Planetary researchers have long thought that the dwarf planet Ceres may have a temporary, thin atmosphere (an exosphere), but mysteries lingered about its origin and why it’s not always present. Now, scientists from NASA’s Dawn mission suggest that the Cerean exosphere appears to be related to the behavior of the Sun, rather than Ceres’ proximity to the Sun.
Occator crater on Ceres. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.
When energetic particles from the Sun hit exposed ice and ice near Ceres’ surface, it transfers energy to the water molecules as they collide. According to the Dawn team, this frees the water molecules from the ground, allowing them to escape and create a thin atmosphere that may last for a week or so.
“Our results also have implications for other airless, water-rich bodies of the Solar System, including the polar regions of the Moon and some asteroids. Atmospheric releases might be expected from their surfaces, too, when solar activity erupts,” said Dawn mission principal investigator Dr. Chris Russell, from the University of California, Los Angeles.
Before Dawn arrived in orbit at Ceres in 2015, evidence for an atmosphere had been detected by some observatories at certain times, but not others, suggesting that it is a transient phenomenon.
In 1991, the International Ultraviolet Explorer satellite detected hydroxyl emission from Ceres, but not in 1990.
Then, in 2007, ESO’s Very Large Telescope searched for a hydroxide emission, but came up empty.
ESA’s Herschel Space Observatory detected water in the possible exosphere of Ceres on three occasions, but did not on a fourth attempt.
As Dawn began its thorough study of Ceres in March 2015, scientists found ample evidence for water in the form of ice.
Dawn’s Gamma Ray and Neutron Detector (GRaND) has found that the uppermost surface is rich in hydrogen, which is consistent with broad expanses of water ice. This ice is nearer to the surface at higher latitudes, where temperatures are lower.
Other research has suggested that persistently shadowed craters are likely to harbor ice.
Because of this evidence for abundant ice, researchers think that the Cerean exosphere is created in a process similar to what occurs on comets, even though they are much smaller.
In that model, the closer Ceres gets to the Sun, the more water vapor is released because of ice sublimating near or at the surface.
But the new study, published in the Astrophysical Journal Letters, suggests comet-like behavior may not explain the mix of detections and non-detections of a weak atmosphere.
“Sublimation probably is present, but we don’t think it’s significant enough to produce the amount of exosphere that we’re seeing,” said lead author Michaela Villarreal, a graduate student at the University of California, Los Angeles.
The team showed that past detections of the transient atmosphere coincided with higher concentrations of energetic protons from the Sun.
Non-detections coincided with lower concentrations of these particles.
What’s more, the best detections of Ceres’ exosphere did not occur at its closest approach to the Sun.
This suggests that solar activity, rather than Ceres’ proximity to the Sun, is a more important factor in generating an exosphere.
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M.N. Villarreal et al. 2017. The Dependence of the Cerean Exosphere on Solar Energetic Particle Events. ApJL 838, L8; doi: 10.3847/2041-8213/aa66cd
This article is based on a press-release from the National Aeronautics and Space Administration.
