Forget rocky worlds like Earth and Mars.
New discoveries about icy worlds like Europa and Enceladus make them the ideal candidates
for the search for life in the Universe.
In fact there could be hundreds, or even thousands of times more worlds out there with ability
to support life.
Of course, there’s a problem, how do we search for life when it’s hidden beneath
kilometers of ice?
Decades ago, Mars seemed like the most viable place to search for life in the Solar System.
The Red Planet is cold, dry and airless today, but it certainly seemed to have liquid water
there in the ancient past.
Of course, wherever we find liquid water here on Earth, we find life.
At the bottom of the ocean, where the crushing pressures would kill us in a moment.
In steaming volcanic vents.
Beneath glaciers, deep underground, even huddled in nuclear reactor cooling tanks.
NASA’s Mars exploration program has been following the story of water.
Opportunity and Spirit discovered evidence that Mars had liquid water in the ancient
past.
And the Curiosity Rover doubled down on that, finding minerals that indicate there was water
on the surface of Mars for a long time.
But then, long ago, the conditions changed, Mars lost its atmosphere, became cold, dry
and inhospitable to life.
It’s possible that life could still be there on Mars, huddled underground in salty reserves
that prevent the water from freezing or evaporating.
But so far, scientists haven’t found it yet.
This shows that life on rocky worlds is tenuous at best.
Too close to the star, or too far.
Not a thick enough atmosphere, or too thick, creates a world that’s inhospitable to life.
And even if a world was, briefly a place worth calling home, main sequence stars are constantly
putting out more radiation, shifting the habitable zone farther out.
Think about how long Earth will be habitable.
Life crawled out from the oceans 430 million years ago, and planetary scientists estimate
we’ve only got another 500 million to a billion years before the Sun gets too hot
and boils the oceans dry.
But now we’re discovering there are other places in the Solar System to look for life
- water worlds.
In fact, the number of these places, and the amount of liquid water on them is difficult
to wrap your brain around.
The Earth is a desert compared to the amount of liquid water that’s out there in the
Solar System.
Europa alone has 2-3 times as much water on Earth.
And this life could be safe and sound, protected from radiation, meteor impacts and nearby
supernovae for billions of years.
Long after the Sun has burned out and faded away.
The first flyby of an icy ocean world came with the Pioneer missions to the outer planets.
Pioneer 10 captured the first rudimentary images of Jupiter’s Europa in 1973, but
it wasn’t enough to make out any surface features.
This was followed up by NASA’s Voyager missions, which captured the moons of Jupiter in great
detail.
Io, Ganymede, Callisto, and especially Europa.
By the time the Voyagers arrived at Jupiter and imaged its moons, planetary scientists
were already starting to consider the implications that the tidal flexing might have on them.
The push and pull with Jupiter might keep these worlds warmer than would be expected,
for small worlds which should have frozen solid billions of years ago.
Voyager 1 passed the world at a few hundred thousand kilometers distant, but it was Voyager
2 that gave planetary astronomers their first close-up look at the world.
They saw a world, covered in ice, without the impact craters common on every other solid
world in the Solar System.
Something was constantly resurfacing the crust of Europa.
The brown streaks across the surface looked like cracks in the ice, where regions moved
around like plate tectonics here on Earth.
Could there be a liquid ocean beneath this icy crust?
If there’s liquid water, is there life down there?
The discoveries made on Europa have been made again and again across the Solar System.
Enceladus, is the best example.
NASA’s Cassini spacecraft discovered huge geysers of water ice spewing out into space,
and more recently, it turned up evidence that hydrogen gas is present in the water.
This hydrogen is the ideal nutrient for bacteria here on Earth, so it seems even more likely
that an ecosystem could be there, beneath the ice on many of these icy worlds.
Even more recently, astronomers discovered that Europa has the same geysers of water
ice found on Enceladus.
We’re starting to see a pattern here.
One big mystery about ocean worlds was recently solved.
The smaller worlds like Enceladus should have frozen solid, years ago, even with the tidal
interactions with Saturn and the decaying radioactive material inside it.
In a new study in the journal Nature, a worldwide team of researchers including folks from NASA
have used data from Cassini to develop a model for how it could have kept liquid.
It appears that Enceladus has a porous, sponge-like core, which water is squeezed through by the
tidal interactions with Saturn.
By constantly cycling its water through its rocky core, Enceladus keeps its oceans liquid.
Maybe this mechanism is behind liquid oceans on many worlds.
