| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data product describes a map of the high-energy gamma-ray (HEGR)
count rate across the surface of Vesta. As shown in PEPLOWSKIETAL2013,
HEGRS are sensitive to the bulk elemental composition of the near
surface material, particularly its heavy major-element content.
Parameters
==========
Each row of the table provides the high-energy gamma-ray (HEGR) count
rate information for a single rectangular pixel. The format for each
row is '(i10,6(f7.1),f8.2)'. 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 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 HEGR_COUNTING_RATE
(E14.4) HEGR count rate c/s
8 HEGR_COUNTING_RATE_UNCERTAINTY
(E14.4) HEGR count rate uncertainties c/s
9 HEGR_SAMPLES (I10) Number of samples per pixel c/s
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
The data set is a high order data product derived from gamma-ray
counting data acquired by GRaND in LAMO as described by PEPLOWSKIETAL2013.
Review
======
This data set is in review by NASA Planetary Data System.
Data Coverage and Quality
=========================
The data set contains a global map of the high-energy gamma ray count rate
contributions from Vesta's regolith. There are no gaps in coverage. The
depth sampled by high-energy gamma rays is a few 10s of decimeters.
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,
LAWRENCEETAL2013, 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 the leakage flux of
high-energy gamma rays produced within Vesta's regolith are described by
PEPLOWSKIETAL2013. We note that the tabulated values give the high-energy
gamma ray count rates after correcting for background signals and
variability due to solid angle, orientation, and galactic cosmic rays. The
data are subsequently smoothed over the footprint of the GRaND instrument.
The resuling values are proportional to the heavy major-element composition
of the surface.
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