September 24, 2009—A series of meteor strikes on Mars has uncovered pure water ice—and maybe liquid water—in the red planet's relatively balmy mid-latitudes.
Sharp-eyed spacecraft discovered the ice when several meteors slammed into Mars's surface and created five craters some 1.5 to 8 feet (0.5 to 2.5 meters) deep and 26 feet (8 meters) in diameter.
A single meteor shower created the craters, which revealed bright blue ice that vaporized in about 200 days. Above, the left column shows four separate craters right after the impact, and right, the same craters after the ice had mostly vaporized.
If the instruments had photographed the same areas just a few months later, they would have missed the surprising discovery altogether.
"Given how dry Mars's atmosphere is, we wouldn't expect buried ice that far south," said University of Arizona astronomer Shane Byrne, who works with the HiRISE camera on NASA's Mars Reconnaissance Orbiter.
"The fact that we see it means that recently, maybe only a few thousand years ago, there was a much more humid climate on Mars than we have today."
99 Percent Pure
Surprisingly the ice appears to be 99 percent pure, much like glacial ice on Earth. That means it's solid ice, rather than ice-laced soil, for example, Byrne said. He suggested two intriguing theories for its origin.
(Read: "Mars—Planet Ice" in National Geographic magazine.)
One is that the ice may have formed by accumulating snowfall, though that seems unlikely.
"The last time we would have expected snowfall in that area of Mars was a very long time ago, maybe half a million years ago," Byrne said.
Alternatively, a type of "frost heave" may have been at work. This occurs when a thin film of liquid water develops inside the porous soil, and pressure causes the water to flow to the coldest areas and freeze into a pure ice layer.
If Mars does have frost, could the tiny amounts of liquid water possibly harbor microscopic, extraterrestrial life? No one knows.
"But some water is better than nothing," Byrne said.
The Viking Lander 2 narrowly missed discovering this ice in the mid-1970s, when it landed near the impact crater area and dug some 5 inches (12 centimeters) into the surface—perhaps 4 inches (10 centimeters) short of the pure ice layer, Byrne added.
"It's strange to think that this lander went tens of millions of miles and was within 10 centimeters of possibly discovering this water ice.
"If it had, the Mars program would have looked a lot different over the past 30 years."