from Sci-Hub Website
the search for life
in the solar system's other
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:
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.
which orbits Saturn, both have vast oceans secreted beneath their
frozen outer shells.
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.
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.
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.
When the Galileo spacecraft returned in the 1990s, it saw another clue:
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.
That turned out to be an astrobiologist's fantasy:
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
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.
have got astrobiologists excited
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"
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.
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.
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
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.
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.
Recent estimates suggesting that the
ocean itself is rather alkaline, which would be expected after eons
of serpentinisation, add to the case.
COULD BE THE DEFAULT STATE
FOR LIFE TO ARISE,
WITH EARTH THE REAL OUTLIER"
That would be important because where there are free molecules of hydrogen gas in the deep sea, there tends to be life.
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
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.
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.
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.
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.
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.
In that case, there would be great
swathes of rock surface with which water can react to release lots
Lost City, under the Atlantic,
could exist on Europa
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.
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.
Only then will we know if its vast ocean
gets enough oxidants to create the ratio of elements for life.
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
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.