The Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) mission is designed to orbit Mercury following one Earth flyby, two flybys of Venus and three of Mercury. It launched in August 2004 and will use these flybys to achieve an orbit insertion around Mercury in March 2011. Initial Mercury data collection will begin during the three flybys of Mercury, and will primarily consist of global mapping and measurements of the surface, atmosphere and magnetosphere composition. In addition, data will be collected during the flybys of Earth and Venus. MESSENGER will remain in orbit for the rest of the nominal mission, which is scheduled to end in March 2012. Once in orbit around Mercury it will begin a series of observations using multiple instruments. These observations will provide data to answer questions about the nature and composition of Mercury's crust, tectonic history, the structure of the atmosphereand magnetosphere, and the nature of the polar caps.

The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) is comprised of a small Cassegrain telescope with a 257-mm effective focal length and a 50-mm aperture that simultaneously feeds both an UltraViolet and Visible Spectrometer (UVVS) and a Visible and InfraRed Spectrograph (VIRS). MASCS will investigate Mercury's exosphere by measuring altitude profiles of known species as well as searching for previously undetected species. MASCS will investigate the mineralogical composition of the surface of Mercury by obtainingmaps of surface reflectance spectra on spatial scales of 5 km.

UVVS is a scanning grating, Ebert-Fastie monochromator with a focal length of 125-mm and is equipped with three photomultiplier tube detectors. The three detectors cover the wavelength ranges of the far ultraviolet (115-180 nm), middle ultraviolet (160-320 nm), andvisible (250-600 nm) with an average spectral resolution of 0.6 nm.
Two of the three photomultiplier tubes can be operated simultaneously;any combination of two is possible.

The UVVS instrument will help determine the composition of Mercury's exosphere by measuring the spatial and vertical distribution of known species (H, O, Na, K, and Ca) as well as previously undetected but predicted species (e.g. S, Si, Al, Mg, Fe, and OH). In addition to determining the composition and structure of the exosphere, these data will provide the basis for determining exospheric processes, studying the relationship between surface and exospheric composition,and studying surface-exosphere-magnetosphere interactions.

VIRS is a fixed concave grating spectrograph with a 210-mm focal length. A beam splitter simultaneously disperses the spectrum onto two solid-state array detectors: a 512-element silicon photodiode array, with a sensitivity to visible wavelengths (300-1050 nm), and 256-element indium-gallium-arsenide photodiode array, to measure near infrared wavelengths (850-1450 nm). It is optimized to measure visible and near infrared surface reflectance, and VIRS will obtain data with a resolution of 5 nm. Together, the VIRS and UVVS detectors will measure surface reflectance at middle ultraviolet to visible to near infrared wavelengths to search for ferrous bearing minerals, Fe- Ti bearing phases, and ferrous iron. These measurements will be made with a spatial resolution of 5 km orbetter.

The MASCS instrument is described in full detail in[MCCLINTOCK&LANK2007].