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A huge heart-shaped formation on Pluto’s surface has intrigued astronomers since NASA’s New Horizons probe captured it in a 2015 image. Now researchers think they have solved the mystery of how the distinctive heart came to be – and may reveal new clues about the origins of the dwarf planet.
The structure is called Tombaugh Regio in honor of astronomer Clybe Tombaugh, who discovered Pluto in 1930. But the heart is not a single element, scientists say. And for decades, details about Tombaugh Regio’s elevation, geological composition and distinctive shape, as well as its highly reflective surface that is a brighter white than the rest of Pluto, have defied explanation.
A deep basin called Sputnik Planitia, which makes up the “left lobe” of the heart, houses much of Pluto’s nitrogen ice.
The basin covers an area that spans 745 miles by 1,242 miles (1,200 kilometers by 2,000 kilometers), equivalent to about a quarter of the United States, but is also 1.9 to 2.5 miles (3 to 4 kilometers) lower in altitude than most of the planet’s surface. Meanwhile, the right side of the heart also has a layer of nitrogen ice, but it is much thinner.
Through new research on Sputnik Planitia, an international team of scientists has determined that a cataclysmic event created the heart. After an analysis involving numerical simulations, researchers concluded that a planetary body about 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.
The findings are part of a study of Pluto and its internal structure published Monday in the journal Nature Astronomy.
Recreating an ancient ‘splat’ on Pluto
Previously, the team studied unusual features throughout the Solar System, such as those on the far side of the Moon, which were likely created by collisions during the early, chaotic days of the system’s formation.
The researchers created the numerical simulations using smoothed particle hydrodynamics software, considered the basis for a wide range of planetary collision studies, to model different scenarios for potential impacts, speeds, angles and compositions of the theorized planetary body’s collision with Pluto.
The results showed that the planetary body likely collided with Pluto at a tilted angle, rather than head-on.
“Pluto’s core is so cold that the (rocky body that collided with the dwarf planet) remained very hard and did not melt despite the heat of the impact, and thanks to the impact angle and low speed, the impactor’s core did not sink. in Pluto’s core, but remained intact as a splash in it,” study lead author Dr. Harry Ballantyne, a research associate at the University of Bern in Switzerland, said in a statement.
But what happened to the planetary body after it collided with Pluto?
“Somewhere beneath Sputnik is the remnant core of another massive body, which Pluto never digested,” study co-author Erik Asphaug, a professor at the University of Arizona’s Lunar and Planetary Laboratory, said in a statement.
Sputnik Planitia’s teardrop shape is a result of the frigidity of Pluto’s core, as well as the relatively slow speed of the impact itself, the team found. Other types of faster, more direct impacts would have created a more symmetrical shape.
“We are used to thinking of planetary collisions as incredibly intense events where we can ignore the details except things like energy, momentum and density. But in the distant Solar System, speeds are much slower and solid ice is strong, so you need to be much more precise in your calculations,” said Asphaug. “That’s where the fun begins.”
Pluto’s Dark Origins
While studying the heart’s 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 due to the fact that the basin is less massive than its surroundings, according to the laws of physics, the researchers explained in the study.
However, Sputnik Planitia is close to the dwarf planet’s equator.
Previous research has suggested that Pluto could have a subsurface ocean, and if so, the icy crust over the subsurface ocean would be thinner in the Sputnik Planitia region, creating a dense bulge of liquid water and causing a mass migration toward the equator, said the study authors.
But the new study offers a different explanation for the feature’s location.
“In our simulations, Pluto’s entire primordial mantle is excavated by the impact, and as material from the impactor’s core spreads over Pluto’s core, it creates a local excess mass that could explain the equatorward migration without an underground ocean, or at most a very thin one,” said study co-author Dr. Martin Jutzi, senior researcher for space research and planetary sciences at the Institute of Physics at the University of Bern.
Kelsi Singer, principal scientist at the Southwest Research Institute in Boulder, Colorado and co-deputy principal investigator on 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 she would have liked to have seen “a closer link with the geological evidence”.
“For example, the authors suggest that the southern portion of Sputnik Planitia is very deep, but much of the geological evidence has been interpreted as pointing to the south being shallower than the north,” Singer said.
Researchers believe the new theory about Pluto’s heart could shed more light on how the mysterious dwarf planet formed. Pluto’s origins remain unclear, as it exists at the edge of the solar system and has only been studied closely by the New Horizons mission.
“Pluto is a vast wonderland of unique and fascinating geology, so more creative hypotheses to explain this geology are always useful,” Singer said. “What would help distinguish between different hypotheses would be more information about Pluto’s subsurface. We will only achieve this by sending a space mission into Pluto’s orbit, potentially with a radar that can peer through the ice.”
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