An international team of astrophysicists led by the University of Bern and members of the National Center for Research (NCCR) PlanetS has finally solved the mystery of how Pluto got the giant heart shape on its surface. The team was the first to successfully reproduce the unusual shape through numerical simulations, attributing it to a giant slow impact at an oblique angle.

Ever since cameras on NASA’s New Horizons mission discovered a large heart-shaped structure on the surface of the dwarf planet Pluto in 2015, this “heart” has baffled scientists with its unique shape, geological composition and height. A team of scientists from the University of Bern, including several members of NCCR PlanetS and the University of Arizona in Tucson, used numerical simulations to investigate the origin of Sputnik Planitia, the western teardrop-shaped portion of the surface of Pluto’s heart.

According to their research, Pluto’s early history was marked by the catastrophe that formed the planet Sputnik: a collision with a planetary body about 700 km in diameter, roughly twice the size of Switzerland from east to west. The team’s recently published findings Nature AstronomyIt also suggests that Pluto’s internal structure is different than previously thought, indicating the absence of a subsurface ocean.

A divided heart

The heart, also known as Tombaugh Regio, attracted public attention immediately after its opening. However, it also immediately attracted the attention of scientists because it was covered with a high-albedo material that reflects more light than its surroundings and creates its white color.

However, the heart does not consist of a single element. Sputnik Planitia (western part) covers an area of ​​1,200 x 2,000 kilometers, equivalent to a quarter of the size of Europe or the United States. However, it is noteworthy that this region is 3-4 kilometers lower than the height of most of Pluto’s surface.

“Sputnik Planitia’s bright appearance is due to the fact that it is mostly filled with white nitrogen ice that is constantly moving and convectively flattening the surface. This nitrogen most likely accumulated rapidly after the low-altitude impact,” explains Dr. Harry Ballantyne, lead study author from the University of Bern. author.

The eastern part of the heart is covered by a similar but much thinner layer of nitrogen ice; The origin of this layer is still unclear to scientists, but it is probably related to Sputnik Planitia.

oblique blow

Dr. from the University of Bern, who initiated the study. “The elongated shape of Sputnik Planitia strongly suggests that the collision was not a direct head-on collision but an oblique one,” says Martin Jutzi.

So the team, like many others around the world, used smoothed particle hydrodynamics (SPH) simulation software to digitally recreate such collisions; It changed the composition of both Pluto and the impactor, as well as the speed and angle of the impactor. This simulation confirmed scientists’ suspicions about the oblique impact angle and determined the composition of the impactor.

“Pluto’s core is so cold that the rocks remained very hard and did not melt despite the heat from the impact. Because of the impact angle and low speed, the impactor’s core did not sink into Pluto’s core but remained intact, “like a beacon on it,” explains Ballantyne.

“Somewhere under the Moon there is a remnant of the core of another large body that Pluto never digested,” adds co-author Eric Asfaug of the University of Arizona. This core power and relatively low speed were key to the success of these simulations: A lower power would result in a very symmetric residual surface that does not resemble the teardrop shape observed by New Horizons.

“We’re used to thinking of planetary collisions as incredibly intense events where you can ignore details other than things like energy, momentum and density. But in the distant solar system, speeds are much lower and solid ice is strong, so you need to be much more precise in your calculations, Asfaug says.

The two teams have a long history of working together, starting in 2011, exploring the idea of ​​planetary “bubbles” to explain features on the far side of the Moon, for example. After the Moon and Pluto, the University of Bern team plans to study similar scenarios for other outer Solar System bodies, such as the Pluto-like dwarf planet Haumea.

There is no underground ocean on Pluto

The current study also sheds new light on Pluto’s interior structure. In fact, a giant impact like the one modeled most likely occurred very early in Pluto’s history. However, this creates a problem: A giant collapse like Sputnik Planitia is expected to slowly drift towards the pole of the dwarf planet over time, due to the laws of physics, due to its mass deficit. However, it is paradoxical that it is close to the equator.

The previous theoretical explanation was that Pluto, like some other planetary bodies in the outer solar system, has a subsurface ocean of liquid water. According to the previous explanation, Pluto’s icy crust will be thinner in the Sputnik Planitia region, which will cause the ocean to burst there, and since liquid water is denser than ice, you will have excess mass, which will encourage migration towards the equator.

But new research offers an alternative perspective. “In our simulations, the entire primitive mantle of Pluto has been excavated by the impact, and when the impactor’s core material hits Pluto’s core, it creates a local mass excess that could explain equatorward migration without a subsurface ocean, or at most a very thin ocean,” explains Martin Jutsi. .

Dr. from the University of Arizona, who is also one of the study’s co-authors. Adin Denton is currently conducting a new research project to estimate the speed of this migration. “This new and innovative origin of Pluto’s heart-shaped feature may help better understand Pluto’s origin,” he concludes.