[meteorite-list] Mystery Haze Appears Above Ceres' Bright Spots

[Ron Baalke]

http://www.nature.com/news/mystery-haze-appears-above-ceres-bright-spots-1.18032

Mystery haze appears above Ceres' bright spots

Discovery bolsters idea that intriguing marks are made of ice, not salt.
Alexandra Witze
nature.com
21 July 2015

Bright spots in Ceres' Occator crater have intrigued scientists for months.

Ceres, the biggest asteroid in the Solar System, has a haze that appears 
occasionally in a crater above some of its mysterious white spots.

The phenomenon, observed by NASA's Dawn spacecraft, suggests that the 
bright spots "could be providing some atmosphere in this particular region 
of Ceres", says Christopher Russell, a planetary scientist at the University 
of California, Los Angeles.

Dawn has been orbiting Ceres since March. Russell, the mission's principal 
investigator, described its initial findings at a NASA exploration meeting 
at the Ames Research Center in Moffett Field, California, on 21 July.

Haze on Ceres would be the first ever observed directly in the asteroid 
belt. In 2014, researchers using the European Space Agency's Herschel 
Space Observatory reported seeing water vapour spraying off Ceres, which 
suggested that it was a geologically active world1. Ceres is made of at 
least one-quarter water, a far greater proportion than seen in most asteroids.

Bright spots pepper Ceres' surface, but the haze has been seen in only 
one location so far - a crater named Occator, which has a large bright 
area at its centre and a bunch of smaller spots nearby. Mission scientists 
have been trying to figure out whether the bright spots are made of ice, 
evaporated salts or other minerals, or something else entirely.

Some team members had been leaning towards the salt explanation, but the 
discovery of haze suggests the presence of sublimating ice. "At noontime, 
if you look at a glancing angle, you can see what seems to be haze," Russell 
says. "It comes back in a regular pattern." The haze covers about half 
of the crater and stops at the rim.

Ceres' surface is pockmarked by craters and the occasional mysterious 
patch of highly reflective material.

Eyes on the prize

The Dawn spacecraft carries an infrared spectrometer that should be able 
to discriminate easily between ice or salt. But that instrument, which 
is mapping Ceres' surface, has not had a chance to study the spots properly 
yet. It has been out of commission for small periods of time, leaving 
blank spots in the map, Russell says. The same spectrometer experienced 
problems back when Dawn was orbiting a different asteroid, Vesta, in 2011-12.

Dawn has also found that Ceres is slightly smaller than expected, making 
it roughly 4% more dense than scientists had thought, Russell reports. 
And the obliquity, or tilt, of its orbit is the reverse of what scientists 
had anticipated: "Summer is winter and winter is summer," he says.

Ceres' striking 5-kilometre-high mountain, informally dubbed "the pyramid", 
may be similar to the mountains seen last week on Pluto by the New Horizons 
spacecraft, Russell adds.

At nearly 1,000 kilometres across, Ceres is designated as a dwarf planet, 
and understanding it could help scientists better understand the relationship 
among all the objects in the Solar System. 'Ceres is so big compared to 
all the other asteroids that it's really different," says Andrew Rivkin, 
a planetary scientist at the Johns Hopkins University Applied Physics 
Laboratory in Laurel, Maryland. 'It's sort of the penultimate step before 
a planet."

Following a brief interruption in normal operations on 30 June, Dawn is 
now once again spiraling closer to Ceres. It is less than 4,000 kilometres 
above the asteroid's surface, aiming for less than 1,500 kilometres by 
next month.

Nature
doi:10.1038/nature.2015.18032