Pluto’s heart-shaped basin might not hide an ocean after all

Planetary scientists propose an alternative theory to explain why Sputnik Planitia has stayed put across Pluto’s equator.

Pluto’s heart-shaped basin might not hide an ocean after all
Pluto’s heart-shaped Sputnik Planitia is probably an impact basin that formed billions of years ago. Scientists have proposed that the impactor left behind a dense rocky remnant that has kept the basin from moving toward the poles. NASA, JHUAPL, SWRI Share this: Email Facebook Twitter

Pluto's heart may be concealing a massive, weighty treasure rather than a big ocean.
Researchers write in Nature Astronomy on April 15 that computer models indicate that an object roughly 730 kilometers wide—slightly larger than the asteroid Vesta—might have collided with the dwarf planet billions of years ago, producing the well-known Sputnik Planitia and leaving behind a rocky remnant.

When NASA's New Horizons probe flew by Pluto in 2015, photos of Sputnik Planitia were captured (SN: 7/15/15). The heart-shaped structure, which is filled with frozen nitrogen, is located three to four kilometers below the rest of Pluto's surface and nearly equal in size to the Democratic Republic of the Congo.

“We think it’s an impact basin, because that’s the easiest way to make a giant hole in the ground,” says planetary scientist Adeene Denton of the University of Arizona in Tucson.

However, it is confusing where the basin is—across Pluto's equator. A rotating object, such a dwarf planet or moon, would experience unstable wobbles that would cause the object's tilt to change over millions of years if a massive hole were to be punched in one side of it. This explains, for example, why the massive Aitken basin currently lies close to the lunar south pole on Earth's moon.

According to some experts, the impact that formed Pluto's heart also produced an underground ocean of liquid water that is thick and keeps Sputnik Planitia at the equator (SN: 3/27/20). 

But explaining how the purported ocean could have survived over geologic time has proved challenging. Pluto’s surface is a frigid –230° Celsius and even the base of Sputnik Planitia is probably far below water’s freezing point.

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

To explore this possibility, she and her colleagues used computer simulations to see what would happen if rocky objects of different sizes crashed into Pluto. A space rock that’s roughly 730 kilometers in diameter is large enough to have a dense, solid core surrounded by lighter-weight materials. As a simulated object of such size plowed into Pluto, the impactor’s exterior vaporized but its heavy center remained intact. The core eventually settled below Sputnik Planitia’s surface, where it could keep the heart from straying. 

“This is an important idea for us to be thinking about and exploring,” says Carver Bierson, a planetary scientist at Arizona State University in Tempe who wasn’t involved in the work. Other researchers have raised doubts about cold, tiny Pluto having an ocean, so he’s happy to see an alternative model that can explain Sputnik Planitia’s properties.

Definitively determining which idea is correct will probably require placing an orbiter around Pluto that can measure the dwarf planet’s gravitational field, Denton says. Such a mission has been proposed, though would take decades to reach its goals.