This week’s image shows a canyon system on Charon, the
largest moon of Pluto. This is a very interesting solar system body, as it is
only recently that we have gotten a good look at its surface. This image comes from
the New Horizons probe which visited the Pluto system in 2015. New Horizons has
provided us with a huge amount of data about these small worlds, which will
keep planetary scientists busy for years to come, however it had relatively
little time to observe the Pluto system, as it never went into orbit around
either Pluto or Charon. All of the images were recorded during a brief flyby,
and so the observations had to be carefully planned in order to maximise the
amount of information acquired. The distance to the Pluto system means that
follow up observations aren’t likely any time soon. New Horizons is still
operational, and will go on to examine other objects in the Kuiper Belt,
however it will be decades before another spacecraft can reach Pluto, even if
one were sent in the next few years.
Charon is one of several small objects to orbit Pluto, but
stands out due to its large size. It is almost half the size of Pluto, making
it one of the largest moons relative to the planet it orbits, it is likely that
it evolved independently of Pluto and was later captured by it. The full image
shows how large this canyon is relative to the size of the moon. It is one of
several canyons which scar the surface of this moon, another canyon can be seen
in teh image below, but this one was imaged at an oblique angle, showing how
deeply it cuts into the surface of Charon. NASA’s description of this image
notes that this canyon seems to be 9 km deep in places, and that its cliffs
might be the tallest in the solar system.
In their description of the first image the new Horizons
team note that this canyon lies on the boundary between the highlands to the
top of the image and a large plain to the south. Both terrains are heavily
cratered, and this highlights something very interesting about this small moon,
which makes it very different to the dwarf planet it orbits; Charon is heavily
cratered, whereas Pluto is not.
Pluto has large regions with very few impact craters. Charon
on the other hand is pitted with craters, and looks much more like Mercury or
Earth’s Moon. This striking contrast is quite surprising. Since both objects
are in close proximity they would be expected to be hit by the same number of
meteorites and have the same density of craters. The fact that this is not the
case indicates that Pluto has a surprisingly young surface. The icy surface of
Pluto has clearly been resurfaced regularly, and this suggests that the planet
could be far more active than expected given that it is so small, and so
wouldn’t be expected to retain much heat. Precisely why Pluto is so active
remains to be determined, but it makes these small planets quite an intriguing
target for future missions. New Horizons’ observations definitely raised as
many questions as they answered.
Image Credits
NASA/JHUAPL/SwRI
NASA/JHUAPL/SwRI
Further reading
Differences between Pluto and Charon
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