Sky & Telescope - April 2022 - 20

Astrobiology
THREE WORLDS The surfaces of Europa, Ganymede, and Callisto (left to right) show different degrees of tectonic and geologic activity, as well as
cratering. An unknown dark material covers Callisto's pockmarked surface. Europa's surface looks the youngest, but just how young is it?
below layers of water and ice would be a challenge, to say the
least. " In the case of Ganymede and Callisto, their ice shells
appear to be very thick, quite old, and thus don't provide
much of a window into the ocean below, " says astrobiologist
Kevin Hand (Jet Propulsion Laboratory).
This is why Hand and other scientists working on NASA's
Europa Clipper are pinning their hopes on Europa. The
spacecraft, which launches in the mid-2020s, will join JUICE
in the Jovian system to begin its 40 to 50 close sweeps over
Europa at the start of the next decade.
Europa has two key advantages for life detection over
its Galilean siblings. First, it has a relatively thin ice shell
- roughly tens of kilometers thick, as opposed to a hundred
or more for Ganymede and Callisto - through which
liquid water and biosignatures might spew. Second, evidence
strongly suggests Europa's ocean is in direct contact with the
seafloor. This means the deepest recesses of Europa's ocean
world may be dotted with hydrothermal vents. But if not,
Hand says that even water leaching through mineral-laden
rocks might be enough to spark and sustain life.
And he should know. Hand took part in an expedition
that sent acclaimed film director James Cameron on
a record-breaking dive down to the depths of the Mariana
Trench in 2012. As part of the expedition, a robotic vehicle
descended to a region called Sirena
Deep, where cameras captured a
rocky outcrop covered in peculiar
filaments. " These filaments are
basically dead ringers for microbial
mats, " says Hand. " We think that
this microbial population feeding
on nutrients coming out of
rocks is what then feeds a larger
biosphere. " The same process may
have kickstarted and maintained
microbes on Europa.
Further work by Hand suggests
life on Europa could be more
exciting than one-celled microbes.
Observations and experimental
evidence indicate that charged
particles raining down on Europa
20 APRIL 2022 * SKY & TELESCOPE
from Jupiter's magnetic field are creating oxygen and other
oxidants on the surface. Hand thinks that if these chemical
modifiers are somehow delivered to the ocean below and
get dissolved into the water, they could essentially power
more complex organisms. " That's a big 'if,' " he says. " But
I've published some numbers showing that you could get up
to enough oxygen within Europa's ocean such that, not only
could microbes exist, but multicellular life could also potentially
survive. "
Though tantalizing, all of this is moot if we cannot
confirm life on Europa. Luckily, signs on the surface suggest
there may be ways for Europa Clipper and JUICE to peer into
the depths below. For example, an important and unique
surface feature is Europa's chaos regions, explains planetary
geophysicist Lynnae Quick (NASA), who is co-investigator on
the Europa Imaging System aboard Europa Clipper. " These
are areas where Europa's ice shell has broken up into plates
to form humongous icebergs in an ice-slush water matrix, "
she says. Directly below these icebergs and slush might be
Europa's subsurface ocean. In such regions, sea salts, organics,
and other telltale materials could get churned up onto
the surface, ready to be spied by JUICE or Europa Clipper's
spectral imagers.
Another promising avenue of investigation is Europa's
cryovolcanism. On Europa and
other outer solar system bodies
harboring dark oceans, conditions
are so cold that ice can act like
rock and liquid water as lava to
produce frigid versions of the fiery
volcanism we see on Earth (S&T:
Aug. 2020, p. 32). Galileo images
hinted that in certain regions
on Europa, water seeps through
the ice shell and then spreads
like lava, eventually solidifying
and forming smooth areas on
the surface (just as McKinnon
MICROBIAL MATS An expedition
found these strange, filamentary life
forms on outcrops at Sirena Deep, more
than 10 km below the ocean's surface.
TERRAIN COMPARISONS: NASA / JPL / DLR; MICROBIAL
MATS: K. HAND ET AL. / DEEP SEA RESEARCH 2020

Sky & Telescope - April 2022

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