Pluto's Ice Heart ....underneath the frost

Pluto's most iconic feature -- its "icy heart" -- may have been responsible for tipping the dwarf planet over. Scientists believe the 600-mile-wide region of frozen plains known as Sputnik Planitia gained enough mass over the years, causing Pluto to tilt to its current orientation. And that could mean there's a subsurface ocean lurking underneath the dwarf planet. The cracks and faults on Pluto's surface tell the story of its rollover, according to two new studies published today in Nature. Researchers used computer models to simulate Pluto's reorientation, which would have put a lot of stress on the crust and created these cracks. Those models match up pretty well with the patterns of canyons and mountains that NASA's New Horizons spacecraft saw when it flew by Pluto last year. As for how the flip occurred, the two Nature studies offer complementary arguments. Isamu Matsuyama's study says that the low-lying Sputnik Planitia filled up with a bunch of nitrogen ice, gaining mass that pushed Pluto over. But the second study says the nitrogen ice wasn't enough to completely change Pluto's orientation. 

Even more weight was needed, and a dense ocean lurking just underneath Sputnik Planitia would have been enough to do the trick. Nimmo's study is just further evidence that liquid may be teaming underneath Pluto, making this dwarf planet one of a growing group of objects in our Solar System that harbor oceans. Sputnik Planitia is located in a very special place on Pluto, right next to something called a tidal axis -- the imaginary line that connects Pluto and its largest moon Charon. This axis dictates how Pluto moves if its mass changes. If you were to add extra weight to a certain point on Pluto, the entire dwarf planet would reorient itself so that the weighted point would end up next to this axis.