Mars Revisted

To Bruce Jakosky, life’s demonstrated
ability to weather almost anything Earth can dish out makes a strong
argument that life probably does exist elsewhere in the universe.
One likely spot, he suggested, is an old favorite: Mars.

Given the fertility of our collective imaginings about the red planet over the years,
Jakosky, professor of geology at the University of Colorado at Boulder
and a member of the Laboratory for Atmospheric and Space Physics
there, wisely began his talk with a few ground rules. His first
slide was a cover from the tabloid Weekly World News, with
a prominent photo of a shiny silver saucer hovering above a line
of trees. “This,” he said with deadpan aplomb, “is what I’m not
going to talk about.”

Mars, Jakosky went on to acknowledge, is a stone
that’s already been turned. Twenty-four years ago, two Viking landers
touched down on the planet’s surface, dug some soil samples, and
headed home. Subsequent analysis turned up no trace of organic molecules,
the bare-minimum evidence that would have pointed toward life. The
search for extraterrestrials was dealt a stinging setback. But recent
findings here on Earth, Jakosky said, warrant taking a second look. “Over the last couple
of decades, our understanding of terrestrial life has evolved dramatically.

First
of all, we know now that life originated quickly.” Earth’s early
history, he explained, was exceedingly violent, with frequent catastrophic
bombardments by asteroids not letting up until about four billion
years ago. “Not until then could life have gained a foothold.” Yet
carbon-dating evidence shows that life was already firmly established
by 3.8 billion years ago. “Life sprang up almost overnight once
the right conditions were present,” Jakosky concluded. “To me, this
suggests that anywhere these same conditions exist, the odds are
good that life could be – and probably is.”

Second, he said, “We’ve found out that life on Earth
is incredibly robust and capable,” existing not only in surface
hot springs and around thermal vents but deep within the planet’s
interior. “Twenty years ago we didn’t know about life below the
surface. Today we think that half of Earth’s biomass exists there,
inside rocks. We were missing half of the life on Earth!”

In short, “Life doesn’t require much for its support,”
Jakosky said. The basic necessities are only three: a liquid medium,
an energy source, and the presence of a few choice elements. Here
on Earth that means water, sunlight, and an atmosphere shot through with carbon, hydrogen, nitrogen,
and oxygen. “Of these elements,” Jakosky said, “carbon is probably
the most important,” not just because of its abundance – it exists
all over the universe – but also because of its versatility. “Carbon
combines with oxygen to form a gas – carbon dioxide – that can be
dissolved in water, so it’s transportable. It can precipitate out
and be stored as limestone when it’s not needed. People ask, ëDoes
life have to be carbon-based? What about silicon?’ But carbon is
so much more capable.”

Does
Mars meet the three basic criteria? From this distance, it’s difficult
to say. But “we can learn a lot,” Jakosky said, “by looking at pictures.”
Present-day Mars is much colder than Earth, too cold to sustain
liquid water on its surface. But photographs depicting what looks
like erosion of crater rims and other features suggest that abundant
water has been present there even very recently. Other photos show
networks of branching lines that look like river tributaries; still
others, broad channels up to 100 kilometers wide. “That’s an hour’s
drive here on Earth. That much water couldn’t have come from just
rainfall; there must have been some catastrophic release.” Yet tracked
to their sources, these channels reveal nothing. “It looks like
water burst forth from beneath the crust,” Jakosky said. “Almost
certainly there is still water down there.”

What about an energy source? Granted, the sun is
too far off to power Earth-style photosynthesis, but geochemical
energy – from volcanoes, and even from mineral weathering – is a
viable alternative, Jakosky suggested. He showed a picture of Olympus
Mons, a volcanic Martian peak that is twice as tall as Earth’s Everest,
with a summit area 100 kilometers across. “With volcanism and liquid
water,” he said, “there’s a possibility of hydrothermal vents, like
the ones we see at Yellowstone.”

As for those life-building elements – carbon, hydrogen,
oxygen, and nitrogen – they are all present in the Martian atmosphere. According to the
recent Pathfinder mission, magnesium, iron, aluminum, and phosphate
– all potential role-players, as well – are components of Martian
rocks. “So life could have originated on Mars,” Jakosky said. “That doesn’t mean
that it did, or that it’s there now. But it’s reason enough to look.”

Oh, and there’s one more reason: whatever it is
that’s embedded in the small set of Martian meteorites that have
been recovered over the last 20 years. From a pocket Jakosky produced
a sliver of dark mineral cased in clear plastic, and held it aloft.
“This is part of one of about 15 rocks that have been picked up
on the Antarctic ice sheets,” he said, “where if you find a rock,
the only place it can have come from is out of the sky. These rocks
are young, volcanic, which means they came from a planet with recent
geologic activity: Earth, Venus, or Mars.” Gases trapped within
the samples show that they’re unearthly: there’s not enough oxygen
present for them to have been trucked down from New Zealand, say.
More positively, the levels of argon, xenon, and krypton are identical
to what is present in the Martian atmosphere – “and nowhere else,”
Jakosky said. “If these rocks didn’t come from Mars, we don’t
know anything about the solar system.”

In 1996 NASA created a splash by reporting that
one of the Martian meteorites, known as ALH84001 (for its discovery
in the Allan Hills region of Victoria Land, in 1984), contains some
rather interesting tidbits. Lodged within limestone deposits formed
in cracks in the rock were tiny tube-shaped structures that just
might be fossilized life-forms. Make that extremely tiny: The largest of them is less than
1/100th the width of a human hair. “Nano-fossil-like structures,”
NASA has called them. “They look like terrestrial bacteria, except
they’re a thousand times smaller” in volume, Jakosky said. Apparently
they formed, whatever they are, the same way fossils occur in limestone
on Earth. But could they really be remnants of life?

“We don’t have enough data to tell,” Jakosky said.
Researchers at Johnson Space Center, he noted, have also identified
organic molecules in ALH84001 and some of the other fragments: polycyclic
aromatic hydrocarbons, to be precise. “These could be precursors
of life, but they are also typical of decay products from the earthly
combustion of fossil fuels.” They could be simple contamination,
in other words. Again, “We will only find out by getting more samples
from the Martian surface and bringing them back to study.”

The
chief difference between now and the Viking mission days, Jakosky
said, is that, “We know better what to look for now. Twenty years
ago, we didn’t know to look for hydrothermal vents.” He and his
colleagues at NASA also have a better idea of where to look: “In
river channels and canyons, places where there has been liquid water.”
Or at crater rims, some of which appear from photographs to be rimed
with ice.

“It’s possible that we won’t find any evidence
of life,” Jakosky said. “But that would also be an important result.
It would lead us to question again what we have learned about life’s
origin here on Earth.”

“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.