More Clues Emerge On Mars' Watery Past
Earth might not be the
only planet in our solar system to have hosted salty seas. NASA's
Opportunity has uncovered evidence that the rocks near its landing
site on Mars not only were once wet, but likely formed at the
bottom of a body of gently flowing saltwater.
"We think Opportunity is now 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.
Dr. Ed Weiler, NASA associate administrator for space science,
said, "This dramatic confirmation of surface 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."
At a press conference at NASA Headquarters announcing the
findings, NASA Administrator Sean O'Keefe said the discovery would
have "profound implications for future exploration."
Rippled patterns in the rocks at Meridiani Planum suggest that
the land there was once a salt flat or playa, sometimes covered by
shallow water and sometimes dry, said Dr. John Grotzinger, rover
science-team member from the Massachusetts Institute of Technology,
Cambridge. 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.
According to Grotzinger, the sand-sized grains of sediment
making up the rocks were shaped into ripples by water at least 5
centimeters (2 inches) deep -- possibly much deeper -- and flowing
at a speed in the range of 10 to 50 centimeters (4 to 20 inches)
per second.
Telltale patterns called crossbedding and festooning, in which
some layers within a rock lie at angles to the main layers, led
scientists to the conclusion that the rippled shapes formed under a
current of water -- and not wind. Festooned layers have
smile-shaped curves that are produced when loose sediments are
shifted by 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.
So far, the findings do not specify how long liquid water
covered the area, or how long ago, but controllers at NASA's Jet
Propulsion Laboratory in Pasadena, Calif., plan to send Opportunity
out across the a plain toward a thicker exposure of rocks in the
wall of a crater that should add to the story.
A second line of evidence, observations of chlorine and bromine
in the rocks, also suggests that water covered the surface when
these rocks were forming. Rover scientists presented some of that
news three weeks ago as evidence that 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 that rock-forming particles precipitated from evaporating
water as salt concentrations climbed past saturation.
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 Mars 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."
NASA's Mars Reconnaissance Orbiter, due to launch in 2005, will
survey the whole planet for sites with water-related minerals and
help identify sites for future landings.
JPL engineers now expect Opportunity and Spirit to operate
several months longer than the rovers' initial 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."