Figure 10: Pitted terrain
on Sputnik Planum. Credit:
NASA/JHUAPL/SwRI
Elsewhere, large bright regions are seen where frozen volatiles from
the atmosphere have been deposited, especially across more heavily cratered higher latitudes. In one region strange ‘halo’ craters were
observed, the haloes apparently created by methane ices deposited
on the crater rims, whereas the darker crater floors and intermediate
terrain show signs of being dominated by water ice. Possible reasons for
this distribution of ices, shown in Figure 12, are yet to be determined.
Figure 11: Bladed terrain in Tartarus Dorsa. Credit: NASA/JHUAPL/SwRI
Not all of Pluto’s surface materials are ices. Deposition from the atmosphere has covered large areas of Pluto with reddish-brown material, first detected from Earth, and imaged in detail by New Horizons.
Ultraviolet radiation from the Sun causes chemical reactions in atmospheric methane and nitrogen which create complex reddish-brown
hydrocarbon molecules, known as tholins, like those already observed
in the atmosphere of Saturn’s largest moon, Titan. These fall from the
atmosphere, covering the ground with a layer of ‘gunk.’ This blankets
pre-existing relief – mountains, valleys, troughs, craters - especially along
the equatorial regions, as Figure 13 shows. Cthulhu Regio is the most extensive such region, stretching almost halfway round the planet.
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