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High-Resolution Mapping of Ceres using the HST Advanced Camera for Surveys

Joel W. Parker
Southwest Research Institute, Boulder, CO

Ceres is the largest minor planet in the main asteroid belt with a volume about eight times that of the next two largest asteroids Vesta and Pallas. Its mass is about four times that of Vesta and perhaps six times that of Pallas. Ceres has a semi-major axis of 2.7 astronomical units. Ceres’ low inclination to the ecliptic plane allows it to be one of the two targets of the Dawn mission. Its low density (~2100 kg/m³) indicates a large percent water content, perhaps similar to that of Ganymede.

The sensitivity and resolution of the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope provides the opportunity to obtain higher-quality images of Ceres than have so far been obtained in previous observations. These images can lead to a better understanding of Ceres and help improve the planning for the Dawn mission. These two general goals in turn lead to the following four measurement objectives of our HST proposal: to achieve dense rotational phase coverage with multi-spectral imaging over the complete rotation period of Ceres to map its surface; to resolve and track surface features in order to unambiguously determine its pole position; to measure the three- dimensional shape of Ceres better than any existing shape model in order to significantly refine density measurements; and to map Ceres’ color variegation and photometric parameters in order to identify and characterize possible surface units for the first time.

In December 2003 and January 2004, nine HST orbits were used to obtain 259 images of Ceres, averaging close to 50 minutes of observations on each orbit, using the ACS High Resolution Camera (HRC). Six consecutive orbits were used to provide full coverage of the 9.1 hr rotation period. Each orbit had the following order of exposures: V-U-UV-V-U-UV-U-V where V is the F555W filter, U is the F330W filter and UV is the F220W filter. Figure 1 shows the pass-bands of these three filters. The exposures on these six orbits used a 12x12 arcsec (512x512 pixels) subarray of the HRC to provide shorter readout time and in order to fit more images into the HST memory. At the end of four of these six orbits a long-exposure V image was taken using the full 25x25 arcsec field of view in order to search for possible satellites of Ceres. The remaining three orbits were separated by 120^o in rotational phase and the images were “dithered” (using small pointing offsets) to obtain higher spatial resolution. Each of these three orbits had the following order of exposures: V-U-UV-U-V. In all cases, each exposure consisted of several subexposures in succession, which will be combined to improve the signal to noise ratio after removing cosmic rays. Analysis of the images has begun. See accompanying report from P. C. Thomas.

Figure 1. Throughput versus wavelength of the three filters (UV, U and V respectively) that were used in this Click for larger view.