Sky and Telescope - June 2016 - 10

News Notes

PLUTO I Floating Hills of Nitrogen

Challenger
Colles

Sputnik Planum
cellular terrain

Rugged
uplands
Hill
clusters
Hill chains
Glaciers

Clusters and chains of hills on Pluto appear to be blocks of water ice "ﬂoating" in a higher-density
"sea" dominated by frozen nitrogen. The zoom spans about 500 by 340 km (300 by 210 miles).

But these Pluto-bergs are unlikely to be
solid throughout. A "reasonable" porosity
of 15% would make them signiﬁcantly
more buoyant, explains Jeﬀ Moore (NASA
Ames Research Center). Even so, he continues, "We suspect that the N2 deposit
of Sputnik Planum is several kilometers
deep - maybe on order 10 km in places."
Interestingly, one large cluster, nicknamed Challenger Colles (honoring those

lost aboard the Space Shuttle Challenger
in 1986; see image above), measures 60
by 35 km (37 by 22 miles). This grouping
isn't out in the middle of Sputnik Planum
but rather located near its eastern margin,
near the peaks of central Tombaugh Regio
(another informal name). So perhaps these
hills became "beached" once the nitrogen
ice got too shallow.
■ J. KELLY BEATTY

SUPERNOVAE I Brightest Supernova Bafﬂes Astronomers
The most luminous supernova ever
discovered, ASASSN-15lh, challenges
popular theories for blazingly bright
exploding stars.
The supernova appeared in June 2015
in Southern Hemisphere images taken
by the All-Sky Automated Survey for
Supernovae (ASAS-SN). Its light traveled
for 2.8 billion years to reach Earth, giving
it a redshift of 0.2326. Due to its distance,
the explosion's glow peaked at only 17th
magnitude (S&T: Nov. 2015, p. 12).
But its intrinsic luminosity was twice as
bright as any supernova yet discovered. Six
months later, this single object continued
10

June 2016 sky & telescope

to emit a level of energy that rivaled that of
all the stars in the Milky Way combined.
Subo Dong (Peking University, China)
and colleagues released an update in the
January 15th Science. ASASSN-15lh was
more than 10 times brighter than Type Ia
supernovae (which come from exploding
white dwarfs), putting it in the class of socalled superluminous supernovae (SLSNe).
The explosion appears to be much hotter
than normal. Its home is unusual, too: a
bright but otherwise unassuming galaxy
that forms less than one star per year.
Astronomers think SLSNe are the deaths
of massive stars, which live fast and

die young. Such stars would be rare in
ASASSN-15lh's stagnant environment.
Common explanations for SLSNe don't
work for ASASSN-15lh. The explosion's
spectrum shows no sign of circumstellar material, which would boost the glow
when the dying star's outer layers collided
violently with it. And the supernova
has already emitted more energy than
could be provided if the dying star's
core became an extremely magnetized,
spinning neutron star called a magnetar.
Astronomers must now explore more
exotic options.
■ MONICA YOUNG

S&T: GREGG DINDERMAN, SOURCE: NASA / JHU-APL / SWRI

Within days of New Horizons' historic
ﬂyby of Pluto last July 14th, mission scientists released snapshots showing unexpectedly tall mountains partially rimming a vast and very ﬂat plain. The plain,
informally named Sputnik Planum, is
dominated by frozen molecular nitrogen
(N2) and some frozen carbon monoxide
(CO), whereas the surrounding uplands
are mostly frozen water.
But recently the team unveiled an
image of Sputnik Planum that reveals
clusters of hills that stick up through the
plain's surface. The plain is made up of
large, polygonal slabs, and the hills, which
are up to a few kilometers across, appear to
be bobbing along in the icy ﬂoes and getting trapped where the slabs meet.
Perhaps the mysterious hills are fragments of water ice from the mountains
that partially surround Sputnik Planum.
Importantly, these water-ice "islands"
appear to be analogous to ocean-going
icebergs here on Earth - and, as such,
they might oﬀer a hint of the depth of
Sputnik Planum's frozen nitrogen "sea."
So how deep might that be? Assuming
that these hills are truly free ﬂoating and
in what geologists call isostatic equilibrium, something like 90% of the mass
(and thus volume) of each ﬂoating hill
lies beneath the surface.

To get astronomy news as it breaks,
visit skypub.com/newsblog.

http://www.skypub.com/newsblog

Sky and Telescope - June 2016

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