Pluto may harbour a slushy water ocean beneath its most prominent surface feature, known as the "heart". This could explain why part of the heart-shaped region – called Sputnik Planitia – is locked in alignment with Pluto’s largest moon Charon. An ocean beneath the icy crust could have acted as a heavy, irregular mass that rolled Pluto over.
The findings are based on data from Nasa’s New Horizons spacecraft. The space probe flew by the dwarf planet in July 2015 and is now headed into the Kuiper Belt, an icy region of the Solar System beyond Neptune’s orbit.
Sputnik Planitia is a circular region in the heart’s left "ventricle" and is aligned almost exactly opposite Charon. In addition, Pluto and Charon are tidally locked, which results in Pluto and Charon always showing the same face to each other.
"If you were to draw a line from the centre of Pluto’s moon Charon through Pluto, it would come out on the other side, almost right through Sputnik Planitia. That line is what we call the tidal axis" said James Keane, from the University of Arizona, co-author of one of a pair of papers published on the subject in Nature journal.
This is strongly suggestive of a particular evolutionary course for Pluto. The researchers contend that Sputnik Planitia formed somewhere else on Pluto and then dragged the entire dwarf planet over – by as much as 60 degrees – relative to its spin axis.