Sky and Telescope - September 2018 - 12

COSMIC RELIEF by David Grinspoon

Nature loves to riff on existing motifs, even
in the most unearthly of environments.
THE MOST INTERESTING WORLDS

are those that are most varied and
vibrant. Consider Earth. Lying between
its churning interior and solar-driven
atmosphere, its surface is constantly
reworked, producing inﬁnite diversity
and beauty. Maybe I'm biased, but that
winning combination of vigorous internal and atmospheric activity makes for
the best planetary exteriors.
Fortunately, we keep ﬁnding more
places where the fertile interface
between geology and meteorology cooks
up marvels. Such features are often
a beguiling mix of the familiar and
the exotic, with recognizable forms
assembled from available materials
that are malleable in the extreme (to
us) conditions found on other worlds.
On Titan, for example, a hydrological
cycle creates recognizable fractal rivers
and lakeshores, but the working ﬂuid is
methane, not water.
Pluto, as revealed to us by the New
Horizons encounter of 2015, sports
precipitous water-ice mountains capped
with methane snow, ringing a vast
nitrogen glacier called Sputnik Planitia.
Upon seeing that massive smooth plain,
clear and fresh and free of craters, we
were consumed by the mystery of its
self-erasing surface. What is happening
below to drive that sea of solid nitrogen
to turn itself over? It may simply be
that the radioactive decay of the rocky
interior generates enough heat to churn
the soft nitrogen ice.
There's no new physics here - simply common materials found in such
otherworldly scales and conditions that
our pre-encounter imaginations were
not quite up to the task. This is why we
explore rather than simply stay home
12

S E P TE M B E R 2 018 * SK Y & TELESCOPE

and construct models.
Yet it's not just internally driven activity that
gives Pluto its dazzling
variety and complexity.
Our favorite Kuiper Belt
orb, we ﬁnd, is another
place where many of the
grandest enigmas and
most exquisite features
arise from the interaction between surface
and atmosphere.
The wonderful
discovery of dunes on
the western ﬂanks of
Sputnik (see page 8)
conﬁrms this. Among
the discoverers is Jani
Radebaugh, a planetary
geologist at Brigham
Young University, who
was instrumental in
gathering the international, multi-disciplinary team that produced
the June 1st Science
paper that made the case
for Plutonian dunes.
I admit I was skeptical when I ﬁrst heard
about potential dunes
on Pluto. I assumed
its wispy atmosphere,
100,000 times thinner than Earth's at the
surface, would be far too
rareﬁed to blow around
material in the way
needed to fashion dunes.
But Jani knows dunes
through and through,
on our planet and

■ DAVID GRINSPOON
recently completed a
book tour for Chasing
New Horizons, which
he coauthored with
Alan Stern, the Pluto
mission's principal
investigator.

LE AH TISCIONE / S&T

Variations
on a Theme

elsewhere. Applying
her physical intuition,
honed from years of
hiking over, measuring, and comparing
geologic features, she
recognized the telltale forms of surface
landscapes formed by
windblown deposits of
ﬁne materials. "When
I ﬁrst saw the highresolution images of
those areas, I realized
that these were dunes,"
she told me. "I knew
there had to be an
atmosphere that could
produce them."
The rare Plutonian air hosts strong
breezes, but what
generates the initial
force to lift the small
particles of methane
snow that seem to be
forming these drifts?
One possibility: As
the solid nitrogen
surface heats up in
the Sun, it vaporizes,
launching particles
skyward. Alternatively,
the dunes could have
formed in the past
when Pluto's changeable atmosphere was
thicker.
Nitrogen glaciers,
ice mountains, methane snowcaps, and
now - apparently -
methane "sand" dunes.
Wonders never cease.
Sky and Telescope - September 2018

Table of Contents for the Digital Edition of Sky and Telescope - September 2018
