Data Set Information
DATA_SET_NAME ODY MARS GAMMA RAY SPECTROMETER 5 ELEMENT CONCENTRATION V1.0
DATA_SET_ID ODY-M-GRS-5-ELEMENTS-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview : The Mars Odyssey Gamma-Ray Spectrometer (GRS) is a suite of three instruments working together to collect data that permits the mapping of elemental concentrations on the surface of Mars. The suite of three instruments, the gamma-ray sensor head (GS), the Los Alamos National Laboratory (LANL) neutron spectrometer (NS) and the Russian Academy of Science Institute for Space Research (IKI) high-energy neutron detector (HEND), are a complementary set of instruments in that the neutron instruments have better counting statistics and sample to a greater depth than the GS, but the GS determines the abundance of many more elements. A full description of the Mars Odyssey Gamma-Ray Spectrometer instrument can be found in [BOYNTONETAL2004]. The ODY MARS GAMMA RAY SPECTROMETER 5 ELEMENTS data set consists of a set of maps and underlying data products originally released to the public in March 2006. They contain data that was collected by the 2001 Mars Odyssey Gamma-ray Spectrometer between 06/04/02 and 04/03/05. These data are highly processed, and represent the fully corrected, CO2 frost free elemental concentrations. Data for all non-radioactive species have been masked to exclude polar regions where were we currently cannot adequately deal with dilution by the large amounts of water ice. The following paragraphs summarize the process by which the CO2 frost free elemental concentrations were determined. Determination of CO2 Frost-Freeness In order to determine the spatial and temporal ranges when Mars is free of Carbon Dioxide frost, observed measurements of Hydrogen and Potassium are used (due to the fact that they are among the strongest gamma ray emitters and provide statistically significant accumulations in relatively short periods of time). Seasonal 5x360 (latitude by longitude) sums are generated and run through our peak measurement algorithms (either by Gaussian fit plus tail component or by simple window counting between channels X and Y, in both cases, subtracting the continuum baseline contribution beneath the given peak of interest). Use of smaller grids on a seasonal basis was attempted, but results were not as easy to interpret due to statistically poorer accumulations. The 5x360 system seems to work quite well, and though it is acknowledged that there are certainly heterogeneities within a particular latitude band (i.e. the planet is certainly not identical, CO2 frost-free-wise through all 360 degrees of longitude), the general assessment of CO2 frost-freeness planet-wide is believed to be quite accurate. In this manner, plots are created showing each individual 5 degree latitude band vs. time (in terms of season, or Ls, where season 0 represents Ls 0 through 15 (center Ls 7.5)). The relative increase and decrease in the H and K signals is then used to determine when CO2 frost is present or not throughout the mission in each particular 5 degree latitude band. For instance, when both H and K agree in terms of apparent maximum counts or calculated fluxes or concentrations, this indicates a period when CO2 frost is absent or at an extreme minimum, such that little or no H or K signal is being attenuated. This happens to be the case in the northern region of the planet between seasons 4-6 and seasons 12-14, depending on Mars year, and exact latitude (such as +70 to +80 or +85 to +90, etc.). Conversely, when the apparent H and K signals are at observed minima, this does not indicate that the H and K concentrations are actually dropping, but rather that the CO2 frost is accumulating and attenuating the underlying H and K gamma ray emissions. In this way, then, by observing apparent increase and decrease (and subsequent plateaus during CO2 frost-free or CO2 frosty durations) in measured counts, calculated fluxes, or calculated concentrations, it is possible to constrain the spatial and temporal ranges of CO2-frost-freeness. The significance, of course, is that then, spectra collected over parts of the planet during times of CO2 frost-freeness may be co-added together over a variety of regions or grids, and the true, unattenuated elemental signatures can be observed, and the true concentrations may be calculated. CO2 Frost Free Mars 060802 to 012206 -90 to -80 MY01_20-MY01_23, MY02_20-MY02_23 -80 to -70 MY01_18-MY02_00, MY02_18-MY02_23 -70 to -65 MY01_01, MY01_16-MY02_01, MY02_16_MY02_23 -65 to -60 MY01_01-MY01_02, MY01_15-MY02_01, MY02_15-MY02_23 -60 to -55 MY01_01-MY01_03, MY01_13-MY02_02, MY02_13-MY02-23 -55 to -50 MY01_01-MY01_04, MY01_10-MY02_02, MY02_12-MY02_23 -50 to 55 MY01_01-MY02_23 55 to 60 MY01_01-MY01_17, MY02_00-MY02_17 60 to 70 MY01_06-MY01_14, MY02_04-MY02_13 70 to 75 MY01_05-MY01_13, MY02_05-MY02_13 75 to 80 MY01_06-MY01_13, MY02_05-MY02_13 80 to 90 MY01_06-MY01_12, MY02_06-MY02_12p Calculation of Concentration (wt%) The