by Joshua Sokol
13 August 2016

from Sci-Hub Website

Ocean worlds,

the search for life

in the solar system's other seas.
Our best chance to find alien life lives

in the vast oceans inside the

icy moons of Saturn and Jupiter

and we don't have to leave Earth

to start looking for.

Oceans inside distant icy moons

are the best prospects for finding

life beyond Earth.
 

Suddenly, out of darkness, a ghostly city of gnarled white towers looms over the submersible.

As the sub approaches to scrape a sample from them, crew-member Kevin Hand spots something otherworldly:

a translucent, spaceship-like creature, its iridescent cilia pulsing gently as it passes through the rover's headlights.

This is not a dispatch from an alien world, but it could be.

Hand is a planetary scientist at NASA's Jet Propulsion Lab in Pasadena, California, and one of a select few to have visited the carbonate chimneys of the Lost City at the bottom of the Atlantic Ocean.

It is the site of an extraordinary ecosystem - one that Hand suspects might be replicated on icy moons orbiting distant gas giants.

"In my head, I was saying to myself: this is what it might look like," he says.

Jupiter's moon Europa, and Enceladus, which orbits Saturn, both have vast oceans secreted beneath their frozen outer shells.

As such, many astrobiologists consider them our best bet in the search for life beyond Earth. NASA is plotting life-finding missions there. But we don't have to wait to dip our toes in extraterrestrial waters.

Having explored extreme ecosystems on our own ocean floor - places like Lost City, where life is fuelled by nothing more than the reaction between rock and water - we know what to look for.

Now the race is on to spot signs of similar geochemical rumblings on Europa and Enceladus, and so discover whether we truly are alone in the solar system.

"Follow the water" has long been the mantra in the search for life, and for good reason:

every known organism needs water to survive.

Most prospecting has been done on Mars, but the Red Planet's water is either long gone or locked in the ground as ice.

These days, even Mars buffs would struggle to deny that the best prospects for finding living extraterrestrials lie further from Earth.

It might seem odd to search for liquid water in places far from the sun's warmth. And yet it looks as if there are sloshing oceans beneath the surfaces of Europa and Enceladus, thanks to tidal flexing as a result of their eccentric orbits.

As the gravity of their host planets pushes and pulls at the moons' interiors, they warm from the inside out - and that heating is enough to maintain a layer of liquid between their rocky mantles and icy crusts.

The first hints of Europa's concealed sea came from the Voyager probes, which explored Jupiter back in the 1970s. Voyager II spotted cracks in Europa's icy surface crust, suggesting active processes below.

When the Galileo spacecraft returned in the 1990s, it saw another clue:

Jupiter's magnetic field lines were bent around Europa, indicating the presence of a secondary field.

The best explanation is the presence of a global vat of electrically conductive fluid, and seawater fits the bill.

We now think this ice-enclosed ocean reaches down 100 kilometers. If so, it contains enough salty water to fill Earth's ocean basins roughly twice over.

The case for a sea on Enceladus washed in more recently. In 2005, the Cassini probe showed that the moon leaves a distinct impression in Saturn's magnetic field, indicating the presence of something that can interact with it.

That turned out to be an astrobiologist's fantasy:

a plume of ice particles and water vapor shooting into space through cracks near Enceladus's south pole.

Cassini has since flown through these plumes several times.

First its instruments revealed the presence of organic compounds. They seemed to be coming from a liquid reservoir - and the particles collected from lowest part of the plumes were rich in salt, indicative of an ocean beneath. Cassini detected ammonia, too, which acts as an antifreeze to keep water flowing even at low temperatures.

All the signs suggested this was a sea of liquid water, stocked with at least some of the building blocks of life.

 

Treasure plumes

"After decades of scratching around Mars to find any organics at all, this was an embarrassment of riches," says Chris McKay, an astrobiologist at NASA's Ames Research Center in Moffett Field, California.

The treasures kept coming.

In March 2015, Cassini scientists detected silicate grains in the plumes - particles that most likely formed in reactions at hydrothermal vents. By September, measurements of how Enceladus's outer crust slips and slides had convinced them that it contains a global ocean between 26 and 31 kilometers in depth.

That's a paddling pool compared with Europa's, but way deeper than Earth's oceans.
 

Enceladus’s plumes have got astrobiologists excited
NASA/JPL-Caltech/Space Science Institute
 

So when do we visit?

NASA has already selected instruments for a new mission to Europa, set for launch in June 2022. It will feature a magnetometer to probe the ocean's saltiness and ice-penetrating radar to show where solid shell meets liquid water.

It might even include a Lander to fish for amino acids, the building blocks of the proteins used by every living thing on Earth.

 

"What could be bending

Jupiter's magnetic field around Europa?

Seawater fits the bill"


 

Two of Saturn's moons may host life:

Enceladus (with its geysers) and Titan.

 

The space agency has also invited proposals for a trip to Enceladus.

One option is the Enceladus Life Finder, a probe that will sample plumes using instruments capable of detecting larger molecules and more accurately distinguishing between chemical signatures.

Other plans have even suggested carrying samples back to Earth for analysis.

With any luck, NASA probes will be arriving at these ocean worlds by the twilight years of the 2020s. Until then we just have to sit tight, daydreaming about what fresh wonders we might find once we get there. Or do we?

In fact, there is plenty we can do in the meantime to plumb Europa and Enceladus's hidden depths. We can survey their surfaces using ground-based telescopes, gawping at the fissures where water might bubble through and leave telltale deposits from the oceans beneath.

We can model the geophysics that keeps them liquid so far from the sun, and may generate conditions that could support life. And we can use the closest analogues on our own planet to guide our search.

On Earth, deep-sea vents at the boundaries between tectonic plates, where magma breaches the sea floor, have long been recognized as hotbeds for life. Around geysers of scalding, murky water - known as black smokers - bacteria feed on chemicals, and all manner of organisms make their living on those microbes.

Europa or Enceladus might just draw enough energy from the tidal push and pull of their host planets to have molten interiors that can fuel similar vents. We don't know. The good news for life hunters, however, is that we're now aware of another possibility.

When we discovered the Lost City vents beneath the Atlantic in 2000, we saw that you can have a hydrothermal ecosystem with resident microbes and the occasional visit from a comb jelly - the otherworldly creature spotted by Hand during his visit - without the faintest rumble of tectonic activity.

Lost City is powered by a chemical reaction called serpentinisation.

 

The Improbable Promise of Titan

Saturn's largest moon, Titan, boasts glorious bays and beaches, but not a drop of liquid water.

 

With temperatures hovering around -180°C, it is far too cold for that.

The lakes and seas that dot its surface are instead filled with methane and ethane, which are gases here on Earth but slick, oily liquids in Titan's frigid climes.

 

This makes Titan an unlikely focus in the search for new forms of life in the solar system.

Astrobiologists have speculated that any life there might run on an entirely alien chemistry. Some suggest that microbes could make a living by breathing hydrogen and eating organic molecules like acetylene and ethane.

 

The Cassini probe has spied evidence of chemical activity in Titan's atmosphere that seems consistent with the idea.

There could, of course, be non-biological explanations for this activity, but the only way to know what causes it is to visit Titan. No such mission has yet been signed off, but recent work has given us fresh impetus by suggesting that the moon's ice-cold chemistry would offer the toolkit required to make weird analogues of the molecules that support life on Earth.

In 2015, a team at Cornell University in Ithaca, New York, constructed a flexible, cell-membrane-like structure using only the ingredients and conditions available on Titan.

 

Earlier this year, Martin Rahm, also at Cornell, and colleagues did some modeling to show that Titan should possess the chemicals required to create even more complex molecules.

Hydrogen cyanide is abundant in Titan's atmosphere and should rain down on the surface, but it doesn't appear to build up there.

 

Instead, Rahm suggests, hydrogen cyanide combines with other molecules when it lands, forming larger ones made of carbon, nitrogen and hydrogen called polyimines - and these could form the backbone of an alternative biology.

At terrestrial temperatures, these chemical structures would fall apart. in the cryogenic seas of Titan, however, they would be preserved and could take on a wide array of forms, some of which could carry out primitive versions of the reactions in living cells here on Earth.

 

Rahm says they might even float to the surface of tidal pools as membrane-like films, or as mats of stacked, crystalline molecules. It's another leap for any such system to be truly alive - to metabolize, replicate or evolve.

 

Even so, given that these chemicals can absorb light at the very wavelengths that penetrate Titan's cloudy atmosphere, any hungry organisms lurking in its methane seas would at least have a few rays of sunshine to snack on.

When alkaline rocks from Earth's mantle meet a more acidic ocean, they generate heat and spew out hydrogen, which in turn reacts with the carbon compounds dissolved in seawater.

It is these reactions that slowly built the towers of carbonate, some 60 meters tall, that disgorge organic-rich alkaline fluids into the water and make methane for microbes to snack on.

According to Michael Russell, a geologist turned astrobiologist at JPL, Lost City is just the sort of place where life on Earth might have begun.

Russell thinks that the imbalance between the alkaline fluid flooding cell-like pores inside carbonate chimneys and the relatively acidic seawater beyond created electrochemical potential that the molecular precursors of life found a way to tap. If he's right, then wherever alkaline hydrothermal vents exist life may have followed.

Astrobiologists like Hand think there is a good chance we'll find them on Europa and Enceladus. Now they are attempting to confirm their suspicions from afar.

One way to do that is to look for molecules whose presence would betray ongoing serpentinisation. Cassini's discovery of silicate grains in Enceladus's plumes suggests this reaction has at least happened there in the past.

Recent estimates suggesting that the ocean itself is rather alkaline, which would be expected after eons of serpentinisation, add to the case.
 

