ADD a note to the edge of a map of our solar system: here be ice zombies.
Before NASA’s New Horizons probe flew past Pluto in July, those behind the project were worried that the dwarf planet would be boring.
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“In your worst nightmare, you convinced NASA to go all the way to the end of the solar system and you would have a cratered ball,” says team member of Washington University in St Louis.
Thankfully, Pluto played along. The fly-by showed that the dwarf planet is more alive than anyone predicted, full of weird geology from glacier-carved valleys to broad mounds that look like ice volcanoes. Last week, planetary scientists began to share their first guesses about these features’ origins at a meeting of the American Astronomical Society’s Division for Planetary Sciences in National Harbor, Maryland.
But as they work to understand how the pieces fit together, two questions linger: what powers Pluto? And will other worlds in the cold reaches of the outer solar system be the same?
Even by themselves, the signs of recent geology on Pluto are giving us plenty to puzzle over. While some areas look dead and scarred by aeons of craters, the west side of Pluto’s famed heart feature sports not a single blemish. The lack of craters means that the region, a glacial field of frozen nitrogen and carbon monoxide called Sputnik Planum, has repaved itself with fresh ice within the last 10 million years. But how?
The round centre of Sputnik Planum may be a basin hollowed out by an impact billions of years ago, New Horizons team member argued at the meeting. Later, the cavity was filled either as ices settled out of the atmosphere, or oozed out as a slushy lava from the crust.
In the photo above are highlighted signs of glacial flow (blue); Sputnik Planum, a basin filled with churning ice (yellow); mountains formed by floating shards of water ice (red); possible ice volcanoes (green).
A range of jagged mountains on the west edge of Sputnik Planum could be blocks of water ice that were released from the crust beneath the basin when it was cracked open during the impact. They may have floated to the top of denser nitrogen ice and collected in a logjam at the basin’s edge, the team says.
Giant polygons that rise a few tens of metres above the Sputnik plains may be rising bubbles of soft nitrogen ice, perhaps from a few kilometres below – maybe even warmed up or melted at that depth. “There’s this churning cauldron of the volatile ices,” McKinnon says. The ices may even evaporate into the atmosphere, fall as snow on the surrounding peaks, and slide into the basin as glaciers, McKinnon says.
The most dramatic signs of recent geological activity are the two putative volcanoes, Wright and Piccard Mons. Each is a few kilometres tall and about a hundred kilometres wide. With depressions at their tops and nubby deposits on their slopes, both look like they were built where material – mysteriously heated from underneath – has been coming to the surface.
But the mountains are too fat to be made of firm water ice, and too sharp to be made of pliable nitrogen ice. “If it’s volcanic, it’s a new kind of construct,” McKinnon says. “Maybe it’s something more interesting, like ammonia in water ice or methanol.”
That sort of mixture might be the key to explaining not just Pluto’s volcanoes, but its overall activity. Pluto is small, just one-third the volume of our moon, which limits the heat it could hold on to since its formation. It also has no giant planet nearby to build up warmth by tugging at its insides, like Neptune’s gravity does to its moon Triton.
For Sputnik Planum to stir like we think it does, Pluto must have found a way to stretch its meagre heat budget. One idea presented at the meeting is that Pluto’s water ice is spiked with something that acts like antifreeze: ammonia.
Antifreeze
Though water ice is frozen so solid it acts like rock on Pluto, an infusion of ammonia, which is abundant in the outer solar system, can make it melt at much lower temperatures. That could let the material under Pluto’s crust churn, with hotter, liquid ice rising to the top, says Alex Trowbridge of Purdue University. “Once you start getting melt, that decreases the viscosity and allows things to flow much easier,” he says.
That could give Pluto life without needing much heat. “If that material is low viscosity, it doesn’t have to be really warm,” says of the US Geological Survey. Far from being frozen solid, a circulatory system of slush might be keeping Pluto frosty but undead.
“Far from being frozen solid, a circulatory system of slush might be keeping Pluto frosty but undead”
Surprised that geology on Pluto has somehow defied the cold, Bland and others are now curious about the other lone dwarf planets out there.
“The common conception is that they don’t have enough energy to drive geologic activity into the present day,” Bland says. But visiting Pluto has proved that idea wrong. “The interesting thing is: if that’s true on Pluto, what are the implications for other bodies in the solar system as well?”
The rest of the dwarf planet family gives plenty of food for thought. Some might be less active: Eris is even colder, and doesn’t have much atmosphere, and thus might not have glacial transport. Haumea has far fewer volatile ices. On the other hand, Makemake and others might move in a similar way to Pluto. “Makemake is like Pluto’s cousin with much more methane,” McKinnon says. We might even be exploring another long-lost member of the family right now (see “Separated at birth?“).
The outermost regions of the known solar system have also just acquired another likely dwarf planet: V774104. Discovered by of the Carnegie Institution in Washington DC, and presented to the meeting, not much is known about the new world – except that it is roughly half the size of Pluto and about three times as far from the sun.
“What Pluto has shown us is that the object itself is generating the activity, and there’s no reason that many of the others, maybe all of the others, shouldn’t do the same,” says New Horizons team leader Alan Stern. New Horizons is already aimed at another distant body (see “Beyond Pluto“), which is probably too small to be alive. But who knows about its larger cousins? “We really need to go find out,” Stern says.
Discover more about Pluto on our New Horizons round-up page.
(Image: NASA/Johns Hopkins University ApL/SWRI)
Separated at birth?
Pluto’s long-lost sibling from the outer solar system could have been under our noses the whole time.
The asteroid belt-dwelling dwarf planet Ceres has ammonia baked into the clays on its surface, according to research from NASA’s Dawn mission presented at last week’s Division for Planetary Sciences meeting. Models predict that the asteroid belt is too warm for ammonia, so that could mean Ceres originated in the same neighbourhood as Pluto.
Previously, Bill McKinnon of Washington University in St Louis argued that Ceres has more water than other asteroids, so it may have moved in from the outer solar system.
Now of Brown University in Rhode Island argues that Ceres’s surface can be best explained by minerals with ammonia locked in. That ammonia could date back to a time when Ceres, fresh from colder climes, still had ice on its surface, which boiled off once it was dragged closer to the sun.
“It would have been spectacular for a while – the ultimate comet,” McKinnon says.
This article appeared in print under the headline “Pluto, king of the ice zombies”
