This representation of Ceres’ Occator Crater in false colors shows differences in the surface composition. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
2 new studies from NASA’s Dawn spacecraft show insights about mysterious bright features found all over the dwarf planet’s surface. In one study, scientists identify this bright material as a kind of salt. The 2nd study suggests the detection of ammonia-rich clays, raising questions about how Ceres formed.
Ceres has more than 130 bright areas, and most of them are associated with impact craters. The bright material is consistent with a type of magnesium sulfate, hexahydrite.
Nathues and colleagues, using images from Dawn’s framing camera, suggest these salt-rich areas were left behind when water-ice sublimated in the past. Impacts from asteroids would have unearthed the mixture of ice and salt, they say.
“The global nature of Ceres’ bright spots suggests that this world has a subsurface layer that contains briny water-ice,” Nathues said.
The surface of Ceres, 584 miles diameter, is generally dark, like fresh asphalt. The bright patches that pepper the surface represent a large range of brightness, with the brightest areas reflecting about 50% of sunlight shining on the area. But there has not been unambiguous detection of water ice on Ceres; higher-resolution data are needed to settle this question.
Oxo Crater, which is about 6 miles (9 kilometers) in diameter, is the second-brightest feature on Ceres. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
The inner portion of a crater called Occator contains the brightest material on Ceres. Occator itself is 60 miles diameter, and its central pit, covered by this bright material, measures about 6 miles wide and 0.3 miles deep. Dark streaks, possibly fractures, traverse the pit. Remnants of a central peak, which was up to 0.3 miles high, can also be seen. With its sharp rim and walls, and abundant terraces and landslide deposits, Occator appears to be among the youngest features on Ceres., about 78 million years old.
Views of Occator appear to show a diffuse haze near the surface that fills the floor of the crater. This may be associated with observations of water vapor at Ceres by the Herschel space observatory that were reported in 2014. The haze seems to be present in views during noon, local time, and absent at dawn and dusk, study authors write. This suggests that the phenomenon resembles the activity at the surface of a comet, with water vapor lifting tiny particles of dust and residual ice. Future data and analysis may test this hypothesis and reveal clues about the process causing this activity.
An image of Occator Crater draped over a digital terrain model provides a 3-D-like perspective view of the impact structure. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/ID
In the 2nd Nature study, members of the Dawn science team examined the composition of Ceres and found evidence for ammonia-rich clays. They used data from the visible and infrared mapping spectrometer, a device that looks at how various wavelengths of light are reflected by the surface, allowingminerals to be identified. Ammonia ice by itself would evaporate on Ceres today, because the dwarf planet is too warm. However, ammonia molecules could be stable if present in combination with (i.e. chemically bonded to) other minerals.
Ammoniated compounds raises the possibility that Ceres did not originate in the main asteroid belt between Mars and Jupiter, where it currently resides, but instead might have formed in the outer solar system where NH3 and N are abundant. Another idea is that Ceres formed close to its present position, incorporating materials that drifted in from the outer solar system – near orbit of Neptune, where M ices are thermally stable.
A group of scientists from NASA’s Dawn mission suggests that when sunlight reaches Ceres’ Occator Crater, a kind of thin haze of dust and evaporating water forms there. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
In comparing the spectrum of reflected light from Ceres to meteorites, scientists found some similarities. Specifically, they focused on the spectra, or chemical fingerprints, of carbonaceous chondrites, a type of carbon-rich meteorite thought to be relevant analogues for the dwarf planet. But these are not good matches for all wavelengths that the instrument sampled, the team found. In particular, there were distinctive absorption bands, matching mixtures containing ammoniated minerals, associated with wavelengths that can’t be observed from Earth-based telescopes. Nnother difference is that these carbonaceous chondrites have bulk water contents of 15- 20%, while Ceres’ content is as much as 30%. “Ceres may have retained more volatiles than these meteorites, or it could have accreted the water from volatile-rich material,” De Sanctis said.
The study also shows that daytime surface temperatures on Ceres are from -136 to -28F (180 to 240 Kelvin). The maximum temperatures were measured in the equatorial region. The temperatures at and near the equator are generally too high to support ice at the surface for a long time, study authors say, but data from Dawn’s next orbit will reveal more details.
In mid-December, Dawn will begin taking observations from this orbit, including images at a resolution of 120 feet per pixel, infrared, gamma ray and neutron spectra, and high-resolution gravity data.
http://www.jpl.nasa.gov/news/news.php?feature=4785
Recent Comments