PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = "2013-07-31 Prettyman Original" OBJECT = DATA_SET DATA_SET_ID = "DAWN-A-GRAND-5-VESTA-ABSORPTION-V1.0" OBJECT = DATA_SET_INFORMATION DATA_SET_NAME = "DAWN GRAND MAP VESTA NEUTRON ABSORPTION V1.0" DATA_SET_COLLECTION_MEMBER_FLG= "N" ARCHIVE_STATUS = "LOCALLY ARCHIVED" DATA_OBJECT_TYPE = "TABLE" START_TIME = 2011-12-08T01:18:52 STOP_TIME = 2012-01-14T00:19:52 DATA_SET_RELEASE_DATE = 2012-08-01 PRODUCER_FULL_NAME = "THOMAS H. PRETTYMAN" DETAILED_CATALOG_FLAG = "N" DATA_SET_TERSE_DESC = "Global map of a compositional parameter that varies linearly with the thermal neutron absorption cross section of Vesta's regolith determined from thermal and epithermal neutron counting data acquired by Dawn GRaND from low altitude [PRETTYMANETAL2013]." ABSTRACT_DESC = "A global map of a unitless compositional parameter, delta-C_perpendicular (DCP), and propagated 1-sigma uncertainties is provided. DCP varies linearly with the macroscopic thermal neutron absorption cross section of Vesta's regolith. An equation for converting tabulated DCP values to absorption units is provided in this document. DCP was determined from thermal and epithermal neutron counting rates measured by the NASA Dawn mission's Gamma Ray and Neutron Detector (GRaND) while in low altitude mapping orbit, about 210 km from Vesta's surface. The measurements are representative of Vesta's bulk regolith composition to depths of a few decimeters with a spatial resolution of about 300-km full-width-at-half-maximum of arc length on the surface. The methods used to determine neutron absorption are described by PRETTYMANETAL2013." CITATION_DESC = "Prettyman, T.H., Dawn GRaND absorption map, DAWN-A-GRAND-5-VESTA-ABSORPTION-V1.0, NASA Planetary Data System, 2013." DATA_SET_DESC = " Data Set Overview ================= A global map of a compositional parameter, delta-C_perpendicular (DCP) and its 1-sigma statistical uncertainty are provided as an ASCII table. DCP varies linearly with the macroscopic thermal neutron absorption cross section of Vesta's regolith. The map data are given for 15-degree quasi-equal area pixels. Equations for converting DCP and uncertainties to physical absorption units are provided (below). The data reduction and analysis methods used to determine corrected neutron counting rates, sensitive to surface compositional variations, are described by PRETTYMANETAL2012. Epithermal and thermal+epithermal (TPE) neutron counting rates measured in Dawn's circular, polar, low-altitude mapping orbit (LAMO), at a mean distance of about 210 km from Vesta's surface were corrected for variations in the flux of galactic cosmic rays (GCR) and solid angle. The data were acquired in early LAMO (Time-interval IA, described in the Supplementary Information of PRETTYMANETAL2012, Fig.1 and Table 2) during a long period (30 days) of continuous instrument operation and steady GCR counting rates. A method to determine neutron absorption variations from thermal+epithermal (TPE) and epithermal counting rates is presented by PRETTYMANETAL2013. A scatter plot of normalized epithermal vs. TPE counting rates is used to derive the compositional parameter, DCP (unitless). The perpendicular distance (DCP, denoted ' delta-C_perpendicular' in PRETTYMANETAL2013) of each data point from a trend line, approximating the variation of epithermal vs. TPE counting rates with hydrogen abundance, is sensitive to neutron absorption. Assuming Vesta's global regolith is well represented by howardite (e.g. see PRETTYMANETAL2012), then the following relationship can be used to convert DCP to physical absorption units: Sigma_Eff = 216.4 * DCP + 66.3, where Sigma_Eff is the macroscopic thermal neutron absorption cross section with units of 10E-4 square-centimeters per gram. The propagated 1-sigma uncertainty in Sigma_Eff is given by Unc_Sigma_Eff = 216.4 * SIGMA_DCP The dependence of Sigma_Eff on elemental composition is described by PRETTYMANETAL2013. Sigma_Eff is a weighted, linear sum of the abundance of neutron absorbing elements in Vesta's regolith. Sigma_Eff is shown to vary linearly with the percentage of eucritic material in howardite. Parameters ========== Each row of the table provides the DCP parameter and 1-sigma uncertainty for a single rectangular pixel. The format for each row is '(i10,6(f7.1),e11.2,e10.1)'. East longitude convention is used (-180 to 180 degrees). The longitudes are given in the Claudia Double Prime coordinate system (see Confidence Level Note). Each pixel spans a separate and unique portion of Vesta's surface, and together, the pixels provide full global coverage. The BROWSE directory displays the data as a global map. The column descriptions follow: COLUMN NAME FORMAT DESCRIPTION