NASA’s Dawn
spacecraft gave scientists extraordinary close-up views of the dwarf planet
Ceres, which lies in the main asteroid belt between Mars and Jupiter. By the
time the mission ended in October 2018, the orbiter had dipped to less than 22
miles (35 kilometers) above the surface, revealing crisp details of the
mysterious bright regions Ceres had become known for.
Scientists had figured out that the bright areas were
deposits made mostly of sodium carbonate — a compound of sodium, carbon, and
oxygen. They likely came from liquid that percolated up to the surface and
evaporated, leaving behind a highly reflective salt crust. But what they hadn’t
yet determined was where that liquid came from.
By analyzing data collected near the end of the
mission, Dawn scientists have concluded that the liquid came from a deep
reservoir of brine, or salt-enriched water. By studying Ceres’ gravity,
scientists learned more about the dwarf planet’s internal structure and were
able to determine that the brine reservoir is about 25 miles (40 kilometers)
deep and hundreds of miles wide.
Ceres doesn’t benefit from internal heating generated
by gravitational interactions with a large planet, as is the case for some of
the icy moons of the outer solar system. But the new research, which focuses on
Ceres’ 57-mile-wide (92-kilometer-wide) Occator Crater — home to the most
extensive bright areas — confirms that Ceres is a water-rich world like these
other icy bodies.
The findings, which also reveal the extent of geologic
activity in Occator Crater, appear in a special collection of papers published
by Nature
Astronomy, Nature Geoscience, and Nature
Communications on Aug. 10.
“Dawn accomplished far more than we hoped when it
embarked on its extraordinary extraterrestrial expedition,” said Mission
Director Marc Rayman of NASA’s Jet Propulsion Laboratory in Southern
California. “These exciting new discoveries from the end of its long and
productive mission are a wonderful tribute to this remarkable interplanetary
explorer.”
Solving the Bright Mystery
Long before Dawn arrived at Ceres in 2015, scientists
had noticed diffuse bright regions with telescopes, but their nature was
unknown. From its close orbit, Dawn captured images of two distinct, highly
reflective areas within Occator Crater, which were subsequently named Cerealia
Facula and Vinalia Faculae. (“Faculae” means bright areas.)
Scientists knew that micrometeorites frequently pelt
the surface of Ceres, roughing it up and leaving debris. Over time, that sort
of action should darken these bright areas. So their brightness indicates that
they likely are young. Trying to understand the source of the areas, and how
the material could be so new, was a main focus of Dawn’s final extended
mission, from 2017 to 2018.
The research not only confirmed that the bright
regions are young — some less than 2 million years old; it also found that the
geologic activity driving these deposits could be ongoing. This conclusion
depended on scientists making a key discovery: salt compounds (sodium chloride
chemically bound with water and ammonium chloride) concentrated in Cerealia
Facula.
On Ceres’ surface, salts bearing water quickly
dehydrate, within hundreds of years. But Dawn’s measurements show they still
have water, so the fluids must have reached the surface very recently. This is
evidence both for the presence of liquid below the region of Occator Crater and
ongoing transfer of material from the deep interior to the surface.
The scientists found two main pathways that allow
liquids to reach the surface. “For the large deposit at Cerealia Facula, the
bulk of the salts were supplied from a slushy area just beneath the surface
that was melted by the heat of the impact that formed the crater about 20
million years ago,” said Dawn Principal Investigator Carol Raymond. “The impact
heat subsided after a few million years; however, the impact also created large
fractures that could reach the deep, long-lived reservoir, allowing brine to
continue percolating to the surface.”
Active Geology: Recent and Unusual
In our solar system, icy geologic activity happens
mainly on icy moons, where it is driven by their gravitational interactions
with their planets. But that’s not the case with the movement of brines to the
surface of Ceres, suggesting that other large ice-rich bodies that are not
moons could also be active.
Some evidence of recent liquids in Occator Crater
comes from the bright deposits, but other clues come from an assortment of
interesting conical hills reminiscent of Earth’s pingos — small ice mountains
in polar regions formed by frozen pressurized groundwater. Such features have
been spotted on Mars, but the discovery of them on Ceres marks the first time
they’ve been observed on a dwarf planet.
On a larger scale, scientists were able to map the
density of Ceres’ crust structure as a function of depth — a first for an
ice-rich planetary body. Using gravity measurements, they found Ceres’ crustal
density increases significantly with depth, way beyond the simple effect of
pressure. Researchers inferred that at the same time Ceres’ reservoir is
freezing, salt and mud are incorporating into the lower part of the crust.
Dawn is the only spacecraft ever to orbit two
extraterrestrial destinations — Ceres and the giant asteroid Vesta — thanks to
its efficient ion propulsion system. When Dawn used the last of a key fuel,
hydrazine, for a system that controls its orientation, it was neither able to
point to Earth for communications nor to point its solar arrays at the Sun to
produce electrical power. Because Ceres was found to have organic materials on
its surface and liquid below the surface, planetary protection rules required
Dawn to be placed in a long-duration orbit that will prevent it from impacting
the dwarf planet for decades.
