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Availability: Archived at NSSDC, accessible from elsewhere


This description was generated automatically using input from the Planetary Data System.

Data Set Overview ================= A Mars atmospheric opacity data product consists of two files, an ASCII formatted detached PDS label file and an ASCII formatted data file. The data file contains values of the Mars atmospheric opacity or optical depth derived from PHX SSI images of the Sun acquired with the two solar filters. The effective wavelengths of these filters are 451, 671, 887 and 991 n. Each data file contains an ASCII table of the derived atmospheric opacity for the given SSI solar filter. Each table contains columns with the source SSI image identifier, the time of image acquisition, the Mars season (Ls), Sun-Mars distance, airmass, observed solar flux, and opacity.

Processing ========== PHX SSI Atmospheric Opacity RDRs are considered Level 5 or Derived Data (equivalent to NASA Level 2). The Mars atmospheric opacity data products are generated from analysis of PHX SSI images.

The Mars atmospheric opacity data products are produced by the Surface Science Imager team using processing procedures and software developed by Mark Lemmon, Texas A&M University.

The data product is generated after each sol in which opacity data is acquired. The generation is done in four steps. First, the input parameters are set up. A list of SSI data products to be processed is read and associated values are determined for Ls, the distance of Mars from the Sun, the sol that the data were acquired, and the actual elevation angle of the Sun. It is likely that standard tools such as the NAIF toolkit will be used for these computations. For each SSI data product, the airmass is computed by integration through a spherically symmetric atmosphere with a scale height equivalent to the gas scale height of the Martian atmosphere.

Second, the solar flux is extracted from each calibrated SSI data product. To do this, the background is determined within an annulus at a fixed radius from the center of the Sun in the image. That background is subtracted, as it would lead to a significant departure from Beers' Law at high airmasses. After background subtraction, the solar flux is integrated over the image. The presence of a few missing pixels (e.g., a Phobos transit or a missing packet that only partly overlaps the Sun) can be accommodated by the integration algorithm. The presence of a large number of missing pixels or any saturated pixels will result in the rejection of an image (returning a flux and opacity of -1.000).

Third, a relative calibration is derived. Data from the afternoon of all sols during which more than 1 image was acquired are considered, together with instrumental uncertainties. The published calibration is considered as a single datum with associated uncertainty. The instrument response is varied, and a single best-fit opacity is derived for each afternoon using Beers' Law (I_observed = I_0 exp (-t h), where h = airmass). A best-fitting responsivity is chosen by minimizing the reduced chi-squared of the fit.

Fourth, the relative calibration is used to derive opacities. All images are considered, and Beers' Law is applied to every pair of I_observed and airmass. The relative calibration method ensures that (1) substantial calibration uncertainty is not propagated into uncertainty in opacity once sufficient surface data are obtained, and (2) that the processing transfers smoothly from using the laboratory calibration when the first datasets are obtained to using the relative calibration when enough surface data exist.

Data ==== Each Mars atmospheric opacity data product is structured as two files; a detached PDS label file and a separate data file. Both components are stored as ASCII text. Data within the opacity data file is organized by time with the most recent measurement being appended to the end of the file.

Each Mars atmospheric opacity data product consists of two parts. The first part of the data file contains header information, which includes parameter values used in the opacity computations and column names for the data rows. The second part of the file, starting at line 10 consists of a PDS table object. The table has eight columns and a variable number of rows. There is one row for each opacity measurement. The number of rows in a data product will increase as new measurements of atmospheric opacity are made. Each row is 88 bytes long including the carriage return and line feed characters. All columns are fixed-width as described in the PDS label and are also delimited with commas. Text columns are surrounded by double-quotes and are left-justified. Numeric columns are right-justified.

Software ======== The ASCII format of the Mars atmospheric opacity data product means that the data can be displayed using a text editor. In addition, the use of the PDS table structure for this data product means the data can be readily imported into spreadsheet and plotting programs.

PDS-labeled tables can be viewed with the program NASAView, developed by the PDS. NASAView is available in versions that run on SUN/SOLARIS, Windows, and LINUX operating systems. NASAView can be obtained from the PDS web site There is no charge for NASAView.

These data are available on-line from the Planetary Data System (PDS) at:

Alternate Names



  • Planetary Science: Atmospheres

Additional Information



Questions and comments about this data collection can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Dr. Mark LemmonData ProviderTexas A&M
Dr. Mark LemmonGeneral ContactTexas A&M
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