Could Life Exist Near Europa's Surface?
Published: 2000 April 14
2:56 pm ET (1856 UT)
by Bruce Moomaw
Special to SpaceViews
Life on Europa may be surprisingly close
to the surface, despite differing opinions of
the thickness of the Jovian moon's icy
crust, two groups of planetary scientists
reported at an astrobiology conference this
One of the main revelations at the First Annual Conference
on Astrobiology, held at NASA's Ames Research Center in early
April, was that a startling new idea is starting to take significant
hold among Europa researchers: that life on Europa may not be
limited to a hypothesized subsurface ocean but could be
astonishingly near the surface.
The two teams agree on this concept even though they
disagree on the thickness of Europa's icy crust. Richard
Greenberg of the University of Arizona told conference
attendees that he believes the ice layer is just a few kilometers
thick, while Brown University's Robert Pappalardo believes the
ice crust may be much thicker, with an upper, brittle layer 15-30
km (9-18 mi.) thick, and a potentially much thicker lower layer
of warmer, more ductile ice.
The Case for Thin Ice
Greenberg's group has several lines of evidence to support
their claim for a thin ice layer. A spectacular surface network of
cracks and ridges across its icy surface -- most of which consist
of a band of light-colored "fresh" ice between two strips of
darker ice -- have been seen in Galileo images and have
apparently been produced by the in-and-out tidal flexing of
Europa's surface ice crust.
An odd pattern of rotation of those cracks leads Greenberg to
conclude that the icy crust is rotating separately from the
interior of the moon, completing one revolution in 10,000 to
several million years. That rotation, caused by tidal effects from
Jupiter's gravity, is seen by Greenberg as evidence that the crust
is sitting on top of a more fluid layer, like a liquid ocean.
Other evidence for a thin ice layer include "strike-slip" faults,
regions in which one surface ice plate has apparently slid
sideways along the edge of another, like California's San
Andreas Fault, as well as "cycloidal" cracks -- chains of
repeated, connected curving arcs that can stretch for hundreds
of kilometers across the surface.
The cycloidal cracks are particularly convincing to
Greenberg. For Jupiter's tides to form cycloidal cracks in this
way, the ice layer must flex up and down as much as 30 meters
as the force of the tides changes -- and since Greenberg is
convinced that such great flexing can only occur if a layer of
liquid is sloshing around underneath the ice, he regards the
cycloidal cracks as the single strongest piece of evidence yet for
a subsurface liquid ocean.
Greenberg also believes that regions of "chaotic terrain" --
vast areas where the surface ice layer seems to have been
shattered into small blocks that then rotated and tilted in a sea
of soft liquid or slush which then refroze and locked them in
their new positions -- were created by "melt-through" events, in
which either localized tidal heating in the ice layer itself or a
volcanic heat source in the underlying rock completely (though
briefly) melted through the ice layer.
The tidal forces that create such cracks and ridges could
bring liquid water, and hence possibly life, to the surface.
Liquid water is pumped up and down in the crack every few
days, for periods of tens of thousands of years -- until the
creeping rotation of Europa's surface causes the focus of tidal
stress to move on and the crack finally freezes solid again.
During that time, this circulation of liquid water can pull
salts and other chemicals dissolved in the subsurface ocean up
to Europa's surface (perhaps including organic compounds
formed by any volcanic vents that may exist on the rocky floor
of Europa's ocean) -- accounting for the dark material that colors
Perhaps more importantly, it also pumps downward various
biologically useful chemicals that are formed in the upper
fraction of a meter of Europa's ice by Jupiter's intense radiation
belts. These would include some organics such as
formaldehyde (as well as organic compounds dumped on
Europa's surface by impacting comets) -- but they would also
include radiation-formed oxidant chemicals such as sulfur,
hydrogen peroxide, and free oxygen itself, providing Europan
microbes with sources both of food and of energy.
Microbes carried into the cracks by water would have tens of
thousands of years to live (and evolve) comfortably there; but
once the cracks finally froze again, they would either die or live
in suspended animation. However, the population of similar
germs seeping all the way down into Europa's main ocean
would supply each new crack with a new supply of microbes.
