Search for the putative dust belts of Mars: The late 2007 opportunity

The putative dust belts of Mars, a thin equatorial Phobos ring and a thick tilted Deimos torus, whose existence was predicted several decades ago, remain undiscovered. The previous attempt of direct observational detection, undertaken with the Hubble Space Telescope (HST) during the Mars equatorial plane crossing in May 2001, set an upper limit on the normal optical depth to ∼3×10-8∼3×10-8 for the Phobos ring and ∼10-7∼10-7 for the Deimos torus. This paper analyzes possible reasons for non-detection of the belts and focuses on the next, and the last for three decades to come, natural opportunity to search for the dust belts during the Mars ring plane crossing in December 2007. We have extended our dynamical models to predict the appearance of both dust belts and to estimate the distribution of their optical depth and brightness. Our new calculations show that at least the Deimos dust torus may have escaped HST detection in 2001 only marginally. A thoroughly prepared observational attempt in 2007 with HST, Keck or another comparable telescope will have good chances to discover the Deimos torus, if a detector has a sensitivity by about one order of magnitude better than the one used in 2001. Photometric detection of the Phobos ring appears to be more difficult.