Mars Pathfinder APXS Oxides
Data Set Description

Table of Contents

Data Set Overview

The APXS_oxides is a listing of weight percent oxide abundances derived from the X-ray portion of the APXS_EDR data from the Alpha Proton X-ray Spectrometer (APXS). This data is preliminary and will be later refined as new calibration data become available. Currently, there are oxide results for five rocks and six soils.

Geochemists usually express the abundance of elements in rocks and minerals as weight percents of the oxides. It is a convenience that is followed because most rock forming minerals are stoichiometric compounds and it makes comparison and calculations easier. It does not mean that those specific oxides are necessarily found as minerals or compounds in the sample analyzed; it is only a way to recast the element abundances. Nor does it mean that Fe in the sample is 2+ rather than 3+. It is simply a way of expressing the chemical abundances by stoichiometric assignment to oxides. For geochemical convenience, we have recast the Pathfinder APXS elemental abundances of Na, Mg, Al, Si, S, K, Ca, Ti, Fe, and Cl to weight percent Na2O, MgO, Al2O3, SiO2, SO3, K2O, CaO, TiO2, FeO and Cl.

Further calibration is necessary to determine oxide abundances for the one other soil and four other rock measurements obtained at the Pathfinder landing site. The APXS bumper ring did not make good contact with the soil during A-9 measurements, so more testing is needed to determine the uncertainties on the oxides for this. The X-ray spectra for A-19, A-20, A-23, and A-27 rock measurements were degraded due to the rover battery death on Sol 56. The APXS_EDR data for these are available now, and oxide abundances will be determined at a later date.


The parameters are (1) weight percent abundances of chlorine and the oxides: Na2O, MgO, Al2O3, SiO2, SO3, K2O, CaO, TiO2, and FeO; (2) uncertainties on those abundances; and (3) the original sum of the oxides before normalization. The original sum varies in response to the measurement geometry and is closer to 100% if good contact is made between the sample and the APXS bumper ring.


The only input necessary for processing was APXS EDR data. These data files were the cumulative sums of data acquired from the beginning of the measurement cycle until the final reading. Readings occurred several times for each rock and soil that was analyzed. The first step in processing the data is to subtract subsequent spectra from one another and examine the deconvolved spectra to check for the possibility of any damaged data or instrument drift (e.g. by changing environmental temperature). The next step is to establish any changes in energy calibration, mainly due to changes in environmental temperature and shift the data to correct gain and zero-offset drift. Then the individual shifted spectra are summed together. The summed composite go into a least-squares fitting program that subtracts the background, finds all the peaks in the spectra, and calculates the peak areas and their uncertainties. Calibration curves (peak area versus concentration) for each element are used to derive abundances of each element. These calibration curves were obtained by analyzing terrestrial samples of known composition during the APXS calibration in the laboratory. Some corrections for matrix effects for a few of the elements are made after this. Then, to express the element abundances as oxide abundances, oxygen is assigned stoichiometrically (Fe as FeO, S as SO3, etc.) and the analyses are renormalized to an arbitrary value, in this case 98.0%. P2O5, Cr2O3, and MnO are not included in these preliminary results because they have large errors and the final calibration for these has not been completed.


All of the data in this data set are contained in an ASCII tabular file, ('OX_PERC.TAB') with a detached PDS label ('OX_PERC.LBL').

The tabular file is formatted so that it may be read directly into many database management systems (DBMS) or spreadsheet programs on various computers. All fields in the table are separated by commas; text fields are left justified and numeric fields are right justified. The 'start byte' and 'bytes' values listed in the PDS label do not include the commas between fields. The records are of fixed length, and the last two bytes of each record contain the ASCII carriage return and line feed characters. This allows the table to be treated as a fixed length record file on computers that support this file type and as a normal text file on other computers.

