Sky & Telescope - December 2020 - 8

NEWS NOTES

SOLAR SYSTEM

Asteroid Ceres Is a Water World
SINCE NASA'S DAWN SPACECRAFT

arrived at Ceres, the largest main-belt
asteroid, scientists have thought that
the world once hosted water (S&T:
Dec. 2016, p. 16). But new analyses of
Dawn's data, presented on August 10th
in seven papers in Nature Astronomy,
Nature Geoscience, and Nature Communications, indicate that the world might
still have an ocean, underground.
Dawn's ﬁrst images of Ceres revealed
what turned out to be reﬂective salts
on the ﬂoor of Occator, a fresh-looking
crater 92 km (57 miles) across. But
scientists continued to question how
the spots had remained so bright in
the 22 million years since Occator's
creation, rather than being darkened by
micrometeorite impacts over time.
Then, as Dawn mapped the asteroid's surface and topology, attention
turned to Ahuna Mons, an oddly
shaped mountain some 4,000 meters
(13,000 feet) tall. Its wide top and steep
sides marked it as a likely cryovolcano,
spewing briny ice rather than lava.
Global density estimates also indicate a crème brûlée world, with a tough
shell surrounding a mushy interior.
All this evidence at ﬁrst hinted at an
ancient subsurface ocean that had long
frozen over. But new analysis of data
8

DECEMBER 2 02 0 * SK Y & TELESCOPE

S This simulated perspective view based on
Dawn images shows salty white spots on the
crater floor of Occator.

that Dawn collected in the last phase
of its extended mission, from June to
October 2018, indicates that the ocean
is still there, feeding cryovolcanism and
other geologic activity.
The ﬁrst line of evidence comes from
Dawn's Visible and Infrared Mapping
Spectrometer, which mapped minerals across the asteroid's surface. Maria
Cristina De Sanctis (National Institute
of Astrophysics, Italy) and colleagues
report in Nature Astronomy the discovery of hydrated sodium chloride among
the salts in the bright spots of Occator.
Geological processes must have exposed
this mineral recently, because it's not
stable on Ceres' surface.
A second line of evidence comes
from low-altitude gravity measurements made when Dawn ﬂew as close
as 35 km from the surface. In Nature
Astronomy, Carol Raymond (JPL) and
colleagues demonstrated negative density variations in and near Occator. In
other words, material below the surface
is less dense than it ought to be.
X Recently exposed brine, pushed up from
Ceres' depths, is highlighted in red in this falsecolor view.

■ MONICA YOUNG

* See more images at https://is.gd/
Cereswater

N ASA / JPL- CA LTECH / UCL A / MPS / DLR / IDA (2)

Occator Crater

To explain the anomaly, Raymond's
team suggests that a brine-rich reservoir
lies under the surface, melted millions
of years ago by the impact that created
the crater. The reservoir has stayed ﬂuid
due to the presence of salts and other
minerals, which lower water's freezing
point and also provide a layer of thermal insulation.
Analysis of the gravity data supports
the idea that not only did the impact
melt water in an underground chamber, but it also likely created fractures
connecting Occator's ﬂoor to a larger,
deeper, and older reservoir, one that is
capable of driving very recent or even
current activity.
The results place Ceres in the realm
of ocean worlds, says Dawn project
scientist Julie Castillo-Rogez (JPL). But
she notes that the asteroid's underground ocean is unique among the
solar system's other ocean worlds, such
as Jupiter's moon Europa and Saturn's
Enceladus. Ceres' "ocean" is more of a
slurry, Castillo-Rogez explains: 25-30%
saltwater mixed with a high concentration of rocky particles.
Whether such an ocean would be
habitable is still an open question, but a
fascinating one to consider. As CastilloRogez writes, the new data show that
Occator is "an obvious target for a
future mission."
https://www.is.gd/
Sky & Telescope - December 2020

Table of Contents for the Digital Edition of Sky & Telescope - December 2020
