Click on the thumbnails to see images of Mars from the past 30 years.
Sunset over the rim of the Gusev Crater on Mars, captured by NASA's Mars Exploration Rover Spirit on May 19, 2005. Credit: NASA/JPL/Texas A&M/Cornell
Before NASA's Mars Global Surveyor stopped calling home in November, the satellite, which had been orbiting our neighbor planet since 1997 and was the source of the Google Mars data, captured a compelling image. Relayed back to Malin Space Science Systems in San Diego, CA, was a photograph of what looked like a newly formed streambed that flowed down a gully into the base of a crater.
Researchers were stunned because the exact location had been photographed five years prior by Surveyor and had revealed no such feature. The image itself is remarkable: It shows the flow—which appears lighter against the darker, older terrain around it—emerging from the Martian surface several hundred meters up a steep incline along the inside edge of a crater. It traces a course downhill until reaching the nearly flat bottom, where it spreads out like the fingers of the Mississippi Delta.
The Grand Canyon of Mars, Valles Marineris, compiled by the Thermal Emission Imaging System (THEMIS), a visible-light and infrared-sensing camera on NASA's Mars Odyssey orbiter, Credit: NASA/JPL/Arizona State University
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Mike Malin, the chief investigator and president of Malin Space Systems that built and operated Surveyor's Mars Observer Camera, authored a paper in Science hypothesizing that what Surveyor had captured was in fact evidence of a brief, explosive flow of liquid water. It could only have been brief, because while the surface of Mars is around ‑63°C, the atmospheric pressure is so low that water boils even at that temperature. Malin suggested that water forcibly erupted onto the surface and raced down the slope before evaporating and leaving only the visible etching of shifted dust and rock.
It is a suggestion of water that leads to the suggestion of life. But the question is raised: Do we know what we're looking for? In January we heard a hypothesis that gave us a new reason to look up in anticipation: Scientists at the American Astronomical Society meeting suggested that the Viking Landers of 1976 may have overlooked a form of microbial life that could, perhaps, exist on Mars.
The surface of Mars, including a thin coating of water ice, at the Utopia Planitia landing site, taken by Viking Lander 2 on May 18, 1979. Credit: NASA/JPL-Caltech
When the Viking missions were conceived, we had yet to find and identify here on our own planet forms of life that exist in almost unimaginably harsh environments: extreme cold, extreme pressure, extreme heat, extreme acidity. Conditions that approach the sort found on Mars have been colonized here on Earth by these extremophiles. Dirk Schulze-Makuch of Washington State University and Joop Houtkooper of Justus-Liebig University of Giessen in Germany looked back at the Viking missions and pointed out that the landers' experiments (designed to find H2O-based life forms) would have failed to find signs of life that evolved the ability to use a water/hydrogen peroxide (H2O2) mixture—which could be well suited to Mars's harsh climate. Extremophiles on Earth have adapted to use hydrogen peroxide—one organism, Acetobacter peroxidans, for instance, uses it as part of its metabolism. Schulze-Makuch and Houtkooper argued that if H2O2 biochemistry evolved on Mars, the Viking landers wouldn't have detected it—in fact, the Viking experiments would have destroyed H2O2 biochemistry in whatever sample they collected. Which means, of course, that we now need to go back and look again, this time with a better appreciation for the ingenuity of life.
The south polar ice cap, as it appeared on April 17, 2000 to the Mars Global Surveyor Mars Orbiter Camera. NASA/JPL/MSSS