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When considering the possibility of life beyond Earth, we look for three main ingredients:
the first one is key elements such as carbon, hydrogen, oxygen, and sulfur.
The second one is a source of energy,
and the third, and perhaps most important, is the existence of liquid water.
Water is a necessary solvent in all chemical reactions that have to do with life.
Energy is required to drive these chemical reactions
and organic matter is the material from which all life that we know of is made.
Life as we know it requires liquid water.
Scientists believe that life on Earth started in our oceans.
Now through our exploration of the Solar System, we’ve realized that the moons
around the giant planets have the right conditions that there could be liquid water
underneath their surfaces.
So that really sort of expands our whole concept of where you could have a habitat
where we might find life.
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Water is fairly common in the universe.
We’ve seen traces of water in large molecular clouds between stars.
We’ve seen traces of water in protoplanetary disks.
We’ve also seen traces of water as water vapor in the atmospheres of giant planets
around other stars, and we know that water is in the atmospheres and interiors
of our Solar System’s giant planets.
So, we know that water is ubiquitous throughout the universe.
As far as liquid water, that’s a little less common.
Earth is the only planet in the Solar System where we see water at our surface.
Moons such as Enceladus and Europa may have liquid water beneath layers of ice.
We’re really expanding our understanding of what makes a place habitable.
Instead of just looking for an Earthlike, terrestrial planet that’s a very specific distance
from its star, we’re learning that there can be hidden habitats
that are underneath icy layers,
and they can be a lot further out from the Sun.
So we believe icy moons in the Solar System actually harbor kilometers thick oceans
underneath their icy surfaces.
These icy moons and their subsurface oceans may be some of
the best places to search for life elsewhere in our Solar System.
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Enceladus is one of Saturn’s many moons, and it’s a very small moon
that people tend to kind of ignore because it is so small
about 500 kilometers in diameter.
But decades ago, in the 1980s from ground-based observing,
we found out that the location of Enceladus relative to Saturn
happened to coincide nicely with Saturn’s E ring
and so we were thinking that Enceladus had something to do with the E ring
particulates, that icy material, but we weren’t sure.
What we later find from Cassini was that we directly determined
that there are indeed plumes jetting out of the south polar region from cracks
in the south pole of Enceladus in the crust.
And it’s dominantly water-rich material just jetting out into space.
And so the way we saw it, Cassini happened to be located where Enceladus was backlit
from the Sun, and so you saw this curtain of beautiful, diffuse material
jetting out of the south polar region – quite breathtaking actually.
Even more, we were able to use the different compliments of instruments
onboard Cassini to go after the chemical composition of the plumes,
and that’s where things got really interesting. So number one, that’s because of liquid water.
There’s definitely a liquid water reservoir subsurface below the icy crust that is there.
Number two, the chemical composition of the plumes told us that there is a lot of organics
things that make up amino acids and things on life that are very interesting.
And number three, what we were really looking for was a source of energy.
On Enceladus, photons from the Sun aren’t going to work because you can’t penetrate
the tens of kilometers of icy crust to get down to where the liquid water reservoir is.
But, what Enceladus does have is hydrothermal vents.
It’s very hot with the liquid water, that has a lot of analogies with the ocean floor
where we have a form of releasing chemical energy via something called serpentinization.
And so we think that Enceladus might have that potential
to have an energy source being chemical, not sunlight.
And so you put all that together and Enceladus has all the ingredients,
or most of what we need for life.
. That makes it a very astrobiologically interesting object to study.
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Europa is one of the largest moons of Jupiter, and we believe that Europa has
a subsurface ocean tens to hundreds of kilometers thick
And so this ocean may be one of the best places to search for life in the Solar System.
There’s been three space missions that have provided evidence for
Europa harboring liquid water.
The first one is Voyager in the late 70's, the second one is the Galileo mission
in the late 90s, and most recently Hubble, which detected plume-like emission
from hydrogen and oxygen which is closely related to the existence of water
beneath its surface.
These plumes may be directly ejected through cracks in the surface of the moon,
and therefore what we are seeing in water vapor plumes
is the actual ocean water from the subsurface of the moon.
As these plume particles are ejected to space, solar radiation is going to excite these
water particles, creating vibrational modes. Now, these vibrational modes are signatures
that can be detected at infrared wavelengths by the Keck Observatory.
So, we observe Europa on seventeen dates. What we found is that the majority
of observations have no presence of water; however, on one of those dates
we detected water. We detected H2O.
In the past, Hubble provided indirect measurements of water by detecting hydrogen and oxygen,
but now we have directly detected water for the first time.
Both the Webb Telescope and the Europa Clipper mission will give us
us a much more detailed picture of the surface of Europa, its cracks and crevices,
detailed pictures of the water vapor, as well as other molecules that may also
be emanating from the subsurface of Europa.
So both of these missions will give us a great picture of whether Europa is truly habitable.
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Titan is a moon of Saturn.
It’s the second largest moon in the Solar System and it is about two times larger
than Earth’s Moon and actually bigger than the planet Mercury.
Titan is also interesting – it’s the only moon in our Solar System with an atmosphere.
