1969 Mariner 7 IRS: Data Set Recovery and Calibration

Laurel E. Kirkland¹, Kenneth C. Herr², Paul B. Forney³, and Donald K. Stone²

 ¹Lunar and Planetary Institute, Houston, TX; ²The Aerospace Corporation, El Segundo, CA; ³Lockheed Martin Missiles and Space, Palo Alto, CA

Introduction.  In 1969 the Mariner 7 Infrared Spectrometers (IRS) returned a unique set of approximately 140 infrared spectra of the planet Mars covering the wavelength region from 1.8 to 14.4 µm (5550 - 690 cm-1).  Meanwhile a third IRS was actively measuring lab spectra of gases, minerals and ices thought to be present on the "Red Planet."  Unfortunately, over the following three decades, all the IRS inflight calibration data, much of the preflight calibration data, and all of the IRS lab data were lost to the planetary science community.

The IRS spectra contain a wealth of information, but in many ways the data set remains an untouched resource.  In main this is because the IRS spectra of Mars were never released in a version calibrated in wavelength and intensity, and because the lab spectra were never released at all.  Also, computers have improved to the point that it is now practical to manipulate the data more extensively.  Therefore, we desired to recover and calibrate this unique data set.

We located the original IRS data tapes, recovered the spectra measured of Mars, collected instrument and calibration information from the original IRS team, and proceeded with the calibration [Forney and Kirkland,1997; Kirkland et al., 1998].  Thus for the first time since the 1970's, we have IRS spectra that are calibrated in wavelength and intensity using the original data set and calibration information and expertise from the original IRS team.

Our examination shows the calibrated IRS spectra are of high quality.  They have a very good signal to noise ratio, and the extended wavelength coverage has proven informative.  Here, we present information on the recovery and calibration of IRS-7 spectra for the thermal region, and show a preliminary comparison to spectra recently returned by the 1997 Mars Surveyor Thermal Emission Spectrometer (TES), which measures from ~6 to 50 µm (1670 - 200 cm^-1).  Table 1 lists the instrument parameters for IRS, the 1971 Mariner Mars Infrared Spectrometer (IRIS), and TES.  Appendix 1 lists publications that have utilized IRS data.

Table 1: Instrument parameters for IRS, IRIS, and TES.

Instrument.  IRS used two circular variable interference filters (CVIF) with ~1% resolution to scan continuously two channels, from 1.8 to 6.0 µm (channel 2) and 4.0 to 14.3 µm (channel 1) [Herr et al., 1972].  The two channels measured contiguous, overlapping swaths, with the long wavelength channel offset north of the short wavelength channel.  IRS had a spatial resolution of 130 x 80km at closest approach, and each spectrum contains approximately 1340 discrete measurements which were scanned every ten seconds.   Figure 2 and figure 3 show the optical layout.

Uniqueness of IRS data.  Even after 30 years the IRS data set remains unique in the following ways:  (1) it is the only data set which contains both spectra of Mars and also Mars analog minerals, ices, and gases measured with identical instruments and with the laboratory instrument held under simulated flight conditions;  (2) it contains the only spacecraft spectra of Mars which cover both reflected light and thermal emission regions. This is important because each spectral region provides complementary information;  (3) it contains the only laboratory spectra measured of possible atmospheric gases at Mars ambient conditions. Furthermore, these were measured with an instrument identical to the ones which returned the spectra of Mars;  (4) currently it can provide the only internally calibrated spacecraft spectra of the 1.8-3.1 µm region, which includes the important 3 µm hydration band.  The 1989 Phobos 2 Imaging Spectrometer for Mars (ISM) measured from 0.77-3.1 µm, but as a result of problems with its internal calibration, its spectra must rely on a calibrated that forces them to match telescopic spectra (from 0.77-2.5 µm) [Mustard et al., 1993], or a laboratory analog of Phobos (from 2.5-3.1 µm) [Erard et al., 1994];  (5) it contains the only spacecraft coverage from 3.1 to 5 µm;  (6) it contains the only spacecraft coverage from 5 to ~7.5 µm measured with good SNR;  (7) it contains the best spectral resolution coverage of the 9 µm band measured while the atmosphere was clear. Also of note are IRS polar cap, post-terminator, and limb scans.

IRS and TES provide the only spacecraft coverage of the informative 9 µm region measured while the martian atmosphere was very clear.  The only other spacecraft instrument to cover this region was the 1971 Mariner 9 IRIS, which measured spectra during and just after a dust storm.  IRS has a better spectral resolution than TES, but TES has better spatial resolution (3 vs. 130 km), and should ultimately have far better spatial coverage.

