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.
See also...
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the orbits of Jupiter and Neptune), which was first discovered in 1997
and subsequently found to be the largest such body, with an...
Assessing the composition of ices on the surface of Makemake. The Dwarf Planet Makemake is one of the three largest
Trans-Neptunian Objects, with a diameter of about 1400 km, along with Pluto and
Eris (2370 km 2330 km respectively). Makemake is thought to have a...
For over a century Pluto was the only known bright object in the
Solar System beyond Neptune, but in the past two decades a large number of such
objects...
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