Our knowledge of Mercury is quite limited, compared with our knowledge of the other terrestrial planets. It rests on ground-based observations and on images and particles-and-fields data acquired by Mariner 10 during its flybys of the planet in 1974 and 1975 . The former have established the presence at Mercury of a tenuous exosphere of sodium and potassium and may also provide evidence for the existence of water ice beneath the hermean regolith. The latter offered a first look at Mercury's heavily cratered surface and revealed that Mercury has a strong magnetic field and an active magnetosphere, with intense particle bursts and magnetic field disturbances suggestive of the occurrence of magnetospheric substorms.
The brief, tantalizing glimpses of Mercury offered by Mariner 10 raised fundamental questions about Mercury's geological history and about the interaction of the solar wind with the planet's magnetosphere, atmosphere, and surface. What, for example, is the origin of Mercury's magnetic field? Is it a remanent field, or does it result from dynamo processes occurring in a fluid core? If the latter, what does the existence of fluid core rather than a solid core imply in terms of the origin, composition, and thermal history of Mercury? What do the unique surface features observed by Mariner (e. g., large-scale thrust faults) tell us about Mercury's crustal evolution? What is the shape and structure of the hermean magnetosphere, and what are the properties of its constituent plasmas? How does the absence of an Earth-like ionosphere affect the interaction of Mercury's magnetosphere with the solar wind? These and other questions remain unanswered, despite their importance for our knowledge of Mercury as well as their relevance to comparative studies of solar system plasma processes.
A Mercury Orbiter mission would address both magnetospheric physics and planetary science objectives and would, as well, support solar and heliospheric physics investigations. Mercury Orbiter's primary magnetospheric physics and solar physics/heliospheric science objectives would be 1) to map in three dimensions the magnetic structure and plasma environment of the "miniature" hermean magnetosphere; 2) to study in detail the principal physical processes taking place during magnetospheric substorms at Mercury, with an emphasis on differences from terrestrial substorms resulting from Mercury's lack of a highly conducting ionosphere; 3) to assess the role of interplanetary conditions in determining the rate at which Mercury's magnetosphere draws energy from the solar wind and the manner in which it is later dissipated; 4) to investigate heliospheric structure and dynamics inside of 0.5 AU; and 5) to utilize the proximity of Mercury to the Sun to achieve fundamental solar physics objectives by measuring neutrons and charged particles emanating from flare regions. The primary planetary science objectives would be: 1) to complete the global surface mapping initiated by Mariner 10; 2) to obtain global geochemical terrain maps of the occurrence of such elements as Fe, Th, K, Ti, Al, Mg, and Si; 3) to measure the intrinsic magnetic field in sufficient detail to allow for the detection of magnetic anomalies; and 4) to map Mercury's gravitational field and associated anomalies.