Sky and Telescope - October 2016 - 18

New Horizons: Part 1

50 km

BEAUTIFUL BADLANDS Left: The rugged peaks of al-Idrisi Montes sit against the northwestern shore of Sputnik Planitia. Right:
Even though much of Pluto never gets warmer than 50 kelvin, New Horizons scientists have spotted evidence of massive, recent
downslope glacial ﬂow (arrowed) from eastern Tombaugh Regio, at right, into Sputnik Planitia. Altitudes span 6 km in this color-coded
view and range from pink (low) to orange (high).

has to escape to the surface somehow. Second, frozen
nitrogen melts at just 63K and should ﬂow readily even
when solid at Sputnik Planitia's surface temperature of
37K (-393°F).
The polygons provide the key evidence: they're the
tops of solid-state "bubbles" percolating up from below.
Sputnik Planitia is, in eﬀect, a giant solid-state lava
lamp that uses convection to deliver interior heat to the
surface. Recent calculations by Alexander Trowbridge
(Purdue University) and others show that convection will
occur if the layer of nitrogen ice has a depth of at least
435 m (¼ mile).
Also, the bigger the polygons, the deeper the ice
should be. On this basis, Trowbridge's team argues that
the plain's convecting layer of frozen nitrogen should
be at least 10 km thick in its center and thinner along
its outer margin. But that would require the underlying
basin to be implausibly deep, counters McKinnon. He
and others believe the convection cells should get sluggish as they encounter the cold ice near the surface, an
eﬀect that would widen the polygons. So they maintain
the ice layer need only be 3 to 6 km deep.
Whichever model is correct, both groups agree that
the upwelling convection pushes the topmost ice along at
rates of a few centimeters per year, meaning that the surface of Sputnik Planitia renews itself very rapidly - in

( A LIT TLE ) M O R E T O C O ME
Roughly 6 gigabytes (80%) of the New Horizons results are
in hand. But there's still a smattering of low-priority data to
retrieve from the spacecraft, now some 3 astronomical units
past Pluto and 5.4 billion kilometers from Earth.

October 2016 sky & telescope

just 500,000 to 1,000,000 years. No wonder there aren't
any craters!
En route to the surface, the heat from deep inside
should have melted Pluto's water-ice mantle at its base,
creating a global layer 50 to 150 km thick. But does that
deep-seated reservoir still exist? Two lines of evidence
argue that it does.
First, if the mantle were solid throughout, overlying
pressure should gradually transform the frozen water to
a higher-density variant, known as Ice II. This conversion reduces the needed volume for all that ice, and Pluto
should have contracted by up to 13 km in radius. We'd
see that expressed topside as a dramatic series of compressional fractures - but they're aren't any.
Second, just the opposite seems to be happening. An
analysis by Noah Hammond (Brown University) and two
colleagues argues that the assorted cracks seen in Pluto's
surface result from extension, not compression, created
because the globe has gradually expanded. "A subsurface
ocean that was slowly freezing over would cause this
kind of expansion," Hammond explains.
The New Horizons images reveal another pair of
features that could have resulted from internal stirrings.
Each is big, round, and tall, with a deep hole at its center.
One, dubbed Wright Mons, is about 150 km across and
3 or 4 km high, with a central depression at least 5 km
deep. To its south, lurking in the shadows along the
day-night terminator, is a similar mound, Piccard Mons,
that's half again wider and taller.
On Earth, we'd call these volcanoes. But Pluto's
interior isn't hot enough to melt rock. Instead, a type of
eruption called cryovolcanism could have occurred if a
buried reservoir of liquid water, mixed with ammonia
to lower its melting temperature, were squeezed under

NASA / JHU-APL / SWRI (2)

5 km

Sky and Telescope - October 2016

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