Sky & Telescope - April 2022 - 21

predicted for Ganymede). And in 2013, a team led by planetary
astronomer Lorenz Roth (now at KTH Royal Institute
of Technology, Sweden) announced it had used Hubble data
to spot potential cryovolcanic eruptions, sending plumes
of water vapor roughly 200 kilometers into space. Further
hints of huge plumes spewing out of different locations were
reported in 2016.
Europa Clipper team members are crossing their fingers
that the spacecraft spots one of these chilly eruptions. " We've
seen these beautiful chaos regions, we've seen what appear to
be smooth cryolava flows on Europa's surface, " says Quick.
" But I hope that we're actually able to catch geyser-like
plumes in the act. " Tempering expectations, she warns that
the huge plumes reported by Roth and others might be the
exception rather than the rule. " It could be that most are 20
to 30 kilometers tall, " she explains. Moreover, plumes may
not even originate from the Europan ocean, instead erupting
from shallow pockets of briny water in the ice shell. " We're at
a point where we have to plan for anything. "
McKinnon, who is a member of the sounding radar
teams for both JUICE and Europa Clipper - jokingly given
the acronyms RIME (Radar for Icy Moons Exploration) and
REASON (Radar for Europa Assessment and Sounding: Ocean
to Near-surface) - is circumspect about the evidence for,
and chances of, spotting plumes on Europa. However, he says
that Europa Clipper offers the best hope of finding plumes if
they do indeed exist. " We'll be passing really close by, and if
there's any kind of even small eruptions from the surface, it
will show up in the data. " Better yet, if a larger geyser happens
to erupt on one of the spacecraft's close approaches,
Europa Clipper has instruments to " taste " the vapor as it flies
through the thin spray, just as Cassini did with Enceladus'
plumes (though Europa Clipper's instruments are far better
equipped to analyze the kind of complex chemistry that
would suggest life).
Of course, from their vantage point high above the surface,
Europa Clipper and JUICE are highly unlikely to provide
incontrovertible proof that life lies below the icy Galileans'
surfaces. For that, we need a lander. " You've got to get down to
the surface and grab a chunk of material and put that through
instruments that are specifically designed to search for complex
organics and various parameters that can help us assess
whether or not we've actually found life itself, " says Hand.
Hand - who was heavily involved in the conceptualization
of Europa Clipper and is now co-investigator on the
spacecraft's mass spectrometer instrument - is co-leading a
pre-project team to do just that. The Europa Lander concept,
if approved, will dig up samples shielded from damaging
radiation about 10 centimeters beneath the surface and then
analyze them in a miniature onboard laboratory to look for
biosignatures.
DIVING DEEP The Sensing With Independent Micro-swimmers
(SWIM) mission proposal would deploy centimeter-scale robotic fish into
an icy moon's subsurface ocean. Each fish would carry its own sensing
and communication systems.
sk yand tele scope .o r g * APRIL 2022 21
And if signs of life are found, what then? Drill down to
seek it out, of course! Already, NASA mission concepts are
exploring weird and wonderful technologies to dive through
the ice shell and explore the ocean underworld - including
a swarm of tiny robotic fish and a larger, instrument-laden
robotic snake.
Other icy worlds in the solar system - such as Saturn's
moons Titan and Enceladus, Neptune's moon Triton, or even
Pluto - may yet prove to offer a better chance of finding alien
life in our backyard. But for now, the icy Galilean moons
seem like some of our best bets. In decades to come, who
knows, we may be watching in awe as a serpent robot makes
first contact with a Europan squid.
¢ BENJAMIN SKUSE is a science writer based in Somerset,
United Kingdom.
FURTHER READING: For a deeper dive into the dark oceans of
the solar system and beyond, read Kevin Hand's new book Alien
Oceans: The Search for Life in the Depths of Space.
NASA / JPL-CALTECH
http://skyandtelescope.org
Sky & Telescope - April 2022

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