Sky and Telescope - July 2017 - 10
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Recent Briny Eruptions on Ceres?
EVER SINCE NASA'S Dawn spacecraft
reached asteroid 1 Ceres in early 2015,
mission scientists have been ﬁ xated on
bright patches inside the 92-km-wide
(57-mile-wide) crater Occator (above).
The spots, forming a bright area called
Cerealia Facula at Occator's center and
a complex of secondary spots, collectively called Vinalia Faculae, on its
eastern ﬂoor, appear to be deposits of
carbonate-rich salts - residue from
briny ﬂows that gurgled up from a ﬂuid
reservoir (perhaps a global ocean) deep
in the asteroid's interior.
Occator was gouged into the landscape about 34 million years ago, but
the whitish dome at its center is much
younger - just 4 million years old.
That's the conclusion of a new analysis
published in the March 2017 Astronomical Journal by Andreas Nathues
(Max Planck Institute for Solar System
Research, Germany) and colleagues.
The team focused a lot of its attention on the bright, high-standing dome
in the crater's center (right). Some 3
km across and 400 m high, it's not the
classic "central peak" that many large
craters get when they form. Instead, the
dome sits within a broad pit, about 11
km across, that's rimmed by fractures.
The dome isn't really white, despite
what images suggest. It's about 30%
reﬂective - compared to 2% to 4%
J U L Y 2 0 1 7 * SK Y & TELESCOPE
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.
for the dark surrounding terrain. Last
year another Dawn team, using the
spacecraft's infrared spectrometer,
found infrared absorption bands due to
carbonates in Cerealia Facula. The carbonate deposit must be fairly thick, too,
because the dome bears a dozen small
impacts, 80 to 300 m across, and 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 constrain 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
An enhanced-color close-up reveals a bright dome sitting within a smooth-walled pit in
the center of Occator. Numerous linear features and fractures crisscross the dome's top
SOL A R SYSTEM TOP: ASA / JPL / UCL A / MPS / DLR / IDA / PSI, BOT TO M: ASA / JPL / UCL A / MPS / DLR / IDA / PSI / LPI