The PDS label is object-oriented. The object to which the label refers (the TABLE) is denoted by a statement of the form:

^object = location

in which the carat character ('^', also called a pointer in this context) indicates that the object starts at the given location. For an object located outside the label file (as in this case), the location denotes the name of the file containing the object. For example:


indicates that the TABLE object is in the file OX_PERC.TAB, in the same directory as the detached label file. The detached label file is a stream format file, with a carriage return (ASCII 13) and a line feed character (ASCII 10) at the end of each record. This allows the file to be read by the MacOS, DOS, Unix, and VMS operating systems.

Ancillary Data

Calibration APXS measurements obtained in the laboratory.

Coordinate Systems

Estimates of the locations of the 5-cm diameter spots measured on rocks and soils on Mars are reported as XYZ coordinates in the Martian Local Level Coordinate Frame. Only those for which the oxide abundances have been determined are listed:

X Y Z Rock/Soil APXS Target
A-2 1.89 -1.95 0.31 soil off the end of the ramp
A-3 1.30 -2.45 0.18 Barnacle Bill rock
A-4 2.79 -2.64 0.28 soil near Yogi
A-5 3.29 -2.48 0.28 soil near Yogi
A-7 4.58 -2.91 -0.18 Yogi rock
A-8 2.85 1.13 0.32 Scooby Doo indurated soil or rock
A-10 3.74 -0.43 0.28 dark soil next to Lamb
A-15 -5.87 2.80 0.52 Mermaid dune
A-16 -3.79 -1.31 0.12 Wedge rock
A-17 -5.56 -3.25 -0.35 Shark rock
A-18 -4.81 -3.81 -0.54 Half Dome rock, first location

The Mars Pathfinder Lander (L) Coordinate Frame

The Mars Pathfinder Lander is a tetrahedral structure. One of its faces, the one upon which it sits, is called the base petal and houses most of the lander equipment. The other three faces, or petals, open after surface impact to expose these systems. The rover is mounted on one of these petals. The Mars Pathfinder Lander Coordinate Frame, or 'L' Frame, has the lander base petal as its reference plane and its center coincident with the geometric center of the base petal. The YL-axis of this coordinate system passes through the geometric center of the rover petal, and defines the reference direction. The ZL-axis is normal to the reference plane and coincident with the nominal spacecraft spin vector. When the lander is upright on the surface, the ZL-axis is directed positively downward into the ground.

The Martian Local Level (M) Coordinate Frame

The Martian Local Level Coordinate Frame is a right handed, orthogonal, frame whose origin is co-incident with the origin of the Lander Coordinate Frame. The XM axis points north, the YM axis points east, and the ZM axis points down.

For more information on Mars Pathfinder coordinate systems, see the [MELLSTROM&LAU1996], [WELLMAN1996B], and [VAUGHAN1995] references. However, please note that as of the time this APXDDRDS.CAT file was written, [WELLMAN1996B] had not yet updated his discussion of elevation measurements to match that agreed upon by the Project. Where he used elevation ranges of 0° to 180°, the MPF Project used -90° to +90°.


The APXS oxides data can be displayed on UNIX, Macintosh, and PC platforms using any ASCII editor.

Media / Format

The APXS oxides data will be stored on compact disc-read only memory (CD-ROM) media. The CDs will be formatted according to ISO-9660 and PDS standards. The data files will not include extended attribute records (XARs), and will therefore not be readable on some older VMS operating systems.

Confidence Level Overview

These oxide abundances are preliminary results for the X-ray portion of the APXS_EDR data. The confidence is indicated by the uncertainties that are assigned to each of the oxides in the APXS_oxides ASCII table. These uncertainties were derived from the range in differences found between recommended and measured values for eight reference standards.


Prior to release, the data will be reviewed by the APXS instrument team and the Planetary Data System.

