Beneath Pluto’s “heart” lies a cold, slushy ocean of water ice, according to data from NASA’s New Horizons mission. In a paper published in the journal Nature [http://www.nature.com], the New Horizons team reports that the dwarf planet’s most prominent surface feature—a heart-shaped region named Tombaugh Regio—may harbor a bulging, viscous, liquid ocean just below its surface.
The existence of a subsurface ocean may solve a longstanding puzzle: For decades, astronomers have observed that Tombaugh Regio, which is Pluto’s brightest region, aligns almost exactly opposite from the dwarf planet’s moon, Charon, in a locked orientation that has lacked a convincing explanation.
A thick, heavy ocean, the new data suggest, may have served as a “gravitational anomaly,” or weight, which would factor heavily in Pluto and Charon’s gravitational tug-of-war. Over millions of years, the planet would have gradually turned itself, aligning its subsurface ocean and the heart-shaped region above it, almost exactly opposite along the line connecting Pluto and Charon.
“Pluto is hard to fathom on so many different levels,” says New Horizons co-investigator Richard Binzel, professor of Earth, atmospheric and planetary sciences at MIT. “People had considered whether you could get a subsurface layer of water somewhere on Pluto. What’s surprising is that we would have any information from a flyby that would give a compelling argument as to why there might be a subsurface ocean there. Pluto just continues to surprise us.”
During its flyby of Pluto, New Horizons collected measurements of surface features, including the dimensions of Pluto’s bright, heart-shaped region. The researchers determined that the heart-shaped region is aligned directly opposite from the direction of Charon.
The massive basin also appears extremely bright relative to the rest of the planet, and the reason, the New Horizons data suggest, is that it is filled with frozen nitrogen ice.
Previously, Binzel and the New Horizons team had found evidence that this liquid nitrogen may be constantly refreshing, or convecting, as a result of a weak spot at the bottom of the basin. This weak spot may let heat rise through Pluto’s interior to continuously convect the ice, bubbling it over “like boiling oatmeal,” Binzel says.
To the New Horizons team, a weak spot in Sputnik Planitia’s basin suggests that the planet’s crust, particularly in this region, must be quite thin. If a massive impactor indeed created the basin, it may have also triggered any material beneath the surface to push the thin crust outward, causing a “positive gravitational anomaly,” or a thick, heavy mass, that would have helped to align the region relative to Charon.
But what sort of material would create enough of a gravitational weight to reorient the planet relative to its moon? To answer this, the team turned to a geophysical model of Pluto’s interior, working in measurements from the New Horizons spacecraft.
“Pluto is small enough that it’s just about almost cooled off but still has a little heat, and it’s about 2 percent the heat budget of the Earth, in terms of how much energy is coming out,” Binzel says. “So we calculated Pluto’s size with its interior heat flow, and found that underneath Sputnik Planitia, at those temperatures and pressures, you could have a zone of water-ice that could be at least viscous. It’s not a liquid, flowing ocean, but maybe slushy. And we found this explanation was the only way to put the puzzle together that seems to make any sense.”
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