A Cracked Egg Moon

One of the first things you hear when learning about the states of matter is about good old H2O: Steam, Water, Ice.  Naturally you are asked “Why does Ice float?” The answer is a simple matter of density, frozen water is less dense because water expands when it freezes.  You can do a bit of an experiment by filling a balloon with water.  Paint the balloon and put it in the freezer.  When it freezes the water will expand, and so will the rubbery balloon, but the dried paint will crack at weak points.

This is exactly the same thing that happened to Pluto’s moon Charon, and it has the scars to prove it.

Multiple views taken by New Horizons as it passed by Charon allow stereo measurements of topography, shown in the color-coded version of the image. The scale bar indicates relative elevation. Credit: NASA/JHUAPL/SwRI

The photos from the New Horizons spacecraft show a cracked surface on Charon, with incredibly deep chasms from a crust that was pulled apart.  But how did it get to be like this? Does this mean Charon was warmer in the past?

The answer is almost certainly yes. Charon is mostly made of water ice, and would have been warmer in the first few eons after it formed due to radioactive decay of elements in it’s crust.  This heat could have warmed deeper layers, creating a subsurface ocean.  The cold of space would certainly freeze the exterior first, but as the interior ocean froze and expanded, the pressure cracked and expanded the crust, leaving the scars that mark the surface today.

The region pictured above, called Serenity Chasma, is 1,800 Km long and 7.5 Km deep, making it much longer and deeper than the grand canyon.  Though it’s not as long or deep, its size is approaching the great Valles Marineris on Mars.

Understanding the history of Pluto and Charon gives deeper insights to the formation and evolution of the solar system, and can help scientists characterize how planets form around stars in general.  Charon’s past is just one more piece of the great puzzle of the universe.

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