Just because a planet is out of sight doesn't mean it's out of mind. Consider Mars, which passed behind the Sun a few weeks ago. Although it won't return to view again for a couple of months, as many as a half-dozen spacecraft are operating there, beaming back pictures and much more about the Red Planet.
One of their main goals is to find evidence of water on Mars, either today or in the past. And they've found quite a bit. Earlier this year, in fact, three studies that relied on pictures snapped from orbit suggested that regular rainfall carved rivers and filled lakes on Mars as recently as three billion years ago.
The studies looked at valleys, rock layers, and fan-shaped structures inside impact craters.
All three studies concluded that networks of valleys on Mars were carved by running water, probably fed by rainfall. The rain most likely came in waves -- wet seasons followed by long dry spells. Rivers flowed across the surface, sometimes emptying into bowl-shaped impact craters. As the rivers slowed and dried up, they left fan-shaped deltas like those found at the mouths of rivers here on Earth.
The studies didn't agree on when the rains ended, though. One said they must have dried up by three and a half billion years ago. But another said the rains kept falling for another 500 million years.
The fleet of Mars spacecraft will continue to snap pictures to help answer the questions about wet days on a dry planet.
Script by Damond Benningfield
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