Sky and Telescope - November 2015 - 20

New Horizons
Hydra

16:00

Pluto-Earth
occultation

15:00

14:00

Charon
13:00

Charon-Sun
occultation

PATH TO SUCCESS During its July 14th pass
through the Pluto system at nearly 14 km (8.6
miles) per second, New Horizons executed
hundreds of preprogrammed twists and turns
to record everything it could. Mission scientists
will have to wait more than a full year to get all
the results in uncompressed form.

Kerberos

Charon-Earth
occultation

Styx
12:00

50,000 km

Pluto-Sun
occultation
Closest to
Charon
(28,840 km)

Pluto
11:00

30,000 miles
0.24$

Closest to
Pluto
(12,504 km)

Nix

Sun
Earth

10:00

oriz

ons

traj

ect

ory

9:00

S&T: GREGG DINDERMAN, SOURCE: NASA / JHU-APL / SWRI

Rugged cratered terrain

Nitrogen ice ﬂow

Polygonal cells

SPUTNIK PLANUM

30 km

ON THE MOVE Along the northern boundary of Pluto's
Sputnik Planum, swirl-shaped patterns of light and dark suggest
that frozen nitrogen and other exotic ices have ﬂowed around
obstacles and into depressions, much like glaciers on Earth.
SPUTNIK PLANUM

Polygonal cells
Norgay Montes

Hillary Montes

NASA / JHU-APL / SWRI (3)

Inﬁlled crater
Thin ice
sheet
CTHULHU REGIO

60 km

STARK CONTRASTS The southern region of Pluto's Sputnik
Planum contains surprisingly tall mountain ranges, consisting
mostly of water ice, that rise up to 3½ km (11,000 feet) above the
surrounding terrain. They're called Hillary Montes and Norgay
Montes to honor Sir Edmund Hillary and Tenzing Norgay, the
ﬁrst two humans to reach the summit of Mount Everest in 1953.
A bright veneer of exotic ices covers much of the terrain.

November 2015 sky & telescope

suspected by cosmochemists to exist on Pluto but undetected in spectra of its surface.
The spacecraft's color images and near-infrared spectra reveal that Pluto's surface has a coating of red-tinged
material that's most likely a mixture of complex organic
compounds. Scientists suspect that the darkest spots are
coated with deposits of tholins, a catchall term coined in
the 1970s for complex, carbon-rich organic compounds.
One basic characteristic that has eluded astronomers
since Pluto's discovery 85 years ago is its precise diameter. Based on previous occultation work, researchers
deduced that it was at least 2,302 km but couldn't pin
down the true value. Now, thanks to the spacecraft's
imagery, they realize that it's 2,370 km (1,475 miles).
This simple reﬁnement means that Pluto is slightly
larger than Eris (2,336 ± 12 km). The two had been in
competition for "king of the Kuiper Belt" since the latter's discovery a decade ago. (Yet, despite being slightly
smaller, Eris has a mass 28% greater than Pluto's.) It also
means that Pluto's overall density must be lower than
thought, somewhere around 1.90 g/cm³. So the interior
must be slightly more ice rich (35% or 40% by volume)
and its rock fraction slightly lower (60% or 65%).
Floating above what Stern calls a "wonderland of
diverse geological expression" is Pluto's extremely tenuous, nitrogen-dominated atmosphere. As depicted in the
diagram above, the spacecraft brieﬂy watched both the
Sun and Earth duck behind the dwarf planet's limb, and
these dual occultations provided unique probes of the
atmosphere's cryogenic wisps. For example, the surface pressure was far lower than models predicted, no
more than 10 microbars - what you'd experience at an
altitude of 96 km above Earth. As recently as two years
ago, ground-based occultations yielded pressures nearly
twice as high. Pluto swung through the perihelion of
its strongly elongated orbit in 1989. So some researchers
have speculated that, in the 2½ decades since, everything has gotten colder and that the atmosphere is liter-

Sky and Telescope - November 2015

Table of Contents for the Digital Edition of Sky and Telescope - November 2015