Mystery Solved? Odd Bright Spots on Ceres Explained as Salt Deposits

False color view of Occator crater on Ceres, showing the unusual bright spots. The image was taken by the framing camera on NASA’s Dawn spacecraft from a distance of about 2,700 miles (4,400 kilometers). Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

There now might be a definitive answer to a puzzle which has intrigued both scientists and the public for some time: What are those odd bright spots on the dwarf planet Ceres? A new study suggests they are a type of salt, originating from a subsurface layer of briny water-ice. Another study points to the existence of ammonia-rich clays on Ceres.

The two studies, based on data from the Dawn spacecraft which is still orbiting Ceres, have been published in the journal Nature—here and here.

 The bright spots have been a subject of much speculation. Are they ice? Salt? Something else? The brightest ones, in Occator crater, appeared the most enigmatic, almost looking like lights of an alien city. The answer may not be quite that exciting, but it is still a fascinating clue as to how Ceres formed and evolved.

There are over 130 such bright spots in all, with most of them being inside impact craters on Ceres, which is only about 587 miles (945 kilometers) in diameter. The new study, led by Andreas Nathues at Max Planck Institute for Solar System Research in Göttingen, Germany, has shown that the bright deposits are consistent with a type of magnesium sulfate called hexahydrite. Epsom salt is another type of magnesium sulfate. So how did they form? Nathues suggests that the salt deposits were left behind in places where the water-ice sublimated off the surface, since Ceres has no atmosphere to speak of, after being exposed by meteorite or asteroid impacts.

“The global nature of Ceres’ bright spots suggests that this world has a subsurface layer that contains briny water-ice,” Nathues said. He notes, however, that “The whole picture we do not have yet.”

The brightest deposits seen are in the Occator crater, one of the larger craters on Ceres. A central pit, about 6 miles (10 kilometers) wide and 0.3 miles (0.5 kilometers) deep, is covered in the bright stuff. Occator itself is 60 miles (90 kilometers) in diameter. Other than the bright spots, Ceres’ surface is quite dark overall, similar to fresh asphalt. The brightest spots reflect about 50 percent of the Sun’s light that hits them. Occator is also thought to be quite young, geologically speaking (estimated to be 78 million years old), with its still-sharp rim and walls. There are also other terraces, landslides, dark streaks, and the remnants of a central peak visible.

Occator crater is also interesting because what appears to be a diffuse, misty haze has been observed which hovers near the surface and fills the inside of the crater. How this haze forms isn’t known yet, but it might be associated with previous observations from the Herschel Space Observatory in 2014. The haze is visible at noon on Ceres, but is not seen at dawn or dusk. The process may be similar to what happens on comets, when water vapor lifts tiny particles of dust and ice off the surface.

From the new paper:

“These unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible. Of particular interest is a bright pit on the floor of crater Occator that exhibits probable sublimation of water ice, producing haze clouds inside the crater that appear and disappear with a diurnal rhythm. Slow-moving condensed-ice or dust particles, may explain this haze. We conclude that Ceres must have accreted material from beyond the ‘snow line’11, which is the distance from the Sun at which water molecules condense.”

Oblique view of Occator crater and the bright spots on Ceres. Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

According to Chris Russell, principal investigator of the Dawn mission, at the University of California, Los Angeles, “The Dawn science team is still discussing these results and analyzing data to better understand what is happening at Occator.”

He added, as reported in Gizmodo: “We believe this is a huge salt deposit. We know it’s not ice and we’re pretty sure it’s salt, but we don’t know exactly what salt at the present time.”

While the spots may be better understood now, there is also another finding of importance to planetary scientists: evidence for ammonia-rich clays, detected by the visible and infrared mapping spectrometer on the spacecraft. In order to remain on Ceres’ surface, ammonia ice would need to be chemically bound to other minerals, otherwise it would just evaporate. The finding suggests that Ceres may not have formed where it is now, in the main asteroid belt. Instead, it may have originated from farther out in the Solar System, where nitrogen ices are abundant.

“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, at the National Institute of Astrophysics in Rome.

Indeed, the spectrum of reflected light from Ceres is similar to that of some meteorites. There were differences, however, too. Carbonaceous chondrites, a type of carbon-rich meteorite thought to be similar in composition to Ceres, were not good matches for all wavelengths that were sampled by Dawn’s instruments. Some distinctive absorption bands were found to match mixtures containing ammoniated minerals, but those wavelengths can’t be seen by Earth-based telescopes. It was also noted that carbonaceous chondrites have bulk water contents of 15 to 20 percent, while Ceres’ content can be as much as 30 percent.

“Ceres may have retained more volatiles than these meteorites, or it could have accreted the water from volatile-rich material,” De Sanctis said.

As for ice, there doesn’t seem to be much, if any, on Ceres’ surface. Even at its distance from the Sun, the temperature at and near the equator is too warm for ice to last long, from about -136 degrees to -28 degrees Fahrenheit (180 to 240 Kelvin). Ice was previously a leading contender for explaining the bright spots, but now there seems to be little of it on the surface.

Color-coded map of Ceres. Occator crater, with the brightest spots, is middle-right of center. Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

From the new paper:

“Our measurements indicate widespread ammoniated phyllosilicates across the surface, but no detectable water ice. Ammonia, accreted either as organic matter or as ice, may have reacted with phyllosilicates on Ceres during differentiation. This suggests that material from the outer Solar System was incorporated into Ceres, either during its formation at great heliocentric distance or by incorporation of material transported into the main asteroid belt.”

This week is also very significant for the Dawn mission, since Dawn has now reached its final lowest orbit, called the Low-Altitude Mapping Orbit (LAMO), at an average altitude of 240 miles (385 kilometers) above the surface. Dawn will next use its ion engine to make a small adjustment to the orbit, a “trajectory correction maneuver,” so that the spacecraft’s new orbit matches observational plans. The opportunity window for this adjustment is from Dec. 11-13. Dawn will begin taking the highest resolution images of Ceres yet, starting in mid-December, with a resolution of 120 feet (35 meters) per pixel. The spacecraft will also make observational measurements of infrared, gamma ray, and neutron spectra, as well as high-resolution gravity data.

The highest-resolution view so far of the bright spots in Occator crater. New evidence suggests they are salt deposits. Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

As reported earlier on AmericaSpace, Dawn’s previous third orbit was called the High-Altitude Mapping Orbit (HAMO), which it reached last Aug. 17. and surveyed Ceres from an altitude of approximately 915 miles (1,470 kilometers). From that orbit, NASA release a stunning video of a 3-D “cruise” over Ceres’ surface, which included views of the odd solitary mountain, dubbed “The Lonely Mountain,” which looks like a massive cone about 4 miles (6 kilometers) tall.

As Dawn science team member Paul Schenk, a geologist at the Lunar and Planetary Institute in Houston, Texas, described it: “This mountain is among the tallest features we’ve seen on Ceres to date. It’s unusual that it’s not associated with a crater. Why is it sitting in the middle of nowhere? We don’t know yet, but we may find out with closer observations.”

The Dawn team had also previously released a new map of Ceres, with new feature names, all eponymous for agricultural spirits, deities, and festivals from cultures around the world, including Jaja, after the Abkhazian harvest goddess, and Ernutet, after the cobra-headed Egyptian harvest goddess. A 12-mile-diameter (20-kilometer) mountain near Ceres’ north pole is now called Ysolo Mons, for an Albanian festival that marks the first day of the eggplant harvest.

The isolated conical mountain nicknamed “The Lonely Mountain,” which is about 4 miles (6 kilometers) tall. How did it form? Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Ceres is an unusual little world, which is starting to reveal its mysteries for the first time, providing new clues as to how dwarf planets, such as Ceres and Pluto, formed and evolved in our Solar System.
“Ceres continues to amaze, yet puzzle us, as we examine our multitude of images, spectra and now energetic particle bursts,” said Chris Russell, Dawn principal investigator at the University of California, Los Angeles.
Dawn was the first spacecraft to visit a dwarf planet (before New Horizons reached Pluto) and the first to orbit two different Solar System bodies, initially the asteroid Vesta from 2011-2102 and now Ceres in 2015.