MOUNTAIN VIEW, CA – Compelling evidence for a liquid water
ocean beneath its icy crust makes Jupiter’s moon Europa an
attractive target for scientists seeking life in distant
regions of our solar system. Recent work by Dr. Elisabetta
Pierazzo, currently at the Planetary Science Institute, and
Dr. Christopher Chyba of the SETI Institute, sheds light on
the question of whether enough “biogenic elements,” the raw
ingredients for life, including carbon, nitrogen, sulfur and
phosphorus, could be present to support Europan life.
Because Europa’s formation conditions are uncertain,
scientists do not know the exact composition of the moon’s
ocean and overlying ice. Some models suggest a Europa
depleted of life-essential carbon and other important
biogenic chemicals at birth. Pierazzo and Chyba explored
comets as an alternate source for biogenic materials,
applying complex modeling methods to set the lower limits
for a Europan inventory. In the May edition of the journal
Icarus, Pierazzo and Chyba present a paper that concludes
the Europan inventory to be “substantial.”

“We now know that enough of the right materials should have
been present to support a Europan biosphere,” says author
Chyba, who in addition to studying Europa, also oversees a
broad spectrum of astrobiological research conducted at the
SETI Institute’s Center for the Study of Life in the

“If these chemicals find their way into the ocean,” said
Pierazzo, “and if there exists a mechanism that could take
them through the formation of increasingly complex organic
molecules, those elements could ultimately evolve into
living cells.”

In their model, Pierazzo and Chyba used typical cometary
sizes, densities, and impact velocities throughout Solar
System history to calculate how much biogenic material would
remain on the moon’s surface after impact events. Unlike
the more massive Earth, which has a much higher escape
velocity and can therefore retain a higher percentage of
cometary impact material, Europa has a very low escape
velocity, thus losing a significant portion of material from
any projectile that hits its surface.

Earlier studies of cometary impacts on Earth and Mars by the
authors suggested substantial amounts of prebiotic chemicals
including amino acids would have survived cometary impacts,
especially at very low, grazing angles, and thus contributed
to the planets’ inventories of complex organic materials.
While Europan models also predict significant post-impact
survival rates for similar impacts, the low escape velocity
of the moon would allow the vast majority of this complex
organic material to be lost; with the rest of the projectile
material, it would disappear in space.

Nevertheless, cometary impacts would provide billions of
tons of carbon, and somewhat less nitrogen, sulfur and
phosphorus to the surface of Europa. These amounts are
significant, and correspond to about 1% of the biomass of
prokaryotic life (cells lacking nuclei and believed to be
representative of early life) in today’s Earth oceans.
Knowing that, at a minimum, Europa has enough of the
elements needed to sustain a biosphere offers further reason
for scientists to feel hopeful about the search for
extraterrestrial life within our own solar system.

Dr. Chyba is the Carl Sagan Chairholder and Director of the
Center for the Study of Life in the Universe at the SETI
Institute in Mountain View, California, and is also an
Associate Professor of Geological and Environmental Sciences
at Stanford University in Stanford California.
The SETI Institute, a private nonprofit organization
dedicated to scientific research, education and public
outreach, seeks to explore, understand and explain the
origin, nature and prevalence of life in the universe.

Dr. Pierazzo is a research scientist at the Planetary
Science Institute of Tucson Arizona where her work focuses
on impact cratering of planetary surfaces and their effects
on the evolution of biospheres.

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