urn:nasa:pds:context:instrument:xrs.mess 1.0 1.7.0.0 Product_Context 2016-10-01 1.0 extracted metadata from PDS3 catalog and modified to comply with PDS4 Information Model urn:nasa:pds:context:instrument_host:spacecraft.mess::1.0 instrument_to_instrument_host Schlemm, C.E., II, R.D. Starr, G.C. Ho, K.E. Bechtold, S.A. Benedict, J.D. Boldt, W.V. Boynton, W. Bradley, M.E. Fraeman, R.E. Gold, J.O. Goldsten, J.R. Hayes, S.E. Jaskulek, E. Rossano, R.A. Rumpf, E.D. Schaefer, K. Strohbehn, R.G. Shelton, R.E. Thompson, J.I. Trombka, and B.D. Williams, The X-Ray Spectrometer on the MESSENGER spacecraft, Space Science Reviews, 131, 393-415, 2007. reference.SCHLEMMETAL2007 XRAY SPECTROMETER Spectrometer not applicable not applicable 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 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. 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 atmosphere and magnetosphere, and the nature of the polar caps. The X-Ray Spectrometer (XRS) experiment is comprised of three identical gas proportional counters (GPC) that measure X-rays emitted from the surface of Mercury in the energy range from about 1 to 10 keV. X-rays in this energy range sample the planetary surface to depths of a few tens of microns. The GPCs each have a 10-cm2 active area and use both anti-coincidence wires and pulse shape discrimination to minimize background. Balanced filters are used to resolve the lower energy X-ray lines from Mg, Al, and Si. This technique has been used previously on other orbital X-ray experiments flown on the Apollo 15, 16 and NEAR-Shoemaker missions. One GPC has a thin aluminum filter, which filters out Si photons, one has a thin Mg filter, which filters out Al and Si photons, and the third GPC has no filter. The energy resolution of the gas counters is sufficient to resolve the higher energy lines from S, Ca, Ti, and Fe. A small Si- PIN detector is used as a solar monitor, because the Sun is the source of the planetary X-ray fluorescence. The field-of-view (FOV) of the GPCs is 12 degrees. Both the GPCs and the Si-PIN detector count individual photons, producing an electronic pulse that is proportional to the energy of the absorbed photon. Each valid count is stored in energy histograms (spectra) that are 244 channels for the GPC and 231 channels for the Si-PIN. The accumulation time for these histograms will vary depending upon proximity to the planet. At periapsis the accumulation time will be 40 seconds and at apoapsis 450 seconds. The XRS is comprised of three physical units. The GPCs make up the Mercury X- ray Unit (MXU). The Solar Assembly for X-rays (SAX) includes the Si-PIN diode, preamplifier, shaping electronics and thermal electric cooler (TEC). The analog and digital electronics are contained in the Main Electronics for X-rays (MEX) unit. The SAX is located at the ''top center'' edge of the solar shield (see Figure 1 in the XRS EDR SIS document). The MXU and MEX are co-located within the payload adapter ring. The XRS instrument is described in full detail in [SCHLEMMETAL2007].