New findings support
prospect of life on Jupiter's moon Europa
BY MARK SHWARTZ
 If alien creatures exist elsewhere in our
solar system, they're most likely to be found on Europa,
one of 16 moons orbiting Jupiter.
There is strong evidence
that beneath Europa's frozen exterior of ice lies an
ocean of liquid water -- one of the essential ingredients
for all living organisms.
Many scientists believe
that this vast subterranean sea could host living
microorganisms similar in size and complexity to bacteria
found on Earth. Others question whether a frozen moon
with a surface temperature of 260 o F (170o C) can
produce sources of energy useful for the basic chemical
reactions necessary for life.
But a new report in the
Jan. 27 issue of the journal Nature concludes that
Europa does indeed contain plenty of biological fuels,
thanks to billions of charged particles that constantly
rain down from neighboring Jupiter.
This relentless
bombarbment of radiation "should produce organic and
oxidant molecules sufficient to fuel a substantial
Europan biosphere," writes Christopher Chyba,
associate professor (research) of geological and
environmental sciences.
On Earth, all organisms
use carbon as a basic building block of life to construct
everything from cells to DNA. Many organisms obtain their
energy from carbon-based molecules like sugar, and some
form of energy is required to free the carbon atoms from
their chemical bonds.
Plants and algae use
energy from sunlight to produce their own organic
molecules out of carbon dioxide gas taken from the
atmosphere or the ocean. The process is known as
photosynthesis.
According to Chyba,
sunlight would not provide enough energy to sustain life
on Europa since its ocean appears to lie "beneath an
ice layer too thick to permit photosynthesis."
Pam
Engelbretson, courtesy of SETI/JPL
An artist's
conception depicts the liquid ocean that may exist
beneath the icy surface of Europa, one of Jupiter's 16
moons. Scientists believe that microbes can survive in
Europa’s sea if they have an adequate source of
food, such as organic molecules that are formed on the
frozen surface.
A likelier
source of energy, he concludes, may come from
fast-moving, charged particles that pound Europa from the
atmosphere of Jupiter. Jupiter has the strongest magnetic
field of any planet," Chyba says, more than 10 times
stronger than Earth's. When protons, electrons and other
particles from space get trapped in Jupiter's
magnetosphere, they are accelerated to extremely high
velocities.
Europa's orbital path
around Jupiter lies deep within this powerful magnetic
field, so it receives a continuous barrage of electrified
particles or ions.
According to Chyba, when
these ions slam into the icy surface of the moon,
chemical reactions are likely to occur, transforming
frozen molecules of water and carbon dioxide into new
organic compounds such as formaldehyde.
It turns out that one of
the most common bacteria on Earth, Hyphomicrobium,
survives on formaldehyde as its sole source of carbon,
and Chyba believes that similar formaldehyde-feeding
microbes could be alive and swimming in Europa's
subsurface ocean.
In addition to creating
organic fuels, radiation from Jupiter also may drive
chemical reactions that produce oxidants -- molecules
such as oxygen and hydrogen peroxide that can be used to
burn formaldehyde and other carbon-based fuels.
But Chyba notes that the
oxidant and organic molecules formed on Europa's frigid
surface "are biologically relevant only if they
reach the ocean."
The problem is that, if
there is a liquid ocean on Europa, it's hidden beneath an
ice sheet about 50 to100 miles (80 to 170 km) thick. So
if extraterrestrial creatures are going to feast on
formaldehyde, there has to be a way to get that compound
through the dense layer of ice and into the liquid sea
below.
Recent photographs taken
by NASA's Galileo spacecraft reveal evidence of sudden
melt-throughs in the ice that could allow oceanic
microbes to come into quick contact with oxidants and
organic food sources. The result could be a dramatic
increase in population similar to "microbial
blooms" that periodically occur in the Earth's
oceans. Chyba points out that Europa's surface ice
appears to get naturally recycled into the ocean every 10
million years -- a process that would allow a very
gradual delivery of life-giving molecules to any
submerged organisms. And just how many microbes might
exist in Europa's sea? Chyba's conservative estimate: one
per cubic centimeter -- a far cry from the hundreds of
thousands of organisms that occupy each cubic centimeter
of water on Earth.
Could life on our planet
have its origins on Europa? Probably not, according to
Chyba.
"Europa is as old as
our solar system," he says, "but it's probably
too far, too deep inside Jupiter's gravity well to have
inoculated Earth with life-bearing debris."
Chyba emphasizes that all
theories about life on Europa hinge on proof that a
liquid body of water actually exists between the moon's
surface and its rocky core.
"The point is to go
there and find out," Chyba says, noting that in
three years NASA plans to launch the Europa Orbiter
satellite that will use radar to detect the presence of
large bodies of subsurface water. The Orbiter should
reach Europa in 2008, and NASA hopes to follow that with
a remote landing.
"We'll know in the
next 10 years if there's an ocean," Chyba predicts.
"If there is, Europa will be the site of a series of
new space missions."
As a student, Chyba's
interest in extraterrestrial life led him to the Cornell
University laboratory of famed astronomer Carl Sagan, a
long-time advocate of planetary exploration. Chyba
received his Ph.D. in astronomy under Sagan's guidance in
1991. Today, in addition to his post on the Stanford
faculty, Chyba holds the Carl Sagan Chair for the Study
of Life in the Universe at the SETI Institute in Mountain
View, Calif. "SETI" is the acronym for the
Search for Extraterrestrial Intelligence.
From 1993 to 1995, Chyba
served as a White House adviser on national security.
Beginning Feb. 1, he will become co-director of the
Stanford Center for International Security and
Cooperation, an organization dedicated to finding
innovative solutions to worldwide security problems such
as arms control and ethnic conflict. SR
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