What’s up with Pluto’s biggest moon? A lot, according to scientists.
NASA’s New Horizons spacecraft, the first to travel to Pluto and the Kuiper Belt of which it is a part, have yielded the first detailed color images of Charon, the largest of Pluto’s five moons. At 750 miles in diameter, Charon is about half as wide as Pluto (1,430 miles) itself.
“We thought the probability of seeing such interesting features on this satellite of a world at the far edge of our solar system was low,” Ross Beyer, an affiliate of the New Horizons Geology, Geophysics and Imaging (GGI) team from the SETI Institute and NASA Ames Research Center in Mountain View, Calif., said in a statement. “But I couldn't be more delighted with what we see."
The images from New Horizons, taken on July 14 and finally transmitted back to Earth on Sept. 21, reveal a belt of fractures and canyons slightly north of the moon’s equator – a canyon system four times as long as the Grand Canyon and in some places twice as deep.
“It looks like the entire crust of Charon has been split open,” John Spencer, deputy lead for GGI at the Southwest Research Institute in Boulder, Colo., said in the statement. “With respect to its size relative to Charon, this feature is much like the vast Valles Marineris canyon system on Mars.”
Scientists at NASA are still looking into possible explanations for why these geological events occurred – or may, in fact, still be ongoing. One possibility is cyrovolcanism, in which water, ammonia, or even methane erupts instead of molten rock. Researchers are still determining whether that process happens on Pluto as well:
Stunning images of Pluto and Charon: Not just ice balls anymore (+video)
Images of Pluto and its largest moon Charon, released Wednesday, show complex worlds with spectacular surface features that rival anything found elsewhere in the solar system.
By Pete Spotts, Staff writer JULY 15, 2015
LAUREL, MD. — 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?" asks Alan Stern, at the Southwest Research Institute and the New Horizons mission's lead scientist. 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.
New Horizons will continue to bring back even more detailed images of Charon over the next year, and mission project scientist Hal Weaver, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., predicts that with those transmissions, “Charon’s story will become even more amazing!”