calculation of concentration for any given element involves several steps, some of which are quite involved. Forward modeling, for instance, is a process with extensive documentation, to which the reader is referred for details [BOYNTONETAL2007], but for the purposes of this summary, the goal is simply to explain how one can proceed from accumulation of gamma rays to concentration (wt%) of a particular element on the surface (and near subsurface) of Mars. The first step is measurement of gamma ray counts over a particular region on the planet, whether it be geologic, or gridded in nature. That is, one may be interested in a geologic region such as Arabia Terra and measuring the counts of a particular gamma ray line collected within the boundaries of said geologic region; one might also be interested in making a 5x5 degree map and measuring the counts in each 5x5 degree bin in order to establish the basis for the map. In either scenario, the first step is measurement of counts. There are two fundamental methods of measuring counts from a continuous spectrum; one is a Gaussian fit plus tail component, and the other is a simple counting method essentially integrating the counts that fall method, as counts in a window between X and Y are counted). In both cases, a fit to the continuum baseline is made, and those counts are subtracted so the resultant counts are simply those contained within the peak or peaks of interest. In some cases, such as H, K, Si, Th, U, etc., there are spacecraft or instrument interferences or backgrounds to take into account, so those contributions are quantified and subtracted from the observed counts in the peak or peaks. Once a background/interference adjusted area is measured, then a concentration may be calculated. For each peak of interest, there is a forward modeled expected contribution of counts from Mars for the given peak of interest in the given region or grid of interest. There are a number of factors that go into this modeled calculation, including atmospheric thickness, detector efficiency as a function of angle, footprint of the detector, production values of the elements based on an educated component model of Martian soil, etc. The reader is referred to the forward model documentation [REF?] for an in-depth understanding of these contributions. Essentially, though, a particular model of martial soil and rock is determined based on ground truth from previous and current missions. For the given model, there is a mass fraction for each element in the soil mixture, and there is a corresponding forward modeled number of counts for each particular gamma ray line of interest. So, for example in the case of Cl, while there may be nine individual lines, each one will have a unique expected number of model counts, but the mass fraction on the soil model would be the same for all nine. From this point, determination of concentration is rather simple. The background adjusted (if the element in question had any particular instrument, spacecraft, or additional gamma line interference) or the raw observed counts themselves are compared to the modeled counts (i.e., divided by the forward counts) and then multiplied by the mass fraction for the particular model soil that was chosen for the planet surface. The calculated value is a unitless ratio of measured counts over expected counts times a weight percent, which we now refer to as the 'raw concentration'. In order to transform the value into a true wt% concentration, a correction factor (distinct depending on whether the line is neutron capture or inelastic scatter), based on water and iron content, as well as comparison of Si capture to scatter values (as this is the only element that gives us ground truth measurements for both types of reactions) is applied to the flux to obtain true wt% concentration. Table of Peaks Used to Determine Elemental Concentrations Energy (keV) Element Z Source Method Process ------------ ------- - ------ ------ ------- 2223.25 H 1 1H(n,g)2H fit Capture 1778.97 Si 14 28Si(n,ng)28Si window Scatter 3538.97 Si 14 28Si(n,g)29Si window Capture 4933.89 Si 14 28Si(n,g)29Si window Capture 4422.89 Si 14 28Si(n,g)29Si window Capture-esc-1 1951.14 Cl 17 35Cl(n,g)36Cl fit Capture 1959.35 Cl 17 35Cl(n,g)36Cl fit Capture 6110.84 Cl 17 35Cl(n,g)36Cl fit Capture 5599.84 Cl 17 35Cl(n,g)36Cl fit Capture-esc-1 7790.33 Cl 17 35Cl(n,g)36Cl fit Capture 1460.82 K 19 40K window Radioactive 1381.75 Ti 22 48Ti(n,g)49Ti window Capture 7638 Fe 26 56Fe(n,g)57Fe window Capture-doublet 7127 Fe 26 56Fe(n,g)57Fe window Capt.