Lab data.  The complete IRS data set includes transmission, reflection, and emission-absorption spectra of gases, minerals, ices, and mineral-ice combinations thought to be possible Mars analogs.  The lab measurements were made with a third, flight qualified IRS, called Flight C, which was identical to the instruments which flew on the twin Mariner spacecraft.  To minimize the thermal stray light problems, and to simulate the deep space background, Flight C was placed in an environmental chamber which had liquid nitrogen cooled walls and high emissivity baffles [Herr et al., 1998].   Figure 4 shows the small environmental test chamber that was used to measure the minerals and ices. The large chamber (figure 1 and figure 5) was used for the long path cell and calibration measurements.

Of particular interest are laboratory spectra measured of gases such as CO₂ at temperature and pressure conditions thought typical for Mars.  Flight C measured gas samples using a temperature controlled long path cell, which used multiple reflections to provide path lengths up to 2540 meters [Horn and Pimentel 1971; Horn et al. 1972; McAfee 1974].  The long path length is necessary to allow a spectrometer to measure spectra of gases at the low pressures typical of Mars. The cell simulated martian conditions and produced an accurate characterization of martian atmospheric absorptions.  This type of measurement has never been repeated, so these spectra remain unique.  Gases measured include CO₂, CO, CH₄, NO₂, N₂O, OCS, O₃, and NH₃.

Figure 2:   IRS optical layout.   This was scanned from an original blueprint from the Principal Investigator's archive, (Dr. George Pimentel), courtesy of Jeanne Pimentel.

Figure 5:   IRS large ETC configuration.   This shows IRS on the large ETC mount. The gold plated radiator "fence" is visible under IRS, and the two cooling gas bottles are on the right. IRS was optically coupled to the long path cell for gas measurements while IRS was held in the large ETC.

IRS data set loss.   From 1969 to 1972 the original IRS team at UC Berkeley used IRS spectra to derive information about the surface and atmosphere of Mars, but the group disbanded after their NASA contract ended in 1972.  Although they had given NASA tapes of the flight data, these were later misplaced, and this resulted in the complete loss of this data set to the planetary science community.

However, the IRS group had given a researcher at UCLA a data tape containing part of the IRS data set [Pleskot and Kieffer, 1977], and in the mid 1980s, Dr. Terry Martin archived this partial data set with NASA [Martin and Kieffer, 1985].  Unfortunately, the tape was missing all the flight calibration spectra, most of the preflight calibration information, and all of the lab spectra. Nonetheless, the availability of the data inspired a new round of Mars research using IRS spectra.  However, the lack of calibration spectra, combined with uncertain knowledge about the original data format and processing that had been applied to the data, resulted in calibration problems which eventually caused most of the newer research to end in frustration.  The data set had the potential to provide unique information on the surface and atmospheric composition of Mars, but much of the information remained inaccessible without a solid calibration.

Data recovery and calibration.  We located the original set of 7-track data tapes that contained the complete IRS data set, and recovered from these tapes all the IRS spectra of Mars, and began recovering the laboratory spectra. Members of the original IRS team participated in the data recovery and calibration [Forney and Kirkland, 1997; Kirkland et al., 1998].  The thermal wavelength spectra of Mars have been calibrated in wavelength and intensity, and the reflected wavelength spectra have received a preliminary wavelength calibration.  Once the recovery and calibration is complete, we plan to archive the complete data set with the NASA Planetary Data System.

One of the chief difficulties encountered by earlier researchers was calibrating the spectra in wavelength [Pleskot and Kieffer, 1977; Martin, 1990; Calvin, 1990].  These early efforts calibrated each spectrum by hand using atmospheric absorptions as wavelength references.  Uncertainties about the processing that had been applied to data on the tape given to Martin apparently caused the main source of difficulty, so we began by recovering the original, unprocessed data, and then drawing on the knowledge of IRS team members for the calibration.  The person who originally calibrated IRS, Paul Forney (IRS Project Scientist), provided without hesitation his original IRS wavelength calibration.  His method proved quick, simple, and effective, and can be applied to all the spectra, thus eliminating the time-consuming hand calibration.

This step-wise approach produced a methodical reconstruction and calibration of the IRS spectra of Mars from ~5 to 14.4 µm, which includes the spectral region where TES and IRS spectra overlap (~6 to 14.4 µm).  Calibration of the shorter wavelength IRS data is currently in progress.