According to Manasvi Lingam and Abraham Loeb from the Harvard Smithsonian Center for Astrophysics,
there are hundreds and maybe thousands of times more of these worlds than the terrestrial
planets we’re looking for in the habitable zones of other stars.
The habitable zone is the region around a star where liquid water can be present.
Too close and water boils away.
Too far and water freezes and stops being useful to life.
That’s why it’s the Goldilocks zone, where it’s just right.
But this habitable zone doesn’t mean that any worlds within this zone are actually habitable.
In our own Solar System, Venus and Mars are within the habitable zone, like Earth.
But Venus had a runaway greenhouse effect, while Mars lost its atmosphere eons ago.
For a world to actually be habitable, it needs to have that perfect temperature where the
water is actually a liquid.
But the icy worlds discovered here in our Solar System already have that prerequisite
for life.
They’re largely made of water, and they all seem to have some kind of liquid ocean
surrounded by a crust of ice.
Stars blow out solar winds of charged particles.
There’s constant radiation coming from galactic cosmic rays.
Planets like Jupiter can generate deadly magnetospheres of trapped radiation around them.
But for these ice worlds, that’s no problem.
Radiation can’t pass more than a few meters down.
Again, according to Lingam and Loeb, the lack of sunlight is certainly a problem, so you’re
not going to get the density of a biosphere that we have here on Earth.
There’ll just be fewer organisms than a place like Earth.
And those lifeforms are more likely to be the simpler, single-celled bacteria than the
larger, complex lifeforms we have here.
But if you’re looking for raw numbers, statistically speaking, they estimate that there could be
1000 ocean worlds for every rocky planet that exists within the habitable zone of a star.
Of course, detecting these worlds is going to be difficult.
You could search for hotspots on ocean worlds, but these could be volcanic hot spots.
The best solution is to go and drill down through the ice to sample the water beneath.
One of NASA’s next flagship missions is known as Europa Clipper.
Scheduled to launch in the 2020s, it’ll fly to Jupiter and its moons.
It’ll then make regular flybys past Europa, as many as 45 over the lifetime of its mission.
During this time, it’ll capture close up images and scientific data in the dangerous
radiation environment of Europa, and then speed off to safety.
It’ll have the standard suite of cameras, mass spectrometers and other scientific sensors.
But one of the most interesting instruments will be an ice penetrating radar that will
be able to peer several kilometers through the ice shell to figure out how thick it is,
and if there are pockets of liquid water closer to the surface.
The potential for life to flourish on an ocean world has serious implications for the spread
of life in the Universe.
And we’ll get to that in a second, but first I’d like to thank:
Domanic Santerre Geoff Trueman
Glenn Hall
And the rest of our 785 patrons for their generous support.
If you love what we’re doing and want to get in on the action, head over to patreon.com/universetoday.
If there are thousands of times more water worlds out there than terrestrial planets,
that’s really where the life is.
And these worlds will have completely isolated biospheres, cut off from their star.
In fact, a world like Europa, orbiting around a star like Jupiter, wouldn’t care whether
there was a star at all.
The tidal interactions with the planet would keep the world melted, allowing life to survive
for eons.
In their study, Lingam and Loeb note that these kinds of worlds, with thick atmospheres
and subsurface oceans could float through interstellar space, continuing to support
life.
They calculated that if Earth was ejected from the Solar System into deep space without
the Sun, the oceans would freeze down to a depth of 4.4 kilometers.
But below that, in places like the Mariana Trench, life would go on in those subsurface
lakes, for eons.
Earth could drift through space for millions or even billions of years until it encounters
another star system and gets warmed up again.
Life for us might be relatively short lived, but life on Earth is here to stay.
We think about Europa and Enceladus as places for liquid water, but researchers think there
could be liquid oceans at Pluto, Eros, Sedna, any place that has a large amount of water.
Just there, there could be life on Pluto right now.
There’s so much water out there in space, and it’s the ideal place to search for life
elsewhere in the Solar System.
Of course, because it could be locked down beneath the ice, we’ll need to develop exotic
new techniques to measure and observe it.
That means new missions with new technologies that can peer through the ice to find pockets
of water underneath, or fly through Europan geysers and sample the chemicals.
It turns out our search for life in the Universe has just begun.
Where do you think we should be searching for life in the Universe?
Do you have some clever ideas to detect live on ocean worlds?
Let me know your thoughts in the comments.
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In our next episode we’ll talk about biosignatures.
What kinds of chemicals would we need to detect in the atmosphere of an extrasolar planet
to know that there’s life there.
It’s a surprisingly tricky problem.
That’s next time.