X-ray fluorescence spectra obtained by the MESSENGER spacecraft orbiting Mercury indicate that the planet's surface differs in composition from those of other terrestrial planets. Relatively high Mg/Si and low Al/Si and Ca/Si ratios rule out a lunarlike feldspar-rich crust. The sulfur abundance is at least 10 times higher than that of the silicate portion of Earth or the Moon, and this observation, together with a low surface Fe abundance, supports the view that Mercury formed from highly reduced precursor materials, perhaps akin to enstatite chondrite meteorites or anhydrous cometary dust particles. Low Fe and Ti abundances do not support the proposal that opaque oxides of these elements contribute substantially to Mercury's low and variable surface reflectance.

The Jupiter Energetic Particle Detector Instruments (JEDI) on the Juno Jupiter polar-orbiting, atmosphere-skimming, mission to Jupiter will coordinate with the several other space physics instruments on the Juno spacecraft to characterize and understand the space environment of Jupiter's polar regions, and specifically to understand the generation of Jupiter's powerful aurora. JEDI comprises 3 nearly-identical instruments and measures at minimum the energy, angle, and ion composition distributions of ions with energies from H:20 keV and O: 50 keV to > 1 MeV, and the energy and angle distribution of electrons from < 40 to > 500 keV. Each JEDI instrument uses microchannel plates (MCP) and thin foils to measure the times of flight (TOF) of incoming ions and the pulse height associated with the interaction of ions with the foils, and it uses solid state detectors (SSD's) to measure the total energy (E) of both the ions and the electrons. The MCP anodes and the SSD arrays are configured to determine the directions of arrivals of the incoming charged particles. The instruments also use fast triple coincidence and optimum shielding to suppress penetrating background radiation and incoming UV foreground. Here we describe the science objectives of JEDI, the science and measurement requirements, the challenges that the JEDI team had in meeting these requirements, the design and operation of the JEDI instruments, their calibrated performances, the JEDI inflight and ground operations, and the initial measurements of the JEDI instruments in interplanetary space following the Juno launch on 5 August 2011. Juno will begin its prime science operations, comprising 32 orbits with dimensions 1.1 × 40 RJ, in mid-2016.

The Energetic Particles and Ion Composition (EPIC) instrument, flown onboard the GEOTAIL satellite, is designed to measure the characteristics ofparticle populations important to understanding the make-up and dynamics of the earth's geomagnetic tail. To do this, EPIC, a joint endeavor between the Technical University of Braunschweig (TUB), the University of Maryland (UM), and The Johns Hopkins University Applied Physics Laboratory (JHU/APL), is made up of five subassemblies: the SupraThermal Ion Composition Spectrometer (STICS) sensor, the STICS analog electronics, the Ion Composition System (ICS) sensor, the ICS analog electronics, and the Data Processing Unit (DPU). The STICS sensor provides -4ir angular coverage, composition and spectral observations, with charge state determination for all ions from 30 keV to 230 keV/e, and mass per charge measurements >_7.5 keV/e. The ICS sensor provides flux, composition, spectra, and angular distributions over two polar angles of the elemental species protons through iron from >_50 keV to 3 MeV along with angular distributions in one plane of electron fluxes >32 keV and >1 10 keV. The DPU provides the capability of numerous operating modes from which a small number will be selected to optimize data collection throughout the many phases of the GEOTAIL mission. To date the EPIC instrument performance has been very successful. In this paper we describe the instrument, its operation, and show some of our early results.

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NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission will further the understanding of the formation of the planets by examining the least studied of the terrestrial planets, Mercury. During the one-year orbital phase (beginning in 2011) and three earlier flybys (2008 and 2009), the X-Ray Spectrometer (XRS) onboard the MESSENGER spacecraft will measure the surface elemental composition. XRS will measure the characteristic X-ray emissions induced on the surface of Mercury by the incident solar flux. The Kα lines for the elements Mg, Al, Si, S, Ca, Ti, and Fe will be detected. The 12°field-of-view of the instrument will allow a spatial resolution that ranges from 42 km at periapsis to 3200 km at apoapsis due to the spacecraft's highly elliptical orbit. XRS will provide elemental composition measurements covering the majority of Mercury's surface, as well as potential high-spatial-resolution measurements of features of interest. This paper summarizes XRS's science objectives, technical design, calibration, and mission observation strategy.