Standing Body of Water Left it Mark in Mars Rocks
NASA’s Opportunity rover has demonstrated some rocks on
Mars probably formed as deposits at the bottom of a body of
gently flowing saltwater.
"We think Opportunity is parked on what was once the
shoreline of a salty sea on Mars," said Dr. Steve Squyres of
Cornell University, Ithaca, N.Y., principal investigator for
the science payload on Opportunity and its twin Mars
Exploration Rover, Spirit.
Clues gathered so far do not tell how long or how long ago
liquid water covered the area. To gather more evidence, the
rover’s controllers plan to send Opportunity out across a
plain toward a thicker exposure of rocks in the wall of a
crater.
NASA’s Associate Administrator for Space Science Dr. Ed
Weiler said, "This dramatic confirmation of standing water in
Mars’ history builds on a progression of discoveries about
that most Earthlike of alien planets. This result gives us
impetus to expand our ambitious program of exploring Mars to
learn whether microbes have ever lived there and, ultimately,
whether we can."
"Bedding patterns in some finely layered rocks indicate the
sand-sized grains of sediment that eventually bonded together
were shaped into ripples by water at least five centimeters
(two inches) deep, possibly much deeper, and flowing at a
speed of 10 to 50 centimeters (four to 20 inches) per
second," said Dr. John Grotzinger, rover science-team member
from the Massachusetts Institute of Technology, Cambridge,
Mass.
In telltale patterns, called crossbedding and festooning,
some layers within a rock lie at angles to the main layers.
Festooned layers have smile-shaped curves produced by
shifting of the loose sediments’ rippled shapes under a
current of water.
"Ripples that formed in wind look different than ripples
formed in water," Grotzinger said. "Some patterns seen in
the outcrop that Opportunity has been examining might have
resulted from wind, but others are reliable evidence of water
flow," he said.
According to Grotzinger, the environment at the time the
rocks were forming could have been a salt flat, or playa,
sometimes covered by shallow water and sometimes dry. Such
environments on Earth, either at the edge of oceans or in
desert basins, can have currents of water that produce the
type of ripples seen in the Mars rocks.
A second line of evidence, findings of chlorine and bromine
in the rocks, also suggests this type of environment. Rover
scientists presented some of that news three weeks ago as
evidence the rocks had at least soaked in mineral-rich water,
possibly underground water, after they formed. Increased
assurance of the bromine findings strengthens the case rock-
forming particles precipitated from surface water as salt
concentrations climbed past saturation while water was
evaporating.
Dr. James Garvin, lead scientist for Mars and lunar
exploration at NASA Headquarters, Washington, said, "Many
features on the surface of Mars that orbiting spacecraft have
revealed to us in the past three decades look like signs of
liquid water, but we have never before had this definitive
class of evidence from the martian rocks themselves. We
planned the Mars Exploration Rover Project to look for
evidence like this, and it is succeeding better than we had
any right to hope. Someday we must collect these rocks and
bring them back to terrestrial laboratories to read their
records for clues to the biological potential of Mars."
Squyres said, "The particular type of rock Opportunity is
finding, with evaporite sediments from standing water, offers
excellent capability for preserving evidence of any
biochemical or biological material that may have been in the
water."
Engineers at NASA’s Jet Propulsion Laboratory (JPL),
Pasadena, Calif., expect Opportunity and Spirit to operate
several months longer than the initial rover’s three-month
prime missions on Mars. To analyze hints of crossbedding,
mission controllers programmed Opportunity to move its
robotic arm more than 200 times in one day, taking 152
microscope pictures of layering in a rock called "Last
Chance."
JPL, a division of the California Institute of Technology in
Pasadena, manages the Mars Exploration Rover Project for
NASA’s Office of Space Science, Washington. For images and
information about the project on the Internet, visit:
