Water Rich World

High-resolution observations from NASA’s Dawn spacecraft of mysterious bright spots (faculae) in Occator crater on the dwarf planet Ceres suggest the existence of a brine reservoir — which is about 40 km (25 miles) deep and hundreds of km wide — that emerged to the surface through long-lived cryovolcanic activity as a consequence of the impact that created the crater.

This false-color image shows the dwarf planet Ceres. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.

This false-color image shows the dwarf planet Ceres. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.

“Dawn accomplished far more than we hoped when it embarked on its extraordinary extraterrestrial expedition,” said Dawn’s mission director Dr. Marc Rayman, a researcher at NASA’s Jet Propulsion Laboratory.

“These exciting new discoveries from the end of its long and productive mission are a wonderful tribute to this remarkable interplanetary explorer.”

Long before Dawn arrived at Ceres in 2015, astronomers had noticed diffuse bright regions on the dwarf planet 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.

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 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 researchers 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 Dr. 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.”

Dawn captured pictures in visible and infrared wavelengths, which were combined to create this false-color view of a region in 92-km- (57-mile-) wide Occator crater on the dwarf planet Ceres. Recently exposed brine in the center of the crater was pushed up from a deep reservoir below Ceres’ crust. In this view, it appears reddish. In the foreground, is Cerealia Facula, a 15-km- (9-mile-) wide region with a composition dominated by salts. The central dome, Cerealia Tholus, is about 3 km (1.9 miles) across at its base and 340 m (1,100 feet) tall. The dome is inside a central depression about 900 m (3,000 feet) deep. The area depicted in this scene is about 2.1 km (1.3 miles) wide in the foreground, about 11 km (7 miles) wide across the dome, and 56 km (35 miles) wide in the background, where the crater rim rises against the black sky. The distance from the near point (at the bottom) to the far point (at the top) is about 52 km (32 miles). Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.

Dawn captured pictures in visible and infrared wavelengths, which were combined to create this false-color view of a region in 92-km- (57-mile-) wide Occator crater on the dwarf planet Ceres. Recently exposed brine in the center of the crater was pushed up from a deep reservoir below Ceres’ crust. In this view, it appears reddish. In the foreground, is Cerealia Facula, a 15-km- (9-mile-) wide region with a composition dominated by salts. The central dome, Cerealia Tholus, is about 3 km (1.9 miles) across at its base and 340 m (1,100 feet) tall. The dome is inside a central depression about 900 m (3,000 feet) deep. The area depicted in this scene is about 2.1 km (1.3 miles) wide in the foreground, about 11 km (7 miles) wide across the dome, and 56 km (35 miles) wide in the background, where the crater rim rises against the black sky. The distance from the near point (at the bottom) to the far point (at the top) is about 52 km (32 miles). Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.

In the 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, the Dawn 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.

The authors inferred that at the same time Ceres’ reservoir is freezing, salt and mud are incorporating into the lower part of the crust.

The findings were published in a series of papers in the journals Nature Astronomy, Nature Geoscience and Nature Communications.

_____

C.A. Raymond et al. 2020. Impact-driven mobilization of deep crustal brines on dwarf planet Ceres. Nat Astron 4, 741-747; doi: 10.1038/s41550-020-1168-2

M.C. De Sanctis et al. 2020. Fresh emplacement of hydrated sodium chloride on Ceres from ascending salty fluids. Nat Astron 4, 786-793; doi: 10.1038/s41550-020-1138-8

B.E. Schmidt et al. Post-impact cryo-hydrologic formation of small mounds and hills in Ceres’s Occator crater. Nat. Geosci, published online August 10, 2020; doi: 10.1038/s41561-020-0581-6

J.E.C. Scully et al. 2020. The varied sources of faculae-forming brines in Ceres’ Occator crater emplaced via hydrothermal brine effusion. Nat Commun 11, 3680; doi: 10.1038/s41467-020-15973-8

A. Nathues et al. 2020. Recent cryovolcanic activity at Occator crater on Ceres. Nat Astron 4, 794-801; doi: 10.1038/s41550-020-1146-8

R.S. Park et al. 2020. Evidence of non-uniform crust of Ceres from Dawn’s high-resolution gravity data. Nat Astron 4, 748-755; doi: 10.1038/s41550-020-1019-1

P. Schenk et al. 2020. Impact heat driven volatile redistribution at Occator crater on Ceres as a comparative planetary process. Nat Commun 11, 3679; doi: 10.1038/s41467-020-17184-7

This article is based on text provided by the National Aeronautics and Space Administration.

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