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.

Let’s learn about Pluto
“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.


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