When NASA's New Horizons spacecraft flew by Pluto in 2015, the first images it sent back to Earth contained an unexpected surprise: a strange, huge, bright, heart-shaped region, standing out strongly against the dwarf planet's dark surface. The area is known as the “Tombo Zone” in honor of the astronomer who discovered Pluto in 1930. Its area is 1,800 km from east to west and 1,500 km from north to south, and from the moment it was sighted, scientists have been trying to find out why. It was made and why it looks so different from the rest of the planet. Even more interesting is why the plain that forms the western part of the region, known as Sputnik Planitia, lies about 4 kilometers below the surrounding terrain.
Now, a team of researchers from the universities of Bern, Switzerland, and Tucson, Arizona, believe they have found the keys to the origin of this massive cosmic core. The core is mainly composed of nitrogen ice, and was formed after the “slow impact” of a large ice boulder about 730 kilometers in diameter. The study has just been published in “Natural Astronomy”.
The researchers reached this conclusion after using computer models to simulate different types of impacts on Pluto's surface and studying the resulting formations. They confirmed that the light colors of the Tombo region are due to the presence of nitrogen ice, very different from that covering the rest of the planet.
“While the vast majority of Pluto's surface consists of methane ice and its derivatives on a crust of water ice,” says Harry Ballantyne, lead author of the study, “Sputnik Planitia is mostly filled with nitrogen ice, which probably 'accumulated very substantially' quickly after the impact.” The eastern part of the core is also covered by a similar, but much thinner, layer of nitrogen ice, but its origin is still unclear to scientists, although it is also likely related to Sputnik Planitia.
In the illustration, a large icy object collides with Pluto
Roger Thibault/University of Bern
Slow effect
According to Eric Asfaugh, a professor at the University of Arizona's Lunar and Planetary Laboratory and co-author of the article, impacts between icy bodies that occur at the boundaries of the solar system are not the same as those that occur near the Sun. “We are accustomed to thinking of planetary collisions as incredibly intense events whose details can be ignored except for aspects such as energy, momentum and density,” the researcher says. But in the distant solar system, speeds are much slower and solid ice is strong, so you have to be more precise in your calculations. “This is where the fun starts.”
“Pluto's core is so cold that the rocks remained very solid and did not melt despite the heat of the impact,” Ballantyne says. “In addition, thanks to the low impact angle and velocity, the impactor's core did not sink below the surface of Pluto, but rather remained intact as if it had been struck.”
Under the supervision of Martin Goetze of the University of Bern, the team used a simulation method called “smooth particle hydrodynamics” to test different impact angles and sizes of colliding objects to see what kind of impact could lead to the formation of Sputnik Planitia, the western planet. Part of Pluto's core, it has an area of about 2,000 square kilometers and is located at an altitude of about 4 kilometers above its surroundings. It really looks like a massive dent caused by a massive planet-wide strike.
Simulations revealed that the formation arose very early in Pluto's history, perhaps after an oblique impact, which gave rise to its elongated shape. According to the authors, the icy rocks that struck Pluto were about 730 kilometers in diameter, but because of the planet's icy core, the impact did not melt or liquefy any of its parts, which would have happened near the Sun, causing the object to sink below the surface.
Far from it, the object collided with the solid floor of Pluto, and so far, scientists believe, it could be under the thin layer of nitrogen ice covering Sputnik Planitia. As Asphaug puts it, “Somewhere beneath Sputnik lies the core of the remains of another massive body that Pluto never fully digested.”
There is no underground ocean
The simulations also indicate that Pluto does not have an underground ocean beneath its icy crust, as some previous studies have suggested. Since the mass of the “core” is less than the mass of the rest of the planet's surface, the entire region is supposed to have gradually migrated poleward due to Pluto's rotation over millions of years. But instead, the formation is located near the equator: an odd location that previous studies have suggested may be due to the dynamics of a huge mass of subterranean liquid water.
However, the new research shows that an underground liquid ocean is not needed to explain the location of Pluto's core. “In our simulations, Pluto's primordial mantle has been completely excavated by the impact, and when material from the impactor's core collides with Pluto's core, excess local mass is created that could explain the migration toward Pluto,” Goetzi says. The equator without the need for an underground ocean, or at most a very shallow one.
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