MOFFETT FIELD, Calif., April 4 — Planetary scientists have a hunch that the building blocks of life just might lurk within an ocean on Europa, a moon of Jupiter. The problem is, that ocean is completely sealed off beneath miles and miles of icy crust. Or is it?
WATER AND EUROPA were among the themes at the first Astrobiology Science Conference, a standing-room-only event that runs Monday through Wednesday at NASA’s Ames Research Center here. The meeting brought together top researchers on Europa and Mars, as well as scientists seeking planets beyond our solar system, signals from extraterrestrial civilizations, the secrets behind life’s origins on Earth and its persistence under extreme conditions.
The rise of astrobiology has come about discoveries about life on Earth
as well as worlds in space:
Microscopic organisms have been found living in pockets of water contained
within polar ice as well as within the
boiling-hot volcanic vents on the bottom of Earth’s oceans. That
has led scientists to believe life can be sustained in any
clime where three conditions are present: sufficient energy, organic
chemicals to serve as fuel and liquid water.
On Earth, water plays a key role in nearly every facet of life — from
climate control to the movement of continental plates to the chemistry
of the cell, said Philip Ball of the British journal Nature. Ball, who
has written a book on the wonders of water titled “Life’s Matrix,” said
the complex chemistry of life “can perhaps only be conducted in such a
compassionate medium of water.” Water’s the thing that has driven much
of the recent interest in Europa, a moon about as big as
Earth’s own. Analysis of Europa’s surface, its density and even its
magnetic properties have led researchers to conclude that it has a 90-mile-deep
(150-kilometer-deep) top layer of frozen and liquid water. These scientists
believe that volcanic
activity sends heat rising from the depths of the Europan ocean. Patterns
in the surface ice — long ridges, broken-up
regions and bulging domes — indicate that warmer material occasionally
breaks through.
Is that material slush or water? How far down do you have
to go before you get to the liquid stuff? Those are key
questions, because the answers will influence where you’d focus
the search for signs of life. Two experts on Europa set
forth their opposing views on the issue Monday, but they agreed
that if life exists on Europa, you wouldn’t have to
drill through miles of ice to find the evidence. They both said
the answers could be lying right on the surface.
THIN ICE
Richard Greenberg of the University of Arizona’s Lunar and Planetary
Laboratory pointed to Europa’s characteristic patterns of cracks
as evidence that the moon’s global shell of ice might be only a few miles
deep — and that water might be welling up through cracks in that ice
even today. As Europa follows its elliptical orbit around Jupiter, it’s
subject to gravitational tides that flex and fracture the ice, he explained.
Images from the Voyager and Galileo probes indicate that the orientation
of the cracks changes over time, sometimes creating swooping arcs, or criss-crosses,
or odd triple tracks across the ice. He said Europa’s tides could
generate such patterns only if the top layer of ice was relatively
shallow - no more than a few kilometers deep. Greenberg said the
triple tracks may have been formed by water welling up through a
tidal crack, then pushing up onto the surface as the crack closes.
After repeated cycles, it would build up on each side of the
crack, like the walls of snow that line a freshly plowed street.
Greenberg argued that active cracks could open and
close on a daily basis for tens of thousands of years — perhaps
giving organisms at least a temporary foothold on Europa’s icy crust. But
he said such organisms would have to adapt to shifts in the cracking
patterns — perhaps by
moving on to new cracks as the old ones froze up. Since
some of the cracks appear to be of recent origin, he said, “it seems
plausible to imagine that they’re continuing,” he said.
When it comes to life on Europa, “the real action is at
these ridges,” Greenberg said.
THICK SLUSH
Robert Pappalardo of Brown University has a different model for Europa: As he sees it, the moon most likely has an uneven shell of solid ice, with slushy “warm ice” extending down six to 15 miles (10 to 25 kilometers) before you get to the liquid water. Pappalardo focused on the pits, domes and dark spots on Europa’s icy surface — characteristics known as “lenticulae.” Such pock marks on the ice could only be formed by warm ice welling up toward the surface, creating partial melting or deformation in the upper layers of the ice, he said. “You can’t do that in a melt-through model,” he said. His analysis of Europa’s thermal properties and ice flow models led him to conclude that the world is “a planetary Lava Lamp,” with soft, salty slush blurping up and even occasionally breaking through to the surface. He and a colleague at Brown, James Head, argue that “the most accessible near-surface niches in which to find extant or dormant Europan biology are brine-rich liquid inclusions within the ice, analogous to those recently identified in terrestrial sea ice.”
THE NEXT STEP
So what’ll it be? The domes of slush, or the cracks in the ice? There’s
still not enough information to confirm what
scientists think they know about Europa’s ocean — even to confirm that
the ocean actually exists. That will have to wait
for the next mission to Europa, an orbiter due to be launched
in 2003.
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