When the New Horizons spacecraft returned the first closeup pictures of Pluto's moon Charon last year, scientists were surprised to see a huge chasm gouging across the moon's middle. Now they have a good guess as to what caused it: Charon is bursting at the seams.
Or at least it used to be. When Charon was young, it had a radioactive core to keep it warm. Beneath its icy shell, that warm center may have melted a watery subsurface ocean. Later, when the moon's radioactive fuel ran out and the center cooled, that ocean may have frozen and expanded, splitting Charon across the middle.
The New Horizons team arrived at this hypothesis after finding evidence that parts of the surface ice had melted and re-frozen.
Charon's chasms are about 1,100 miles long, or about four times longer than the Grand Canyon, and 4.5 miles deep in some places.