Sky and Telescope - January 2016 - 12

News Notes

New Horizons recorded this oblique, backlit panorama of Pluto from 18,000 km (11,000 miles) away just 15 minutes after its closest approach
on July 14, 2015. The rugged Norgay Montes are at left, and the smooth plain Sputnik Planum is at right.

It's going to take a year for NASA's
New Horizons spacecraft to send back all
the images and other data it collected as it
swept past Pluto on July 14, 2015, and the
mission team has repeatedly promised
that watching those observations trickle in
will be like getting birthday presents every
week. Two of those "gifts" appear here.
The spectacular backlit panorama of
Pluto provides an oblique perspective that
accentuates this little world's surprisingly rugged and varied terrain. "This
image really makes you feel you are there,
at Pluto, surveying the landscape for
yourself," comments principal investigator
Alan Stern (Southwest Research Institute).
Yet, more than a pretty picture, the
high-phase-angle lighting provides
important insights into all that's going
on down below. For example, even
though Pluto's atmosphere is incredibly
tenuous, it's stacked with more than a
dozen thin haze layers that extend from
the icy ground up to altitudes of 100 km
(60 miles) or more. The angular peaks
informally dubbed Norgay Montes, which
dominate the left side of the panorama,
rise to elevations of about 3½ km (11,000
feet) - far higher than anyone expected.
"The randomly jumbled mountains
might be huge blocks of hard water ice
ﬂoating within a vast, denser, softer
deposit of frozen nitrogen," suggests
12

Januar y 2016 sky & telescope

lead geologist Jeﬀ Moore (NASA Ames).
"Think Silly Putty."
Meanwhile, planetary geologists don't
yet know how or when Pluto's largest
moon came to exist. But this much is
certain: Charon has had a rough time
of it in the eons since. Exhibit A is an
enormous system of fractures girding the
moon's midsection for at least 1,600 km
- four times the Grand Canyon's length
and twice as deep in spots - and it likely

Canyons gird Charon's midsection, breaking up the moon's cratered terrain. To
their south, the fractures give way to the
smoother plain informally named Vulcan
Planum. This scene, a combination of
blue, red, and infrared images, resolves
details as small as 0.8 km (0.5 mile).

continues onto Charon's farside. This
crevasse chain has the look of a huge tear,
like crustal rending seen on Earth (the
East African Rift, for example) and Mars
(Valles Marineris).
"It looks like the entire crust of Charon
has been split open," observes John
Spencer (Southwest Research Institute),
who serves as deputy lead for the mission's
geology, geophysics, and imaging team.
Such wholesale extension might have
occurred, the New Horizons team speculates, if Charon once had an interior ocean
of water that expanded as it froze long ago.
The overlying crust would have cracked
wide open to accommodate the increased
volume. Whatever the cause, the faults
and canyons indicate some kind of global
geologic upheaval in Charon's past.
The enigmatic, red-stained depression
at Charon's north pole also deﬁes explanation. One early speculation held that
the reddish material is a veneer of organic
compounds, synthesized from methane
that escaped from Pluto. But the new
images suggest that most of the reddish
terrain is partly bounded by a high-standing rim. That wouldn't happen if the red
material came from transplutonian space.
Instead, some or all of it might have come
from Charon's interior. Read the team's
Science paper at http://is.gd/plutosci1015.
■ J. KELLY BEATTY

NASA / JHU APL / SWRI (2)

PLUTO I Close-up of a Binary Dwarf Planet

http://www.is.gd/plutosci1015

Sky and Telescope - January 2016

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