If Mars ever supported life, it must have had liquid water, something that is now precluded on the surface by sub-freezing temperatures and a low atmospheric pressure. One of the main objectives of the current Mars Exploration Rovers is to find evidence on the surface of what might have been a warmer, wetter planet in the past.
Three weeks ago the Mars science team announced evidence of aqueous chemical alteration of a rock outcropping in the Meridiani area of Mars. This evidence suggested that the rocks had either formed in a wet environment of subsequently been altered by liquid water percolating through the soil. Now the team has new evidence to suggest that these rocks were formed on the floor of a lake or shallow sea, where currents created distinctive ripple patterns called crossbedding. These are thus shown to be true sedimentary rocks, in contrast to the majority of rocks on the surface of Mars, which have a volcanic origin. (The famous Mars meteorite, ALH 84001, is an example of a volcanic rock with subsequent aqueous alteration that included formation of small carbonate deposits).
These sedimentary rocks in Meridiani are part of the outcrop of rock visible on the wall of the small crater where the Opportunity rover landed. This is the first piece of martian bedrock to be analyzed on the surface of the planet. Based on this single example, it is not possible to infer whether similar wet conditions once existed over much of the surface. Also, since the exposed rock layer is small, we have no idea of the history that might be recorded in thicker layers of sedimentary rock. However, finding any direct evidence of ancient seas is an important discovery, which elevates the importance of Mars as a possible abode of life earlier in its history. If we wished to find fossil traces of microbial life, this area of Mars would be an obvious candidate for more detailed analysis by future mobile surface labs or for return of a sample to Earth.
Following is the NASA news release describing this new evidence of ancient water on Mars.
NAI Senior Scientist
NASA NEWS RELEASE: 2004-090 March 23, 2004
Standing Body of Water Left Its 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 deep, possibly much deeper, and flowing at a speed of 10 to 50 centimeters 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."
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 that 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, Pasadena, Calif., expect Opportunity and Spirit to operate several months longer than their 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."