How Pluto got one of its most notable features has been up for debate ever since the New Horizons spacecraft laid eyes upon it in 2015. The heart-shaped region, named Sputnik Planitia, is a deep basin full of ice, but its origin is a bit of a mystery.
Now a study published in Nature has proposed a new answer. Led by Douglas Hamilton from the University of Maryland, it suggests that the heart formed early in Pluto’s life, and its modern appearance is the result of the evolution of the dwarf planet.
Previous studies had suggested this basin, 1,000 kilometers (620 miles) wide, was the result of an impact 4 billion or so years ago. Hamilton and his team, though, propose that the heart has always been there, and the weight of the ice alone has caused the depression.
“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,” said Hamilton in a statement. “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.”
They came to their conclusion using computer simulations. Sputnik Planitia is particularly of interest because it is perennially under the moon Charon, which is positioned at always the same point above Pluto in its orbit, being tidally locked.
According to the model, 1 million years after Charon formed – likely itself by an impact on Pluto – ice deposits began to build up. Pluto’s spin axis is unusual in that it is tilted by 120 degrees to the orbital plane, whereas Earth’s is 23.5 degrees.
Consequently, in its 248-year orbit, the coldest places on Pluto are not at its poles, but at 30 degrees north and south latitudes. The former is almost bang in the middle of Sputnik Planitia, which is found at 25 degrees north latitude.
As ice built up in the region, it would have reflected more sunlight and heat, keeping temperatures low and allowing more ice to form, creating a positive feedback loop known as the runaway albedo effect – where “albedo” is a measure of the reflectivity of a material.
The weight of this ice would have been enough to create the basin in the region, according to Hamilton. And in fact, such was the weight that it may have shifted the dwarf planet’s center of mass. Just a few million years ago, this caused Pluto to become locked to Charon – and Sputnik Planitia had a 50:50 chance of being either facing Charon, or on the opposite side of the dwarf planet. Hamilton likened this to “a Vegas slot machine with just two states”.
This is not the only theory for the basin’s formation. Other theories suggest it was formed by an impact, perhaps the same one that formed Charon. If this is correct, it hints at a subsurface ocean on Pluto being the cause of the large amount of nitrogen ice in Sputnik Planitia.
One other point of note is that the basin is larger than the amount of ice in it, which suggests that the heart has been wasting away over time.
But, at the very least, it re-emphasizes how fascinating Pluto is. Aside from Earth and Mars, it is the only world we know to have an ice cap.