April 15 () –
The mystery of how Pluto got a giant heart-shaped feature on its surface has finally been solved by an international team of astrophysicists.
The team, led by the University of Bern and members of the PlanetS National Competence Center for Research (NCCR), is the first to successfully reproduce the unusual shape with numerical simulations, attributing it to a giant and slow impact at an oblique angle.
Since cameras from 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 due to its unique shape, geological composition and elevation. A team of scientists from the University of Bern, including several members of the NCCR PlanetS, and the University of Arizona in Tucson, have used numerical simulations to investigate the origins of Sputnik Planitia, the teardrop-shaped western part of the surface of Pluto's core.
According to their research, Pluto's early history was marked by a cataclysmic event that formed Sputnik Planitia: a collision with a planetary body about 700 km in diameter, about twice the size of Switzerland from east to west. The team's findings, which were recently published in Nature Astronomyalso suggest that Pluto's internal structure is different from what was previously assumed, indicating that there is no ocean beneath the surface.
The heart, also known as Tombaugh Regio, caught the public's attention immediately after its discovery. But it also immediately caught the interest of scientists because it is covered in a high-albedo material that reflects more light than its surroundings, creating its whiter color.
However, the heart is not composed of a single element. Sputnik Planitia (the western part) covers an area of 1,200 by 2,000 kilometers, which is equivalent to a quarter of Europe or the United States. What is surprising, however, is that this region It has an elevation three to four kilometers lower than most of Pluto's surface.
“Sputnik Planitia's bright appearance is because it is predominantly filled with white nitrogen ice that moves and convects to constantly smooth the surface. This nitrogen probably accumulated quickly after impact due to the lower altitude,” he explains. it's a statement Dr. Harry Ballantyne of the University of Bern, lead author of the study.
The eastern part of the heart is also covered by a similar, but much thinner, layer of nitrogen ice, the origin of which is still unclear to scientists, but is probably related to Sputnik Planitia.
““The elongated shape of Sputnik Planitia clearly suggests that the impact was not a direct head-on collision but rather an oblique one,” notes Dr. Martin Jutzi of the University of Bern, who initiated the study.
So the team, like many others around the world, used its Smoothed Particle Hydrodynamics (SPH) simulation software to digitally recreate such impacts, varying both the composition of Pluto and its impactor, as well as the speed and angle of the impactor. These simulations confirmed scientists' suspicions about the oblique angle of the impact. 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 of the impact, and thanks to the impact angle and low velocity, the core of the impactor did not sink into Pluto's core, but that remained intact. like a touch,” explains Ballantyne.
“Somewhere beneath Sputnik lies the remnant core of another massive body, which Pluto never fully digested,” adds co-author Erik Asphaug of the University of Arizona. This core force and relatively low velocity were key to the success of these simulations: a lower force would result in a very symmetrical leftover surface feature that does not resemble the teardrop shape observed by New Horizons.
“We are used to thinking of planetary collisions as incredibly intense events where the details can be ignored except for things like energy, momentum and density. But in the distant solar system, the speeds are much slower and the ice solid is strong, so you have to be much more precise in your calculations. That's where the fun begins“says Asphaug.
The two teams have a long history of collaborations together, since 2011 exploring the idea of planetary “splats” to explain, for example, the characteristics of the far side of the moon. After our Moon and Pluto, the University of Bern team plans to explore similar scenarios for other outer solar system bodies, such as the Pluto-like dwarf planet Haumea.
