Billions of years ago, a piece of interplanetary debris smashed into Pluto and left an 825-kilometer-wide crater. But instead of turning into an ugly pockmark, that mighty scar may be responsible for one of Pluto’s most charismatic features: Its icy heart.
New data from NASA’s New Horizons spacecraft, which flew past Pluto in July, are helping scientists better understand the incredibly varied features on the world’s surface, which include smooth icy plains, parallel blades known as “snakeskin terrain,” and potential ice volcanoes.
Recently, observations revealed a circular feature surrounding the western ventricle of the striking, heart-shaped region known as Tombaugh Regio.
“The question is, could this be a relic giant impact basin?” asked Paul Schenk, at the 47th meeting of the American Astronomical Society’s Division for Planetary Sciences. “The key to that question is, is it deep? And the answer is yes.”
The basin is about 4 kilometers deep and stretches roughly one-third of the way across the icy world, said Schenk, of the Lunar and Planetary Science Institute. Finding a similarly sized gouge means going all the way to the other end of the solar system – to Mercury, where the Caloris impact basin stretches roughly one-third of the way across that roasted, dense world. (To put this in Earthly perspective, it would be as if a crater obliterated everything between Mexico and Canada, from California to North Carolina — aka, the majority of the United States.)
On Pluto, the smooth icefield known as Sputnik Planum sits within the basin. It’s a region that — unlike the possibly 4-billion-year old scar — is relatively young, at about 10 million years old. The edges of the basin are steep in the north but degraded in the south, where a large portion of the rim is missing. But the general shape is still obvious.
“It is indeed circular, except for the southern extension,” Schenk said. “The floor is basically flat.”
The impact created a depression that may have been perfect for accumulating flowing ices, especially given its location on the side of Pluto that never sees its large moon, Charon, said Douglas Hamilton of the University of Maryland, College Park. Pluto and Charon are locked in a whirling dance in which they keep the same face pointed at one another all the time. So Charon never fills the skies over Sputnik Planum – and none of the meager sunlight it reflects as Charonshine ever warms the cockles of Pluto’s heart.
That geometry helps make the region an efficient cold trap, or an area where ices can congregate. And that does seem to be what’s going on here. Will Grundy, from the Lowell Observatory, reported at the meeting that Sputnik Planum is full of basically every type of ice that has been spotted on Pluto, including carbon monoxide, nitrogen, and methane. The only species that’s conspicuously absent is water ice, which makes up the planet’s soaring mountains.
In fact, Sputnik Planum is more like an ice cap than anything else.
“We see ice caps throughout the solar system — ice caps on Earth, ice caps on Mars,” Hamilton said. “What we have to do is explain why this ice cap is at 30 degrees north on Pluto.”
It will take a bit of work to sort out exactly what happened, but scientists are well on their way to solving the mystery of Pluto’s heart.