APPLIED PHYSICS LABORATORY, Laurel, Md.—Etched by canyons, crinkled by mountains, and cleansed of craters, the surface of Pluto and its largest moon, Charon, are unexpectedly dynamic, according to the first high-resolution images downloaded by the New Horizons team this morning. Mission scientists describe the findings as almost paradoxical, because the two worlds had been thought too small to sustain the internal heat that drives geologic activity on Earth, and they do not experience the tidal heating that drives such activity elsewhere in the outer solar system. “The team has been abuzz,” deputy project scientist Cathy Olkin told a press briefing. “Look at this! Look at that! This is amazing!”
It is tempting to say that textbooks will need to be rewritten, but when it comes to Charon, so little was known that textbooks had barely even been written. Just yesterday scientists were describing it as an ancient, cratered terrain like Earth’s moon. Today, presenting a global image with a resolution of about 2.3 kilometers per pixel, Olkin drew attention to troughs and cliffs extending for 1,000 kilometers and canyons as deep as 10 kilometers. Some regions have few craters, suggesting they are so geologically young they have scarcely been battered by impacts. The dark, red polar cap—informally named “Mordor”—appears to have been shattered in places by impacts, suggesting that the dark covering is but a thin veneer.
As for Pluto, the team zoomed in on the southern fringe of the famous “heart”—now called Tombaugh Regio in honor of Pluto’s discoverer—with a resolution of about 390 meters. Devoid of craters, the region must be less than 100 million years old, according to John Spencer of the Southwest Research Institute. Moreover, mountains perhaps 3 kilometers in height tower over the region. Their size implies solidity that could only be provided by water ice, which would be hard as rock upon Pluto's frigid surface. Though circumstantial, this is the first evidence that Pluto has water ice.
Up close, Pluto's surface features suggest ongoing geologic activity, although the team has not found any smoking gun in the form of geysers, erupting ice volcanoes or surface movement. Pluto and Charon are locked into a synchronized orbit, always presenting the same face to each other, so they do not exert strong tidal forces. Therefore they lack the most obvious source of power for internal activity. They irrefutably demonstrate that “you do not need tidal heating to power ongoing geologic activity on icy worlds,” Spencer said.
“We now have an isolated, small planet that's showing activity after four and a half billion years,” said New Horizons mission lead Alan Stern, also of the Southwest Research Institute. This “sends geophysicists back to drawing boards,” he added.
Already, theories are emerging about the mysterious heat sources within Pluto's and Charon's interiors. Spencer and other team members speculated that the bodies could be powered by radioactive isotopes or by heat left over from their formation. A tantalizing possibility is that the reservoir for that remnant heat is a subterranean water ocean. Heat loss would cause the ocean to freeze gradually, buffering the interior temperature. Scientists have speculated for years that dwarf planets could harbor such oceans.
Today’s images do not max out the spacecraft’s resolution. Yet-to-be-seen images of Pluto's sunlit hemisphere will pick out details as small as 70 meters, and the team also awaits stereo views to confirm the world's topographical relief.
So far, the New Horizons mission has been brought to you by the word “diversity.” No one quite expected Pluto, let alone Charon, to be so lively and variegated. “Normally you expect differentiation by latitude … but that doesn’t explain the huge heterogeneity within a latitude,” William Grundy of Lowell Observatory told a press briefing earlier today. Even Pluto's now-famous heart, as unified as it might look in visible light, is actually broken; color maps reveal its two lobes have substantially different compositions, for reasons as yet unknown.
All the geographic diversity the New Horizons team is finding in the Plutonian system is forcing them to work against intuitions powerfully shaped by familiar Earthly experience. For instance, water ice plays the role of relatively inert rock or dirt on Pluto because it’s usually too cold to change form into liquid or vapor. Pluto has three gases that can cycle among solid and gas phases—nitrogen, carbon monoxide, and methane—rather than just the one (water) of Earth, potentially creating unprecedentedly complex and cryogenic seasons. Cycles of evaporation and condensation might well be the dominant way that material sloshes around the surface. But the released images show no signs yet of fluid flows, such as sinuous riverbeds. It could be that these substances also cycle among multiple crystalline phases.
Though overshadowed by the images, the low-resolution infrared spectra released today confirm the surprising compositional diversity. Methane ice covers Pluto’s north pole as well as one of its dark equatorial splotches, albeit the ice is mixed with as yet unidentified other material. Older Earth-based telescope observations had lacked the sensitivity and resolution to spy such subtleties.
The data release came at the end of a very different day than yesterday at the Applied Physics Laboratory. Crowds of family members, NASA officials, and other supporters have cleared out, the parking lot suddenly seemed much bigger, and rousing manifestos of exploration have given way to intense discussions of spectroscopy and engineering. Scientists mill around the press office, offering opinion and reactions with the giddy abandon that would normally come only after a few stiff drinks. The fact that no one can really be sure of anything, that every remark is just the start of a line of investigation, has changed the rules of discourse, expanding the possibilities for playful inquiry. Lacking the scholarly language for these new features, scientists have been left struggling for words. “It looks like piles of stuff with grooves on it,” Spencer said. “To be technical.”