PROVIDENCE,
R.I. (3/5/98)- The latest
pictures beamed from the Galileo spacecraft provide further suggestions
that there is ample slushy water on Jupiter's moon Europa (pictured).
Meanwhile, American and Russian astrobiologists are looking closer to home,
Antarctica, for hints on what life-forms might inhabit such conditions.
The newly released images were taken in December 1997 by the Galileo
spacecraft and just received on Earth. The new images provide three key
pieces of evidence showing that Europa may be slushy just beneath the icy
crust and possibly even warmer at greater depths. The evidence includes
an unusually shallow impact crater; chunky textured surfaces like icebergs;
and gaps where new icy crust seems to have formed between continent-sized
plates of ice.
Some of the new images focus on the shallow center of the impact crater
known as Pwyll. The images indicate that a meteorite slammed into Europa
relatively recently, about 10-100 million years ago. Darker debris around
the crater suggests the impact excavated deeply buried material. But the
crater's shallow basin and high set of mountain peaks may mean that subsurface
ice was warm enough to collapse and fill in the deep hole, according to
Brown University researcher Geoffrey Collins, a member of the Galileo research
team.
The existence of a subsurface ocean warm enough to be slushy also may
explain the origins of an area littered with fractured and rotated blocks
of crust the size of several city blocks, called "chaos" terrain. The new
images show rough and swirly material between the fractured chunks, which
may have been suspended in slush that froze at the very low surface temperatures,
says Robert Pappalardo, another member of the Galileo team.
The new images also suggest that large plates of ice seem to be sliding
over a warm interior on Europa, similar to the way Earth's continental
plates move around on our planet's partly molten interior. New crust welling
up between the separating plates on Europa was probably slushy ice or possibly
liquid water that has frozen and fractured.
"Together, the evidence supports the hypothesis that in Europa's most
recent history, liquid or at least partially liquid water existed at shallow
depths below the surface of Europa in several different places," says James
Head, Brown University professor of geological sciences and a group leader
of the Galileo research team.
"The combination of interior heat, liquid water, and infall of organic
material from comets and meteorites means that Europa has the key ingredients
for life," Head says. "Europa, like Mars and the Saturn moon Titan, is
a laboratory for the study of conditions that might have led to the formation
of life in the solar system."
Antarctic
Research
American and Russian scientists have now teamed up to study deep ice
from the Antarctic, looking for indications that fungi, bacteria, and even
diatoms could survive conditions found on Europa and other bodies in the
solar system.
Click on the image for
the location of the Vostok Station
(images courtesy of USGS,
Columbia University).
"It's possible to say that ancient impacts of asteroids on the Earth
could have ejected soil, rocks, and seawater containing terrestrial microorganisms
into space, and that they may have made it to other places in the solar
system," explained Richard Hoover at NASA's Marshall Space Flight Center.
Hoover is an X-ray astronomer who is also is internationally known for
his work on diatoms. Hoover believes that living microorganisms locked
in ice have a chance of remaining viable for long periods in outer space.
The debate over whether the Antarctic Allan
Hills meteorites brought life from Mars (or were contaminated by life
on Earth) is a well known issue in the exobiology debate. Other evidence
for the possibility of bacterial space travel includes: asteroids striking
the Earth or Mars and blasting materials into space; the survival of streptococcus
bacteria on the Surveyor 3 moon lander; and the survival of microorganisms
inside Antarctic ice.
Recent discoveries
on our own planet have broadened the conditions under which biologists
might expect to find life. Life forms have been discovered in volcanic
vents deep in ocean trenches, in geysers, and in ice more than 400,000
years old.
This week, Russian and American scientists are examining ancient ice
drilled at Russia's Vostok (East) Station about 1,000 km (1,600 mi) from
the South Pole. Eventually, they hope to examine water taken from inside
a lake - liquid, not ice - discovered under Vostok Station in 1996.
While Lake Vostok holds clues about life on Earth, it also is a good
model for conditions on Europa. The lake is about 140 miles long and 30
miles wide, and 1,600 ft deep. Recent data indicate that it has about 50
meters (165 ft) of sediment at the bottom.
"Recent research [shows] that extremely severe conditions of cosmic
environments do not exclude the possibility that microorganisms may exist in anabiotic states at high
altitudes in interplanetary space," wrote Dr. S. S. Abyzov of Russia's
Institute of Microbiology of the Russian Academy of Sciences in Moscow
in a recent paper. The only way to resolve the question is to use the Antarctic
as a model for conditions in comets, the Martian ice caps, and other icy
moons orbiting Jupiter and Saturn.
At Vostok station in 1975, Abyzov discovered bacteria, fungi, diatoms,
and other microorganisms which were blown to Antarctica by winds from lower
latitudes. The numbers of the organisms at different depths, and thus different
ages of the ice, change with major climate changes on the Earth. Thus,
the ice also serves as a time capsule, preserving specimens of life as
far back as 500,000 years. This offers the potential for studying how genetic
material changes over the centuries.
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