When NASA’s New Horizons spacecraft sailed past Pluto in 2015, it captured an image of the dwarf planet’s heart-shaped geography. But while it was there, New Horizons also caught a glimpse of Pluto’s dark side.
An image of the back of the dwarf planet shows black ripples of rock exactly opposite Sputnik Planitia, the basin that makes up the left lobe of Pluto’s heart. Now, researchers report that those ripples could be evidence that Pluto has a 93-mile-thick ocean sandwiched between its crust and its core. The findings were presented the virtual Lunar and Planetary Science Conference, which had its in-person event canceled due to the new coronavirus pandemic.
The impact that carved the Sputnik Planitia basin could have sent shock waves around Pluto that ripped up the landscape on the other side, according to the results first pre-published in October, which have not yet been peer reviewed. Signs a similar event on Mercury are also apparent where a 950-mile-long impact crater sits at the exact opposite side of the planet to a region of chaotic geography, reports Robin George Andrews for Scientific American.
“If the impact is large enough, the planet itself can act like a lens, and focus the wave energy at the exact opposite point on the planet from the impact,” Purdue University planetary scientist Adeene Denton tells Science News’ Lisa Grossman.
When a large projectile—250 miles wide, in Denton’s simulations—crashes into a dwarf planet like Pluto, it creates a shock wave followed by a stress wave. As the pair ripple across the surface of the distant world, the waves also travel through its center. But waves would move at different speeds in different materials: quickly through the dwarf planet’s dense core, slowly through the icy crust, and slowest through a liquid ocean.
For Brigham Young University planetary scientist Jani Radebaugh, who wasn’t involved in the study, the research shows how insightful New Horizons’ brief visit to Pluto has been. “It’s amazing how we squeeze every little bit out it,” she tells Scientific American.
The model is still in early development, but it adds to a growing collection of hypotheses that suggest there could be water on Pluto. In 2016, analyses of its surface fissures and spherical shape pointed experts to conclude that a subsurface ocean might be to blame. And research published last year suggested that the impact on Sputnik Planitia may have broken through Pluto’s crust and brought the ocean to the surface, where it froze and tipped Pluto into its current orientation.
The images of Sputnik Planitia’s antipode is relatively low resolution, so it’s difficult to tell what exactly they’re showing. And other explanations have been offered for its chaotic state.
“There’s a lot of weird stuff on that far side of Pluto,” says James Tuttle Keane, a planetary scientist at NASA’s Jet Propulsion Laboratory, who was not involved with the study, to Scientific American. “And there are a lot of different ways that you can imagine creating some of those odd patterns that we see.”
For example, it’s possible that volatile ices, made of methane, carbon dioxide, and nitrogen, could have dug up the landscape by cycling between gaseous and solid states.
The New Horizons spacecraft is now exploring further into the Kuiper Belt, a ring of icy objects including Pluto, so evidence needed to confirm or deny theories about Pluto’s subsurface ocean will have to wait for another mission.
But if it’s true, “we might need to think a little differently about the Kuiper belt itself and all of these distant worlds in it and maybe look at them a little bit closer. They could all host such incredible, rich geologic histories,” Denton tells Scientific American. To Science News, she says, “It’s certainly not exactly a smoking gun. But it’s exciting.”