By many measures Mercury is an intriguingly odd member of the planetary family [1]. It is the smallest of the terrestrial planets, but its density (corrected for self-compression) and by inference more common 1 :1 ratio, as for the Moon). Mer- cury’s atmosphere is both the most tenuous and the most strongly variable among the terrestrial planets and the only atmosphere in which such crustally derived elements as sodium, potassium, and calcium are major constituents.
A further context in which to regard Mercury’s
unusual attributes is provided by the recent dis- covery of planets and planetary systems in orbit about other stars [2]. While the extrasolar planets documented to date are all analogues to the gas- giant planets of our solar system, the orbital pa- rameters of Mercury fall within those of known extrasolar planets (Fig. 2), and Mercury provides our nearest laboratory for studying planetary sys- tem processes in the vicinity of a host star. Fur- ther, a variety of e¡orts are under way to detect and characterize extrasolar Earth-like planets [3], and Mercury will provide a relevant point of com- parison with those less than 1 AU from their par- ent star