Data Coverage and Quality

The quality of the preliminary X-ray oxide abundances is indicated by the results for Murchison C2 meteorite and the Martian meteorite Zagami [RIEDERETAL1997B]:

  (1) (2) (3) (4) (5)
Na2O 2.3 0.7 to 1.2 1.5 0.7 0.2
MgO 8.8 8.6 to 11.6 18.2 18.2 19.9
Al2O3 7.1 4.8 to 6.2 2.4 2.3 2.3
SiO2 49.6 48.4 to 50.9 31.0 31.0 28.5
SO3 0.3 0.15 to 0.29 7.8 7.8 7.9
K2O 0.25 0.13 to 0.24 0.06 0.04 0.04
CaO 10.9 9.7 to 11.1 2.0 1.8 1.9
TiO2 1.0 0.74 to 1.4 0.04 ---- 0.06
FeO 17.4 18.0 to 24.5 30.2 30.2 27.1

(1) Zagami Martian meteorite rock slice, APXS analysis for a counting time of 127,470 seconds. (2) Zagami Martian meteorite, five individual chips of about 0.5 g each, measured using instrumental neutron activation analysis (INAA), X-ray fluorescence (XRF) and carbon-sulfur analyzer (GSA) at Max-Planck Institut fur Chemie. (3) powdered Murchison C2 meteorite, measured using an APXS for a counting time of 242,030 seconds. (4) powdered Murchison C2 meteorite, measured using an APXS for a counting time of 20,360 seconds. (5) powdered Murchison C2 meteorite, measured using INAA, XRF and GSA at Max-Planck Institut fur Chemie.

Results for the soil A-2 are not as good as the others, due to poor contact with the sample by the APXS deployment mechanism and lower counting rates for alpha particles, protons, and X-rays. This is reflected in the lower original sum of the oxides for A-2.

When measuring rock and soil samples, the desire was to obtain at least 10 integrated hours. Only 3 hours of nighttime measurement were needed for a good X-ray analysis. X-ray spectra obtained during the night, when ambient surface temperatures were low, were unaffected by electronics noise. Ten hours of measurement during the day or night provide good alpha and proton analyses. Shorter times still provide useful results. The measurement times for the 11 APXS measurements that have been converted to oxide abundances are shown below:

Measurement initial start
time and final stop time
(Local True Solar Time)
meas. time
APXS spectra
accumulation time
A-2 Sol 2 14:53 - Sol 3 10:00 19.6 15.9 soil
A-3 Sol 3 15:00 - Sol 4 07:01 16.5 13.6 rock
A-4 Sol 4 16:59 - Sol 5 01:32 8.8 8.1 soil
A-5 Sol 5 16:01 - Sol 6 06:55 15.3 9.2 soil
A-7 Sol 10 14:17 - Sol 11 02:37 12.7 5.7 rock
A-8 Sol 14 14:03 - Sol 15 02:55 13.2 5.7 soil
A-10 Sol 20 14:03 - Sol 21 02:59 8.3 7.0 soil
A-15 Sol 28 14:05 - Sol 29 02:44 8.0 5.3 soil
A-16 Sol 37 14:07 - Sol 38 03:05 8.2 6.5 rock
A-17 Sol 52 14:18 - Sol 53 03:05 8.0 7.0 rock
A-18 Sol 55 14:06 - Sol 56 00:05 7.2 5.9 rock

Measurement initial start and stop times were obtained from the SCLK times in the downlink telemetry for the acknowledgement of the exact commands that were issued to trigger the start and stop of each APXS measurement (usually Meas_Start, Meas_Stop, Reset, or Shutdown). SCLK times were converted to Local True Solar Time using the script sclk2ltmst (see Ancillary Data discussion). This time is only as accurate as the rover's clock, and is a close approximation to the exact initial start and stop time of each APXS measurement. In a few cases, the downlink was lost, and uplink predictions were used instead. Some of the cumulative APXS measurements were interrupted by other rover activities, in which case, the first accumulation start time and last stop time are indicated. Integrated measurement time, as indicated by ALPHA_SAMPLING_DURATION, PROTON_SAMPLING_DURATION, and XRAY_SAMPLING_DURATION in the data file headers, is always less than the accumulation final stop time minus the initial start time, because the sampling durations do not include quiet periods when the APXS was powered off and detector 'dead time'.


Further calibration and special processing is underway to improve the conversion of X-ray data to elemental and oxide abundances, and to calibrate the alpha and proton portions of the data and combine them with the X-ray data.

Related Information


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