Sky and Telescope - November 2016 - 18

New Horizons: Part 2

J. KELLY BEATTY Thanks to New Horizons, planetary scientists realize most
of what they thought they knew about Pluto's climate was wrong.

S E C O N D IN A S E R IE S
October's issue devoted eight pages to New Horizons
results about Pluto's surface. This article explores Pluto's
atmosphere and its solar-wind interactions. December's
issue will examine Charon and its four smaller siblings.

Ever since the discovery of Pluto's tenuous atmosphere
in June 1988, outer-planet specialists have pondered how
it has managed to stick around. Its only possible source
is surface ice, sublimating (converting directly to gas) as
sunlight warms it. Yet this small, distant world is so cold,
averaging about 45 kelvin (-380° Fahrenheit), that the
frozen nitrogen and methane on its surface can sublimate only very slowly. Moreover, given that Pluto swung
closest to the Sun in 1989 along its strongly eccentric
orbit, many researchers thought that conditions wouldn't
be warm enough to heat the ice for long, and what little
atmosphere surrounds this little world would all soon
freeze and literally drop from the sky onto the surface. If
we didn't get there soon, mission proponents had argued
to gain NASA's approval, there'd be no gas left to study.
Then a recent series of stellar occultations by Pluto,
recorded by ground-based astronomers, showed an unexpected trend: even as Pluto edged farther from the Sun,
its atmospheric pressure started rising, not falling. That
wouldn't happen if the gases were freezing out.
The explanation seems to be rooted in Pluto's extreme

axial tilt: its north pole is tipped 120°° downward with
respect to "up" in solar system coordinates. For much
of its 248-year-long orbit around the Sun, Pluto has one
pole constantly in sunlight and the other in shadow.
Back in 2013, Leslie Young (Southwest Research Institute) ﬁgured out that there's enough frozen nitrogen
and methane in the northern hemisphere (the one now
in sunlight) to supply the atmosphere with a sizable
reservoir and keep it from completely collapsing. But that
didn't explain why the pressure might be on the rise.
Many such questions about the state of Pluto's
atmosphere awaited New Horizons, and the mission's
scientists had a carefully designed plan for ﬁnding the
answers. First, the ultraviolet spectrometer, named Alice,
deduced the frigid air's composition. Second, the spacecraft brieﬂy ducked out of view behind Pluto as seen
from both the Sun and Earth, and those occultations
allowed scientists to probe the atmosphere's temperature
and pressure from top to bottom. Finally, instruments
called PEPSSI (a clever acronym for Pluto Energetic Particle Spectrometer Science Investigation) and SWAP (Solar
Wind Around Pluto) detected a huge but vanishingly thin
bubble of gas stripped from Pluto's upper atmosphere by
the solar wind and ionized by ultraviolet sunlight.
HAZY CIRCLE Despite the thinness of Pluto's atmosphere,
New Horizons scientists were startled to ﬁnd dozens of thin
haze layers in this view taken while looking back in the Sun's
direction minutes after the spacecraft's ﬂyby on July 14, 2015.
NASA / JHU-APL / SWRI

November 2016 sky & telescope

Sky and Telescope - November 2016

Table of Contents for the Digital Edition of Sky and Telescope - November 2016