Scientists have finally solved the mystery of how a giant heart-shaped object appeared on the surface of Pluto. An international team of astrophysicists has for the first time successfully reproduced the unusual shape with the help of numerical simulations and attributed it to a huge but slow-moving impact at an acute angle.
Since cameras on NASA’s New Horizons mission discovered a large heart-shaped structure on the surface of the dwarf planet Pluto in 2015, the find has puzzled scientists with its unique shape, geological composition and height. So scientists used numerical simulations to investigate the origin of the western part of Pluto’s heart-shaped surface object, called Sputnik Planitia.
According to research, a cataclysm occurred in Pluto’s early history that influenced the formation of this region. It was a collision with a planetary body about 700 km in diameter, roughly twice the size of Switzerland from east to west. The team’s findings also suggest that Pluto’s internal structure is different from what was previously thought and that it does not have a subsurface ocean.
Pluto’s heart, also known as the Tombo Region, has attracted public attention and scientific interest because it is covered in high-albedo material that reflects more light than the surrounding environment, giving it a whiter colour. However, the heart does not consist of one element. Its western part occupies an area of 1200×2000 km, which is equivalent to a quarter of Europe. However, it is striking that this region is 3-4 km lower in altitude than most of Pluto’s surface.
“The bright appearance of the Satellite Plain is explained by the fact that it is mostly filled with white nitrogen ice, which moves and convects, constantly levelling the surface. This nitrogen, most likely, quickly accumulated after the collision due to the lower altitude,” the scientists say. The eastern part of the heart is also covered with a similar but much thinner layer of nitrogen ice, the origin of which is still unclear to scientists.
“The elongated shape of the region convincingly indicates that it was not a direct head-on collision, but rather an oblique one,” the scientists add. The team used smoothed particle hydrodynamics (SPH) simulation software to digitally recreate such collisions, varying both the composition of Pluto and its impactor, as well as the collision speed and angle. The simulations confirmed the scientists’ suspicions about the oblique collision angle and determined the composition of the impact body.
“Pluto’s core is so cold that the rocks remained solid and did not melt, despite the heat from the impact, and due to the impact angle and low speed, the impactor core did not sink into Pluto’s core but remained intact as a splash on it,” the scientists said. “Somewhere under the surface, there is a remnant of the core of another massive body that Pluto never digested.” The strength of the core and the relatively low speed were key to the success of these simulations because less strength would have resulted in a very symmetrical shape of the residual surface, which is not like the droplet shape seen by New Horizons.
This study also shares new information about Pluto’s internal structure. A giant impact like the one modelled is much more likely than one that occurred very early in Pluto’s history. However, this creates a problem: a giant depression like the one in this region should, according to the laws of physics, slowly move toward the dwarf planet’s pole over time because it has a mass deficit. However, it is paradoxically located near the equator.
The previous theoretical explanation was that Pluto has a subsurface ocean of liquid water. According to this explanation, Pluto’s icy crust would be thinner in the region of the Sputnik Plain, which would cause the ocean to bulge, and since liquid water is denser than ice, the result would be an excess of mass that would cause it to migrate equatorward.
However, a new study offers an alternative view. “In our simulations, all of Pluto’s primordial mantle has been destroyed by the collision, and as the core material falls onto Pluto’s core, it creates a local mass excess that could explain equatorward migration without a subsurface ocean, or at most with a very thin ocean,” – scientists note.
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