Geology of Pluto

Sputnik Planitia appears to be composed of ices more volatile, softer and more dense than the water-ice bedrock of Pluto, including nitrogen, carbon monoxide and solid methane.[16] A polygonal convection cell structure is visible over much of the planitia. No craters have been found, indicating that its surface must be less than 10 million years old.[17] A number of mechanisms are proposed to explain the absence of craters, including cryovolcanism (volcanoes erupting volatiles instead of magma), convective overturn, and viscous relaxation – processes that would erase negative topography.[17] Glaciers of what is probably solid nitrogen can be seen flowing from the planitia into adjacent depressions and craters. Nitrogen from the plain appears to have been carried via the atmosphere and deposited in a thin layer of ice on uplands to the east and south of the plain, forming the large bright eastern lobe of Tombaugh Regio. Glaciers appear to be flowing back into the planitia through valleys from these eastern highlands.

Localization of frozen carbon monoxide in Sputnik Planitia (shorter contours represent higher concentrations).

Polygonal ice patterns in southern Sputnik Planitia (context) due to convection. Dark spots in the troughs at lower left are pits.[18]

Closeup view of sublimation pits (context) in Sputnik Planitia

Additional views of Sputnik Planitia sublimation pits (context); some (left image) have dark material within

Mountains several kilometres high occur along the southwestern and southern edges of Sputnik Planitia. Water ice is the only ice detected on Pluto that is strong enough at Plutonian temperatures to support such heights.

Cthulhu Regio and other dark areas have many craters and signatures of solid methane. The dark red color is thought to be due to tholins falling out of Pluto's atmosphere.

The mid-northern latitudes display a variety of terrain reminiscent of the surface of Triton. A polar cap consisting of solid methane "diluted in a thick, transparent slab of solid nitrogen" is somewhat darker and redder.[20]

Snakeskin terrain formed by penitentes covering Tartarus Dorsa.

The western part of Pluto's northern hemisphere consists of an extensive, highly distinctive set of 500-meter-high mountains informally named Tartarus Dorsa; the spacing and shape of the mountains looks similar to scales or to tree bark. A January 2017 Nature paper by Dr. John Moores and his colleagues identified these icy ridges as penitentes.[21] Penitentes are icy depressions formed by erosion and surrounded by tall spires. Pluto is the only planetary body other than Earth on which penitentes have been identified. Although penitentes have been hypothesized on Jupiter's satellite Europa, current theories suggest they may require an atmosphere to form. Moores and his colleagues hypothesize that Pluto's penitentes grow only during periods of high atmospheric pressure, at a rate of approximately 1 centimeter per orbital cycle. These penitentes appear to have formed in the last few tens-of-millions of years, an idea supported by the sparsity of craters in the region, making Tartarus Dorsa one of the youngest regions on Pluto.[21]

Cutting through both Tartarus Dorsa and Pluto's heavily cratered northern terrain (and thus formed more recently than both) is a set of six canyons radiating from a single point; the longest, informally named Sleipnir Fossa, is over 580 kilometers long. These chasms are thought to have originated from pressures caused by material upwelling at the center of the formation.[22]

When New Horizons first sent back data from Pluto, Pluto was thought to be losing hundreds of tons of its atmosphere an hour to ultraviolet light from the Sun; such an escape rate would be too great to be resupplied by comet impacts. Instead, nitrogen was thought to be resupplied either by cryovolcanism or by geysers bringing it to the surface. Images of structures that imply upwelling of material from within Pluto, and streaks possibly left by geysers, support this view.[18][23] Subsequent discoveries suggest that Pluto's atmospheric escape was overestimated by several thousand times and thus Pluto could theoretically keep its atmosphere without geological assistance, though evidence of ongoing geology remains strong.[24]

Two possible cryovolcanoes, provisionally named Wright Mons and Piccard Mons, have been identified in topographic maps of the region south of Sputnik Planitia, near the south pole. Both are over 150 km across and at least 4 km high, the tallest peaks known on Pluto at present. They are lightly cratered and thus geologically young, although not as young as Sputnik Planitia. They are characterized by a large summit depression and hummocky flanks. This represents the first time large potentially cryovolcanic constructs have been clearly imaged anywhere in the Solar System.[25][26][27]

Pluto - possible cryovolcanoes

Wright Mons (overall context)

Wright Mons, displaying its central depression ( source image (context))

3D map showing Wright Mons (above) and Piccard Mons

A 2019 study identified a second likely cryovolcanic structure around Virgil Fossae, a series of troughs in northeastern Cthulhu Macula, west of Tombaugh Regio. Ammonia-rich cryolavas appear to have erupted from Virgil Fossae and several nearby sites and covered an area of several thousand square kilometers; the fact that the ammonia's spectral signal was detectable when New Horizons flew by Pluto suggests that Virgil Fossae is no older than one billion years and potentially far younger, as galactic cosmic rays would destroy all the ammonia in the upper meter of the crust in that time and solar radiation could destroy the surface ammonia 10 to 10000 times more quickly. The subsurface reservoir from which this cryomagma emerged may have been separate from Pluto's subsurface ocean.[28]