Mystery solved: Bright areas on Ceres come from salty water below

Date:
August 11, 2020
Source:
NASA/Jet Propulsion Laboratory
Summary:
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?


FULL STORY ========================================================================== [Illustration of dwarf | Credit: (c) Martin / stock.adobe.com]
Illustration of dwarf planet Ceres (stock image).

Credit: (c) Martin / stock.adobe.com [Illustration of dwarf | Credit:
(c) Martin / stock.adobe.com] Illustration of dwarf planet Ceres
(stock image).

Credit: (c) Martin / stock.adobe.com Close 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.

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/

========================================================================== Story Source: Materials provided by NASA/Jet_Propulsion_Laboratory. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Images_of_Occator_Crater,_animations,_orbit_of_Ceres ==========================================================================


Link to news story: https://www.sciencedaily.com/releases/2020/08/200811134900.htm

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