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Tempe, Arizona

Contact:
Lynette Summerill, lsummer{at}asu.edu, (480) 965-4823

February 14, 2003

Europa surface missions necessary step in extraterrestrial search
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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."

Summerill, with Media Relations & Public Information, can be reached 
at (480) 965-4823 (lsummer{at}asu.edu).

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