Data Set Information
DATA_SET_NAME MESSENGER E/V/H GRNS 5 GAMMA RAY SPECTROMETER DAP V1.0
DATA_SET_ID MESS-E/V/H-GRNS-5-GRS-DAP-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview : This data set consists of the GRS advanced products (DAPs). There is one DAP product derived from calibrated data that maps either gamma-ray count rates or count-rate-derived elemental abundances over the northern hemisphere. Instrument Overview : The GRS detector is a high-resolution coaxial germanium crystal 50 mm in diameter and 50 mm in length, chosen for its radiation damage resistance and annealing capabilities. The detector is rigidly clamped in a hermetically sealed Al capsule pressurized with clean, dry nitrogen. The capsule is cooled to an operating temperature in the 80-95 K range by a mechanical cryocooler. A plastic scintillator anti-coincidence shield surrounds the germanium detector in its sides and back, for rejection of cosmic-ray background. Galactic cosmic rays continuously bombard the surface of Mercury, and through interactions with the surface, gamma-rays of discrete energies that are characteristic of specific elements are created. A fraction of these gamma-rays, as well as those from the decay of radiogenic elements escape from the surface, where they can be detected by the orbiting GRS. Gamma-ray fluxes are measurable at altitudes up to 1000 km and for gamma-rays up to about 10 MeV that emanate from depths of up to tens of centimeter beneath the surface. Detected fluxes are generally low and require numerous orbital passes over a specific region to obtain a statistically well-defined energy spectrum. The measurements of elements such as Fe, Si, Mg, Ca, Ti, K and Th by GRS will provide insight into distinguishing between different formation models for Mercury as well as other planetary evolution issues. See the INSTRUMENT.CAT file for more information and [GOLDSTENETAL2007] for full details. Calibration Overview : The DAPs have been corrected for deadtime, efficiency, and attitude-dependent count rate and abundance dependencies. They represent spatially resolved measurements and therefore include spatial processing. Parameters : The principal parameters when observing with the GRS are as follows: * Accumulation Time: The accumulation time, in seconds, of the Ge detector. * Sub-spacecraft latitude/longitude. * Observing geometry. Data : The GRS-derived maps are either gamma-ray count rates or count- rate derived elemental abundances. These were created from calibrated gamma-ray spectra, represent spatially resolved measurements, and have been highly processed to remove variability originating from detector altitude and viewing geometry. The measurements were also corrected for deadtime. Each map pixel corresponds to 0.5 deg. x 0.5 deg. surface elements, and range from -180 deg. to 180 deg. longitude and -90 deg. to 90 deg. latitude. The data are mapped in larger surface elements whose size is variable but are typically larger than 15 deg. x 15 deg. The value for each surface element was derived from multiple data records (GRS_CAL_AC) summed using the process described for deriving GRS_RDR_SUM records. The maps were created using multiple GRS_CAL_AC spectra that were summed according to the sub-spacecraft latitude and longitude and were limited to data acquired when the angle between the detector boresight and the spacecraft-to-planet-center vectors was less than 15 deg. There are a total of 8 GRS_DAP maps: a potassium (K) abundance map, silicon (Si), oxygen (O), and K gamma-ray count rate maps, along with 4 maps of the statistical significance of these measurements. The maps have spatial coverage that is limited to the northern hemisphere as a result of the highly elliptical orbit of MESSENGER about Mercury (with periapse location at mid-to-high northern latitudes) and the strong altitude-dependence of the GRS measurements. The spatial resolution of the measurements as shown in the maps is variable to ensure comparable statistical significance for each measurement pixel [PEPLOWSKIETAL2012B]. The data reduction process used to create these maps is presented in the document MESSGRS_PROCESSING.PDF, located in the DOCUMENT directory. Also see the GRS CDR-RDR-DAP SIS in the DOCUMENT directory for further details.
DATA_SET_RELEASE_DATE 2014-03-07T00:00:00.000Z
START_TIME 2011-03-24T12:00:00.000Z
STOP_TIME 2011-10-11T11:59:59.000Z
MISSION_NAME MESSENGER
MISSION_START_DATE 2004-08-03T12:00:00.000Z
MISSION_STOP_DATE 2015-04-30T12:00:00.000Z
TARGET_NAME CALIBRATION
EARTH
MERCURY
VENUS
TARGET_TYPE CALIBRATION
PLANET
INSTRUMENT_HOST_ID MESS
INSTRUMENT_NAME GAMMA RAY SPECTROMETER
INSTRUMENT_ID GRS
INSTRUMENT_TYPE GAMMA RAY SPECTROMETER
NODE_NAME Geosciences
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE Confidence Level Overview : The GRS DAP data is the advanced data set released for the GRS. There may be further recalibrated deliveries of these data if needed and future data may require calibration changes. Review : The GRS CDR/RDR/DAP SIS and data were reviewed internally by the GRS team prior to release to the PDS. PDS also performed an external review of the GRS CDR/RDR/DAP. The RDR and DAP data are based on the uncalibrated EDR data and the calibrated data, which have been previously reviewed, and GRS science spectra have been analyzed based on the EDR data. However, GRS science analysis based on these data has not yet taken place; when this occurs, it may be found that a new release of these data, possibly recalibrated, is needed. Data Coverage and Quality : Data reported are the derived data received from the spacecraft during the first year of Mercury Orbit operations. This mission phase is defined as: Start time End time Phase Name Date (DOY) Date (DOY) ------------------- ----------------- ----------------- Mercury Orbit 04 Mar 2011 (063) 17 Mar 2012 (077) The specific operational periods during the Mercury Orbit phase were: Start time End time Phase Name Date (DOY) Date (DOY) Sensor -------------------- ----------------- ----------------- ------ Mercury Orbit 23 Mar 2011 (082) 03 Jun 2011 (154) Ge Mercury Orbit 11 Jun 2011 (162) 14 Jun 2011 (165) Ge Mercury Orbit 16 Jun 2011 (167) 05 Jul 2011 (186) Ge Mercury Orbit 20 Jul 2011 (201) 25 Jul 2011 (206) Ge Mercury Orbit 28 Jul 2011 (209) 06 Sep 2011 (249) Ge Mercury Orbit 08 Sep 2011 (251) 17 Sep 2011 (260) Ge In this list, 'Ge' means Ge and shield. During orbital operation, the GRS remained on continuously, except for periods when the high voltage was down, for various reasons, including an X-class SPE (Solar Particle Event) that safed the detector, OCMs (Orbital Correction Maneuvers), and cryocooler temperature setpoint changes. Periods having conditions that lead to broad energy resolution, such as radiation damage revealed by a Ge temperature increase or large very low-energy noise, have also been removed. Not shown in the table of operational periods are periods of SPEs that did not safe the detector: SPE2 08-02-2011, SPE3 08-04/05-2011, SPE4 08-09-2011, and SPE5 09-08/09-2011. These time periods should probably be eliminated from analysis, due to count-rate saturation and strong activation of gamma-ray peaks. Beginning on 11 October, 2011, the HPGe detector began registering anomalous counts in the low-energy (< 300 keV) portion of the spectrum. While these events are lower in energy than the gamma-ray photopeaks of interest, the resulting count rate during these periods is sufficiently high to degrade the energy resolution for the detection of all gamma rays, regardless of energy. The magnitude of the anomalous counts and corresponding degradation of the energy resolution varies, and all data acquired after 11 October, 2011 should be carefully evaluated for its energy resolution prior to analysis. Particular care should be taken prior to summing individual gamma-ray spectra with differing energy resolution to produce summed spectra for analysis. Post 11 October 2011 data was not used for the generation of the DAPs. The GRS operated in anomalous count rate mode for the remainder of its life. On 15 June, 2012 the GRS cryocooler failed after approximately 9,500 hours of operation. This compares favorably to the 8,000 hour mean lifetime for these coolers. The failure occurred during a routine detector cool down following the 8 June 2012 annealing activity. Several unsuccessful attempts to revive the cooler have been made. Without the ability to maintain the HPGe detector at cryogenic temperature, gamma-ray measurements are not possible. Post-June 15 HPGe data will not be generated or delivered to the PDS for archiving. The instrument was repurposed with a software upload to enhance neutron and charged particle measurements from the shield. These new products may be delivered to the PDS in future deliveries. DAPs were created using the highest-quality GRS measurements. Data acquired during solar particle events (SPE) or unusual instrument operations were omitted. These include: 3-16 June, 2011; (SPE, including instrument safing and subsequent recovery) 6-16 July, 2011; (HV off, followed by GRS Annealing) 26 July, 2011; (HV off for Orbital Correction Maneuver) 2 August, 2011; (SPE) 4-5 August, 2011; (SPE) 9 August, 2011; (SPE) 7-8 September, 2011; (SPE and instrument safing) 21-23 September, 2011; (SPE) Data acquired after 28 September 2011, just prior to the onset of the anomalous count low energy count rates was likewise omitted due to its compromised energy resolution. Limitations : Though calibrated, this data set is based on minimally processed data received from the spacecraft telemetry and ingested into the MESSENGER Science Operations Center (SOC). No data gaps or corrupted data have been identified for any of the GRS operational periods. Although there may be some data not identified as missing or corrupted, such data should be minimal and a very small fraction of the available data.
CITATION_DESCRIPTION P. Peplowski (APL), MESSENGER E/V/H GRNS 5 GAMMA RAY SPECTROMETER DAP V1.0, NASA Planetary Data System, 2013.
ABSTRACT_TEXT Abstract : The GRS experiment is a gamma-ray spectrometer designed to observe spectra of gamma rays emitted from Mercury's surface in the energy range from 0.1 to 10 MeV. This data set consists of the MESSENGER GRS advanced data products (DAPs), which are jpeg2000 maps of gamma-ray count rates or elemental abundances derived from summed GRS data, each with a corresponding one standard deviation error map.
PRODUCER_FULL_NAME PATRICK PEPLOWSKI
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