NASA/JPL/DLR

"HIDDEN OCEANS

COULD BE THE DEFAULT STATE

FOR LIFE TO ARISE,

WITH EARTH THE REAL OUTLIER"
 

To figure out if the process is happening today, however, we want to see hydrogen.

That would be important because where there are free molecules of hydrogen gas in the deep sea, there tends to be life.

"Hydrogen is chocolate-chip cookies for microbes," says McKay.

Although Cassini was not built to detect molecules as large as amino acids, the probe could detect small molecules like hydrogen.

In fact, that is precisely what it was attempting to sniff out late last year, during its penultimate dive through the plumes; mission scientists are still analyzing the data. But it will be tricky to distinguish between the possible sources of any hydrogen molecules they find.

The trouble is that hydrogen in the plumes could

Our planet's oceans are clearly visible from afar. But elsewhere in the solar system, seas hide beneath the frozen surface of moons - which might make them friendly to life either be from serpentinisation or from water split apart in the atmosphere, after it was launched from the surface.
 

If it turns out to be the former, it would be a big deal - the strongest indication yet that hydrothermal vents at the bottom of Enceladus's ocean are serving up good amounts of chemical fuel.

There may also be other clues in Cassini's back catalogue.

The probe flew through the plumes so fast that it broke apart larger compounds, and we might be able to use its detection of the fragments to reconstruct the big stuff.

"There are clearly some aromatics in some of these heavier compounds," says Hunter Waite at the Southwest Research Institute in San Antonio, Texas.

But aromatic compounds can be produced through either biological or abiotic processes, so its presence wouldn't be a smoking gun.

Still, it would help us understand what kinds of carbon chemistry can flourish under the ice.

Europa is even more likely to have serpentinisation as it is much larger, meaning it boasts more rock in contact with seawater. There are no confirmed plumes to sample, though. Instead we are learning about its ocean chemistry by peering at its surface from telescopes on Earth.

In October 2015, for example, observations made with the Keck Observatory in Hawaii revealed a strange- looking substance in a region of Europa riddled with cracks. Although the chemical signature suggests it could be dirty water ice, the dirty part has so far defied identification.

Patrick Fischer of the California Institute of Technology in Pasadena, who led the analysis, says the deposits could be potassium chloride or sodium chloride.

Both are normally transparent but could be rendered visible by the shower of energetic particles raining down from volcanoes on Io, Europa's explosive sister moon. If so, we could be looking at salts left behind after underground water breached the surface and then evaporated.

That would suggest the ocean is seasoned not with the sulphate salts from Io, as most people expected, but with chloride - making it perhaps a third as salty as expected and therefore friendlier to life.

This May, Hand and his colleagues made a bigger splash with a study suggesting that Europa's ocean has a chemical balance similar to Earth's. The calculations were based on estimates that fractures in the moon's sea floor could reach as deep as 25 kilometers into the rocky interior.

In that case, there would be great swathes of rock surface with which water can react to release lots of hydrogen.
 

Life-giving chimneys like those at

 Lost City, under the Atlantic,

could exist on Europa

Lost City Expedition/NSF
 

But that is just one part of a cycle required for life as we know it:

electron-grabbing oxidants like oxygen and electron-giving reducing agents like hydrogen have to meet and react, releasing energy that living things rely on in the form of electrons.

Europa has no atmosphere from which to cycle oxygen, as Earth does, but we know that radiation from Jupiter produces oxidizing chemicals on its surface.

To arrive at their conclusions about Europa's sea, Hand and his colleagues assumed that these oxidants are being cycled from surface to sea.
 

Life, cycled

That's a big assumption.

"If you mix the subsurface and the surface, then you get a chemical cycle that life could take advantage of," says Britney Schmidt, an astrobiologist at the Georgia Institute of Technology in Atlanta.

If not, life is unlikely.

And it's not yet clear whether that cycling happens on Europa, never mind Enceladus, where the radiation from Saturn is weaker, leaving fewer oxidants on its surface.

To find out, Britney Schmidt has drilled through Antarctic sea ice and deployed a robotic submarine to study the underside, where fresh ice is constantly forming and melting.

"If we can figure out how the ice and ocean system works here on Earth, then we can extrapolate back to Europa," she says.

Only then will we know if its vast ocean gets enough oxidants to create the ratio of elements for life.

It is possible, of course, that life elsewhere follows a different rulebook, that it is made from a different set of biochemical building blocks.

So what should we be looking for if not organic molecules and amino acids? It is a question that astrobiologists contemplate, but it can probably only be answered by finding alien life forms.

Maybe we never will. Maybe we really are alone in the solar system. If we can detect something akin to deep-sea alkaline vents on faraway moons, however, the odds of finding extraterrestrials would be slashed.

We might also have to entertain the prospect that similarly life-friendly conditions are lurking beneath the shells of other icy worlds:

moons like Ganymede, Mimas and Ceres.

In fact, given how common we now know them to be, oceans concealed by frozen crusts could be the default condition for life - in which case our blue planet, with its peculiar open oceans, is the outlier.