"The Scientist" 15[6]:10, Mar. 19, 2001

    Water, and Life, on Mars?

    New images reveal strongest evidence yet for
    Earth-like features on the red planet

    By A.J.S. Rayl
 

    Recent images of Mars sent back to Earth from the Mars Global Surveyor
(MGS) reveal "strong evidence for a past watery environment" and "evidence
of possible present-day watery environments," Jet Propulsion Laboratory
research scientist Jeffrey J. Plaut told a symposium audience at the
American Association for the Advancement of Science annual meeting in San
Francisco last month.

    The latest high-resolution images from the Mars Orbital Camera (MOC)
provide the strongest evidence yet that the red planet was once host to
great rivers, lakes and perhaps even an ocean. Hauntingly, the colorful
topographic shade map projected on the screen behind Plaut--which was
based on the findings and projections--showed a planet that looked like
Earth.

    Whether Mars was once a watery environment and whether water may still
be present there hidden beneath the upper crust, particularly in the polar
regions, has been a hotly debated topic since Viking returned images in
the late 1970s that identified several areas containing layers of what
appeared to be sedimentary rock in the interior of Valles Marineris. "So
this is not the first evidence we have seen for sedimentary rock, but what
the MGS study really provides, Plaut told The Scientist later, is "a
spectacular, detailed, close-up look at these features that confirm they
really do have a lot of the characteristics of waterborne sedimentary
rocks and that they are widespread," found in a number places, including
crater floors, areas where scientists never thought water was to have
played a big role.

    Sedimentary rock layers are common on Earth and hold the fossil
records of early life, so if Mars was once a watery environment that
hosted life, the obvious places to look for fossil life would be in these
sedimentary rock outcrops, he added. In fact, a number of missions to Mars
are slated to do just that. Three landers are scheduled to touch down in
early 2004. The European Space Agency (ESA) will launch Mars Express--an
orbiter that will jettison a miniature lander--Beagle 2--upon its arrival.
The National Aeronautics and Space Administration will launch twin Mars
Exploration Rovers (MER-A and MER-B), which will also search for signs of
subsurface water with an instrument called MARSIS, for Mars Advanced Radar
Subsurface & Ionospheric Sounding.

    The challenge now is to find a safe landing site; the layers of rock
outcrops are usually on the side of a hill, rather hazardous for a lander.
"We have to find an area that's about 100 kilometers across ... and
whether we're going to be actually able to drive up to a layered outcrop
over such a big target is hit or miss," says Plaut, who is the
co-principal investigator on MARSIS. In any case, there are other places,
he points out, "where it looks like some floods have tended to deliver
material from the hilly areas out into the plains and if we can land in
those plains maybe we will be able to study some of the rock that came
from the layered outcrops."

    Other images from the MOC have revealed that in nearly a dozen
different locations on Mars--all of them far from the equator--there are
signs that water has been seeping out of the walls of valleys and craters,
forming small gullies. While some scientists speculate this activity may
be as recent as 10 years ago, others assert 10 million years ago is more
likely.

    "It's always possible," Plaut admits, that the sedimentary rock layers
could have been made without water. "But if we go with the simplest, most
obvious explanation, it is that these were deposited in standing bodies of
water that were long-lived in some sense," he says. In any case, "unlike
the gullies, these layers ... indicate ... a very early period in Martian
history, perhaps during its first billion years of life, as many as 3.5
billion years ago. It's the question of life--past or present--that keeps
us going back to Mars. "What's changed now, says Plaut, "are our chances
for answering those questions."

A.J.S. Rayl (ajsrayl@loop.com) is a contributing editor for The Scientist.
 

Oryginal: http://www.the-scientist.com/yr2001/mar/rayl_p10_010319.html

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