Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.7105r&link_type=abstract
American Astronomical Society, DPS meeting #38, #71.05; Bulletin of the American Astronomical Society, Vol. 38, p.627
Astronomy and Astrophysics
Astronomy
Scientific paper
Asteroid 1 Ceres is the largest main-belt asteroid. Ceres has been the subject of increased attention of late, with our knowledge of this body increasing rapidly since its identification as a target of NASA's Dawn spacecraft mission. Despite much work, consensus as to the surface composition of Ceres has remained elusive. Its spectrum in the visible and near-IR (0.4-2.5 μm) is typical of C-class and related asteroids, and consistent with carbonaceous chondrites, though not diagnostic of them. An absorption at 3 μm indicative of hydrated (water- or OH-bearing) minerals has been seen on Ceres for nearly 30 years (Lebofsky 1978), though identification of a sub-band at 3.05 μm has been controversial: Lebofsky et al. (1981) proposed water ice frost, and King et al. (1992) later proposed ammoniated clays. Neither of these identifications are cut-and-dried, however: water ice frost is only stable near Ceres’ poles (and only marginally there), while ammoniated clays have never been seen in meteorites.
In order to address this issue, we observed Ceres in the 2-4 μm region over two nights in May 2005 using the IRTF. After thermal correction, Hapke mixture modeling of full-night averages of Ceres’ spectrum indicates that an iron-rich clay, such as those discussed in Calvin and King (1997) and found in carbonaceous chondrites, could be responsible for the 3.05 μm band. A spectrum of Ceres from the KAO strengthens the interpretation of iron-rich clays rather than the competing hypotheses. In addition, the Ceres data show evidence of carbonate minerals near 3.8-3.9 μm with a contribution of 4-6%, similar to what is seen in CI meteorites. We will discuss the observations and modeling and the implications thereof.
We acknowledge NASA Planetary Astronomy Grant NNG05GR60G and the people of Hawai'i for allowing telescopes on their sacred mountain.
Clark Beth Ellen
Rivkin Andrew S.
Volquardsen Eric L.
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