Sky and Telescope - October 2016 - 16

New Horizons: Part 1

North
Pole

Pluto

23°

South
Pole

120°

Sun

ics

i
Trop

p
Tro

SKY & TELESCOPE DIAGRAM / SOURCE: ALISSA EARLE

Earth

Equator

North
Pole

South
Pole
Day tropics

Night tropics

Equator

∗ not to scale

DIFFERENT TILTS The 23° obliquity of Earth's spin axis gives
us predictable and reasonably moderate seasons. But Pluto's
tilt of 120° creates climatic extremes not seen on any other solid
body in the solar system. For example, its tropics (regions from
which the Sun can appear directly overhead) encompass virtually the entire globe.

The Heart of Pluto
Roughly a month before the ﬂyby, the spacecraft's Long
Range Reconnaissance Imager (LORRI) spied a distinctive bright patch on the side of Pluto shaped like a heart.
In fact, this brightening had been evident in Hubble
images acquired during the 1990s. Having cultivated
close relationships with Clyde Tombaugh (Pluto's discoverer) and his wife, Patsy, before they died, Stern and
his team named it Tombaugh Regio - informally so, as
the box below explains.
But when seen up close during the ﬂyby, the heart
clearly was broken in two, having a rougher eastern half
butting against a remarkably smooth plain to its west.
The latter, somewhat larger than Texas, was dubbed
Sputnik Planum as a nod to Earth's ﬁrst artiﬁcial satel-

What's in a Name?
As of early August, all of the names used by scientists for speciﬁc features on Pluto and its moons
remained informal and unoﬃcial. Usually, the International Astronomical Union's Working Group for Planetary System Nomenclature (WGPSN) approves thematically appropriate categories for feature names on a
planetary body well ahead of time. But that didn't happen for New Horizons. Instead, the mission's scientists
conducted a public poll for names (see ourpluto.org),
ostensibly with a nod from the IAU, and the informal
names used to date derived from that eﬀort. WGPSN
oﬃcials were not amused, especially because the various monikers used for speciﬁc features have never
been submitted for formal approval. However, some
degree of détente should have emerged on August 8th,
when the two groups were scheduled to meet.

October 2016 sky & telescope

lite. Spectral scans by New Horizons reveal a composition
dominated by frozen nitrogen and carbon monoxide.
Thanks to stereo imaging, it's now clear that this icecovered plain is not a plateau, as ﬁrst thought, but instead
an enormous depression 3 to 4 km (about 2 miles) deep
at its center. So it's almost certainly the site of a primeval
impact roughly 1,000 km across - a basin big enough
to wrap around the curvature of Pluto itself. There's no
obvious rim, though geologists wouldn't expect one to
remain after billions of years of exposure and erosion.
But it is bordered by clusters of tall water-ice mountains
- al-Idrisi Montes to the west and Hillary Montes and
Norgay Montes to the south - that might be jumbles of
crumpled upper crust or piles of impact ejecta.
(Recently, after realizing that Sputnik is not a plateau
but a depression, mission scientists morphed its classiﬁcation from Planum to Planitia.)
Little did the team know how crucial this low-lying
plain has been to Pluto's geologic history. Even the spacecraft's closest, most detailed images show no impact craters - not one - anywhere within its conﬁnes. If Pluto
were the same kind of eons-old ice ball found frequently
in orbit around the outer planets (Jupiter's Callisto and
Saturn's Rhea come to mind), Sputnik Planitia should
have been heavily peppered with pits.
But given its dearth of craters, the team's geologists initially announced that its surface could be no
older than 100 million years. Later, on inspection of the
spacecraft's most detailed images, they sliced the age to
no older than 10 million years. In geo-speak, this means
Pluto is probably geologically active today.
The plain exhibits a pattern of interconnected
polygons, 10 to 40 km across, with shallow depressions
along their margins. "We saw that cellular pattern right
away," recalls geophysicist William McKinnon (Washington University, St. Louis), and immediately likened
it to a kind of patterned ground that undergoes constant
renewal in Earth's polar regions.
Close inspection shows clusters of hills that stick
up through the plain's surface. Up to a few kilometers
across, they appear to be bobbing along in the icy ﬂoes
and become concentrated where the polygonal slabs
meet. The mission's geologists suspect that the mysterious hills are mountain-size chunks of water ice
transported from the surrounding uplands, especially
Tombaugh Regio. These "islands" appear to be aﬂoat in
Sputnik's sea of frozen nitrogen, analogous to oceangoing icebergs here on Earth. (It's entirely plausible: very
cold pure water ice has a density of 0.934 g/cm3, and that
of frozen nitrogen is 1.027 g/cm3.)
New Horizons' best images show that ﬂoes of nitrogen ice are indeed sliding into Sputnik Planitia from the
higher elevations in neighboring Tombaugh Regio - in
ways very reminiscent of glacial ﬂows on Earth, observes
investigator Orkan Umurhan (NASA Ames Research

http://www.ourpluto.org

Sky and Telescope - October 2016

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