SETI bioastro: Could Life Exist Near Europa's Surface?

From: Larry Klaes (lklaes@bbn.com)
Date: Mon Apr 17 2000 - 10:50:26 PDT


http://www.spaceviews.com/2000/04/14b.html

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
       month.

          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
       the cracks.

          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
       ocean.

       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
       world.



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