Pluto's core was likely created by an ancient collision

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A huge heart-shaped feature on Pluto's surface has intrigued astronomers ever since NASA's New Horizons spacecraft captured it in a 2015 image. Now, researchers believe they've solved the mystery of how the distinctive heart came to be, and could reveal new clues about the dwarf planet's origins.

This feature is called “Tombo Regio” in honor of astronomer Clyde Tombaugh, who discovered Pluto in 1930. But scientists say the core is not all one element. For decades, details about Tombo Reggio's elevation, geology and distinctive shape, as well as its highly reflective surface, which is brighter white than the rest of Pluto, have eluded explanation.

A deep basin called Sputnik Planitia, which forms the “left lobe” of the core, is home to much of the nitrogen ice found on Pluto.

The basin covers an area 745 by 1,242 miles (1,200 km by 2,000 km), which is about a quarter the size of the United States, but is also 1.9 to 2.5 miles (3 to 4 km) lower in elevation than the majority of the United States. Planet surface. Meanwhile, the right side of the core also contains a layer of nitrogen ice, but it is much thinner.

Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/NASA

The New Horizons spacecraft captured an image of Pluto's heart on July 14, 2015.

Through new research on Sputnik Planitia, an international team of scientists has determined that a cataclysmic event created the core. After an analysis including numerical simulations, the researchers concluded that a protoplanetary body about 435 miles (700 kilometers) in diameter, or roughly twice the size of Switzerland from east to west, likely collided with Pluto early in the dwarf planet's history.

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These results are part of a study on Pluto and its internal structure published on Monday in the journal Nature astronomy.

Previously, the team studied unusual features across the solar system, such as those on the far side of the Moon, that were likely created by collisions during the system's chaotic early days of formation.

The researchers created numerical simulations using smooth particle hydrodynamics software, which is the basis for a wide range of planetary collision studies, to model different scenarios of the possible impacts, velocities, angles and compositions of a theoretical planetary body collision with Pluto.

The results showed that the planetary body would likely collide with Pluto at an oblique angle, rather than head-on.

“The core of Pluto is so cold that (the rocky body that collided with the dwarf planet) remained very solid and did not melt despite the heat of the collision, and thanks to the angle of the collision and the low speed, the core of the colliding body did not melt,” said Dr. Harry Ballantyne, the study’s lead author, and a co-researcher at the University of Bern. In Switzerland, in a statement: “It did not sink into the heart of Pluto, but remained intact like a blow to it.”

But what happened to the planetary body after its collision with Pluto?

“Somewhere beneath Sputnik lies the remains of the core of another massive object, one that Pluto never digested,” study co-author Eric Asfaugh, a professor at the University of Arizona's Lunar and Planetary Laboratory, said in a statement.

The team found that Sputnik Planitia's teardrop shape is a result of Pluto's cold core, as well as the relatively low speed of the impact itself. Other types of faster, more direct effects would have created a more symmetrical look.

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“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. But in a distant solar system, the speeds are much slower, and solid ice is strong, so you have to be more precise in your calculations.” This “It's where the fun starts.”

While studying the heart feature, the team also focused on Pluto's internal structure. An impact early in Pluto's history would have created a mass deficit, causing Sputnik Planitia to slowly migrate toward the dwarf planet's north pole over time while the planet was still forming. This is because the basin is less massive than its surroundings, according to the laws of physics, the researchers explained in the study.

However, Sputnik Planitia is located near the dwarf planet's equator.

Previous research has suggested that Pluto could have a subsurface ocean, and if so, the icy crust above the subsurface ocean would be thinner in the Sputnik Planitia region, creating a dense bulge of liquid water and causing mass to migrate toward the equator, the study authors said.

But the new study offers a different explanation for the location of the advantage.

“In our simulations, Pluto's primitive mantle has been completely excavated by the impact, and as the impactor's core material is scattered over 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 subsurface ocean,” said Dr. “It's very thin,” said study co-author Martin Goetze, a senior researcher in space research and planetary science at the Institute of Physics at the University of Bern.

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Kelsey Singer, principal scientist at the Southwest Research Institute in Boulder, Colorado and deputy co-principal investigator for NASA's New Horizons mission, who was not involved in the study, said the authors did a thorough job exploring the modeling and developing their hypotheses, although they would have liked to She sees a “closer connection to the geological evidence.”

“For example, the authors suggest that the southern part of Sputnik Planitia is very deep, but much of the geological evidence has been interpreted to suggest that the south is less deep than the north,” Singer said.

Researchers believe that the new theory regarding Pluto's core could shed more light on how the mysterious dwarf planet formed. Pluto's origins have remained a mystery since it is located at the edge of the solar system and has only been studied closely by the New Horizons mission.

“Pluto is a vast wonderland with unique and fascinating geology, so more creative hypotheses to explain that geology are always helpful,” Singer said. “What would help distinguish between the different hypotheses is to have more information about what is beneath the surface of Pluto. We can only achieve this by sending a spacecraft into the orbit of Pluto, perhaps using a radar that can look through the ice.”

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