New theory proposes weather, not impact, produced Pluto’s Sputnik Planitia
An impacting object might not be the cause of the formation of Sputnik Planitia – the left side of Pluto’s now famous ‘heart’ feature – as previously thought. This is according to a new theory proposed by Douglas Hamilton of the University of Maryland. He argues the basin could have formed as a result of the dwarf planet’s spin axis and unusual climate.
Hamilton, the lead author of one of several Pluto studies published in the Dec. 1 issue of the journal Nature, bases his theory on a computer model.
Since the 2015 discovery of the smooth, craterless region – a deep basin containing nitrogen, methane, and carbon monoxide ices – scientists had presumed it was created by the impact of an object that had hit Pluto while traveling at a high speed.
Now, Hamilton is proposing unusual weather conditions caused by Pluto’s 120-degree axial tilt caused an accumulation of ice in Sputnik Planum that then created the basin.
“The main difference between my model and others is that I suggest that the ice cap formed early, when Pluto was still spinning quickly, and that the basin formed later and not from an impact,” Hamilton explained. “The ice cap provides a slight asymmetry that either locks toward or away from Charon when Pluto’s spin slows to match the orbital motion of the moon.”
Pluto’s unusual axial tilt means the coldest locations on the planet are not the poles but the regions at latitudes 30 degrees north and 30 degrees south.
At these low temperatures, even small ice deposits can grow into larger ones because ice reflects solar heat back into space, keeping the regions cold. This attracts additional ice, resulting in a cycle known as the “runaway albedo effect”. Over time, this cycle produces a single large ice cap that can dominate an entire region.
Sputnik Planitia’s center is located at a latitude of 25 degrees north, putting it in one of the planet’s two perpetually cold regions, and it is covered by a single ice cap.
Although Sputnik Planitia’s basin is larger than its current volume of ice – suggesting the entire heart region known as Tombaugh Regio is losing mass – the weight of its ice could itself have created the basin by pushing down on the surface beneath it.
“Pluto’s big heart weighs heavily on the small planet, leading inevitably to depression,” Hamilton said.
Hamilton also noted a similar phenomenon occurred on Earth, where the Greenland Ice Sheet pushed down the crust it lies on, creating a basin.
The weight of Sputnik Planitia’s ice could also have shifted Pluto’s center of mass at around the same time that Charon’s gravitational force slowed Pluto’s rotation.
With Pluto and Charon both large and close to one another, it took just several million years for the two objects to become tidally locked to one another, meaning both objects always show each other the same face.
When this happened, Sputnik Planitia’s mass assured it had a 50 percent chance of directly facing Charon and a 50 percent chance of facing as far away from Charon as possible. It ended up in the latter position, with a longitude of 175 degrees.
Different computer models were used in the other Pluto studies published in the same issue of Nature.
Francis Nimmo, of the University of California at Santa Cruz, along with Hamilton and several other authors, suggests in another paper that Sputnik Planitia was formed by an impact, with the basin being created later, only after Pluto’s rotation rate had slowed. The basin then moved a small degree to its current location.
A later basin formation, along with Sputnik Planitia’s current properties, suggests the presence of a subsurface ocean beneath its ice.
Hamilton acknowledges either of the two theories of Sputnik Planitia’s origin could be correct.
“Either model is viable under the right conditions,” Hamilton said. “While we cannot conclude definitively that there is an ocean under Pluto’s icy shell, we also cannot state that there is not one.”
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.