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Pluto's 'Unprecedented' Ice
Provinces and Other Surprises
from NASA's New Horizons
Pluto, known for more than eight decades as just
a faint, fuzzy and faraway point of light, is
shaping up to be one of the most complex and
diverse worlds in the solar system.
Pluto's frigid surface varies tremendously from
place to place, featuring provinces dominated by
different types of ices — methane in one place,
nitrogen in another and water in yet another,
newly analyzed photos and measurements
from NASA's New Horizons mission reveal.
"That is unprecedented," said New Horizons
principal investigator Alan Stern, who's based at
the Southwest Research Institute in Boulder,
Colorado.
"I don't know any other place in the entirety of
the outer solar system where you see anything
like this," Stern told Space.com. "The closest
analogy is the Earth, where we see water-rich
surfaces and rock-rich surfaces that are
completely different."
That's just one of the new Pluto results, which
are presented in a set of five New Horizons
papers published online today (March 17) in the
journal Science. Taken together, the five studies
paint the Pluto system in sharp detail, shedding
new light on the dwarf planet's composition,
geology and evolution over the past 4.6 billion
years.
A distant world coming into focus
New Horizons becomes the first probe to
explore Pluto in mid-2015. Pluto was discovered
in 1930 by American astronomer Clyde
Tombaugh. The dwarf planet remained largely
mysterious for many years thereafter, because it
lies so far from Earth.
Pluto orbits in the Kuiper Belt, the icy realm
beyond Neptune, at an average distance from the
sun of about 40 astronomical units (AU). (One
AU is the distance from the Earth to the sun —
about 93 million miles, or 150 million
kilometers.)
On July 14,2015, New Horizons performed the
first-ever flyby of Pluto, coming within just
7,800 miles (12,550 km) of its surface. The
spacecraft saw towering water-ice mountains;
flowing nitrogen-ice glaciers; pebbly
"snakeskin" terrain; a vast, crater-free plain
known as Sputnik Planum; and many other
features that scientists are still trying to figure
out.
One of the new studies dives deeply into the
geology of these features, revealing new insights
about their possible origin and evolution. The
620-mile-wide (1,000 km), nitrogen-icedominated Sputnik Planum, for example,
apparently sits atop a huge and ancient impact
basin, mission scientists say.
Sputnik Planum is smooth and pristine, bearing
no impact scars. This shows that the region was
resurfaced extremely recently — 10 million
years ago at most, and possibly much more
recently than that, researchers said.
But other parts of Pluto harbor lots of visible
craters, and some regions have a middling
(small) number, suggesting that the dwarf planet
has been geologically active on a large scale
over its entire history.
This finding came as a big surprise when it was
first announced last year. Earth remains
geologically active because it has a hot, molten
core. Some icy satellites, such as Saturn's moon
Enceladus and the Jovian moon Europa, also
harbor substantial internal heat, which is
generated by the powerful gravitational tug of
their giant parent planets. But something else is
likely happening at Pluto.
"I think we have to rethink our whole
understanding of geophysics — how you keep
small planets active over time," Stern said.
Stern isn't sure what exactly is going on, but he
has a favorite hypothesis: that a subsurface Pluto
ocean has been slowly freezing over the eons.
predicted, researchers said. In addition, the
dwarf planet is losing its atmosphere at a much
lower rate than scientists had thought.
"As it freezes, it releases latent heat," he said. "It
may be the freezing of this ocean that's powering
all this geology."
Modeling studies had predicted that Pluto would
be outgassing at extremely fast rates, losing lots
of gas molecules to space every second. But
New Horizons' measurements showed a mere
leak rather than a gush — in fact, the model
estimates were off by a factor of about 5,000.
"That shows the power of having a spacecraft
there — actually going to see," Stern said.
More geology insights
The new geology study also discusses and
interprets a number of other Pluto features, such
as the huge, dark-red Cthulhu Regio — which
appears to owe its color to hefty concentrations
of tholins, complex organic molecules that
drifted down out of the dwarf planet's
atmosphere — and the towering peaks Wright
Mons and Piccard Mons.
New Horizons also spotted long tectonic faults
with very steep associated scarps —
characteristics that suggest Pluto has a thick
crust composed of water ice, mission team
members said.
One of the other new Science papers maps out
the distribution of various ices across Pluto's
surface. This material — primarily frozen
methane, nitrogen, carbon monoxide and water
— is deposited in a curiously distinct way,
researchers found; some Pluto provinces are
dominated by nitrogen ice, others by methane
and so on (though there are places, such as
Sputnik Planum, where several different ices are
found in abundance).
This pattern suggests that volatile material
(nitrogen, methane and carbon monoxide) is
being moved around by sublimation,
condensation and glacial flows on both seasonal
and geological time scales, the researchers said.
Surprising atmosphere
New Horizons found that Pluto's upper
atmosphere — the part that's more than 120
miles (200 km) above the surface — is a lot
colder than pre-flyby modeling work had
These twin atmospheric surprises — the cold
high-altitude temperatures and the low escape
rate — are closely related, Stern added. Cold
molecules are less energetic, and are therefore
less likely to break free of Pluto's gravitational
bonds.
Pluto's moons, too
Pluto does not cruise through space by itself.
The dwarf planet has five moons — Charon,
Nix, Hydra, Kerberos and Styx. The latter four
are tiny, measuring just a few dozen miles wide
at most, but Charon is 750 miles (1,200 km)
across — more than half as wide as Pluto itself.
Indeed, scientists regard Pluto and Charon as a
binary system.
More data coming
There is still a lot of flyby data left to analyze.
New Horizons has beamed only half of the
close-encounter images and measurements back
to mission control, Stern said; the entire treasure
likely won't be on the ground until October.
New Horizons may also perform another flyby
that could help scientists better understand
Pluto's neighborhood. The spacecraft is currently
cruising toward a small Kuiper Belt object called
2014 MU69, which lies about 1 billion miles
(1.6 billion km) beyond Pluto. If NASA
approves and funds a proposed extended
mission, New Horizons will study 2014 MU69
up close on Jan. 1, 2019.