The mysterious white spots on Ceres have stepped up their game: from certain angles at certain times of day, they apparently produce a haze that partially covers their crater. If the observations are confirmed, it suggests the odd markings are ice directly sublimating into crater-filling haze.
The bright white spots speckle Ceres, but the haze only appears over Occator Crater. Places on Ceres are named for agricultural gods. Occator Crater is named for a Roman god of the harrowing, a suitably ominous namesake for a crater holding mysterious spots.
Ceres is coated in craters, but under those scars is a history of former geological activity. The spacecraft has found landslides, flows, curving canyons, and structures that collapsed long ago. But the most fascinating features by far are the bright, white spots that have been winking at us for months. The distribution of large and small white spots around the dwarf planet doesn’t match up what we’d expect if they were solely the result of normal craters punching through a thin surface veneer to reveal the underlaying material.
The largest white spot is 9 kilometers (6 miles) wide tucked into Occator Crater, with at least eight smaller irregular splotches clustered in the same crater. Planetary scientist Christopher Russell reported about the suspected haze at the NASA exploration meeting at the Ames Research Center on July 21, 2015, saying:
At noontime, if you look at a glancing angle, you can see what seems to be haze. It comes back in a regular pattern.
Bright spots within Occator Crater seen from an altitude of 4,400 kilometers (2,700 miles) on June 9, 2015 at a resolution of 410 meters (0.25 miles) per pixel. Image credit: NASA/JPL/UCLA/MPS/DLR/IDA
The apparent haze half-fills Occator Crater, stopping at the rim, and has reoccured at the same time of day on multiple occasions. If he’s right, this will confirm 2014 observations by the Herschel Space Observatory of water vapour spraying out of Ceres. It would also support the hypothesis that the white spots are ice, and the haze is sublimating ice creating a miniature, temporary pocket of atmosphere on the dwarf planet.
This isn’t a sure thing yet. In a follow-up interview, Russell clarified:
I was speaking from less than a handful of images, and the interpretation of the images is disputed by some team members. I would like the debate to go on internally before we make a pronouncement one way or the other. I of course have my personal opinion, but I am not always right.
Originally, Russell thought the spots might be ice, but when they turned out to have a lower albedo than anticipated at just 50%, he revised his hypothesis to salt. He continued with:
Eventually I am expecting the spectral data will unambiguously tell us what has happened to the surface, but it is a little too soon to be sure.
The mystery won’t be lasting much longer: researchers will be able to identify specific minerals just as soon as the Dawn spacecraft points its spectrometer at the spots. The spectrometer will probe the spots with visible and infrared light; every mineral reflects light in a specific, diagnostic manner. For now, the best guess is that the spots are ice, salt evaporates, or something else entirely.
The Dawn spacecraft visited the massive asteroid and protoplanet Vesta in 2011 and 2012. Its ion engine redirected the spacecraft to a secondary target in the main asteroid belt Ceres, arriving earlier this year. This means that Ceres beat out Pluto as the first explored dwarf planet by just a few months.
Dawn has discovered a few more normal tidbits about Ceres so far. The dwarf planet is slightly smaller than estimated, which brings up its density slightly. The tilt of the world is also off of where we anticipated, resulting in different lighting conditions resulting in adjusting mission plans.
Dawn has taken a spiralling orbital tour of the solar system since launching in September 2007. Image credit: NASA/JPL-Caltech
Dawn is currently spiralling closer to Ceres, with a target of grazing just 1,500 kilometers above the dwarf planet by August.
Top image: Ceres photographed on May 7, 2015 when the Dawn spacecraft was 13,600 kilometers (8,400 miles) above the dwarf planet. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA