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When you turn up your thermostat, it gets warmer. In the case of Earth and Mars, though, it appears to have been just the opposite. Both worlds are cooler today than they were when they were young, even though their “thermostat” — the Sun — keeps getting cranked higher and higher.
As stars age, changes in their cores cause them to get bigger and brighter. Models of how stars evolve show that the early Sun should have been only about two-thirds as bright as it is today.
With less solar energy reaching their surfaces, Earth and Mars should have been much colder than they are today. But the geological record shows that billions of years ago, Earth was much warmer than it is now. Mars was warmer, too, with a thick atmosphere and water flowing across its surface.
There’s a range of possibilities for the discrepancy, which is known as the “young-Sun paradox.” The Sun could have been slightly more massive than it is now, for example, making it brighter. More about that tomorrow.
Other ideas say the difference is in the planets.
The young Earth, for example, had much less land than it does today. Water reflects less sunlight into space, so it may have held onto more of the feeble solar energy. Or volcanic activity may have belched more greenhouse gases into the atmosphere, or simply made the atmosphere denser than it is today. Either would have trapped more of the Sun’s energy — allowing the young planet to crank up its own thermostat.
Script by Damond Benningfield, Copyright 2013
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