Measuring the plasma environment at Mercury: The fast imaging plasma spectrometer

Abstract— The plasma environment at Mercury is a rich laboratory for studying the interaction of the solar wind with a planet. Three primary populations of ions exist at Mercury: solar wind, magnetospheric, and pickup ions. These pickup ions are generated through the ionization of Mercury's exosphere or are sputtered particles from the Mercury surface. A comprehensive mission to Mercury, such as MESSENGER (MErcury: Surface, Space ENvironment, GEochemistry, Ranging), should include a sensor that is able to determine the dynamical properties and composition of all these plasma components. An instrument to measure the composition of these ion populations and their three‐dimensional velocity distribution functions must be lightweight, fast, and have a very large field of view. The fast imaging plasma spectrometer (FIPS) is an imaging mass spectrometer, part of NASA's MESSENGER mission, the first Mercury orbiter. This versatile instrument has a very small footprint, and has a mass that is ?1 order of magnitude less than other comparable systems. It maintains a nearly full‐hemisphere field of view, suitable for either spinning or three‐axis‐stabilized platforms. The major piece of innovation to enable this sensor is a new deflection system geometry that enables a large instantaneous (?1.5π) field of view. This novel electrostatic analyzer system is then combined with a position sensitive time‐of‐flight system. We discuss the design and prototype tests of the FIPS deflection system and show how this system is expected to address one key problem in Mercury science, that of the nature of the radar‐bright regions at the Hermean poles.     

Muscovite Reactions and Partial Melting in South-eastern Connecticut

A sillimanite-orthoclase isograd mapped in south-eastern Connecticutcuts across the strike of the major Ordovician pelitic schistunit. This isograd separates a sillimanite muscovite zone froma sillimanite-orthoclase zone. Muscovite is not present in thesillimanite-orthoclase zone. Segments of all the Cambrian (?) and Ordovician units in south-easternConnecticut were affected by the reaction: muscovite_ssquartzsodicplagioclase_ss = sillimaniteorthoclase_ssless sodic plagioclase_sswater (Guidotti, 1963), and possibly by reactions such as muscovite_ssbiotite_ss3 quartz – orthoclase_ssgarnet_ssTi-rich biotite_sswater.This metamorphism was a post-Lower Devonian event. The sodiumcontent of orthoclases and data on stability boundaries formuscovite, anthophyllite, and cordierite suggest temperaturesof metamorphism in the sillimanite-orthoclase zone of more than650 C. Migmatites were formed during this metamorphic event,partly by isochemical elimination of muscovite to form 7 percent. orthoclase for every 10 per cent. modal muscovite initiallypresent. Most of the migmatites must have developed by partialmelting of rocks in which muscovite was being eliminated, andby the intrusion of melt from deeper zones in which biotitewas being eliminated. Melting in the lower-temperature partof the sillimanite-orthoclase zone must have been controlledby the amount of water released from muscovite; melting in thehigher temperature part may have been more extensive if thetemperature was sufficiently high to produce melts undersaturatedwith water.