It’s Starting to Look Like Ceres is an Ocean World, Too

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It’s Starting to Look Like Ceres is an Ocean World, Too

A new and thorough analysis of high-resolution images and data from
NASA’s Dawn mission have now provided fresh insights into the dwarf
planet Ceres, with intriguing evidence that Ceres has a global
subsurface salty ocean, and has been geologically active in the recent past.

“Evidence that Ceres has long-lived deep brine reservoirs is an exciting
result,� Dr. Hannah Sizemore told Universe Today. Sizemore is a
co-author on five of seven papers published this week in various
journals in Nature, detailing Dawn’s explorations of Ceres. “That a body
of Ceres’ size can retain both heat and interior liquids over the age of
the solar system implies that small bodies are more geologically active
— may be more ‘habitable’ — than we thought.�

Dawn traveled to the asteroid belt to orbit both the asteroid Vesta and
Ceres. It studied Ceres for over three and a half years – from March
2015 to November 2018 — until the spacecraft’s hydrazine maneuvering
fuel was depleted. At closest approach, the orbiter had dipped to less
than 22 miles (35 kilometers) above the surface. The spacecraft spotted
intriguing landforms and features across Ceres that showed it to be a
unique and varied world.

Images of Occator Crater, seen in false-color, were pieced together to
create this animated view. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
A bright area in a crater called Occator was among the most captivating
features. Hinted at more than a decade ago by the Hubble Space
Telescope, Dawn scientists concluded the mysterious bright spots were
sodium carbonate – a compound of sodium, carbon, and oxygen. The
compounds likely came from liquid that percolated up to the surface and
evaporated, leaving behind a salt crust that was highly reflective.

But by the mission’s end, scientists still hadn’t determined where the
liquids came from: did it come from deep inside the dwarf planet and
bubble up to the surface in a volcanic process? Or did the impact that
created the crater form a shallow melt that refroze to create the bright
features.

The new research, using images and gravity data from Dawn, suggests that
both may have happened.

“The gravity data tells us there is probably a deep reservoir of brine –
salty water – about 40 kilometers below Occator,� said Sizemore, who is
a Senior Scientist at the Planetary Science Institute. “Some of that
deep water likely erupted on the surface and contributed to bright spots
and other features in Occator.�

On the other hand, Sizemore continued, image-based analysis of the large
flows and small hills inside the crater suggests that shallow sheets of
muddy impact melt flowed around the inside of the crater.


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
“This formed interesting features as they refroze,� Sizemore said. “Both
of these are exciting, because it means there were transient and
long-lived sources of water in this area, and extensive mixing. Liquid
water and mixing are always exciting for astrobiology.�



The bright conical hills and knobs are similar to small ice hills in
Earth’s polar regions that are formed by frozen pressurized groundwater.
These features in Occator crate would require the movement of water
and/or ice slurries to form, and this activity must have gone on for a
long time after the impact that created the crater.

But if Ceres has a subsurface ocean, how could it remain warm enough to
keep it liquid, since the dwarf planet doesn’t experience tidal forces
from a large planet like some of the moons around Saturn or Jupiter?


This diagram shows Ceres in comparison to other suspected ocean worlds
in the solar system, including Saturn’s moon Enceladus and Jupiter’s
moons Europa and Callisto.
Top left: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA; Top right: NASA /
JPL-Caltech; Bottom: NASA / JPL
“Most of Ceres’ subsurface ocean is frozen today,� Sizemore said in an
email, “with relatively small amounts of remnant liquid. Retaining any
liquid is challenging, given that tidal heating doesn’t occur. We think
that clathrate hydrates (water ice with gases such as methane trapped in
the crystal lattice) make Ceres’ crust very insulating. Clathrates
effectively act like a blanket that allows the dwarf planet to hang on
its interior heat.�



Ceres has over 130 of these bright areas, most inside impact craters.
The global nature of these bright regions suggested early on to Dawn
scientists that Ceres had a subsurface layer of briny water ice, and the
impacts that formed the craters would have ‘unearthed’ the mixture of
ice and salt.

Ceres has a diameter of about 600 miles (960 km), giving it a size
equivalent to 37% of the land area of the continental United States.
Occator crater is 57 miles (92 kilometers) across, and Sizemore said the
bright spots within the crater formed much more recently, perhaps less
than 20 million years ago.


Bright reflective material in Ceres’ Occator crater, imaged by NASA’s
Dawn spacecraft in Sept .2015. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
The brightest area at the center of the crater, called Cerealia Facula,
was found to have hydrated chloride salts. In a study led by Maria
Cristina De Sanctis, she and her team found that since these salts
dehydrate very quickly, the brines may still be percolating to the
surface, indicating that salty fluids could still exist in the interior
of the dwarf planet. In another paper, Andreas Nathues and colleagues
found that Ceres underwent a period of cryovolcanic activity starting
around nine million years ago, and persisted until very recently.



Another paper, published by Britney Schmidt and colleagues in Nature
Geoscience, show that the mounds and hills in Occator crater may have
formed when impact-induced water flows froze. This suggests that
cryo-hydrologic processes extend beyond Earth and Mars, and were active
on Ceres in the geologically recent past.

“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.�

Dawn at Ceres
An artist’s conception shows NASA’s Dawn spacecraft flying above Ceres.
This view incorporates actual imagery from the Dawn mission. Credit:
NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
You can read the full slate of papers on Ceres published this week,
found he https://www.nature.com/collections/agdgfadcag



Castillo-Rogez, J. C. & Rayman, M. D. Nat. Astron. https://doi.
org/10.1038/s41550-020-1031-5 (2020).
De Sanctis, M. C. et al. Nature 536, 54–57 (2016).
Nathues, A. et al. Nat. Astron. https://doi.org/10.1038/s41550-
020-1146-8 (2020).
De Sanctis, M. C. et al. Nat. Astron. https://doi.org/10.1038/
s41550-020-1138-8 (2020).
Scully, J. E. C. et al. Nat. Commun. https://doi.org/10.1038/
s41467-020-15973-8 (2020).
Raymond, C. A. et al. Nat. Astron. https://doi.org/10.1038/s41550-
020-1168-2 (2020).
Fu, R. R. et al. Earth Planet. Sci. Lett. 476, 153–164 (2017).
Hendrix, A. R. et al. Astrobiology 19, 1–27 (2019).
Sources: Planetary Science Institute, Nature, JPL, email correspondence
with Dr. Hannah Sizemore

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