Well put. Just to backstop that a bit more, Dawn's instruments include a visible/IR spectrometer and a gamma-ray sensor (http://dawn.jpl.nasa.gov/technology/sc

Well put. Just to backstop that a bit more, Dawn's instruments include a visible/IR spectrometer and a gamma-ray sensor (http://dawn.jpl.nasa.gov/technology/science_payload.asp http://dawn.jpl.nasa.gov/technology/science_payload.asp) which will give us a spatially-resolved map of the surface and near subsurface chemical composition of Ceres. The resolution of these maps will increase as the orbiter is able to descend to lower altitudes (~375km from its current ~1400km; in late July it was still at 2400km). Performing the descent properly requires knowing about Ceres' gravity field, the modeling for which is happening now, as we learn more about the gravity field. Navigation for this mission was very challenging. Landing would require even more detailed gravity information, as well as chemical information for site selection. But don't think landers and rovers are not being considered. It's all about ranking in the Decadal Survey, conducted regularly for all planetary missions by the National Academy of Science (https://en.wikipedia.org/wiki/Planetary_Science_Decadal_Survey https://en.wikipedia.org/wiki/Planetary_Science_Decadal_Surv...). As you can see by visiting the link, there is a lot of competition. A brief piece surveying Ceres research submitted to the last decadal survey (http://www.lpi.usra.edu/decadal/sbag/topical_wp/AndrewSRivkin-ceres.pdf http://www.lpi.usra.edu/decadal/sbag/topical_wp/AndrewSRivki...) contains this: "A landed mission is an obvious subsequent step for the study of Ceres. Such a mission, whether a Phoenix-type lander or a MER-type rover, will provide a critical link between the remote sensing available from the ground and from Dawn and geochemical studies performed in earthbound laboratories. [...] The specific case for such an advanced mission will probably have to wait until the return of Dawn data, but Dawn’s Ceres phase occurs early in the time period considered here, and it is not too early to prepare for such an mission so that we may expeditiously take advantage of Dawn’s findings." In other words: what Animats said, but longer. ;-)