Mystery Solved: Bright Areas on Ceres Come >From Salty Water Below
Data from NASA's recent Dawn mission answers two long-unresolved questions: Is there liquid inside Ceres, and how long ago was the dwarf planet geologically active?
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.
This mosaic image uses false color to highlight the recently exposed brine, or salty liquids, that were pushed up from a deep reservoir under Ceres' crust. In this view of a region of Occator Crater, they appear reddish. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
› Full image and caption
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."
This mosaic of Ceres' Occator Crater is composed of images NASA's Dawn mission captured on its second extended mission, in 2018. Bright pits and mounds (foreground) were formed by salty liquid released as Occator's water-rich floor froze after the crater-forming impact about 20 million years ago. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/USRA/LPI
› Full image and caption
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.
JPL, a division
of Caltech in Pasadena, California, manages Dawn's mission for NASA's Science
Mission Directorate in Washington. Dawn is a project of the directorate's
Discovery Program, managed by NASA's Marshall Space Flight Center in
Huntsville, Alabama. JPL is responsible for overall Dawn mission science.
Northrop Grumman in Dulles, Virginia, designed and built the spacecraft. The
German Aerospace Center, Max Planck Institute for Solar System Research,
Italian Space Agency and Italian National Astrophysical Institute are
international partners on the mission team.
For a complete
list of mission participants, visit:
https://solarsystem.nasa.gov/missions/dawn/overview/
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