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An Ocean in Space

By SpaceRef Editor
April 15, 2001
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For
a long time,” said Chris Chyba, the last Frontiers of Science speaker,
“the difficulty with looking for life on other planets was finding
water.” The concept of the “habitable zone,” developed by Stephen
Dole of the Rand Corporation and Michael Hart of NASA’s Goddard
Space Center and further elaborated by Penn State’s Kasting, along
with Ray Reynolds of NASA Ames and Dan Whitmore of the University
of Southwest Louisiana, put this dilemma in black and white. Of
the planets in our Solar System, Earth, Kasting and his colleagues
calculated, is the only one close enough to the Sun to be warm enough
for liquid water, yet not so close that the water boils away. Actually,
Mars is in the ballpark too, except that present-day Mars has too
little atmosphere to retain the necessary heat – at the surface.
But what about down below?

Recent research has heightened interest in “worlds
that may be rich in liquid water below the surface,” said Chyba,
associate professor of geological and environmental sciences at Stanford University and director
of the Center for the Study of Life in the Universe at the SETI
Institute. Mars is one such world. Another, in some ways even more
tantalizing, is Europa.

Fourth
largest of the 16 known satellites of Jupiter, Europa is a chunk
of rock and metal about as big as Earth’s moon, sheathed in ice.
Voyager photographs taken 20 years ago show a smooth surface scored
heavily with cracks, like a favorite skating pond in late winter.
The absence of craters, Chyba said, shows that unlike our moon,
Europa is geologically active. “Its surface is being reworked every
10, or 20, or 30 million years,” by new material churned up from
below.

The reason for this activity, he said, is the strong
tidal pull exerted by Europa’s giant parent, which causes bulging
and shrinking of the satellite’s crust as Europa moves through its orbit. All that movement
creates friction – and heat. “And we can calculate how much,” Chyba
said. Doing so, he added, “enabled one of the few important successful predictions in the history
of planetary science”: that Io, Jupiter’s closest satellite, was
so heated by friction it would be “the most volcanically active
world in the Solar System. And it in fact is – Voyager has taken
pictures of its volcanoes erupting.”

The pull on Europa, farther out, is less than that
on Io. But there’s still enough friction to heat Europa’s core substantially
– enough to melt away most of its icy layer from the inside, Chyba
said. So, although the surface, which has no atmosphere, remains
a rock-solid -170 degrees C (-274 degrees F), beneath Europa’s ice
in all probability lies a vast body of water.

The evidence of resurfacing seems to corroborate
this, Chyba said, with smooth areas suggesting water flowing out
from the interior only to be quickly re-frozen, like the contents
of a bucket spilled across a frigid sidewalk. The wealth of cracks,
he added, “seem to be related to stretching ice as it rides up on
top of an ocean deforming underneath.” But in a way the most compelling
argument for an ice-bound sea is the magnetometer data.

Jupiter has the strongest magnetic field of any
planet in the Solar System. That field sweeps past Europa every
ten hours, as the giant planet spins on its axis. “If there were
a conductor on Europa – salty water, for example – the changing
magnetic field would set up a current in that conductor,” Chyba explained, and that current
would create Europa’s own magnetic field. Such a field has now been
measured – its strength consistent with an ocean 100 kilometers
deep with a salt content about equal to that of the ocean on Earth.

“It’s hard to avoid the conclusion that there’s
a salty conducting ocean on Europa,” Chyba concluded. “But we’re
not completely certain. And we would like to be, because if there
is a second ocean in the Solar System, we’re going to go back and
have a program of exploration on Europa that rivals the Mars program.
I would go so far as to say that if there is an ocean on Europa,
it is the most exobiologically interesting place in the Solar System.
That is to say, there might be life there.”

What
do we mean by life? That’s the first thing that needs to be agreed
on. “There have been many attempted definitions – thermodynamic,
metabolic, biochemical – but all of them seem to either leave something
out that we know is life, or let something in that we know isn’t,”
Chyba said. “So we have to fall back on a simpler idea, that of
life ëas we know it,'” made of liquid water, organic molecules,
and an energy source. On Europa, “there is almost certainly liquid water present.
There are hints that there are organic molecules present.” What
about an energy source?

“It’s hard to say anything at all about this,” Chyba
admitted. “You can’t have photosynthesis. Light couldn’t penetrate
that surface ice.” Might there be hydrothermal vents at the bottom
of that ocean? “We have no idea.”

