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Triple Play II

February 20, 2015

The Moon and the planets Venus and Mars stage a spectacular encounter early this evening. Venus is the brilliant “evening star” to the left of the Moon. And Mars is just a fraction of a degree from Venus. It’s much fainter, though, so you may need to look carefully to spot it.

Venus and Mars flank our own planet Earth. Venus is the next planet in toward the Sun, while Mars is the next planet outward.

Like Earth, both planets are small and rocky. But their surfaces are quite different. That’s partially because of their different locations, and partially because of their different sizes.

Venus is only a bit smaller than Earth is. Early on, it may have had an atmosphere similar to that of Earth. But because it was so much closer to the Sun, it was much hotter. Any water on the planet boiled away into space or combined with other compounds to make rocks. Over time, carbon dioxide baked out of the rocks and into the atmosphere. This trapped heat from the Sun. So today, Venus’s surface roasts at about 860 degrees Fahrenheit, under an atmosphere that’s about 90 times denser than Earth’s.

Mars is only about half as big as Earth, and it’s about twice as far from the Sun as Venus is. It, too, probably had a warm, wet atmosphere early on. But the planet’s gravity wasn’t strong enough to hold on to its air, so most of it leaked away into space. That leaves Mars cold and dry, with an atmosphere less than one percent as dense as Earth’s.


Script by Damond Benningfield, Copyright 2014

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