G. Michael

Timing of optical maturation of recently exposed material on Ceres

On Ceres, multispectral imaging data from the Dawn spacecraft show a distinct bluish characteristic for recently exposed material from the subsurface in, for example, crater ejecta. Ejecta blankets of presumably old craters show a more reddish spectrum. We selected areas in which fresh material from the Cerean subsurface was exposed at a specific time in the past, and no later geologic process is expected to have changed its surface composition or its cratering record. For each area, we determined two color ratios and the crater retention age. The measured color ratios show an exponential diminishment of the bluish characteristic over time. Although the cause of the color change remains uncertain, the time-dependent change in spectral properties is evident, which could help identify the process.

Astrometric observations of Phobos with the SRC on Mars Express - New data and comparison of different measurement techniques

Aims. From April 2008 to August 2011 Mars Express carried out 74 Phobos flybys at distances between 669 and 5579 km. Images taken with the Super Resolution Channel (SRC) were used to determine the spacecraft-centered right ascension and declination of this Martian moon. Methods. Image positions of Phobos were measured using the limb-fit and control-point measurement techniques. Camera pointing and pointing drift were controlled by means of background star observations that were compared to corresponding positions from reference catalogs. Blurred and noisy images were restored by applying an image-based point spread function in a Richardson-Lucy deconvolution. Results. Here, we report on a set of 158 Phobos astrometric observations with estimated accuracies between 0.224 and 3.405 km circular w.r.t. the line of sight to the satellite. Control point measurements yield slightly more accurate results than the limb fit ones. Our observations are in good agreement with the current Phobos ephemerides by the Jet Propulsion Laboratory (JPL) and the Royal Observatory of Belgium (ROB) with mean offsets of up to 335 m. Our data can be used for the maintenance and update of these models.