Sky and Telescope - November 2015 - 21

Haze layer or wave
(~80 km altitude)
Haze layer or wave
(~50 km altitude)

- Pluto's surface
RING OF HAZE New Horizons was ﬂying through Pluto's
shadow on July 15th, roughly 2 million km (1¼ million miles) away,
when it recorded the dwarf planet's thin, hazy atmosphere (backlit
by sunlight) rimming its silhouette like a luminous halo. The boxed
inset, enlarged above and color-coded to enhance subtle diﬀerences, reveals what appear to be layers or waves within the haze.
NASA / JHU-APL / SWRI

ally freezing and falling onto the surface as frost.
Yet, despite the low surface pressure, Pluto's atmosphere extends upward to about 1,600 km - much
higher up than expected. Moreover, those thin wisps
include a gauzy haze at least 160 km high that shows
hints of at least two discrete layers or broad waves. The
haze consists of acetylene (C2H2), ethylene (C2H4),
and other organic compounds created after ultraviolet
photons in the weak sunlight break down molecules of
atmospheric methane.
There's growing evidence that Pluto is losing what
little gas it has to interplanetary space at a rapid rate,
roughly 500 tons per hour. "We know there's methane
in the atmosphere," explains Fran Bagenal (University
of Colorado, Boulder). "It absorbs sunlight, heats up,
and gives the gas the energy it needs to escape." Once
the escaping molecules are in space, ultraviolet sunlight
ionizes them. Then they're swept up by the solar wind
ﬂowing past Pluto at hundreds of kilometers per second,
creating a tail of charged particles that extends outward,
away from the Sun.
Stern calculates that, over its history, Pluto must have
lost enough nitrogen to form a global layer 0.3 to 3 km
deep. So, for nitrogen to exist on the surface today, he
feels that some process must be dredging it up from the
interior. "We haven't found geysers or cryovolcanoes
yet," Stern says, "but now we'll be looking for exactly the
evidence of these phenomena."

Charon: Dark and Ancient
New Horizons didn't spend all of its ﬂyby time trained
on Pluto. It also scrutinized the sizable moon Charon.
With a diameter of 1,208 km (751 miles), Charon is
roughly half the size of Pluto and a little larger than the
dwarf planet Ceres.
Unlike Pluto, Charon appears dark and more uniformly colored. Yet it too displays far more geologic
diversity than anyone expected. "Originally, I thought

Charon would show an ancient terrain covered in
craters," admits deputy project scientist Cathy Olkin
(Southwest Research Institute). But the spacecraft's most
detailed images "blew our socks oﬀ."
The dark, red-tinged polar cap ﬁrst seen from far
away (and informally dubbed Mordor) bears some kind
of structure - perhaps remnants of a large impact
basin. Here and there are deep fractures, and a gash
of interconnected canyons slashes across the southern
hemisphere for some 1,000 km. Some regions look
smooth and nearly crater-free, a telling sign that they're
relatively young.
Has Charon been churning inside to cause all this
geologic calamity? Perhaps, but the kind of tidal heating
seen among the moons of Jupiter and Saturn can't be the
reason. Pluto and Charon are solidly, permanently locked
in a spin-orbit resonance that keeps one hemisphere of
each constantly facing the other (imagine a pair of ﬁgure
skaters holding hands while spinning around each
other). This conﬁguration doesn't generate tidal stress.
Meanwhile, New Horizons conducted extensive scans
to spot undetected moonlets or faint rings but found
nothing (yet another surprise). Nor did the spacecraft
have especially good opportunities to view Pluto's four
smaller moons. Nix turns out to be 42 by 36 km in size
and Hydra 55 km long. Both are smaller and brighter
than expected, so they're likely ice covered. Sizes for
Styx and Kerberos will have to wait until the craft radios
resolved images back to Earth.
In fact, Stern emphasizes that the slow playback will
give waiting scientists a steady diet of new results to work
with. "Expect many more images and spectra and, from
those, many more discoveries in the months ahead," he
says. "New Horizons is a gift that will keep on giving." ✦
Senior Editor Kelly Beatty has covered planetary science for
more than four decades. But his real claim to fame is that
Clyde and Patsy Tombaugh once spent the night at his home.
Sk yandTelescope.com November 2015

http://www.SkyandTelescope.com

Sky and Telescope - November 2015

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