When
5 p.m., Oct. 27, 2014
Where
Kuiper Space Sciences 312
Dr. Michael Bland
Research Scientist
Washington University - St. Louis
Anticipating Ceres: Expectations of surface morphology and methods for determining internal structure
In roughly six months, the Dawn spacecraft will begin its encounter with the dwarf-planet Ceres. Currently, our understanding of Ceres is limited. Whereas the surface of Ceres is rocky, its shape and low density (~2200 kg m-3) suggest that it may contain a near surface layer of water ice 30-80 km thick. Yet, ground based data are unable to unequivocally determine the body's internal structure, and the possibility that Ceres is ice free and composed of porous, low density (i.e., hydrated) material remains. The two possibilities lead to substantially different conclusions regarding Ceres origin and subsequent evolution. Here I describe how the morphology of Ceres' impact craters can be used to determine whether subsurface ice is present within Ceres, even if no surface ice is observed (ice is thermodynamically unstable on the surface). At Ceres' warm diurnally averaged surface temperature, viscous relaxation of impact craters (and other topography) occurs on short geologic timescales if ice is present. In the equatorial region craters as small as 4 km in diameter will completely relax (i.e., flatten) in ~100 Ma. Larger craters relax even faster. At higher latitudes (colder surface temperatures) relaxation is slowed, but mid-sized craters will still relax in less than 1~Ga. Only at polar latitudes are deep craters preserved. In contrast, if ice is absent no crater relaxation will occur. This result is robust to variations in ice viscosity (e.g., due to grain size), particulate contamination, the presence of an high-viscosity surface "crust", or a rigid rock layer at depth (i.e., a "core"). Thus the presence (or absence) of ice within Ceres can be evaluated immediately upon Dawn's arrival.
Research Scientist
Washington University - St. Louis
Anticipating Ceres: Expectations of surface morphology and methods for determining internal structure
In roughly six months, the Dawn spacecraft will begin its encounter with the dwarf-planet Ceres. Currently, our understanding of Ceres is limited. Whereas the surface of Ceres is rocky, its shape and low density (~2200 kg m-3) suggest that it may contain a near surface layer of water ice 30-80 km thick. Yet, ground based data are unable to unequivocally determine the body's internal structure, and the possibility that Ceres is ice free and composed of porous, low density (i.e., hydrated) material remains. The two possibilities lead to substantially different conclusions regarding Ceres origin and subsequent evolution. Here I describe how the morphology of Ceres' impact craters can be used to determine whether subsurface ice is present within Ceres, even if no surface ice is observed (ice is thermodynamically unstable on the surface). At Ceres' warm diurnally averaged surface temperature, viscous relaxation of impact craters (and other topography) occurs on short geologic timescales if ice is present. In the equatorial region craters as small as 4 km in diameter will completely relax (i.e., flatten) in ~100 Ma. Larger craters relax even faster. At higher latitudes (colder surface temperatures) relaxation is slowed, but mid-sized craters will still relax in less than 1~Ga. Only at polar latitudes are deep craters preserved. In contrast, if ice is absent no crater relaxation will occur. This result is robust to variations in ice viscosity (e.g., due to grain size), particulate contamination, the presence of an high-viscosity surface "crust", or a rigid rock layer at depth (i.e., a "core"). Thus the presence (or absence) of ice within Ceres can be evaluated immediately upon Dawn's arrival.