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One Year Later, II

July 19, 2016

At least four billion years ago, Pluto got smacked. Another icy body slammed into the little world, blasting a massive cloud of debris into orbit around Pluto. Over thousands of years, much of that material coalesced to form Pluto’s big moon Charon. And a few of the leftovers made four tiny moons.

Astronomers have long believed that such a “big whack” was responsible for the birth of Pluto’s moons. But confirming evidence was provided by New Horizons, a spacecraft that flew by Pluto one year ago this month. That evidence also provided the date for the collision.

New Horizons took sharp pictures of Charon and two of the smaller moons. Those images allowed scientists to count the number of impact craters on their surfaces. The greater the number of craters, the older the surface. In this case, the crater counts suggest that the moons formed at least four billion years ago.

New Horizons also found that the small moons are brighter than most of the objects in that region of the solar system, suggesting that they formed more recently, from fresher ice.

The New Horizons pictures also suggest that Charon originally had an ocean of liquid water below its icy crust. The ocean quickly began to freeze, which caused it to expand. That fractured parts of the crust, allowing liquid or slushy water to ooze onto the surface, forming relatively smooth plains — plains that have since been smacked by thousands of crater-forming impacts.

More tomorrow.

Script by Damond Benningfield

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