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Imagery from the New Horizons mission was combined to form
this composite image. Different surface features are easily visible in this
image. Figure 3 from Stern et al. (2015).
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Until fairly recently, our knowledge of Pluto has been fairly
limited: fuzzy images taken from Earth or satellite-mounted telescopes.
However, in July 2015, the New Horizons spacecraft passed by Pluto, yielding
valuable insights about the surface and atmosphere of the dwarf planet. These
results are presented by in a new Science article by Stern et al. (2015)… along
with a multitude of new questions raised by the new observations.
Pluto is a piece of the Kuiper Belt, an area of the solar
system that is composed of small dwarf planets, asteroids, and icy debris. The
Kuiper belt is similar to the asteroid belt, except it lies beyond Neptune, and
the objects that constitute it are planetisimal fragments that are frequently
ice-rich. Prior to 2006, Pluto was considered a planet, but recognition of
other Kuiper Belt material sharing the same orbit caused Pluto to be
reclassified as a dwarf planet.
The New Horizons mission
launched on January 19, 2006, and reached Pluto nine and a half years later on
July 14, 2015. It passed 7,800 miles above the surface of Pluto, gathering
imagery and data about the dwarf planet. It also took observations on Pluto’s
moon Charon and other Kuiper Belt objects.
One of the most striking observations made by the New
Horizons spacecraft is the tectonically young surface. The mission found that
Pluto has a thin layer of nitrogen, carbon monoxide, and methane ices that
overlay a stronger substrate. This substrate is thought to be formed of water-ice,
as it is stronger than ices composed of other elements and could form the
topographic features found on Pluto.
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The new imagery from New Horizons allowed Stern et al.
(2015) to divide Pluto into different geomorphic regions. These regions have
not yet been formally named, although researchers gave them informal names to
refer to them. For explanation of colors, see original paper (link below). Figure 2a from Stern et al. (2005).
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These topographic features include scarps, hummocks,
troughs, mountains, and lobate features that resemble glaciers. These features
could have been caused by landslides, cryovolcanism (ice volcanoes), or the
slow, glacial flow of ice. By counting craters to estimate the age of the
surface, researchers have determined that these processes probably occurred
within a few hundred million years. This recent activity suggests the crust of
Pluto is still changing and evolving. Unlike Earth, Pluto does not have plate
tectonics, a source of internal heat, or energy from the Sun. And unlike many
icy satellites such as Europa or Enceladus, tidal forces are not a factor on
Pluto. Thus, the question remains: where is the energy necessary for these
processes coming from on Pluto? Why isn’t it just a frozen rock on the edge of
our solar system?
The New Horizons mission has opened up many other questions
about Pluto and other Kuiper Belt objects. Are features similar to those seen
on Pluto found on other Kuiper Belt objects? Since Pluto and Charon have
similar compositions, what does that imply about the origin of Kuiper belt
objects and the timing of the formation of the Kuiper Belt? How does volatile
cycling on Pluto operate to maintain its atmosphere? The New Horizons mission
has returned amazing results, but these results have created even more exciting
questions.
Link
Citation
Stern, S. A., Bagenal, F., Ennico, K., Gladstone,
G. R., Grundy, W. M., McKinnon, W. B., Moore, J. M., ... Zirnstein, E. (October
15, 2015). The Pluto system: Initial results from its exploration by New
Horizons. Science, 350, 6258.)
