Pluto's giant heart sets it apart from all the known planets. Known as Tombaugh Regio, the enormous impact basin filled with ices dominates the landscape of the tiny, faraway world. But Pluto's heart may be on the prowl, scientists now say, with it slowly wandering from where it first formed to the spot imaged by NASA's New Horizons spacecraft during last July's flyby.
The center of Tombaugh Regio sits close to an imaginary line, called the tidal axis, that wraps around Pluto. Here, the pull of the tides from the dwarf planet’s largest moon, Charon, is the strongest. Sometime in the past, a massive object smashed into Pluto, carving out the giant basin—which probably unbalanced the steady orbit of the tiny world. Seeking stability, the heart of Pluto began to slide across the surface—and the rest of the planet may have followed, researchers said last week at the Lunar and Planetary Sciences Conference in The Woodlands, Texas.
James Keane, a graduate student at the University of Arizona, has created one of the two competing models that have shown Pluto’s heart to be on the move. He compares Pluto to a football. When thrown with a spin, the pigskin cuts smoothly through the sky, as Pluto originally cut through the solar system. But after the impact, the flood of material in the basin unbalanced the dwarf planet, much as a lopsided mass would break up the football's smooth flight.
As New Horizons started sending back images of Pluto last year, Keane noticed a bright spot near the tidal axis. As the spacecraft drew closer, it soon revealed the heart shape. Scientists could also see the nitrogen ice and other material that filled the crater after impact. It’s that icy material that is responsible for the heart’s movement, Keane says. Just a few miles of nitrogen ice would be enough to cause the entire dwarf planet to reorient itself, changing the location not only of the heart but also of the dwarf planet’s poles.
Nitrogen ice may not be the only thing responsible for setting the heart adrift, though. Planetary scientist Francis Nimmo of the University of California, Santa Cruz, dug a little deeper into what could happen below the surface. In addition to the movement of ices, he suggests that part of the extra mass responsible for the shifting heart could be found beneath the surface of the dwarf planet. According to his models, the massive impact could have heated enough of Pluto’s icy crust to melt it. Water from a liquid ocean mantle would have welled up into the newly formed hollow. Because liquid water is denser than ice, the carved-out region would be more massive than the surrounding crust, creating a heavy bulge that tugged the heart toward the tidal axis.
"You stick an extra little lump on Pluto, that lump is going to feel a pull by Charon," Nimmo says. That extra mass then gets slightly pulled toward the moon.
Keane says the heart probably took a roundabout route to its current home. As Pluto travels on its wonky path around the sun every 248 years, temperatures change from frigid to relatively warm and back again. These fluctuations cause Pluto’s atmosphere to change, as well as the movement of the ices on its surface, so the heart would have spiraled toward its current location along a wobbly path.
Whether or not Pluto has an ocean today is a question scientists continue to puzzle over. Nimmo's model suggests that the mantle, Pluto’s middle layer, must still be liquid today if the heart is to be kept pointed away from Charon. The addition of nitrogen to the ocean would act as an antifreeze and could keep the layer liquid today. Keane's model, on the other hand, doesn't require a liquid layer, though it would work if one existed. That's because his simulations require nitrogen ice and other easily evaporating materials to move across the planet to settle in the heart.
Both models are theoretical, but researchers probably don’t yet have enough data to confirm either of them. While the relatively recent movement of ices was observed by New Horizons, measurements of density would require another mission to the dwarf planet.
Jeff Andrews-Hanna of the Southwest Research Institute in Colorado says that the explanations are intriguing, though both are preliminary enough that he hesitates toward preferring either. "They have an interesting observation that the enormous impact basin on the equator and opposite Charon is indicative of some sort of actual control," he says. "It's suggestive, and they've got some interesting ideas to try to explain it."
Other research has shown that the heart is young, only tens of millions of years old, so the movement of the ices may continue today. That means Pluto's heart may still be slowly traveling; a mission arriving at Pluto in a few decades could see the heart in a slightly different position.
While material moves across the surface of Earth and Saturn's large moon Titan, the fact that the rest of the crust follows the ices is unique. "Ice caps don't usually reorient planets," Keane says.