Our friends at the UA Lunar and Planetary Lab's HiRISE camera are in the headlines today:
"I'm going to hear from my colleagues: 'So, you've discovered water on Mars for the thousandth time?' " says planetary scientist Alfred McEwen of the University of Arizona in Tucson. Actually, it's much better than that. Using the most powerful camera ever to orbit Mars, McEwen and his colleagues are reporting the strongest evidence yet for water on Mars that's flowing, not frozen—and the water is flowing today, not a millennium or an eon ago.
At a few spots, the meager warmth of martian summer seems able to coax enough liquid water out of the ground to darken the soil in streaks. The marks, which sometimes number in the hundreds, grow downhill hundreds of meters only to fade with the winter cold. And where there is liquid water, as they say, there could be life.
The newly recognized seasonal streaks are like nothing else on Mars, McEwen and his colleagues reported last March at the Lunar and Planetary Science Conference—a finding that they also publish online today in Science. That's because nothing else—no other streaks or gullies—behaves as if flowing water is forming it today, they say. Seasonal streaks act as if elevated temperatures around the melting point of water ice unleash liquid water. By monitoring the same areas using the HiRISE camera aboard Mars Reconnaissance Orbiter, the team watched the shapes cycle with the seasons. Dark streaks a few meters wide grew from rocky outcrops down steep, equator-facing slopes beginning in the martian spring and continuing until the early fall. In the colder seasons, the streaks faded away.
The HiRISE site has lots of details.