Figure 17: This flat feature in the Al-Idrisi Montes to the west of Sputnik Planum looks very much like a
frozen lake surface. Credit: NASA/JHUAPL/SwRI
suspected that Charon was created by a large collision, some 4 to 4.5
billion years ago, similar to that which is thought to have created the
Moon. The nearly circular orbits of Pluto’s smaller satellites suggest that
they were also formed in this collision, rather than being captured Kuiper Belt objects.
Despite being only half the size of Pluto, Charon also exhibits a surprising variety of landscapes and surface features, and shows evidence
of a varied geological history. Some of these are shown in Figure 18
and described below. The northern polar region, Mordor Macula, is
reddish (A), indicating possible alteration of methane deposits by solar
radiation. Elsewhere, the northern hemisphere is ancient, rugged and
cratered (B). There are unusual localised ammonia (NH3) deposits associated with a few bright craters (such as C). Ammonia ice hasn’t yet
been seen on Pluto, and its presence here is not fully explained as yet,
although it must be fairly recent, since it is eventually destroyed by solar
radiation and cosmic rays. It may a product of impact events or even
local cryovolcanism.
The recent resurfacing on Pluto involving volatiles like nitrogen is not
seen on Charon, perhaps due to its lower gravity and subsequent loss
of such materials to space. Instead its surface is dominated by water ice. There is evidence of ancient resurfacing, especially across its
smoother southern plains (F), but being smaller than Pluto, Charon’s
internal heat source was probably depleted early in its history. Locally,
surface ices may have flowed under pressure, such as at the distinctive
‘mountain in a moat’ seen at G, where a water-ice mountain has sunk
into less viscous crustal material.
Perhaps the most distinctive and spectacular features on Charon are
its faults, often running in parallel series (D), and canyon systems, which
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