Pluto has a problem: Its thin, nitrogen atmosphere shouldn't be there. Ultraviolet rays from the sun should have knocked it away, molecule by molecule, in the dwarf planet's first few thousand years. Four billion years later, Pluto's atmosphere is still there, a gauzy interplanetary mystery.

Okay, it's really scientists who have the problem, because it's not like there are nitrogen-breathing Plutonians down there pacing worriedly over impending suffocation. Right, NASA? Right?!? No, all worrying has been purely academic, expressed in papers like the one published earlier this month by a pair of New Horizons scientists who examined the trio of prevailing theories behind the atmosphere's perplexing replenishment.

High noon on Pluto looks like dusk on Earth. But even that small amount of solar energy is enough to turn frozen nitrogen on Pluto's surface into gas. Once aloft, Pluto's gravity is too weak to keep the particles from being blown away by ultraviolet radiation. (In case you're wondering, Earth's mass is enough to keep its atmosphere mostly safe from getting blown off into space.)

Pluto has been around for about four billion years, and according to the best math, it should have lost about 7,000,000,000,000,000,000,000 grams of nitrogen (give or take a zero) since then. But scientists currently estimate that the planet has about 30,000,000,000,000,000 grams of nitrogen atmosphere, and loses about 1,500,000,000,000 grams each year. "Basically, it would take only a few thousand to tens of thousands of years to lose that atmosphere," says Kelsi Singer, post-doctoral researcher at the Southwest Research Center and co-author of the paper. Basically, that's not a very long time.

Considering that Pluto has been around for more than four billion years, the odds are pretty friggin' slim that humans would meet the dwarf planet during its brief phase of atmosphere-having. No, something is replenishing the supply. Scientists, you got some 'splaining to do.

There are two basic mechanisms for resupplying Pluto's atmosphere: "You can either bring stuff in from the outside or bring stuff up from the inside," Singer 'splains. Comets are a handy mechanism for both categories, as they can deposit nitrogen when they impact, or punch holes through the surface to expose quantities of the frozen gas. Singer is an impact expert, so she did the calculations for how much nitrogen a comet would shed, or dig up, upon impact. Her co-author Alan Stern, studies atmospheres (among other things) and ran the numbers for gas loss. But even with their most conservative estimates—the maximum number of comets, the deepest of impact craters—nothing from off planet could cover the deficit of nitrogen loss.

Instead, they offer that the nitrogen is being replenished through more conventional geologic activity like cryovolcanism or tectonic action. But nobody yet knows what is behind this geologic churning, as Pluto is too small and too old to be holding much remnant energy from its formation. It's also too far from any planetary body that would be large enough to tidally churn its innards. Something else inside the dwarf planet was causing exhalations. Does a Great Old One stir under the dark equatorial region, ominously nicknamed Cthulhu Reggio? It would explain the Sleeper of R'lyeh's long absence...

Perhaps wisely, Singer and Stern do not explore that chilling hypothesis.

The most likely battery for Plutonian heat are huge chunks of uranium or potassium wedged in its rocky core. "Those will emit heat as they break down, and the heat has to go somewhere, so it causes local melting," says Will Grundy, a planetary scientist, ice expert, and French cheese connoisseur at Lowell Observatory in Arizona.

The authors submitted their paper back in May, just as New Horizons was sending home its first images of Pluto. Since then, the probe's cornucopia of data has supported Singer and Stern's geologic hypothesis. "We do seem to be seeing a good deal of recent geologic activity on Pluto, so that's not a bad sign for our paper," says Singer. Things will look even better come September and October, when they can make better estimations of the impact rate based on the lossless data showing the dwarf planet's craters. "We also get data from particle and plasma, that will help us calc the rate of atmospheric escape," she says.

And every bit that arrives will blow away the mystery of Pluto's missing molecules.