Scientists have long considered Europa, the smallest of the
four Galilean moons orbiting Jupiter, as a prime candidate
for life outside Earth because it is one of the few places
in the solar system where liquid water may be found.
Any future Europa exploration should focus on the
identification of sites where signs of past or present life
can be found and studied, says Ron Greeley, an ASU geology
professor.

Greeley, who heads up the Europa Astrobiology research at
ASU, is co-author of an abstract paper on potential Europa
habitats presented at the 2003 NASA Astrobiology Institute
General Meeting held Feb. 10 – 12 at ASU. The meeting
brought more than 500 researchers from throughout the
United States to discuss the latest developments in
astrobiology. The NASA Astrobiology Institute includes a
multitude of diverse disciplines including chemistry,
biology, geology, microscopy and astronomy.

Greeley said assuming life arises quickly under appropriate
formative conditions, life could be present wherever there
is liquid water, a source of energy and essential elements.
Europa is roughly the size of the moon, and is believed to
have a rocky interior and an outer shell of ice — and
possibly liquid water — about 60 to 100 miles thick.
Scientists say mounting evidence for the existence of a
salty liquid ocean beneath Europa’s icy crust is exciting
because that is just the environment that could provide
favorable conditions for present life, or where signs of
past life may be preserved.

Europa has been studied for years by examining data
collected by the unmanned Galileo spacecraft’s onboard
science instruments, but Greeley and his NASA colleagues
believe future studies of Europa will need to focus on
surface units, particularly in areas where geologic
processes have caused the satellite’s icy crust to melt,
and where organisms would be protected from radiation and
provided with an adequate food supply.

“Now that the Galileo mission is nearly completed, it is
time for researchers to sift through the images to shape
the current state-of-knowledge about the satellite and
pose scientific questions to be addressed by future
missions,” said ASU researcher Patricio Figueredo,
Greeley’s colleague, and first author of the Europa
habitat paper. Although it is not clear to researchers how
far a liquid ocean is from the surface, Figueredo says
scientists must now piece together the visible evolution
history of Europa and determine how different pathways of
energy, materials and nutrient interactions would affect
possible ecosystems in the satellite.

A second paper presented at the conference starts from the
idea that a liquid ocean is present on Europa to offer one
explanation as to why sulfate is found on the surface of
the satellite. Sulfate has been readily observed on
Europa’s surface by a stereoscopic instrument aboard
Galileo. If the sulfate is from a liquid ocean, it is
likely to have been formed by high-temperature fluids
released at the oceanic floor from the satellite’s
silicate mantle.

When these high-temperature fluids are cooled quickly, it
would provide the right conditions to support life, says
ASU’s Mikhail Zolotov and Everett Shock, geology
researchers who presented the paper, “Autotrophic Sulfate
Reduction in a Hydrothermally Formed Ocean on Europa.”

The differentiated internal structure of Europa implies
that high temperature interaction of water and rocks
occurred at least once in the satellite’s history. It is
plausible some volcanic activity is also occurring on
present day Europa, driven by tidal forces. The authors
believe high-temperature fluids from the satellite’s
rocky core flow into the icy-cold ocean above.

Similarly, this phenomenon occurs on Earth, under the
ocean floor within mid-ocean ridge volcanoes. These
deep-sea hydrothermal vents — known more commonly as
black smokers — force sulfur-rich, high-temperature
water (about 350-degrees Celsius) out onto the ocean
floor through chimney-like, volcanic rock structures.
As the hot, mineral-rich water rushes out of the chimney
and mixes with cold ocean bottom water, it precipitates
a variety of minerals as tiny particles that, in turn,
provide energy to marine life. When sulfate from seawater
mixes with the vent fluid, it can be a source of energy
for life through a process called autotrophic sulfate
reduction.

“On Earth, sulfates can be reduced through biologic
activity in oxygen-free sedimentary basins or in
organic-rich oceanic sediments,” said Shock. “Although
the amount of energy on Europa could be insufficient to
allow these biologic organisms to persist throughout the
ocean’s history, a periodic supply of organic compounds
or other environmental factors introduced into the ocean
could maintain life over time. If this process is detected
in the chemical composition of Europa’s oceanic water, it
would be highly suggestive of the involvement of ancient
life.”