Big pollywogs wiggle across parts of Mars. But they’re not like pollywogs on Earth. Instead of small tadpoles, they’re craters with channels cut through their rims. They’re called pollywogs because that’s what the outline of the crater and channel resemble.

Present-day Mars is drier than any desert on Earth. Its atmosphere is too thin to allow liquid water to stand on the surface. In the distant past, though, it was much wetter. And some of the pollywogs could have been filled with water billions of years ago.

The craters that form pollywogs range from a few hundred yards to about 10 miles in diameter. At some point, such a crater was filled with water, perhaps topped with ice. When the water filled the crater, it began flowing over the lowest part of the rim. The flowing water carved a notch in the rim. And as it left the crater behind, it carved a channel, like a riverbed.

The channel in the rim didn’t cut all the way to the crater floor, though — one way that scientists know that water was flowing out of the crater, not in.

The water that filled the craters could have fallen as rain or snow any time after about three billion years ago. Or it could have bubbled up from below ground, which seems more likely. Either way, it created some of the most distinctive features on the Red Planet.

And Mars teams up with the Moon tonight. The planet looks like a bright orange star. It stands to the upper right of the Moon at nightfall.

Script by Damond Benningfield

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