UNITS 0 PIXEL_INDEX (I10) N/A 1 MIN_LAT (F7.1) Pixel latitude lower boundary deg 2 MAX_LAT (F7.1) Pixel latitude upper boundary deg 3 DELTA_LAT (F7.1) Width of the pixel in latitude deg 4 MIN_LON (F7.1) Pixel longitude lower boundary deg 5 MAX_LON (F7.1) Pixel longitude upper boundary deg 6 DELTA_LON (F7.1) Width of the pixel in longitude deg 7 DCP (E11.2) Neutron absorption parmeter unitless 8 SIGMA_DCP (E10.1) 1-sigma uncertainty unitless" CONFIDENCE_LEVEL_NOTE = " Confidence Level Overview ========================= The data set is a high order data product derived from neutron counting data acquired by GRaND in LAMO as described by PRETTYMANETAL2013. Review ====== This data set is in review by NASA Planetary Data System. Data Coverage and Quality ========================= The data set contains a global map of a compositional parameter (DCP) that varies linearly with the macroscopic thermal neutron absorption cross section of Vesta's regolith. There are no gaps in coverage. The depth sampled by thermal and epithermal neutrons from which DCP is determined is a few decimeters (for example, see PRETTYMANETAL2011). Coordinate System: The data were analyzed and mapped in the Claudia coordinate system, in which the prime meridian passes close to a small crater named Claudia. In this coordinate system, the prominent Marcia crater is located at 190E longitude. Maps published by the Dawn Geochemistry Working Group in the literature are presented in the Claudia coordinate system [PRETTYANETAL2012, LAWRENCEETAL2013B, PEPLOWSKIETAL2013, PRETTYMANETAL2013, YAMASHITAETAL2013]. Consistent with IAU guidelines, maps of GRaND data in this archive are presented in the 'Claudia Double Prime' (CDP) coordinate system. CDP has the same pole position as Claudia such that latitudes are the same; however, the prime meridian of CDP is shifted by 210 degrees from that of Claudia, passing 20 degrees to the east of Marcia. The coordinate systems are describes in detail by J.-Y. Li in VESTA_COORDINATES_120918.pdf, which accompanies this archive in the DOCUMENTS directory. All of the maps use rectangular pixels with east longitudes within -180 to 180 degrees in the Claudia system. For archiving, the longitudes were converted to CDP as follows: longitude_CDP = longitude_Claudia - 210 degrees. If longitude_CDP is less than -180 degrees, add 360 degrees. For example, if longitude_Claudia = -170 (the location of Marcia crater), then longitude_CDP = -20. Similarly, longitudes in the CDP system can be converted to Claudia as follows: longitude_Claudia = longitude_CDP + 210. If longitude_Claudia is greater than 180 degrees, subtract 360 degrees. Thus, the pixel longitude boundaries, which were tabulated in this archive in the CDP system, can easily be converted back to the Claudia system, in which the data were analyzed, by the user when desired. To further facilitate comparison between the PDS archived maps and those published in the literature, a 'side-by-side' display of each map in the Claudia and CDP systems is included in the accompanying browse file (see BROWSE directory). For further clarification, we note that map pixels in the Claudia system always have MIN_LON < MAX_LON; however, when transformed into the CDP system, not all pixels will meet this condition. For example, in some quasi-equal area maps, the polar pixels have MIN_LON = -180 and MAX_LON = 180 in the Claudia coordinate system. These pixels span 360 degrees in longitude. When transformed to the CDP coordinate system, these pixels have MIN_LON = -30 and MAX_LON = -30, such that MIN_LON = MAX_LON. In addition, for some maps, a portion of the pixels will have MIN_LON > MAX_LON when transformed into the CDP system. To avoid confusion, we included the width of each pixel in longitude as DELTA_LON. For example, for the aforementioned polar pixels, DELTA_LON = 360. Limitations =========== Assumptions and limitations underlying the analysis of neutron absorption by the vestan regolith are described by PRETTYMANETAL2012 and PRETTYMANETAL2013. The parameters of the linear equation that relates DCP to physical absorption units are estimated based on the assumption that Vesta's average regolith composition is similar to that of howardites. While this assumption is supported by spectral reflectance measurements (by telescope and by Dawn) and elemental abundance ratios measured by GRaND [PRETTYMANETAL2012], the mean absorption cross section of Vesta's regolith is not known. The range of absorption determined from GRaND data is expected to be consistent with Vesta's regolith on the ~300-km spatial scales sampled. The 1-sigma errors in DCP reflect the statistical uncertainties in mapped counting rates." 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