Far in the western hemisphere, scientists on NASA's New Horizons mission have discovered what looks like a giant "bite mark" on Pluto's surface. They suspect it may be caused by a process known as sublimation -- the transition of a substance from a solid to a gas. The methane ice-rich surface on Pluto may be sublimating away into the atmosphere, exposing a layer of water-ice underneath.
Cutting diagonally across the mottled plans is the long extensional fault of Inanna Fossa, which stretches eastward 370 miles (600 kilometers) from here to the western edge of the great nitrogen ice plains of Sputnik Planum.
Compositional data from the New Horizons spacecraft's Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument indicates that the plateau uplands south of Piri Rupes are rich in methane ice. Scientists speculate that sublimation of methane may be causing the plateau material to erode along the face of the cliffs, causing them to retreat south and leave the plains of Piri Planitia in their wake.
Compositional data also show that the surface of Piri Planitia is more enriched in water ice than the higher plateaus, which may indicate that Piri Planitia's surface is made of water ice bedrock, just beneath a layer of retreating methane ice. Because the surface of Pluto is so cold, the water ice is rock-like and immobile. The light/dark mottled pattern of Piri Planitia in the left inset is reflected in the composition map, with the lighter areas corresponding to areas richer in methane -- these may be remnants of methane that have not yet sublimated away entirely.
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