Sky and Telescope - February 2016 - 11

PLUTO I Cryogenic Volcanoes Found on Dwarf Planet?
Results from New Horizons' July 14,
2015, ﬂyby of Pluto and its moons continue
to trickle back to Earth, and in November
at the American Astronomical Society's
Division for Planetary Sciences annual
meeting, mission scientists recapped
details from the ﬁrst 20% received.
The spacecraft's images have already
shown that Pluto has an unexpectedly
dynamic geology, with towering ice
mountains, fresh plains of nitrogen ice
that have slowly oozed like glaciers, and a
deeply fractured crust. Now you can add
"giant ice volcanoes" to that list.
According to Oliver White (NASA Ames
Research Center), the spacecraft imaged
two broad, tall mountains that are perhaps
150 km (90 miles) across and topped with
depressions at their summits. They're dead
ringers for the kind of broad shield volcanoes found on Earth (think "Mauna Kea")
and other inner planets - and very unlike
anything ever seen on dozens of icy moons
in the outer solar system. "Whatever they
are, they're deﬁnitely weird," admits White,
"and 'volcanoes' is the least weird explanation at the moment."
The two mounds aren't immediately
obvious in any given image, but they show
up in the 3D topographic maps created by
viewing the surface from multiple angles
(see below). The taller one, informally

NASA / JHU-APL / SWRI (2)

Visible

named Piccard Mons, is 5 km high
(16,000 feet); Wright Mons is 3 km high.
The "lava" in this case was not molten
rock - Pluto's interior is far too cold for
that. Instead, it would have been a slushy
mixture of water mixed with ammonia
and other normally icy compounds that
became liqueﬁed deep down and gushed
out onto the surface.
If these really are cryovolcanoes, their
summit depressions formed when the
source feeding them shut down and the
peaks collapsed. Strange, hummocky textures on their ﬂanks might represent individual ﬂows, though no one has yet oﬀered
details on their composition or origin.
Add "Piccard" and "Wright" to the
growing evidence that Pluto's interior
remained warm - and likely still is warm
- for more than 4 billion years. Ordinarily, objects of this size should have frozen
solid long ago. The interior heat isn't from
tidal stretching and squeezing incited by
another body, as occurs on Io thanks to
Jupiter: Pluto and its big moon Charon are
locked in a spin-orbit coupling that rules
out tidal torquing. The only other plausible
heat source would be the long-term decay
of radioactive isotopes in the rocky core,
which occupies most of Pluto's mass and
about 70% of its diameter.
■ J. KELLY BEATTY

Topographic

Wright Mons
50 miles
80 km

50 miles
80 km

The informally named feature Wright Mons (center feature) is about 160 km (100 miles) wide
and 3 km (10,000 feet) high. Its summit depression is 56 km across, and its sides exhibit a
distinctive hummocky texture. Its shape matches that of shield volcanoes on Earth.

BRIEFS from the DIVISION
for PLANETARY SCIENCES
■ J. KELLY BEATTY

Cracks on Phobos. The enigmatic
grooves on Mars's larger satellite were
long thought to be a consequence of the
formation of Stickney, a 9-km-wide crater
that's a third of Phobos's length. But a
new analysis by Terry Hurford (NASA
Goddard) and others shows that many
of the cracks align well with predicted
tidal stresses induced by Mars. Phobos is
slowly spiraling toward the Red Planet, and
tidal forces can produce more than enough
stress to fracture the surface. Although
no new fractures have appeared in the
decades since spacecraft ﬁrst imaged
Phobos at close range, some grooves are
younger than others - as would occur if
the process that creates them is ongoing.
Deﬁning Planets Redux? Many astronomers have issues with the divisive, Plutodemoting deﬁnition for a planet adopted
by the International Astronomical Union in
2006. Strictly, it doesn't apply to exoplanets, nor does it deﬁne the upper mass limit
for planethood. In a treatise to appear in
Astronomical Journal, dynamicist Jean-Luc
Margot (University of California, Los Angeles) suggests a relatively simple "planet
test" that depends only on the candidate's
mass, orbital period, and the mass of its
host star. The upshot is that a body massive enough to clear its orbit of smaller
interlopers within the star's lifetime is a
planet. (Pluto would still fall short.)
Chance Collision? Astronomers have
recorded what looks like the aftermath of
an impact on the main-belt asteroid 493
Griseldis. Images taken in March 2015
show a faint fan of dust coming from the
asteroid, but the plume is absent from
images taken the following spring and in
2010 and 2012. The feature's orientation is
inconsistent with a cometary tail, and at
the time Griseldis was far from the perihelion of its 5½-year orbit. So David Tholen
(University of Hawaii) and colleagues think
they have evidence of a substantial impact
on the 46-km-wide asteroid's surface.

Sky and Telescope - February 2016

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