Wednesday 9 March 2016

Sputnik Planum: An apparently young feature on the surface of Pluto.

Pluto was the first known and is one of the largest bodies in the Kuiper Belt, a field of Dwarf Planets and smaller bodies beyond Neptune which are thought to have been beyond the zone of true-planet formation, and therefore to reflect the nature of the early Solar System. In July 2015 the New Horizons spacecraft flew past Pluto, making a detailed survey of the surface of the body, the first such study of any Kuiper Belt object. This survey revealed, amongst other features, an area which has been informally named Sputnik Planum, a field of nitrogen ice covering apparently 2.5% of Pluto's surface, which is devoid of impact craters visible at observed resolutions. This is surprising as Pluto sweeps through an area of the Kuiper Belt where it is predicted to encounter many smaller objects which impact on its surface, creating a pattern of craters seen on other parts of the Dwarf Planet. The absence of craters on Sputnik Planum sugests that this area must be relatively young, as no known phenomenon can cause impact events to preferentially avoid one area on the surface of a body.

In a paper published on the online arXiv database at Cornell University Library on 12 January 2016 and in the journal PLoS One on 20 January 2016, David Trilling of the Department of Physics and Astronomy at Northern Arizona University and the Lowell Observatory discuses the nature of Sputnik Planum and attempts to assess how this area could be resurfaced to remove craters.

The entire surface of Sputnik Planum has been imaged at a resolution of 400 m/pixel, at which level of detail it ought to be possible to detect a crater 2 km in diameter, which would be caused by an impactor of roughly 400 m. About 20% of Sputnik Planum has been imaged at the higher resolution of 125 m/pixel, at which resolution it should be possible to detect a crater 625 m in diameter, which would be caused by an impactor approximately 90 m across. No craters were detected on the surface of Sputnik Planum at either resolution.

Low resolution image of the surface of Pluto. Sputnik Planum is the white area at the center of the image. New Horizons.

Trilling estimates that at the predicted gravity of Pluto nitrogen ice with a density of 1000 kg/m2, a crater 2 km across should be removed by viscous relaxation (although a solid ice does flow very slowly) within 10 million years, which he calculates to be the upper age of Sputnik Planum. He also notes that two other possible surface processes could potentially be speeding up this crater removal; active convective overturn, in which the nitrogen ice is being heated from below by some heat-source within Pluto rather than simply by the Sun, of cryovolcanism, in which melted nitrogen is reaching the surface from a buried reservoir, which would also require a heat-source within Pluto.

Trilling also notes that these figures are based uppon current estimates of the number and density of objects within the Kuiper Belt, figures which have a wide margin of error. Potentially the number of objects could be ten times lower, in which case the number of impacts would be ten times lower and Sputnik Planum could have a surface age of about 100 million years, allowing for much slower surface reworking, or ten times as high, in which case the number of impacts would also be ten times as high, and the surface of Sputnik Planum might only be a million years old, strongly suggesting reworking by a process other than viscous relaxation.

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