The geology of Pluto consists of the characteristics of the surface, crust, and interior of Pluto. Because of Pluto's distance from Earth, in-depth study from Earth is difficult. Many details about Pluto remained unknown until 14 July 2015, when New Horizons flew through the Pluto system and began transmitting data back to Earth. When it did, Pluto was found to have remarkable geologic diversity, with New Horizons team member Jeff Moore saying that it "is every bit as complex as that of Mars". The final New Horizons Pluto data transmission was received on 25 October 2016. In June 2020, astronomers reported evidence that Pluto may have had a subsurface ocean, and consequently may have been habitable, when it was first formed.
Surface
More than 98 percent of Pluto's surface consists of solid nitrogen, with traces of methane and carbon monoxide. The face of Pluto oriented toward Charon contains more solid methane, whereas the opposite face contains more nitrogen and solid carbon monoxide. Distribution of volatile ices is thought to be season-dependent and influenced more by solar insolation and topography than by subsurface processes. Maps produced from images taken by the Hubble Space Telescope (HST), together with Pluto's light curve and the periodic variations in its infrared spectra, indicate that Pluto's surface is very varied, with large differences in both brightness and color, with albedos between 0.49 and 0.66. Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as Saturn's moon Iapetus. The color varies between charcoal black, dark orange and white. New Horizons data suggest equally variable surface ages for Pluto, with ancient, dark, mountainous terrain occurring alongside the bright, flat, effectively craterless Sputnik Planitia and various terrains of intermediate age and color. Pluto's surface color changed between 1994 and 2003: the northern polar region brightened and the southern hemisphere darkened. Pluto's overall redness also increased substantially between 2000 and 2002. These rapid changes probably relate to seasonal condensation and sublimation of portions of Pluto's atmosphere, amplified by Pluto's extreme axial tilt and high orbital eccentricity.
Soft-ice plains and glaciers 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. 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. 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. 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.
Water-ice mountains 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.
Ancient cratered terrain Belton 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.
Northern latitudes 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.
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. 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. 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.
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