June 29, 2016

Recent Hydrothermal Activity May Explain Ceres' Brightest Area

The brightest area on Ceres, located in the mysterious Occator Crater, has the highest concentration of carbonate minerals ever seen outside Earth, according to a new study from scientists on NASA's Dawn mission. The study, published online in the journal Nature, is one of two new papers about the makeup of Ceres.

"This is the first time we see this kind of material elsewhere in the solar system in such a large amount," said Maria Cristina De Sanctis, lead author and principal investigator of Dawn's visible and infrared mapping spectrometer. De Sanctis is based at the National Institute of Astrophysics, Rome.

At about 80 million years old, Occator is considered a young crater. It is 57 miles (92 kilometers) wide, with a central pit about 6 miles (10 kilometers) wide. A dome structure at the center, covered in highly reflective material, has radial and concentric fractures on and around it.

De Sanctis' study finds that the dominant mineral of this bright area is sodium carbonate, a kind of salt found on Earth in hydrothermal environments. This material appears to have come from inside Ceres, because an impacting asteroid could not have delivered it. The upwelling of this material suggests that temperatures inside Ceres are warmer than previously believed. Impact of an asteroid on Ceres may have helped bring this material up from below, but researchers think an internal process played a role as well.

More intriguingly, the results suggest that liquid water may have existed beneath the surface of Ceres in recent geological time

The center of Ceres' mysterious Occator Crater is the brightest area on the dwarf planet. The inset perspective view is overlaid with data concerning the composition of this feature: Red signifies a high abundance of carbonates, while gray indicates a low carbonate abundance.

Dawn's visible and infrared mapping spectrometer (VIR) was used to examine the composition of the bright material in the center of Occator. Using VIR data, researchers found that the dominant constituent of this bright area is sodium carbonate, a kind of salt found on Earth in hydrothermal environments. Scientists determined that Occator represents the highest concentration of carbonate minerals ever seen outside Earth. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/ASI/INAF

Surface of Ceres has less water ice to explain craters

Researchers modeled how deep craters would evolve over time on different areas of Ceres (as on Earth, the poles of the dwarf planet are colder than the equatorial regions). While it's possible to have an icy Ceres that supports old craters near the poles, most of the rest of the planet would see the viscosity of water ice allow the interior of the crater to slowly deform. After sufficient time, even deep craters would be reduced to depths of less than 500m if water ice is the dominant material on Ceres. A 100km-wide crater would no longer be visible after just 10 million years.

They then compared that to a catalog of actual crater depths as mapped by Dawn and showed that there's a large discrepancy: many of the actual craters, including some quite old ones, are very deep. In fact, they calculate that the persistence of these craters requires that whatever comprises Ceres' crust has to be over 100 times more viscous than water ice. "Ceres’ outer layer is therefore probably relatively ice poor, with non-ice material constituting 60–70 percent of the volume," the authors conclude.

The typically dark surface of the dwarf planet Ceres is punctuated by areas of much higher albedo, most prominently in the Occator crater. These small bright areas have been tentatively interpreted as containing a large amount of hydrated magnesium sulfate, in contrast to the average surface, which is a mixture of low-albedo materials and magnesium phyllosilicates, ammoniated phyllosilicates and carbonates. Here we report high spatial and spectral resolution near-infrared observations of the bright areas in the Occator crater on Ceres. Spectra of these bright areas are consistent with a large amount of sodium carbonate, constituting the most concentrated known extraterrestrial occurrence of carbonate on kilometre-wide scales in the Solar System. The carbonates are mixed with a dark component and small amounts of phyllosilicates, as well as ammonium carbonate or ammonium chloride. Some of these compounds have also been detected in the plume of Saturn’s sixth-largest moon Enceladus. The compounds are endogenous and we propose that they are the solid residue of crystallization of brines and entrained altered solids that reached the surface from below. The heat source may have been transient (triggered by impact heating). Alternatively, internal temperatures may be above the eutectic temperature of subsurface brines, in which case fluids may exist at depth on Ceres today

Occator crater on Ceres

Nature - Bright carbonate deposits as evidence of aqueous alteration on (1) Ceres

SOURCES- Nature, NASA, JPL, Caltech

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