Researchers at the University of Bern and the National Center of Competence in Research (NCCR) finally solved the mystery of the giant heart-shaped feature on the surface of Pluto, according to a study published in Nature Astronomy. The team created the unusual shape with numerical simulations, explaining that an oblique impact caused the odd shape.
Ever since NASA discovered a large heart-shaped structure on the surface of Pluto in 205, this shape has puzzled researchers. To find the answer, a team of scientists from the University of Bern used computer simulations to investigate how this shape could have appeared. According to the authors, Tombaugh Regio (the heart-shaped feature) resulted from a collision with another planet about 700km in diameter.
The “heart” captured the public’s attention for its novelty. But it also caught the interest of scientists because it’s covered in a high-albedo material that reflects more light than its surroundings, effectively creating a whiter color.
Researchers now know that the “heart” is not a single element. Sputnik Planitia (the western part) covers an area of 1200 by 2000 kilometers (equivalent to a quarter of Europe) and is three to four kilometers deeper than most of Pluto’s surface. “The bright appearance of Sputnik Planitia is due to it being predominantly filled with white nitrogen ice that moves and convects to constantly smooth out the surface. This nitrogen most likely accumulated quickly after the impact due to the lower altitude,” explained Dr. Harry Ballantyne from the University of Bern. The eastern part of the “heart” is also covered by a much thinner layer of nitrogen ice. The origin of this section is still unclear to scientists but is probably related to Sputnik Planitia.
“The elongated shape of Sputnik Planitia strongly suggests that the impact was not a direct head-on collision but rather an oblique one,” said Dr. Martin Jutzi from the University of Bern. The team used computer models to digitally recreate different impacts, varying both the composition of Pluto and the object that impacted, as well as the velocity and angle of the impact. These simulations confirmed that it was an oblique angle of impact and determined the composition of the object that impacted the planet.
“Pluto’s core is so cold that the rocks remained very hard and did not melt despite the heat of the impact, and thanks to the angle of impact and the low velocity, the core of the impactor did not sink into Pluto’s core but remained intact as a splat on it,” explained Dr. Ballantyne. “Somewhere beneath Sputnik is the remnant core of another massive body that Pluto never quite digested,” co-author Dr. Erik Asphaug added. “We are used to thinking of planetary collisions as incredibly intense events where you can ignore the details except for things like energy, momentum, and density. But in the distant Solar System, velocities are so much slower, and solid ice is strong, so you have to be much more precise in your calculations. That’s where the fun starts.”
The current study sheds new light on Pluto’s structure as well. The team revisited the crater of another giant impact like the one simulated, which is likely to have occurred earlier in Pluto’s history. Researchers expected that the giant caused by the early impact would slowly move towards the pole of the dwarf planet over time due. Yet it’s still located near the equator.
In theory, this happened because Pluto has a subsurface liquid water ocean. In this case, Pluto’s icy crust would be thinner in the Sputnik Planitia region, causing the ocean to bulge there, creating a mass surplus that would stop migration toward the equator. However, the authors now offer an alternative perspective. “In our simulations, all of Pluto’s primordial mantle is excavated by the impact, and as the impactor’s core material splats onto Pluto’s core, it creates a local mass excess that can explain the migration toward the equator without a subsurface ocean, or at most a very thin one,” explained Dr. Jutzi.
Ballantyne, H.A., Asphaug, E., Denton, C.A. et al. Sputnik Planitia as an impactor remnant indicative of an ancient rocky mascon in an oceanless Pluto. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02248-1