Pluto’s atmosphere, icecaps and other surprises

New Horizons wowed us with images of giant mountains and glaciers made of frozen gases on Pluto. Now its snapshots have revealed the dwarf planet’s fine hazy upper atmosphere – images that “brought tears to our eyes”, says Michael Summers, a planetary scientist from George Mason University in Virginia and a co-investigator on the mission.

Astronomers have known that Pluto has an atmosphere since 1988, when they watched the dwarf planet as a star passed directly behind it. Just before the star winked out of sight, its light dimmed, indicating Pluto was surrounded by gas. 

Pluto’s haze is probably made of tiny droplets of hydrocarbons, cobbled together from the remnants of atmospheric methane molecules split by the Sun’s ultraviolet light. There’s not much to the haze, though. “If you were on the surface of Pluto, you probably would not see it,” Summers says.

“It’s another mystery we’re going to have to deal with.”

This haze appears to be concentrated in distinct layers, several kilometres thick, the most prominent of which were seen at around 50 and 80 kilometres above the dwarf planet. Nobody is sure why such layers exist, Summers concedes. It’s possible they are created by winds or large-scale patterns of atmospheric circulation, but it’s also possible that they reflect differences in the atmosphere’s chemical reactions at different altitudes. “We’re having to start from scratch, changing what we thought we knew about Pluto’s atmosphere.”

Equally baffling is the measurement of the mass of the atmosphere. Two methods have been used. One is the method that was first used to detect Pluto’s atmosphere. It relies on watching a star passing behind  and observing how much of its light is dimmed by Pluto’s atmosphere – this way, scientists can measure its density and mass. The last time this type of measurement was reported was about two years ago.

The second method involved NASA scientists beaming radio waves toward Pluto, timed so they’d pierce the atmosphere and be picked up by New Horizons passing behind. Surprisingly, this measurement revealed that Pluto’s atmospheric mass had halved since the last measurement. “That’s pretty astonishing,” Summers says. “It’s another mystery we’re going to have to deal with.”

If the new measurement is correct, it  doesn’t necessarily mean the atmosphere has been lost to space. Because of its extreme elliptical orbit Pluto has been moving steadily outward from the Sun and getting colder since its closest approach in 1989. The molecules in the atmosphere can “freeze out”, settling on the surface like frost. Scientists have long speculated that by the time Pluto reaches its furthest point in 2113 — nearly half again as distant from the Sun as its present position — its atmosphere will have frozen out completely. But nobody anticipated the process could occur so rapidly, if that is indeed what has occurred in the last two years. 

Meanwhile, the prize for the most stunning image so far would have to go to Pluto’s equatorial bright spot, a heart-shaped region the size of Texas. The first photo revealed it is flanked on one side by giant  mountains, probably made of water ice, 3,400 metres high. The next showed the heart’s interior appeared to be a broad icecap, informally known as Sputnik Planum (named after the first Russian satellite). And fringing the interior of this icy heart, glaciers of carbon monoxide, methane and nitrogen appear to have flowed (and may still be flowing) into the lowlands.

On Pluto, where the average temperature is around -230 °C, water would be frozen too hard to flow.  But these frozen gases do. “We interpret this to be just like glacial flow on Earth,” says New Horizons co-investigator William McKinnon. No one knows why Pluto has an icecap near its equator. Maybe an ancient impact punctured its mantle allowing liquid nitrogen and other cryogenic fluids to well up from below.

One way scientists might get clues as to what lies beneath, says Ulyana Horodyskyj, a glaciologist at the U.S. National Snow and Ice Data Centre in Boulder, Colorado, is by searching future photos for crevasse fields like those found on earthly glaciers. That’s because crevasses form either in steep sections of the ice, or where the underlying topography causes different parts of a glacier to flow at different speeds. “Thus,” she says, “crevasses are excellent natural mapping tools.”

But for now New Horizons’ dazzling slide show is finished. It switched to beaming the raw data captured by its instruments. But project leader Alan Stern promises that after the quiet patch, “the sky will be raining data” for more than a year. Patience, Pluto-watchers, patience.