The cosmic collision that made Pluto's heart

Secret how Pluto Finally, a giant heart-shaped feature on its surface has been solved by an international team of astrophysicists led by… University of Bern and members of the National Center of Competence in Research (NCCR) PlanetS. The team is the first to successfully reproduce the unusual shape using digital simulations, and attributes this to the effect of the giant, slow tilt angle.

Since cameras NASAThe New Horizons mission discovered a large heart-shaped structure on the surface of the dwarf planet Pluto in 2015. This “heart” puzzled scientists because of its unique shape, geological composition, and height. Scientists from the University of Bern in Switzerland and the University of Arizona used numerical simulations to investigate the origins of Sputnik Planitia, the tear-shaped western part of the surface feature of Pluto's core.

According to their research, Pluto's early history was marked by a cataclysmic event that led to the formation of Sputnik Planitia: its collision with a planetary body just over 400 miles in diameter, roughly the size of Arizona from north to south. The team's findings, which were published in Nature astronomyIt also indicates that Pluto's internal structure is different from what was previously assumed, suggesting that there is no subsurface ocean.

“The formation of Sputnik Planitia provides an important window into the early periods of Pluto’s history,” said Adeniy Denton, a planetary scientist at the Lunar and Planetary Laboratory in Arizona who co-authored the paper. “By expanding our investigations to include more unusual formation scenarios, we have learned some completely new possibilities for the evolution of Pluto, which could apply to other objects.” Kuiper belt Objects as well.”

A view of Pluto taken by NASA's New Horizons space probe on July 14, 2015. Image source: NASA/Johns Hopkins University Applied Physics Laboratory/Southwestern Research Institute.

Divided heart

The “heart”, also known as tombo regio, captured the public's attention immediately after its discovery. But it also immediately caught scientists' attention because it is covered with a high-albedo material that reflects more light from its surrounding environment, creating a whiter color. But the heart does not consist of a single element. Sputnik Planitia covers an area about 750 by 1,250 miles, about a quarter the size of Europe or the United States. But what is striking is that the elevation of this region is about 2.5 miles lower than most of Pluto's surface.

“While the vast majority of Pluto's surface consists of methane ice and its derivatives covering a crust of water ice, Planitia is mostly filled with nitrogen ice, which likely accumulated quickly after the impact due to the low altitude,” the lead author said. From the study was Harry Ballantyne, a research associate in Bern. The eastern part of the core is also covered by a similar but much thinner layer of nitrogen ice, the origin of which remains unclear to scientists, but is likely related to Sputnik Planitia.

Slant effect

Sputnik Planitia's elongated shape and position at the equator strongly suggest that the impact was not a direct collision but rather an oblique impact, according to Martin Goetze of the University of Bern, who initiated the study. Like many others around the world, the team used smooth particle hydrodynamics simulation software to digitally recreate such impacts, varying the configuration of Pluto and its impact body, as well as the speed and angle of the impact body. These simulations confirmed scientists' suspicions about the oblique impact angle and determined the configuration of the impact object.

“Pluto's core is so cold that the rock remained very solid and did not melt despite the heat of the impact, and thanks to the impact angle and low speed, the impact core did not sink into Pluto's core, but rather remained intact as a blow to it,” said Ballantyne. “This was the fundamental force and low speed.” Relativity is key to the success of these simulations: The low force will result in a highly symmetrical surface feature that looks nothing like the teardrop shape observed by NASA's New Horizons probe during its flyby of Pluto in 2015.

“We're used to thinking of planetary collisions as incredibly intense events where you can ignore the details except for things like energy, momentum and density,” said Eric Asfaugh, a Lunar and Planetary Laboratory professor and co-author of the study, whose team collaborated with the research team. Since 2011, Swiss colleagues have been exploring the idea of ​​planetary “explosions” to explain, for example, features on the far side of Earth's moon. “In the distant solar system, velocities are much slower than those closer to the Sun, and solid ice is strong, so you have to be more precise in your calculations. That's where the fun begins.”

There is no subsurface ocean on Pluto

The current study sheds new light on Pluto's internal structure as well. In fact, a giant impact like the one simulated likely occurred much earlier in Pluto's history than in modern times. However, this poses a problem: a giant depression like Sputnik Planitia is expected to slowly drift toward the dwarf planet's pole over time due to the laws of physics, because it is less massive than its surroundings. However, it remained close to the equator. The previous theoretical explanation was based on the existence of a liquid water ocean beneath the Earth's surface, similar to many other planetary bodies in the outer solar system. According to this hypothesis, Pluto's icy crust would be thinner in the Sputnik Planitia region, causing the ocean to bulge upward, and since liquid water is denser than ice, causing a mass surplus that would cause it to migrate toward the equator.

The new study offers an alternative view, according to the authors, pointing to simulations in which Pluto's primordial mantle is completely excavated by the impact, and as the impactor's core material falls onto Pluto's core, it creates a local mass surplus that could explain the migration toward the equator without… A subsurface ocean, or at most a very thin ocean.

Denton, who has already embarked on a research project to estimate the speed of this migration, said the new and innovative origin hypothesis for Pluto's heart-shaped feature may lead to a better understanding of the origin of the dwarf planet.

Reference: “Sputnik Planitia as Impact Remnant Points to Ancient Rock Mass on Oceanless Pluto” by Harry A. Ballantyne, Eric Asfough, and C. Aden Denton, Alexander Emsenhuber, and Martin Goetze, April 15, 2024, Nature astronomy.
doi: 10.1038/s41550-024-02248-1