Astrophysicists solve mystery of heart-shaped features on Pluto’s surface

An international team of astrophysicists, led by members of the University of Bern and the National Center of Competence in Research (NCCR) PlanetS, has finally solved the mystery of how giant heart-shaped features formed on the surface of Pluto. The team is the first to successfully reproduce this unusual shape through numerical simulations, and attributes it to a large and slow oblique impact.

Ever since cameras on NASA’s New Horizons mission discovered a giant heart-shaped structure on the surface of the dwarf planet Pluto in 2015, the “heart” has puzzled scientists because of its unique shape, geological composition and altitude. 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 study the origin of Sputnik Planitia, the western teardrop-shaped portion of the surface feature of Pluto’s heart.

According to their research, Pluto’s early history was marked by a catastrophic event that created Sputnik Planitia: a collision with a planetary body about 700 kilometers in diameter, roughly twice the area of ​​Switzerland from east to west.The team’s findings were recently published in natural astronomyalso showed that Pluto’s internal structure is different from what was previously assumed, showing the absence of a subsurface ocean.

a divided heart

The heart, also known as the “Tombaugh Zone,” attracted immediate public attention upon its discovery. But it also immediately piqued scientists’ interest because it’s covered in high-albedo material that reflects more light than its surroundings, resulting in a whiter color.

However, the heart is not made of a single element. Sputnik Planitia (West) covers an area of ​​1,200 x 2,000 km, equivalent to one-quarter of the size of Europe or the United States. Strikingly, however, this region is three to four kilometers lower than most of Pluto’s surface.

Dr. Harry Ballantyne explained: “Sputnik Planitia’s bright appearance is because it is mainly filled with white nitrogen ice, which is constantly smoothing the surface through movement and convection. Due to the lower altitude, this nitrogen is likely to be in the impact Then it accumulated quickly.

The eastern part of the heart is also covered by a similar but thinner layer of nitrogen ice, the origin of which scientists still don’t know but may be related to Sputnik Planitia.

tilt impact

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

So the team, like others around the world, used smoothed particle hydrodynamics (SPH) simulation software to digitally recreate such an impact, changing the composition of Pluto and its impactor, as well as the speed and angle of the impactor. The simulations confirmed scientists’ suspicions about the impact angle and determined the composition of the impactor.

“Pluto’s core is so cold that despite the heat from the impact, the rock remained very hard and did not melt, and because of the impact angle and low speed, the impactor’s core did not sink into Pluto’s core but remained intact,” Ballantyne explained. .

“Somewhere beneath Sputnik is the remnant core of another massive object that Pluto never fully digested,” added co-author Eric Asphaug from the University of Arizona. This core strength and relatively low velocity are key to the success of these simulations: The lower strength would result in very symmetrical remaining surface features that don’t look like the teardrop shape observed by New Horizons.

“We’re used to thinking of planetary collisions as extremely violent events, and you can ignore the details other than energy and momentum and density and so on. But in the distant solar system, the speed is much slower, and the solid ice is very strong, so you The calculations have to be more precise, and that’s where the fun starts,” Asphaug said.

The two teams have a long record of collaboration and have been exploring the idea of ​​planetary “splats” since 2011 to explain features on the far side of the moon, for example. Following the Moon and Pluto, the University of Bern team plans to explore similar scenarios for other extrasolar objects, such as the Pluto-like dwarf planet Haumea.

There is no underground ocean on Pluto

The current research also provides new clues to Pluto’s internal structure. In fact, a giant impact like the one simulated is more likely to have occurred early in Pluto’s history. However, this poses a problem: Due to the laws of physics, huge depressions like Sputnik Planitia are expected to slowly move toward the poles of the dwarf planet over time because of its mass deficit. But paradoxically, it is located near the equator.

Previous theoretical explanations have been that Pluto, like several other planetary bodies outside the solar system, has underground liquid water oceans. According to previous explanations, Pluto’s icy shell would be thinner in the Sputnik Planitia region, causing the oceans there to expand, and since liquid water is denser than ice, there would eventually be an excess of mass, causing it to migrate toward the equator.

However, this new study offers another perspective. “In our simulations, all of Pluto’s original mantle was excavated by the impact, and when the impactor’s core material splashed onto Pluto’s core, it created a local excess of mass, which could explain the absence of a subsurface ocean. The migration towards the equator is, at best, very thin,” explains Martin Jutzi.

Study co-author Dr. Adeene Denton of the University of Arizona is currently working on a new research project to estimate the speed of this migration. “This novel and creative origin of Pluto’s heart-shaped features may lead to a better understanding of Pluto’s origins,” she concluded.