Pluto and Charon are dead; long Live Pluto and Charon.
Gone forever are notions that the main actors in this binary planet system are relatively unremarkable ice balls beyond Neptune, thanks to NASA's New Horizons mission.
Images released today show complex worlds with spectacular surface features that rival anything found elsewhere in the solar system but with their own unique twists:
• On Pluto, mountain ranges that appear to be built from ultra-hard water ice tens of miles wide vault up to 11,000 feet above the surrounding terrain. For mission scientists, they evoke comparisons with the Rocky Mountains in the western United States. Indeed, the dwarf planet seems to be made largely of water ice, covered by a relatively thin veneer of nitrogen, methane, and carbon monoxide ices.
• On Charon, Pluto’s largest moon, canyons up to 4 miles deep are etched across its face, comparable in depth to Valles Marineris on Mars – one of the red planet's most impressive features. Charon also hosts a set of troughs, and cliffs extend for more than 600 miles across the surface.
• On both, evidence suggests that they have had internal heat sources that allowed these bodies to refresh their surfaces within the past 100 million years. For Charon, the evidence comes in the form of an unexpectedly smooth surface, apparently lacking craters. For Pluto, geophysical activity appears to be ongoing.
This represents a fundamental discovery, mission scientists say.
Often, icy bodies that show such geological activity – think Saturn's moon Enceladus, for example – typically are moons orbiting giant planets.
The heat source driving the activity comes from friction inside a moon generated through gravitational interactions with the host planet as well as with any neighboring moons.
"That can't happen on Pluto," says John Spencer, a planetary scientist at the Southwest Research Institute and a member of the New Horizons science team. "There is no giant body that can be deforming Pluto on an ongoing, regular basis" to heat its interior.
Charon is too small to do that, he adds.
"This is telling us that you do not need tidal heating to power ongoing recent geological activity on icy worlds," he says. "That's a really important discovery that we just made this morning."
"We've settled the fact that these very small planets can be very active after a long time," added Alan Stern, also with the Southwest Research Institute and the New Horizons mission's lead scientist. "This is going to send a lot of geophysicists back to the drawing board to try to understand how exactly you do that."
The science team offers two potential explanations for the heat needed to produce geological activity: the decay of radioactive elements in a rocky core or the slow freeze-up of a subsurface ocean. The act of freezing releases heat. It also causes water to expand, a force that could have contributed to some of the surface features examined so far.
The notion of a thin veneer of other ices atop water ice also could imply active cryovolcanism on Pluto (the volcanic eruption of volatiles such as water, ammonia, or methane, instead of molten rock), although no evidence for that activity has appeared at this early stage of data analysis.
At the pace Pluto is losing its largely nitrogen atmosphere, over the course of the age of the solar system the planet would have lost the equivalent of a layer of nitrogen ice between 300 meters and 3 kilometers thick.
"If we only see a veneer, what's going on?" Dr. Stern asks. The nitrogen ice on the surface that is the source of the atmosphere's nitrogen needs to be replaced somehow.
"There must be internal activity that is dredging nitrogen up through cryovolcanism or geysers or some other process that's active into the present on this planet," he says.
The combination of majestic mountains of water ice and a thin veneer of other ices "is very strong evidence that will send us looking as we get more and more data across the surface of the close-approach hemisphere to look for evidence of exactly these phenomena," Stern says.
If these processes are still shaping Pluto – the largest object in an extended realm of icy worlds known as the Kuiper Belt, which extends billions of miles beyond Neptune – other Kuiper Belt objects of comparable size could be active as well, the researchers suggest.