Australian Sky & Telescope — July 2017

8 AUSTRALIAN SKY & TELESCOPE July 2017

SOLAR SYSTEM TOP: ASA / JPL / UCLA / MPS / DLR / IDA / PSI, BOTTOM: ASA / JPL / UCLA / MPS / DLR / IDA / PSI / LPI

EVER SINCE NASA’SDawn spacecraft

reachedasteroid1Ceresinearly

2015, mission scientists have been

fixatedonbrightpatchesinsidethe

92-km-wide crater Occator (above).

Thespots,formingabrightareacalled

Cerealia Facula at Occator’s centre

and a complex of secondary spots,

collectively called Vinalia Faculae, on

itseasternfloor,appeartobedeposits

of carbonate-rich salts — residue from

brinyflowsthatgurgledupfromafluid

reservoir(perhapsaglobalocean)deep

intheasteroid’sinterior.

Occator was gouged into the

landscape about 34 million years ago,

butthewhitishdomeatitscentre

ismuchyounger—just4million

yearsold.That’stheconclusionofa

newanalysispublishedintheMarch

2017 Astronomical Journalby Andreas

Nathues(MaxPlanckInstitutefor

SolarSystemResearch,Germany)and

colleagues.

Theteamfocusedalotofits

attentiononthebright,high-standing

domeinthecrater’scentre(right).

Some3kmacrossand400metreshigh,

it’s not the classic ‘central peak’ that

many large craters get when they form.

Instead,thedomesitswithinabroad

pit,about11kmacross,that’srimmed

by fractures.

The dome isn’t really white, despite

what images suggest. It’s about 30%

reflective—comparedto2%to4%

for the dark surrounding terrain. Last

year another Dawn team, using the

spacecraft’s infrared spectrometer,

foundinfraredabsorptionbandsdue

to carbonates in Cerealia Facula. The

carbonate deposit must be fairly thick,

too, because the dome bears a dozen

smallimpacts,80to300macross,and

Recent eruptions on Ceres?

White spots dot the floor of

Occator, a prominent crater

on Ceres. NASA’s Dawn

spacecraft has seen haze

inside the crater that appears

to be linked to the spots.

An enhanced-colour

close-up reveals a bright

dome sitting within a

smooth-walled pit in

the centre of Occator.

Numerous linear features

and fractures crisscross

the dome’s top and flanks.

all are bright like their surroundings.

The dome’s relatively young age

suggests that cold, briny eruptions,

known as cryovolcanism, emerged from

a liquid reservoir trapped between a

muddy icy mantle and a silicate-rich

core. Once the slushy stuff breached the

surface, exposing it to the cold vacuum

of space, the brine would have quickly

frozen and its water would have rapidly

boiled or sublimated away, leaving the

salts behind as a solid residue.

Whether the salts now exist as a stiff

layer or as a fine fluffy powder on the

surface isn’t known. In late April the

Dawn project planned to examine the

dome with an illumination phase angle

of 0° — that is, with sunlight coming

from directly behind the spacecraft.

Observations made at this special

geometry should narrow down the grain

sizes in the salt deposits.

Nor is it clear how often eruptions

might have occurred. “A long-lasting

process appears to be prevalent,” the

team concludes, “whereby periodically

or episodically ascending bright

material from a subsurface reservoir

was deposited, expelled from fractures,

and extruded onto the surface, forming

the present-day central dome.”

■ J. KELLY BEATTY

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