on modern Earth

could also produce phosphine as a good robust bio signature.

With that in mind, my team and I

simulated loads of hypothetical planetary systems

with phosphine producing biospheres

and we found that with near future telescopes,

we could find it reasonably easy

to detect these phosphine producing biospheres

on hydrogen-rich and CO2 rich planets, sun-like stars,

and also smaller stars like brown dwarfs.

And this made phosphine a really promising bio signature.

What makes phosphine a really good bio signature

is that it seems to have no significant and false positives

as long as it's found on rocky planets

so definitely not a sign of life on Jupiter.

We came to this conclusion

by considering every false phosphine scenario

we could conceive of.

We looked at standard chemical processes

and we found that in all cases,

the formation of phosphate on rocky planets

is highly thermodynamically disfavored.

So then we looked at more intense systems like lightning

or volcanism and meteors,

and we found that even in the most favorable scenarios,

phosphine could only be produced in teeny tiny quantities

and always many orders of magnitude

below anything we could detect.

So at that point, we looked into

increasingly more implausible formation mechanisms

and found that none could produce any phosphine

that we could ever detect.

So with this finding in mind,

I published an article in January this year

with this tweetable conclusion saying any detectable amounts

of phosphine found on a rocky planet

cannot be explained without life.

But this manuscript spent about a year

and a half in revision.

And when I submitted this paper in 2018,

it was a cool conclusion,

but it wasn't a controversial conclusion

because it was just me describing

a completely theoretical, hypothetical situation

no one was concerned about.

For years, I told many board audiences about phosphine

and how great a BioSphincter it was.

I asked them to imagine these kind of distant planets,

wet, anoxic, tropical paradise

with a rich anaerobic biosphere producing tremendous amounts

of phosphine signaling life ambiguously.

And some of you might now know

that this was a very naive attitude

because a few months later,

I got a weird email from Jane Greaves

and Astronomer Akanda saying,

I think I found phosphine on Venus.

Is that weird, I think it's weird.

I'm paraphrasing here, Jane is very professional,

but Jane was telling me that her

and her colleagues had a detection of phosphine

in the clouds of Venus,

which is the only potentially habitable location

on that planet.

Jane knew that might be a big deal,

but she didn't know anyone

who knew about phosphine in an astrobiological context.

Fortunately, Paul Rimmer, a mutual colleague,

had been a member of one of those board audiences

where I told everyone how amazing phosphine was

as a bio signature.

So Jane got in touch to ask how good a bio signature is it?

And I told Jane what I'm telling you right now,

which is it's a wonderful bio signature

and are you sure you found phosphine

because that's insane.

And that was the beginning of our collaboration.

We got extra observation time with Alma,

a powerful telescope that seemed

to confirm the signal detected

with a weaker telescope, JCMT.

And we worked really hard to figure out A,

is it really phosphine, and B, if it is,

is there really no way of making it without life?

We considered every possible molecule

and we found that phosphine was indeed the best candidate

for the signal.

And we found that the signal corresponded

to very high concentrations of phosphine.

And so at that point, led by William Bains,

we expanded my analysis of phosphine

on hypothetical exoplanets

to apply to the very concrete example of Venus

and we couldn't, no matter how hard we try,

explain the presence of phosphine

in the quantities we founded without life intervening.

So at that point, being faced with the exhaustion

of all abiotic means of production,

we seize on the possibility

that this phosphine might just be produced by life.

Now there's still much we don't understand about Venus.

So all we really know

is that something strange is happening on Venus,

some exotic unknown chemistry.

Now, whether that's exotic unknown biochemistry,

that's something for us to figure out

with the rest of the scientific community.

And there are many next steps

to understanding phosphine in Venus

starting with addressing these primary uncertainties

of the discovery.

And we will need many more observations, more models,

and more work to understand this.

But what I want to highlight right now

is how a virtually unknown

and really quite revolting molecule

became such an important piece in the puzzle

for figuring out life beyond Earth.

And we cannot look for phosphine on planets

all over the galaxy, but the lesson here

is not that we should look for phosphine,

but we shouldn't ignore molecules

just because they're not abundantly produced by life

that is familiar to us.

Phosphine is just one of those thousands of molecules

that could form a biosphere,

and we are currently not able

to detect the majority of them.

In fact, we can only detect a 4% of them.

For the majority of bio signatures,

we are not prepared to detect them.

And I'm currently working to solve this problem

with many students from high school PhD level,

but right now, we are not prepared to both detect

and understand the presence

of a biosphere on an alien planet.

So my biggest professional concern is not that we will fail

to point our telescopes at an inhabited planet

in our lifetime.

My biggest professional concern

is that we'll point our telescopes directly

at an inhabited planet,

but not have the tools to know it.

Together with a large group of students,

my goal is to provide the tools

to understand alien biospheres so that one day,

we will know life when we see it, thank you.