MESSENGER Enhanced Gamma Ray Spectrometer Data Set (MEAP-5)

MEAP5 – Enhanced Gamma Ray Spectrometry Data

PDS Bundle: urn:nasa:pds:izenberg_pdart14_meapDOI: 10.17189/1518648

PDS Collection: urn:nasa:pds:izenberg_pdart14_meap:data_grsspectra

For more information about MEAP5 products, see:

Peplowski et al., 2012. Variations in the abundances of potassium and thorium on the surface of Mercury: Results from the MESSENGER Gamma-Ray Spectrometer. Journal of Geophysical Research: Planets, 117(E12), doi:10.1029/2012JE004141.
Evans et al., 2017. The MESSENGER Gamma-Ray Spectrometer: Calibration and operations. Icarus, 288, pp.186-200, doi:10.1016/j.icarus.2017.01.022.

For more information about MEAP products, see the Messenger Advanced Product Specification.

GRS enhanced data products are PDS4-compliant versions of the GRS HPGe gamma-ray spectra CDR data products, with an additional entry that logs the low-energy count rate as a means to filter data product quality. GRS enhanced products, denoted as RACs to differentiate them from CDRs and RDRs, contain the 16384-channel gamma-ray energy deposition spectra and associated spacecraft ephemeris and housekeeping data contained in the final delivery of MESSENGER GRS CDRs. Data are a time-series of records, collected in a single day, representing the accumulation times over which MESSENGER GRS data were acquired. Spectra products reported in the RAC are the “anti-coincidence” (ANC) spectra – data that corresponds to HPGe events without a coincident event in the GRS ACS. ACS anti-coincidence removes backgrounds from charged particles, providing higher signal-to-background than the raw HPGe spectra.

The additional entry, denoted as “LE_COUNT_RATE”, records the number of events in the spectrum with energy less than 50 keV, in units of counts per second (cps), for each spectral accumulation. This value was calculated by summing the number of events in the energy range and dividing by the live time of the measurement. Live time calculations are discussed in Peplowski et al. (2012). LE_COUNT_RATE is a proxy for low-energy noise present in the GRS measurements (Evans et al., 2017), a problem that started on 11 October 2012 and that resulted in degradation of the energy resolution of the GRS measurements. LE_COUNT_RATE values >200 cps correspond to significantly degraded performance, lower values are considered acceptable for science analysis.

Inclusion of the LE_COUNT_RATE allows for data acquired post onset of the low-energy noise problem to be used in the analysis of GRS spectra products. For nominal data selection criteria (e.g. Peplowski et al., 2012), application of a LE_COUNT_RATE threshold of <100 cps can increase the total amount of GRS data available for science by >25%. Exact gains depend on data selection criteria. As the GRS measurements were count-rate limited, the RAC data products facilitate identification of new elements on Mercury improvement to the statistical precision of existing elements, and/or new spatially resolved measurements of varying surface composition.

The GRS RAC files are binary data files. Each file represents a collection of GRS measurements (energy spectra), instrument housekeeping, and spacecraft ephemeris corresponding to all data collected during a given Earth day. Within each file are entries for every GRS data accumulation within that given day. Unless otherwise noted, the housekeeping and ephemeris for the midpoint time of that data accumulation. Data accumulations are typically ~60 s long while MESSENGER is close to Mercury (altitude < 4000 km), and >> 60 s long while MESSENGER is far from Mercury (altitude > 4000 km). RAC files were only produced for GRS data acquired while the MESSENGER spacecraft was orbiting Mercury. Energy spectral data is recorded for the “anti-coincidence” mode only. This data represents HPGe events without a coincidence event in the Shield, a means to filter out background-inducing charged particle events.

The MEAP-5 RAC files are named GRS_RAC_YYYYDDDZZZ.DAT, where YYYY is the year and DDD is the day of year.

MEAP5 PDS4 Logical Identifiers:

urn:nasa:pds:izenberg_pdart14_meap:data_grsspectra:grs_rac_yyyydddzzz

For more on MEAP5 parameter definitions, see Section 5.4.3.5 in MESSENGER Advanced Product Specification (PDF).