Dawn mission reveals new clues about Ceres’ mysterious bright spots
Two groups of researchers using data from NASA’s Dawn spacecraft have announced new findings about Ceres, including new insights into the mysterious bright spots found on the dwarf planet’s surface. One study indicates that the bright material on the surface is a type of salt. The other study discovered evidence of ammonia-rich clays on Ceres. Both studies were published in the journal Nature.
Ceres has more than 130 bright spots, most of which are associated with impact craters. A team of scientists, led by Andreas Nathues at Max Plank Institute for Solar System Research in Göttingen, Germany, found that the bright material is consistent with a type of magnesium sulfate called hexahydrite. A different type of magnesium sulfate found on Earth is known as Epsom salt.
The researchers, using images from Dawn’s framing camera, suggest that salt-rich areas were left behind when water-ice sublimated in the past. Impacts from asteroids may have unearthed the mixture of ice and salt.
“The global nature of Ceres’ bright spots suggests that this world has a subsurface layer that contains briny water-ice,” Nathues said.
The majority of the surface of Ceres, which measures some 584 miles (940 kilometers) in diameter, is about as dark as fresh asphalt. The bright spots very greatly in brightness, with the brightest areas reflecting approximately 50 percent of the sunlight shining on the area. There has not yet been any unambiguous detection of water-ice on Ceres; higher-resolution data will be required to settle the question.
The brightest material on Ceres is found inside a crater called Occator. Occator is 60 miles (90 kilometers) in diameter and its central pit, which is covered by the bright material, measures about 6 miles (10 kilometers) wide and 0.3 miles deep. With its sharp rim and walls, Occator seems to be one of the youngest features on Ceres. Dawn mission scientists estimate that the crater is approximately 78 million years old.
Some views of Occator seem 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. Further data and analysis may reveal clues about the source of the haze.
“The Dawn science team is still discussing these results and analyzing data to better understand what is happening at Occator,” said Chris Russell, principal investigator of the Dawn mission, based at the University of California, Los Angeles.
In the second study published in Nature, researchers with the Dawn science team examined the composition of Ceres and found evidence of ammonia-rich clays. The researchers used data from Dawn’s visible and infrared mapping spectrometer, a device that looks at how different wavelengths of light are reflected by the surface, allowing minerals to be identified.
Ammonia ice would evaporate on Ceres today because the dwarf planet is too warm. Ammonia molecules could be stable if they were chemically bonded to another mineral.
The presence of the ammonia-rich clays raises the possibility that Ceres did not originate in the main asteroid belt between Mars and Jupiter, where it is currently, but might have formed in the outer Solar System. Another possibility is that Ceres formed close to its current location but incorporated materials that drifted in from the outer Solar System.
“The presence of ammonia-bearing species suggests that Ceres is composed of material accreted in an environment where ammonia and nitrogen were abundant. Consequently, we think that this material originated in the outer cold Solar System,” said Maria Cristina De Sanctis, lead author of the study, based at the National Institute of Astrophysics, Rome.
As of this week, Dawn has reached its final orbital altitude at Ceres, approximately 240 miles (385 kilometers) and the surface of the dwarf planet. In mid-December, Dawn will begin taking observations from this orbit, including images at a resolution of 120 feet (35 meters) per pixel, infrared, gamma ray, and neutron spectra, as well as high-resolution gravity data.
Video courtesy of NASA/JPL
Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.