A look at life on Earth, he continued, shows that
higher life forms – eukaryotes – require something beyond the three
basics: they need oxygen, too, to help metabolize energy. “Even
tubeworms and clams at hydrothermal vents need oxygen; it’s produced
at the surface and finds its way down. If not for photosynthesis
these organisms would die.” Oxygen, whether in Europa’s atmosphere
or in its ice-covered ocean, is likely to be scarce, Chyba said.
So, “as much as I would like to see giant squid swimming in Europa’s
ocean, we probably have to content ourselves with one-celled organisms
analogous to bacteria or archaea.”

On the other hand, he noted, there are some creatures
on Earth that get along fine with no oxygen at all. Methanogens,
for example, are a class of bacteria that digest hydrogen and carbon
dioxide to produce methane. “And they probably get that hydrogen
from rocks. If Earth froze over tomorrow and became a world that
looked like Europa, we would probably continue to have an ecosystem
living underground for billions of years.”

Conceivable, too, are energy sources on Europa that
we simply don’t know about: Chyba offered a suggestion based in
photochemistry. Jupiter’s strong magnetic field, he said, acts like
a particle accelerator, shooting charged particles – radiation –
into Europa’s ice. “We know from Galileo’s observations that there
are carbon dioxide molecules mixed in with that ice. Once you irradiate
carbon-dioxide-bearing ice, you make simple organic molecules, like
formaldehyde. And you can make oxidants from the ice itself. These
molecules are frozen together, and at melt-through events they could
get mixed into the ocean.” Using Earth analogies, Chyba said, “We
can estimate that Europa’s ocean, in this way, could support a bacterial
ecosystem.” Not a very robust one – “only about 1/10,000 as dense
as that in Earth’s ocean” – but, hey, it’s a start.

The
only way to know whether such an ecosystem is out there,” he said,
“is to go look.” That’s the rationale behind NASA’s Europa Orbiter,
planned for launch in 2006. The Orbiter’s primary objectives, which
Chyba helped to draft, are to verify the presence of an ocean, measure
how thick the ice is, and spot evidence of organics. “After the
Orbiter, there is planned a Lander. And after that, maybe a series of missions, to get beneath the ice.”

All of which is a bit more involved than missions
to Mars. “It takes three years just to get there,” Chyba said, “and
another to get into orbit. And once you’re there, you have that
punishing radiation,” conditions so harsh that the Orbiter is expected
to survive for only a month. Then, too, there’s the possibility
of contamination, “both forward and back, but it’s the forward contamination
I’m worried about. We need to be extremely careful that we don’t
introduce organisms that would interfere with Europa’s possible
ecosystem. And if the ocean is sterile, we don’t want to introduce
any false positives.”

Securing answers from far-off Europa will be an
extraordinarily complicated endeavor, as difficult, perhaps, as
humans have ever attempted. To Chyba, however, the effort required
will be well worth it.

“My suspicion is that if we find an ecosystem on
Mars, it’s quite possible that it will share a common ancestor with
life on Earth,” he explained. “Whichever world evolved life first
will have inoculated the other,” through asteroids or other space-borne
debris. “But I think that if we find life on Europa, it’s probably
an altogether different form of life.” Something beyond even our
current power to imagine.


Credits

“Astrobiology: Looking for Life in the Universe,” the 2000 Penn State Lectures on the Frontiers of Science, was organized by the Eberly College of Science and sponsored by the pharmaceutical company Pfizer Inc. The series of six lectures took place on consecutive Saturday mornings from January 22 through February 26, 2000, on the Penn State University Park Campus. This special report was funded by the Penn State Astrobiology Research Center, the Pennsylvania Space Grant Consortium, the Education office of the NASA Astrobiology Institute, Pfizer Inc., and the Eberly College of Science. It was written and produced by Research Publications in the Office of the Vice President for Research, Penn State University, 320 Kern Graduate Building, University Park, PA 16802; 814-865-3477; editor@research.psu.edu.

The Penn State Astrobiology Research Center, directed by Hiroshi Ohmoto, Ph.D., professor of geosciences in the College of Earth and Mineral Sciences, is one of 11 lead members of the NASA Astrobiology Institute. PSARC is affiliated with Penn State’s Life Sciences Consortium, the Environment Institute, and the Pennsylvania Space Grant Consortium. For more information, contact PSARC at 814-863-8761, or lxd1@psu.edu, or see http://psarc.geosc.psu.edu.

SpaceRef staff editor.