Article Review: The Geology of Pluto and Charon through the eyes of New Horizons by Moore et al., (2016)
By Paul Christian Yang-ed
July 14, 2016 marked the first year anniversary of the flyby of the American space probe New Horizons over the dwarf planet Pluto. Until then, it was merely a fleck of pixels, as seen by the Hubble Space Telescope. Even its diameter remained uncertain. What we merely knew about Pluto before the flyby was that it orbits eccentrically, crossing even Neptune’s orbit albeit safely at a tilted angle and in resonance; that it revolves around a common barycenter with its largest moon Charon; and that its atmosphere was made up largely of nitrogen. Everything else was at best hypothetical.
But one year since the momentic close-up of this Kuiper belt object, data beamed from the lonely traveler yielded surprising results, especially with regard to Geology, as revealed in a paper whose lead author is Jeffrey Moore from the New Horizons Team, and is published March this year in the peer-reviewed journal Science. Before Pluto was imaged, it was only the Neptunian moon Triton, which is believed to be a KBO, that has ever been imaged and studied in terms of planetary geology for the class of these objects. In contrast, a great deal more asteroids, planets, moons, and comets have been imaged close up and studied.
For a heavenly object of Pluto’s size, scientists expected that it would be too small to sustain active geological processes. Smaller bodies lack mass, and therefore are not expected to retain internal heat for too long after their formation until which they become geologically dead. Io, a moon of Jupiter which is smaller than Pluto, is geologically active but only due to the tugging Jupiter and its neighboring moons exert on it, just like how a putty warms as it is being flexed over and over. They expected that Pluto would look barren, cratered, and even boring like our own moon, like almost asteroids, and even like Titania, an icy moon of Uranus that is almost the same size as Pluto.
As revealed in the paper, the scientists were in for a shock. Images from the lower-resolution but colored RALPH camera and the higher-resolution but black-and-white (monochromatic) LORRI camera revealed a dynamic surface geology. It has a variety of landscapes as noted in the paper. One of these features are glaciers, whose composition was determined by a spectroscope aboard New Horizons. Glaciers made up of nitrogen slosh (instead of water ice here on Earth) carve and modify the Plutonian surface. These glaciers mostly drain into what they believe is an ancient large crater basin which they call “Sputnik Planum” that forms the “heart” of Pluto. That plain is less than thrice the area of the Philippines combined. The presence of cellular structures in the same plain are hypothesized to have been formed by subsurface upwelling, implying the need for an internal driving force. The paper however does not address what force drives the upwelling. But what the authors are certain is that the absence of craters on these glaciers and other smooth surfaces in Pluto indicate that these surfaces are relatively young and had been modified by recent geological activity.
Another geological feature described in the paper were mountains. Unlike mountains here on Earth which are made up of silicate rocks, the mountains in Pluto were determined to be made up of water ice by New Horizon’s spectroscope. The authors hypothesize that under Pluto’s very cold and low pressure regimes, the water ice mountain could be buoyant enough to float like ice bergs, and can be transported by the glaciers and other ice bergs that push them or by the ices that slide beneath them. The blocky and chaotic texture of the Al-Idrisi mountains northwest of the “heart” of Pluto led the authors to interpret it as a piece of detached crust that has been transported by methane ice to its current position. Noticeably absent again was an explanation for an internal tectonic force that drives the detachment and transport of these water ice mountains.
Eerie blade-like ridges comparable to snakeskin were also described east of Sputnik Planum. These are also made up of methane and nitrogen ices that roughly align north to south. The authors propose that these ridges were made by subsurface collapse, or degradation by sublimation, or by preferential ice deposition. The nearest analogy of these structures on earth are suncups or penitentes, but occurring in scales in terms of kilometers. Close to these ridges, plateaus and alpine terrains also occur.
Short of outrightly calling them the controversial label of “cryovolcanoes” or ice volcanoes, the authors describe two features which they instead call “large mounds with central depressions”. These mounds, found south of Sputnik Planum, both feature concentric rims surrounding a central depression. The two mounds are 4 and 6 kilometers tall respectively, relatively shorter than Mt. Everest which is 8 km tall. The authors note that the symmetric rims around the central depression can be interpreted as having formed as the mounds build themselves — not unlike how typical volcanoes build themselves. To reach such heights, the authors hypothesize that the mounds would have to be made up something stronger than nitrogen ice which is the composition of the lower-level landscapes of Pluto such as the plateaus and glaciers. Should these mounds be proven as cryovolcanoes, the mystery behind Pluto’s otherwise dead geology will further baffle scientists. How else could these cryovolcanoes be active if the planet is dead within? There is also no large planet nearby that can flex and warm Pluto from within, unlike in the case of Io. These mounds, if ever they are cryovolcanoes, could also serve as the replenishment of Pluto’s ices that get sublimated to space.
Other interesting features that indicate Pluto’s dynamic geology include “graben normal fault systems” which may mean that some parts of Pluto’s crust are undergoing thinning and brittle deformation prior to being split and getting separated. This would definitely ring a bell if further verified since only Earth is known to have such processes currently ongoing. The paper however warned that it is out of their scope to explain the actual structural-tectonic mechanisms behind these fault systems. In addition, spectroscopy data reveal the presence of water ice along these long faults. The great length of the faults as well as the steepness of the fault scarps led the authors to believe that Pluto’s crust is very thick, rigid, and cold. Despite this, they propose that the freezing of a subsurface ocean is responsible for its cracking, especially if the subsurface ocean was initially made up of water which tends to expand as it freezes. The spectroscopy data along the fault scarps may tell us that the author’s subsurface ocean freezing hypothesis may not be so farfetched.
Unlike its parent body, Charon displayed a strikingly different appearance. It is gray instead of orange although it sports a smooth dark-red plain over its north pole. It is more rugged and lacks the glaciers that are common in Pluto. Charon has more water ice covering its surface instead of nitrogen or methane ice. Its northern hemisphere, except the red plain at the pole, remains heavily cratered and is interpreted to have remained unmodified for billions of years. In contrast, its southern hemisphere is smoother. The smoother hemisphere could have been possibly reshaped by eventual relaxation of subsurface water ice, just like what happened in Ganymede, Jupiter’s largest moon. But just like Pluto, Charon also has also belts of “graben” and “normal faults”. The authors propose that the presence of a binary rough vs. smooth terrain, craters cutting the fault belts and terrains, and large belts of steep, tall fault scarps indicate that at some undetermined time in Charon’s ancient past, Charon may have expanded on a planet wide scale as its subsurface ocean froze. The last time a model of planetary expansion was seriously considered in geology was in the 19th to early 20th century, way before the plate tectonics paradigm, and Charon may cause the scientific community to look again at this model, but this time from another angle.
As Pluto and Charon was unmasked for the first time, the discoveries there have led planetary geologists to rethink their traditional assumptions and models. Pluto was added to the list of heavenly bodies with active surface geology, which includes Earth, Io, Titan, Mars, Venus, and comets. New Horizon’s unmasking has made the boring small thing into something completely baffling. The authors note that the contrast and diversity of surfaces between separate KBOs like Charon and Pluto, and even Triton, could possibly hint at the diversity of other KBOs out there that are waiting to be explored. In this aspect, we could not agree more.