And the rare but huge "melt-through" events that formed
Europa's chaotic-terrain areas would release far greater amounts
of biologically useful surface chemicals into the ocean.
An Alternative View
Pappalardo, on the other hand, believes that the brittle
super-cold surface layer of ice is separated from any underlying
liquid ocean by a thick layer of warm ice that very slowly flows
and convects. They think that this warm ice is what oozes up to
fill the tidally-produced cracks. They also think that the chaotic
regions are not areas where Europa's ice completely melted
through, but rather areas where "diapirs" -- columns of
relatively warm, buoyant ice heated by local tidal stresses --
have slowly plowed upwards through the surrounding ice to
bulge out and break up the top brittle-ice crust.
Such a different explanation for Europa's surface features,
though, doesn't rule out the possibility for finding life near the
moon's surface. Pappalardo points out that the blocks of solid
crustal ice in the chaotic regions clearly sank into some kind of
soft material -- either liquid or slush -- and he thinks that there
are local areas of Europa's ice (as in pack ice on Earth) where
there are high concentrations of salts.
When the relatively warm ice of a diapir reaches these
patches -- which, being salty, have much lower melting points
than regular water -- they completely melt into pockets of liquid
brine, which can erupt to the surface, or drain away and cause
the solid surface above to collapse into them, or exist under the
surface for a very long time before refreezing.
Such rising ice diapirs could slowly carry frozen nutrients
and frozen, suspended-animation microbes from an underlying
Europan ocean up to the surface brine pockets, where the
microbes could come back to life and live for substantial
periods (especially if useful radiation-produced surface
chemicals mixed with the liquid brine).
Indeed, since Europa in its warmer ancient days certainly did
have a large subsurface liquid ocean in which life could have
evolved, it's possible that it could still have life even if its ocean
is now frozen completely solid -- such microbes could have
evolved to spend thousands or even millions of years in frozen
suspended animation within the ice, coming to full life and
reproducing only on those rare occasions when they find
themselves within a liquid-brine pocket.
One additional piece of hopeful news on this possibility was
reported by Jody Deming, who noted that recent studies have
shown that even very cold Arctic sea ice -- as cold as -15 deg C
(5 deg F) -- has turned out to be filled with tiny interconnected
pores of liquid brine containing not only thriving bacteria but
even photosynthetic diatoms.
In short, we may not need to find Europan life by developing
a "cryobot" probe that would spend years melting its way down
through Europa's ice layer to the underlying ocean.
Near-surface probes that melt only a short distance into the ice,
and then melt it and look for evidence of the frozen remains of
life -- or even frozen hibernating microbes -- may be enough,
and in fact might find more life there than in the underlying
Searching for Life on Europa
How should we go about exploring Europa, though? The
next stage, the Europa Orbiter, is currently scheduled to arrive
in orbit around that world in 2008, and then conclusively settle
the question of what lies beneath its ice layer -- liquid water,
warm ice, or solid ice -- with a variety of instruments.
However, the spacecraft is seriously limited weightwise, and
may not carry an infrared spectrometer or any other instrument
to actually analyze the various chemicals mixed in with
Europa's ice. It is now obvious, though, that -- in order to
understand the chances that Europa has life -- we must carry out
a detailed analysis of that ice as soon as possible.
For these reasons, leading astrobiologist Jack Farmer strongly
favors making every effort to add a small surface lander to the
Europa Orbiter if it is at all possible, to look for organics and
other compounds in Europa's near-surface ices. The need is
accentuated by the fact that NASA -- for several reasons -- is
seriously considering delaying even the Europa Orbiter for one
or two years. Because of those delays and the long flight times
to Europa, "I'll be in my rocking chair before we land anything
on Europa," grumps Farmer.
Such exploration, through, must be done carefully to avoid
contaminating Europa with terrestrial microbes on spacecraft.
NASA planetary protection officer John Rummel told the
convention that a report by a National Research Council group
on how to protect Europa from Earthly contamination will be
released in a few weeks.
Given the continuing growth of scientific interest in Europa
made clear at this conference, we may also, with luck, soon see
an acceleration in the pace of exploration of this strange distant
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