Scientists Offer New Theory About How Pluto’s ‘Icy Heart’ Was Formed

Pluto's frozen heart-shaped feature has attracted scientists ever since NASA's New Horizons spacecraft discovered it in 2015. According to NASA, researchers have taken interest on the heart's western lobe, informally named Sputnik Planitia, which is a deep basin containing three kinds of ice: frozen nitrogen, methane and carbon monoxide.

These unique attributes have been the subject of many research studies, building models about how it was formed, all of which refer to the feature as an impact basin -- a depression created by a smaller celestial body that may have come in extremely high speed and impacted Pluto.

But according to a study recently published in the journal Nature, Sputnik Planitia may have formed earlier in Pluto's history and that its features were consequences of evolutionary processes.

"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," Douglas Hamilton, a professor of astronomy at the University of Maryland and lead author of the study, said in a press release.

"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."

Hamilton found that the initial location of Sputnik Planitia could be explained by Pluto's unusual climate and its spin axis, which is tilted by 120 degrees (Earth is tilted 23.5 degrees). Modeling of Pluto's temperatures showed that when averaged over Pluto's 248-year orbit, the 30 degrees north and south latitudes emerged as the coldest places on the planet, which is colder than either pole. According to Hamilton, ice would have formed naturally around these latitudes, including at the center of Sputnik Planitia, which is located at 25 degrees north latitude.

Moreover, Hamilton's model showed that a small ice deposit naturally attracts more ice to form. Even if the basin is losing mass over time, ice continues to accumulate and affected Pluto's center of mass.

Charon is also slowing the dwarf planet's spin. Since it is almost as big as Pluto itself, it has greater effect on the planet than the moon has to the Earth. As a result, Pluto locks one portion of it to Charon, so the large mass of Sputnik Planitia would have had a 50 percent chance of either facing Charon directly or turning far away from the moon.

"Sputnik Planitia is one of Pluto's crown jewels, and understanding its origin is a puzzle," Alan Stern, New Horizons principal investigator, said in a statement. "These new papers take us a step closer to unraveling this mystery. Whatever caused Sputnik to form, nothing like it exists anywhere else in the solar system. Work to understand it will continue, but whatever that origin is, one thing is for certain-the exploration of Pluto has created new puzzles for 21st century planetary science."