Sputnik planitia, the heart-shaped basin of fresh ice on Pluto, may have caused the entire dwarf planet to roll over, as if the poles decided to go on a world tour. If so, it builds a case for a huge liquid ocean lying beneath Pluto's heart.
The enormous smooth depression's shape captured the public's imagination during New Horizons' flyby, leading to numerous cartoons and even a poetry challenge. Planetary scientists, however, were more concerned with explaining it. Most agreed that it was a partially filled-in impact basin from an asteroid or comet hitting Pluto, but its location and composition were more puzzling.
Earlier this year, graduate student James Keane of the University of Arizona floated a radical theory about how the great stretch of low-lying ice came to lie where it does, namely that the dwarf planet had rolled until the heart arrived at its current location. Now he has backed the idea up with more rigorous analysis, gaining publication in the journal Nature in the process.
The moon Charon's orbit takes one Plutonian day, so it sits permanently over one point of the dwarf planet's surface. The planitia's center, the heart of the heart, is located just 400 kilometers (250 miles) from the spot directly opposite Charon. This could just be a coincidence – a result of a large comet happening to hit at just this location, but Keane thought this unlikely. Instead, he proposed the impact happened elsewhere, and a resulting imbalance in weight distribution caused the dwarf planet to roll until it reached the most stable orientation.
The Earth, with its large equatorial bulge, would resist such rolling, but Pluto spins so slowly it has almost no such bulge. Other slow-turning objects in the Solar System are thought to have undergone similar reorientations, including Earth's own Moon. Keane concludes the impact probably occurred at a location far to the northwest of where the planitia now lies.
A team led by Professor Francis Nimmo of the University of California, Santa Cruz reached similar conclusions to Keane, earning publication in the same edition of Nature. They conclude the chance of Sputnik planitia forming so close to the axis that points to Charon is just 5 percent.
Both teams note the planitia lies 3.5 kilometers (2.2 miles) below its average surroundings. This would be expected to lead to a “negative gravity anomaly”, sadly not something out of Avatar where mountains float, but a lower concentration of mass than elsewhere on the surface. However, for rotation to align the Planitia with Charon requires a positive gravitational anomaly, indicating some form of hidden extra mass.
The mass could be explained if an ocean, primarily made up of liquid water and ammonia, lies beneath the heart. Since water is denser than ice, such an ocean, if sufficiently deep, would more than offset the missing ice and provide the additional gravity. Even before New Horizons' astonishing flyby there was speculation that Pluto contained an ocean underworld, maintained through tidal flexing from the Pluto-Charon interaction.
With time the ocean would slowly freeze from the outside in, and Nimmo's team argue that this would lead to stresses, creating cracks in the dwarf planet's surface. Sadly, it will be a long time before we get to go back and check if they are right.