It’s surrounded by sort of an envelope of gaseous nitrogen
just like our own Earth is.
Titan was first discovered by telescope observations back in the mid-1600s.
The first spacecraft observations were made of Titan during flybys through
the outer Solar System – that was in the late seventies and in the eighties.
But we really were able to explore Titan in-depth with the Cassini-Huygens Mission.
The Huygens probe was dropped into the atmosphere of Titan,
and it made measurements of chemistry, and it took images as it fell to the surface,
and that was back in 2005.
. And since then, the Cassini Orbiter made over a hundred close flybys of Titan.
Cassini in its design with the different instruments – we purposely were picking
instruments that could go into longer wavelengths into the infrared
so we could really understand the moon.
We were able to basically peel back the layers of Titan
to really see what was below, and it was remarkable – very Earthlike.
The landscape is similar to Earth’s in many, many ways, but with a little bit of a twist.
So on Titan, you can find dunes, you find lakes, there are river channels,
the atmosphere is very dense and you can get clouds and smog and you even get rain.
We saw winds, we saw seasons, and one really important thing we saw was liquids
pooling in the polar regions on the surface – a lot of it.
But because Titan is so cold, those features are all made of very exotic materials
compared to what we would find on Earth.
So the lakes and the rain are made of liquid methane, the crust that forms
the surface of Titan is actually water ice, but it’s so cold that it is as hard as rock,
and in the atmosphere we get this organic chemistry that forms large
organic molecules and particulates – they fall down to the surface
and then behave like dust or like sand does.
So it makes us want to go back to really understand the complex,
organic environment of that surface and what it means
for either past life or future life.
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Dragonfly is a mission that was just selected by NASA to fly to Titan
Titan and arrive in the mid-2030’s.
Dragonfly is going to make a whole bunch of measurements to help us understand
the environment on Titan and its potential for habitability.
We’ll be taking measurements of the atmosphere – that includes
things like pressure, temperature, winds.
We’ll probe the surface to try to understand what materials the surface is made out of.
We’ll also be drilling into the surface to look for the types of organic molecules
that are present and to see if we can find any examples of compounds that mimic
the types of building blocks that we know we need for life on Earth.
We don’t really know how life started on Earth. We don’t exactly know what the chemical
was like before life started. So, with Titan we have this really unique opportunity
There are times in Titan’s past where there could be liquid water on the surface.
Impact craters can generate impact melt, and there’s a potential for possible cryovolcanism
cryovolcanism to erupt some liquid water onto the surface.
And so we know that there’s a rich organic chemistry going on in the atmosphere
we know that’s depositing to the surface.
If there were times where those organics in the liquid water environments were mixing,
then there may be some really interesting chemistry taking place.
When you have these processes operating for hundreds of millions of years,
how far can they get you down that path of chemical complexity
and can we see reactions and molecules that start to look something like
what we think of as essential elements for our biochemistry for life on Earth.
In the future, looking forward as opposed to looking back and thinking about Titan
as a chemical laboratory for the pre-biotic Earth,
I like to look forward thinking about what’s going to happen when the Sun evolves
and warms up and the habitable zone actually moves outward to where Titan is, and it will.
You have all the organics, you’re going to have a source of energy,
all we have to do is melt the frozen water and we’re going to have a pool of organics
just embedded in liquid.
Titan might actually have a chance at that point to harbor life.
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So when we think about ocean worlds, it's good to compare them to what
we know about Earth. In total proportion, Earth is about point-one-percent water.
An ocean world is a body that has, in proportion, about ten times more water
than Earth does.
And when we think of the TRAPPIST planets, those planets have about fifty times
more water in proportion to what Earth does.
Ocean worlds do appear to be common in our galaxy. As far back as the early 2000's,
We had astronomers, some of them still here at NASA Goddard who suggested that we would
have ocean worlds orbiting low-mass stars.
Recently we've looked at about fifty-two exoplanets, and these are low-mass exoplanets
and what we've found is, of these fifty-two planets one out of every four
may be an ocean planet.
And when it comes to these ocean planets, over half of them may be ice-covered
ocean worlds, and so Enceladus and Europa may serve as small-scale analogues
of these planets.
So there are a number of different ways to search for life on planets around other stars,
but the key method is the study of the atmospheres.
We can search for signs of life - biosignatures, we call them, things like oxygen,
water vapor, carbon dioxide, even more unusual biosignatures -
things like chlorofluorocarbons, or other things that are only produced by intelligent life.
By looking for these key constituents of planetary atmospheres that signal life,
We can discover lifeforms on other planets that we could never actually visit in our lifetime.
So this is very analogous to how we study the atmospheres of moons and planets
in our own solar system, and really makes the connection between studying the plumes
of Europa, and the atmospheres of planets around other stars.
What I would like to see is the definition of a habitable zone expanded
We don't want to keep thinking too narrow about liquid on the surface, broaden the scope
and really try to embrace other worlds that might seem too far from their host star,
and frozen out, when they really aren't frozen at all. At great depths, they harbor
a warm, hydrothermal-driven, liquid water environment.
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