Pluto’s heart may hide the rocky wreckage of an ancient impact

That could explain the odd location of Pluto’s famous heart-shaped basin, Sputnik Planitia

An image of Pluto's Sputnik Planitia, a large basin likely formed from an impact billions of years ago.

Pluto’s heart-shaped feature is known as Sputnik Planitia. Scientists think it’s the scar left when something smashed into Pluto billions of years ago. The impact may have left behind a dense rocky remnant that has kept the basin from tipping toward the dwarf planet’s poles.

NASA, JHUAPL, SwRI

Pluto seems to have a heavy heart, and scientists are trying to find out why.

The huge, heart-shaped feature on Pluto’s surface is called Sputnik Planitia. For this feature to sit where it does on Pluto’s side, scientists think something dense and heavy must lie beneath it. One idea was a vast ocean. But computer models now hint it may be a huge rocky remnant instead.

Researchers shared this finding April 15 in Nature Astronomy.

Sputnik Planitia first appeared in images taken by NASA’s New Horizons spacecraft in 2015. The formation has roughly the same area as the Democratic Republic of Congo in central Africa. It sits three to four kilometers (2 to 2.5 miles) below the rest of Pluto’s surface and is filled with frozen nitrogen.

“We think it’s an impact basin,” says Adeene Denton. That would be a scar left by something slamming into Pluto billions of years ago. “That’s the easiest way to make a giant hole in the ground.” Denton is a planetary scientist at the University of Arizona in Tucson.

This basin’s site, across Pluto’s equator, is perplexing. Knocking a huge hole in one side of a dwarf planet should make it wobble. And wobbling over millions of years should make the dwarf planet tilt. That would bring the impact basin closer to one of its poles. (This is why the huge Aitken basin on Earth’s moon sits near the lunar south pole.)

Some scientists thought the impact that carved out Pluto’s heart also created a dense, underground ocean of liquid water. The weight of that water could have kept Pluto from tilting after the impact and held Sputnik Planitia near its equator.

But it’s hard to explain how an ocean could survive on Pluto. The surface is a frigid –230° Celsius (–382° Fahrenheit). The bottom of Sputnik Planitia is closer to the warmth of Pluto’s interior. But even that depth is likely far below water’s freezing point.

“What if Pluto didn’t have an ocean at all?” Denton asks.

Using computer models, her team tested what would happen if space rocks of different sizes crashed into Pluto. One scenario really caught their eye. It involved a space rock roughly 730 kilometers (450 miles) across. That’s slightly larger than the asteroid Vesta. And it’s big enough to have a dense, solid core cloaked in less dense materials.

As an object that size plowed into Pluto, its exterior would have vaporized, a computer model now suggests. But its heavy center would have stayed intact. The core eventually would settle below Sputnik Planitia’s surface. There, it could keep the heart from tilting away from Pluto’s equator. 

“This is an important idea for us to be thinking about and exploring,” says Carver Bierson. This planetary scientist, who works at Arizona State University in Tempe, did not take part in the new work.

Other researchers have raised doubts about cold, tiny Pluto having an ocean, too. So Bierson is happy to see a different model that can explain Sputnik Planitia’s features.

Finding out for sure which idea is correct will likely require placing a spacecraft in orbit around Pluto. That spacecraft could measure the dwarf planet’s gravity, Denton says. Such measurements could offer clues about what Pluto is made of. Scientists have proposed this kind of mission. But even if it happens, it wouldn’t take off for decades.

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