The wavelength calibration precision was checked by calculating the standard deviation of the center of the sharp 790 cm-1 atmospheric CO2 band.  This calculation uses all the dayside, non-polar IRS-7 spectra that have good SNR.  The standard deviation is 0.0140 µm (1.75 cm^-1).  The accuracy was checked by comparing IRS band centers to an average IRIS spectrum.

The IRS story emphasizes the importance of obtaining original expertise for recovering and fully utilizing data measured by a complex instrument.  It was crucial to understand how the instrument produced the data, and not just have the data.  With the help of abstract searches and the Internet, finding the IRS crew was not difficult.  Without their help, recovering and fully understanding the IRS spectra would not be possible.

COMPARISON TO TES DATA

The Mars Global Surveyor Thermal Emission Spectrometer (TES) recently returned spectra from Mars covering ~6 to 50 µm (1700 - 200 cm^-1), and the spectra are in the early stages of calibration and interpretation [Christensen et al., 1998].  TES and IRS have comparable spectral resolutions at 10 µm (20 cm^-1 for TES and 10 cm^-1 for IRS), and both measure this spectral region with good signal-to-noise ratios (~400 for TES and ~600 for IRS, both r.m.s.) [Kirkland et al., 1998; Christensen, 1998].  Since TES and IRS make similar measurements, we desired to compare spectra from both data sets.

Table 1 gives instrument parameters for TES. A preliminary TES archive was released in June 1998.  We converted the archived spectra from radiance to brightness temperature and apparent emissivity [Conel, 1969] with respect to 7.75 µm, the wavelength chosen based on our work with IRS spectra.

Instrument.  At the heart of TES is a Michelson interferometer spectrometer that measures ~6 - 50 µm (1667 to 200 cm^-1) [Christensen et al., 1992]. TES also has a broadband thermal channel (5.5 - 100 µm) and a solar reflectance channel (0.3 - 2.7 µm).  In the most commonly used spectral mode, the "10 cm^-1 sampling mode," TES returns spectra containing 143 discrete measurements over a range of ~1470 cm^-1, and so records one measurement approximately every 10 cm^-1.  TES also has a "5 cm^-1 sampling mode," which has better spectral resolution and records 286 measurements, but these spectra are not yet available for comparison to IRS.

In the "10cm^-1 sampling mode," the TES unapodized spectral resolution is 12 cm^-1, which is the full width at half maximum of the theoretical response function of TES (a sin[x]/x function, Beer, 1992; Christensen et al., 1992) for a mirror throw of 0.25 mm.  Optical apodization caused by the off-axis positioning of each detector within the 2x3 detector array degrades the spectral resolution from that value [Christensen et al., 1992]. In region of the IRS - TES spectral overlap, each TES measurement has an optically apodized FWHM spectral resolution of ~15 to 24 cm^-1 [Christensen, pers. comm., 1998].  The "5 cm^-1 sampling mode" measures with higher resolution. The off-axis positioning also causes each detector to measure a slightly different wavelength.

TES spectra.  An archive of TES spectra with a preliminary calibration to radiance was released in June 1998 (Mars Global Surveyor Sampler, MGS_001). TES has 6 detectors in a 2x3 array, and each requires its own, slightly different wavelength file [Christensen et al., 1992], but the preliminary archive contains only one file. Comparison with IRS spectra indicates the wavelength file archived is most accurate for detectors 2 and 5, which are the two center detectors.   Figure 6 shows a comparison of IRS and TES spectra, using TES spectra measured by detector 5.  The most prominent atmospheric features present in both data sets are given below in Table 2. Figure 7 shows representative IRS data of the shorter wavelengths, but these spectra are not yet calibrated.

Table 2: Prominent atmospheric features in the IRS and TES overlap region.

with upper values in cm^-1 and lower in µm.  From McAfee, 1974; McAfee pers. comm., 1998; Herr et al., 1998.

Figure 6:  IRS - TES comparison.   Red trace is an IRS-7 spectrum, and black and green are TES. Each TES spectrum is the average of three spectra from orbit 34, measured by detector 5. Shown with IRS M7-106. Both spectra are converted to apparent emissivity with respect to 7.75 µm.

Figure 7:  IRS short wavelength data.   This shows an example IRS spectrum covering part of the shorter wavelengths. The full spectrum continues to 6 µm. The short wavelength IRS spectra have received a preliminary wavelength calibration, and the intensity calibration is currently in progress. This shows the spectrum before and after a noise filter has been applied, and illustrates the improvement that can be obtained in the signal-to-noise ratio. The lower trace (black) shows spectrum M6-183 before filtering, and the upper trace (gray) the same spectrum after filtering (offset +5 for clarity). Spectra shown in linearized intensity units.

References