Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.g41b0030c&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #G41B-0030
Physics
1241 Satellite Orbits, 6035 Orbital And Rotational Dynamics, 6230 Martian Satellites
Scientific paper
The shadow cast by Phobos on the surface of Mars is small and moves rapidly, and can thus provide important relative position information. The size, shape, direction and speed of motion of the shadow evolve over a range of time scales determined by the orientations of Mars and Phobos orbits, and their relative orbital rates. We develop a simple geometric model for the temporal and spatial patterns of Phobos shadows on Mars, and comment on the possibility of using observations of the shadow of Phobos as a supplemental method for determining lander locations. The orbital parameters for the Mars-Phobos system are sufficiently different from the Earth-Moon system that terrestrial eclipse experience is a poor guide. The lunar orbit, with mean angular rate 13.33 deg/day, mean radius 60.2 Earth radii, and variable inclination (18-28 deg) to the equator plane, makes solar eclipses on Earth relatively rare. With an average of 2.38 eclipses per year, only a small fraction of the globe experiences a total eclipse in any given century. In contrast, the orbit of Phobos, with rapid mean motion of 1152 deg/day, small mean radius of 2.7 Mars radii, and negligible inclination of 1.1 deg, provides a favorable eclipse environment. Although the size and orbital distance of Phobos are such that total solar eclipses never occur on Mars, partial eclipses are common, with an average of 3.22 per day throughout most of each annual cycle. The Viking Lander 1 (VL1) camera system captured three Phobos eclipse events on 20, 24, 28, September 1977. On Earth, one would never see a series of eclipses at one location over such a short time scale. However, on Mars it is possible to see two eclipses in one day at a single location. A goal of the present study is to address the likelihood of observing a sequence of eclipses, similar to that observed by Viking 1, at a single site. A substantial portion of the surface in the latitude band between -60 and 60 degrees experiences an eclipse event over the course of a Mars year, with exceptionally dense coverage at low latitudes. Since the predicted landing sites of Beagle 2, and Mars Exploration Rovers are all at lower latitudes, this study suggests ample opportunity for making Phobos shadow observations. Beyond their potential use in establishing lander location, multispectral measurements of shadow duration and intensity can also act as probes of local atmospheric conditions, and subsurface thermal properties.
Bills Bruce
Comstock Robert
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