This week's discovery of 11,000-foot-high mountains on Pluto — and the implication that they're made of water ice — is stimulating all sorts of wonderings about what may be going on beneath the surface. Could there be liquid water or slush moving beneath those mountains, warmed by radioactive rocks and laced with antifreeze?
"All the geophysicists are dusting off their models," Lowell University's Will Grundy, a co-investigator for NASA's New Horizons mission to Pluto, told NBC News on Thursday.
Grundy cautioned that it's too early to confirm the presence of water. So far, there's been spectroscopic evidence for the presence of frozen nitrogen, methane and carbon monoxide at the surface — but not the chemical signature of water.
The discussion about mountains made of water ice is based on the assumption that you can't make mountains as high as the ones seen by New Horizons with nitrogen or methane. And it makes sense to suggest that water exists on Pluto, even if it hasn't yet been directly observed, Grundy said.
"Water is really the most common 'rock' in the solar system," he said.
Carolyn Porco, who heads the imaging team for NASA's Cassini mission to Saturn at the Space Science Institute, agrees. "It just makes common sense that there's a lot of water. ... New Horizons is finding what we would have expected on the basis of cosmochemistry," she told NBC News.
Over the past decade, Porco and her colleagues on the Cassini science team have made a torrent of discoveries about what appears to be a hidden sea of water beneath the icy surface of Enceladus, one of Saturn's moons. The Cassini orbiter has detected geysers of water spewing out from a network of cracks, also known as "tiger stripes," near Enceladus' south pole.
Could there be ice geysers on Pluto as well? Before New Horizons' flyby, scientists speculated that the spacecraft might detect plumes of material rising from Pluto, just as they do from Neptune's moon Triton. There's been no sign of such plumes in the initial observations sent by the probe, according to principal investigator Alan Stern, a planetary scientist from the Southwest Research Institute. But it's still early in the game.
Porco said the presence of ice mountains doesn't necessarily mean water is flowing like lava beneath Pluto's surface. "Mountains like this look like they could be produced tectonically," she said. "They just don't look to me like the kinds of features that would be produced volcanically."
But Grundy said the preliminary findings have set the New Horizons team to thinking. Pluto is roughly half rock and half ice by mass. Like Earth, the rocky material inside Pluto could contain elements such as potassium, thorium and uranium. Those elements could generate internal warmth through radioactivity, and the antifreeze effect of salts or compounds such as ammonia or methanol beneath the surface just might keep water fluid enough to flow.
"Maybe there's internal fluid circulation that's colder than the freezing point of water," Grundy suggested.
Cold-loving bacteria have been discovered living inside Canadian Arctic permafrost at subfreezing temperatures, in part due to natural antifreeze. So could any kind of life exist on Pluto? That sounds like the wildest sort of science fiction — but 20 years ago, the idea of looking for life beneath the ice of Enceladus or Jupiter's moon Europa might have sounded just as wild.
In any case, the findings from Pluto won't be the end of the story. Porco said it's essential for the New Horizons probe to study at least one more object in the broad ring of icy material beyond the orbit of Neptune, known as the Kuiper Belt. She also favors following up on Cassini's revelations with a mission focusing on Enceladus.
NASA already has started planning an ambitious mission to Europa — a robotic expedition long championed by Robert Pappalardo, a planetary scientist at NASA's Jet Propulsion Laboratory.
"The spectacular Pluto results underscore that the icy worlds of the outer solar system have been dynamic worlds, where water ice plays the role of rock near their frigidly cold surfaces, and warm watery oceans may lurk beneath," Pappalardo told NBC News in an email. "Coming analyses of the Pluto data — along with future missions, including NASA's mission to Europa — will greatly advance our understanding of the role of water in the outer solar system."