HST observed Mars during all 5 oppositions between 1990 and 1999, providing unique new observations of the planet's atmosphere and surface during seasons which are typically poorly-observed telescopically and in wavelength regions or at spatial scales that are not at all observed by spacecraft. HST observations also filled a crucial gap in synoptic observations of Mars prior to 1998, during a time when no spacecraft were observing the planet. HST data have provided important new insights and understanding of the Martian atmosphere, surface, and satellites, and they continue to fulfill important spacecraft mission support functions, including atmospheric aerosol characterization, dust storm monitoring, and instrument cross-calibration.

Introduction

Mars has been the subject of intense telescopic observations for centuries (see, for example, reviews by Martin et al. 1992 and Sheehan 1988). Interest in the red planet stems partly from its prominent appearance in the night sky as a bright extended object roughly every 26 months, and also from historic telescopic observations and more recent spacecraft encounters that have revealed many similarities between Mars and the Earth in terms of surface and atmospheric characteristics and climatic histories. While cold and arid today and probably inhospitable to most forms of life, evidence exists indicating that Mars once may have had a much more clement climate, during a postulated “warm and wet” epoch early in solar system history (e.g. Pollack et al. 1987; Carr, 1998).