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M. R. Showalter (Stanford), D. P. Hamilton (U. Maryland), P. D. Nicholson (Cornell)
It has long been suspected that Mars might be encircled by two faint rings, one originating from each of its moons Phobos and Deimos. Meteoroid impacts into these moons should release clouds of dust that quickly spread out to become rings; similar dust rings have been associate with small inner moons of Jupiter, Saturn and Neptune. On May 28, 2001 Mars' hypothetical ring plane appeared edge-on to Earth within weeks of its opposition, providing the best Earth-based opportunity to detect these rings in several decades. Using the Wide Field/Planetary Camera 2 on HST, we obtained a set of deep exposures off the east and west limbs of Mars to search for these hypothetical rings.
We will describe the results of our analysis of this data set, which is currently ongoing. Our initial analysis shows no trace of ring material in the orbit of Phobos at a level of I/F~0-6 in edge-on brightness. Assuming a dust albedo comparable to that of Phobos' surface, this limits ring normal optical depth \tau < 10-6, below the \tau of many other known dusty rings and a factor of 30 below the limit set by Viking. With further analysis, it should be possible to push this limit still lower. We will also search the data for the presence of small, previously unseen inner moons of Mars; in principle, objects as small as ~00~m should be detectable.
Support for proposal #8579 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.