-doub./esc-1 2614.53 Th 90 Th-208Tl window Radioactive Correction Factors and Data Smoothing Final adjustments to the raw concentrations determined from capture (of thermal neutrons) and inelastic scatter (by fast neutrons) gamma rays are made to account for the interaction of thermal and fast neutrons with the atmosphere and regolith, and to take advantage of the fact that we have ground truth measurements of silicon on Mars. Silicon is used for determining these correction factors since we detect statistically significant numbers for both capture and scatter produced gamma rays, and because silicon varies by less than about 10% over the entire surface of Mars, reducing the sensitivity of this correction to the precision of the spatial or compositional measurements used. We first make an inelastic scatter correction map for Mars by taking our last, best map of H2O and determining a correction factor for water's effect on fast neutrons from the empirically derived formula: fH2O : 1.1165 - 0.04308[H2O] + 0.001401[H2O]^2 where: [H2O] is the mapped concentration of water in wt%. We make a similar determination for iron's effect from the last, best map of iron using the formula: fFe : 0.9406 + 0.005056[Fe] where: [Fe] is the mapped concentration of iron in wt%. The Si scatter data is then divided by the product of the two correction factors, masked to exclude data from high latitudes where large water concentrations, and attendant dilutions of other elements (including iron and silicon), preclude precise determination of the elemental concentration, smoothed with a 2-d, 15 degree boxcar filter, and re-masked. The resulting map is then adjusted by a constant factor such that the value at the Pathfinder landing site is set equal to 20.95 wt% silicon. This is our 'true' silicon content for the regolith of Mars. We then divide our masked and smoothed Si-scatter raw concentration map by the 'true' map to get a map of scatter correction factors, and divide our masked and smoothed Si-capture raw concentration map by the 'true' map to get a map of capture correction factors. As with all our maps, these correction maps are based on a 0.5 by 0.5 degree grid, with a value assigned to each cell within that grid. These values are then binned up onto 2x2, 5x5 and 10x10 degree bins, with each value in the larger bin being the area-weighted average of the 0.5x0.5 degree cells that comprise it. Finally, we use the correction factors determined in this manner to correct the current data for water and iron, producing a new set of starting maps from which to determine and , and iterate until there is no significant change in either the resulting correction factors or maps. In practice, the first iteration is usually sufficient to show that no further iterations are required. The correctly sized capture or scatter correction map, as appropriate, is then applied to each raw concentration file before smoothing to produce a corrected raw concentration file. This data is then smoothed with a 2-d boxcar filter sized for the uncertainty in the raw data. Data with low uncertainty are smoothed with a small filter to preserve spatial information, while data with high intrinsic uncertainty (due to a low number of counts, poorly determined continuum, interfering peaks, etc.) are smoothed with a large filter in order to reduce spatial noise. As with the smoothing performed in the determination of the correction factors, the data is masked before and after smoothing to prevent the inclusion of spurious (very high uncertainty) data from high latitudes. The 2-d boxcar filters we use are circular in plan view with a radius stated in degrees of great circle arc. They are applied to data on 0.5 by 0.5 degree grids. Thus any mapped value that falls within the filter radius of the point being assessed is given a weight equivalent to the area of its 0.5x0.5 degree cell, but no other weighting algorithm (such as Gaussian weighting) is applied. At high latitudes, many more cells will be included in the smoothing for a given point, but the area considered for each point on the planet is a constant.
DATA_SET_RELEASE_DATE 2006-06-01T00:00:00.000Z
START_TIME 2002-06-04T12:00:00.000Z
STOP_TIME 2005-04-03T12:00:00.000Z
MISSION_NAME 2001 MARS ODYSSEY
MISSION_START_DATE 2001-01-04T12:00:00.000Z
MISSION_STOP_DATE N/A (ongoing)
TARGET_NAME MARS
TARGET_TYPE PLANET
INSTRUMENT_HOST_ID ODY
INSTRUMENT_NAME GAMMA RAY/NEUTRON SPECTROMETER/HIGH ENERGY NEUTRON DETECTOR
INSTRUMENT_ID GRS
INSTRUMENT_TYPE SPECTROMETER
NODE_NAME Geosciences
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE N/A
CITATION_DESCRIPTION Boynton, W.V., 2001 Mars Odyssey Gamma Ray Spectrometer Element Concentration Data, NASA Planetary Data System, ODY-M-GRS-5-ELEMENTS-V1.0, 2007.
ABSTRACT_TEXT The ODY MARS GAMMA RAY SPECTROMETER 5 ELEMENTS data set consists of a set of maps and underlying data products originally released to the public in March 2006. They contain data that was collected by the 2001 Mars Odyssey Gamma-ray Spectrometer between June 4, 2002 and April 3, 2005. These data are highly processed, and represent the fully corrected, CO2 frost free elemental concentrations. Data for all non-radioactive species have been masked to exclude polar regions where we currently cannot adequately deal with dilution by the large amounts of water ice.
PRODUCER_FULL_NAME WILLIAM BOYNTON
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