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8 AUSTRALIAN SKY & TELESCOPE JULY 2016

Pluto’s floating hills

of nitrogen

Brightest supernova baffles astronomers

W

ithin days of New Horizons’
historic flyby of Pluto last July
14, 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 (N 2 ) 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
clustersofhillsthatstickupthrough
the plain’s surface. The plain is made up
oflarge,polygonalslabs,andthehills,
which are up to a few kilometres across,
appear to be merrily bobbing along in the
icyfloesandgettingtrappedwherethe
slabs meet.
Perhaps the mysterious hills are
fragmentsofwatericefromthe
mountainsthatpartiallysurround
Sputnik Planum. Importantly, these
water-ice‘islands’appeartobeanalogous
toocean-goingicebergshereonEarth
—and,assuch,theymightofferahint
ofthedepthofSputnikPlanum’sfrozen
nitrogen‘sea’.Sohowdeepmightthat
be?Assumingthatthesehillsaretruly

free floating and in what geologists call

isostatic equilibrium, something like 90%

ofthemass(andthusvolume)ofeach

floatinghillliesbeneaththesurface.

But these Pluto-bergs are unlikely to be

solidthroughout.A‘reasonable’porosityof

15% would make them significantly more

buoyant, explains Jeff Moore (NASA Ames

ResearchCenter).Evenso,hecontinues,

“We suspect that the N 2 deposit of Sputnik

Planum is several kilometres deep —

maybe on order 10 km in places.”

Interestingly, one large cluster,

nicknamedChallengerColles(honoring

thoselostaboardtheSpaceShuttle

Challengerin 1986; see image above),

measures 60 by 35 km. This grouping

isn’toutinthemiddleofSputnikPlanum

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

T

he most luminous supernova ever
discovered,ASASSN-15lh,challenges
popular theories for blazingly bright
exploding stars.
The supernova appeared in June 2015
in the southern sky in images taken by the
All-Sky Automated Survey for Supernovae
(ASAS-SN). Its light travelled for 2.8 billion
years to reach Earth, giving it a redshift
of 0.2326. Due to its distance, its glow
peaked at only 17th magnitude.
But its intrinsic luminosity was twice as
bright as any supernova yet discovered. Six
months later, this single object continued
to emit a level of energy that rivalled that of

ClustersandchainsofhillsonPlutoappeartobeblocksofwaterice‘floating’inahigher-density

‘sea’ dominated by frozen nitrogen. The zoom spans about 500 by 340 kilometres.

allthestarsintheMilkyWaycombined.

Subo Dong (Peking University, China)

andcolleaguesreleasedanupdateinthe

January15issueofthejournal,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 so-called

superluminous supernovae(SLSNe). The

explosion appears to be much hotter

than normal. Its home is unusual, too: a

bright but otherwise unassuming galaxy

thatformslessthanonestarperyear.

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 magnetised,

spinning neutron star called amagnetar.

Astronomers must now explore more

exotic options.

■ MONICA YOUNG

S&T:

GREGG DINDERMAN, SOURCE:NASA/JHU-APL/SWRI

Challenger

Colles

Hill

clusters

Hill chains

Glaciers

Rugged

uplands

Sputnik Planum

cellular terrain

News Notes