Sky and Telescope - March 2017 - 10

NEWS NOTES

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PLUTO

Sputnik Planitia Made Pluto's "Heart" Wander
THE BIG, NITROGEN-ICE-FILLED basin
informally called Sputnik Planitia has
apparently caused Pluto's entire crust to
shift over time.
Sputnik Planitia is the western half
of a larger, heart-shaped bright feature
nicknamed Tombaugh Regio that's
situated on the hemisphere facing away
from the big moon Charon. About 1,000
km (600 miles) across, the plain's surface is crater free (and thus very young),
rimmed by tall water-ice mountain
ranges, and exhibits glacier-like ﬂows.
Many suspect it's an ancient impact
basin that, over time, has been partly
ﬁlled by a thick layer of frozen nitrogen,

p Nitrogen, carbon monoxide, and methane
ices appear to be concentrated in this bright,
craterless expanse, informally called Sputnik
Planitia. Colors are close to natural.

methane, and carbon monoxide.
Now two analyses conclude that Sputnik Planitia lies almost directly opposite
Charon not by accident but instead
because the feature literally dragged the
crust of Pluto around to the current
arrangement. Geophysicists refer to this
crustal shifting as true polar wander.
It's the tendency of spinning objects to
reorient themselves so that locations
with mass excesses end up on the equator and mass deﬁciencies at the poles.

In the December 1st Nature, four geophysicists led by James Keane (University of Arizona) make the case that the
big basin initially existed much farther
north, near latitude 60°, and gradually
migrated equatorward. They modeled
how this shift, coupled with the gradual
freezing of a global subsurface ocean
early in Pluto's history, would stress and
fracture the world's surface. These fractures match the ones found on Pluto.
To trigger the basin's equatorward
shift, Francis Nimmo (University of
California) and other New Horizons
scientists calculate in an accompanying Nature article that the nitrogen ice
ﬁlling Sputnik Planitia would have to be
at least 40 km (25 miles) deep. That's
implausibly thick, so Nimmo's team
argues that the excess mass needed to
drag the feature toward the equator
comes from a broad, near-surface bulge
in the subsurface ocean that underlies the basin. Such a bulge could have
welled upward when the impact carved
the initial hole in Pluto's ice crust.
Not everyone is convinced that
Sputnik Planitia is an impact basin,
though. Douglas Hamilton (University
of Maryland) and others suggest in the
same Nature issue that the ice formed
ﬁrst, slowly producing a basin beneath it
as it built up. In this scenario, the plain
initially formed at its current location,
not near the pole, because Pluto's highly
tilted spin axis makes these lower latitudes the coldest on the little world.
■ J. KELLY BEATTY

Near-Earth Asteroid Tally Surpasses 15,000
THANKS TO TWO aggressive search
programs, the count of near-Earth
asteroids (NEAs) has surpassed the
15,000-object milestone. Most NEAs
are too small to be spotted far away,
and it's only within the week or so
when they skim near our planet that
they make their existence known. These
days almost all NEAs are swept up by
the Catalina Sky Survey in Arizona or by
the wide-ﬁeld Pan-STARRS 1 telescope
in Hawai'i. Even with all those space

M A R C H 2 0 1 7 * SK Y & TELESCOPE

rocks ﬂying around, there's very little
risk that a known, sizable asteroid will
strike Earth for the foreseeable future.
The object with the highest cumulative probability of impact (that is, the
risk over dozens of close approaches) is
2010 RF12, which has a 1-in-15 chance
of hitting in 2095 - but it's only about
7 m (23 ft) across, almost certainly too
small to make it to the ground.
Meanwhile, Vishnu Reddy (University of Arizona) and colleagues report in

Artist's concept.

the December Astronomical Journal their
discovery of one of the smallest NEAs
yet: 2015 TC25, about 2 m wide. The little
rock is highly reﬂective and bare of dust,
probably in part because of its fast spin.
■ J. KELLY BEATTY

PLU TO: N ASA / JHU -A PL / SWRI; ASTEROID: N ASA / JPL- CA LTECH

SOLAR SYSTEM

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Sky and Telescope - March 2017

Table of Contents for the Digital Edition of Sky and Telescope - March 2017