Abstract

We used the NSFCAM 256×256 InSb array camera at the NASA Infrared Telescope Facility to gather near-infrared (NIR) spectral image sets of Mars through the 1995 opposition. In previous studies with these data [1-6] we noted several interesting spectral features, some of which are diagnostic volatile absorption bands that allow the discrimination between CO₂ or H₂O ices. Band depth maps of these regions show polar and morning and evening limb ices composed of water and some indication of polar CO₂ ices. Other features, near 3.33 and 3.4µm, appear to be confined to particular geographic regions; specifically Syrtis Major. However, the images used in these previous studies were calibrated to either the disk average or only to a rough scaled reflectance by simple division by solar-type star data gathered at the same time as the images. This only allowed determinations of spectral features either relative to some global average of the feature, or to some unit not directly comparable to other published data. For at least three of our observation nights the conditions and data are sufficient to absolutely calibrate the images to radiance factors. For this work we reinvestigate the spectra and band depth mapping results using these absolutely calibrated images. In general we find that bright regions have peak radiance factors of 0.5 to 0.6 at 2.25µm and 0.3 to 0.4 at 3.5µm; dark regions have radiance factors of 0.2 to 0.25 at 2.25µm and 0.1 to 0.15 at 3.5µm. Overall, precision errors are about 0.025 in radiance factor and absolute errors are at the 10 - 15% level. These results are consistent with previous studies that found radiance factors of 0.35 in Tharsis, 0.47 in Elysium, and 0.26 in dark regions at 2.25µm [7,8] and 0.3 in bright regions and 0.1 in dark regions at 3.5µm [8]. These absolute flux values will allow direct comparison of these results to radiative transfer models of the behavior of the surface and atmosphere and also provide an independent measurement for comparison to, and calibration of, imaging and spectral data acquired by spacecraft orbiting Mars.

The Data Set

These data are a subset of images gathered over the 1994-95 opposition using the NSFCAM and its 1% spectral resolution circular variable filter (CVF) at the NASA Infrared Telescope Facility (IRTF). Details on the three nights presented here are summarized in the table below:

	1995 FEB 01	1995 FEB 04	1995 MAR 14
LS	53.2°	54.5°	71.1°
Sub- Longitude	331°-337° 352°-360°	323°-328°	301°-306°
Sub- Latitude	19.5°	19.3°	16.8°
Size	13.6"	13.7"	12.1"
Data mode	32 color Nyquist	Nyquist	32 color

The two data modes differ by the number of images per spectral scan. In 32 color mode, Mars is in 32 wavelengths between 1.56 and 4.1µm chosen to be diagnostic of volatiles and climatically important minerals. In Nyquist mode, Mars is imaged at 48 wavelengths from 3 to 4.15µm in steps that are half the CVF spectral resolution.

Reduction and Calibration

The Mars images were reduced using standard IR techniques: linearization, sky-subtraction, flat-fielding, bad-pixel replacement. Standard star (BS4030) images taken on the same nights were similarly reduced. Since the star was imaged several times on each night, the atmospheric extinction can be calculated. The standard star is a G2IV [9,10,11] star with infrared colors that match both the sun and the solar-type spectral standard µHer [12] to within 0.02 magnitudes. Using color ratios of BS4030 to µHer we can calculate a flux spectrum for our standard star and thus calibrate the DN count aperture photometry to true flux for all observed wavelengths.

Using these DN-to-Flux calibrations we then convert the Mars images from DN counts to radiance factor using the same techniques as Bell, et al. [7] and Roush, et al. [8]:

Where is the appropriate flux per pixel in the Mars images, F is the appropriate stellar flux, F is the solar flux at 1AU, D is the Sun-Mars distance in AU, W is the angular size of a pixel (i.e. "aperture" size), and the IR stellar ratio is the average flux ratio of BS4030 to µHer in the infrared J, H, K, L and M bands (which is equal to 0.0879±0.0004).

Extracted Regional Spectra

Regional spectra have been extracted from the radiance factor images. Regions were chosen to cover a wide range of classical albedo and a wide range of atmospheric pathlength. The radiance factors plotted are averages over an area that is roughly the size of the seeing limit of about 0.6-0.8". Error bars plotted are the 1 standard deviation of the radiance factors in the region averaged and are about 0.025. In general the bright regions have peak radiance factors of 0.5 to 0.6 at 2.25µm and 0.3 to 0.4 at 3.5µm which is consistent with previous studies that found bright region radiance factors of about 0.47 at 2.25µm and 0.3 at 3.5µm [7,8]. Dark regions in our images have radiance factors of 0.2 to 0.25 at 2.25µm and 0.1 to 0.15 at 3.5µm compared to previous studies' values of 0.26 at 2.25µm and 0.1 at 3.5µm [7,8]. Overall absolute errors are at the 10 - 15% level.

Differences in radiance factors could be due to differences in viewing geometries; it is seen in these spectra that the radiance factor of Arabia changes by 0.1 from when it is sub-Earth on 950201 to when it is on the morning limb on 950314. These changes could also be affected by the amount of ice cloud cover, which would be higher near the morning limb or perhaps due to changes in the intrinsic brightness of the regions over time.

Key features seen in the spectra are the broad, deep absorption at about 2.00µm due in part to Mars atmospheric CO₂. The downturn beyond 4.00µm, also due to the Mars atmosphere, can be seen quite well in the topographically low Hellas basin. We also note the much lower 3.25-4.00µm radiance in the north polar region due to the water ices there. This behavior of the Hellas spectra would indicate that it is covered by an ice cloud.

The high resolution spectra of 950201 and 950204 show some strange behavior from 3.2 to about 3.4µm. This "wiggle", not seen previously in relatively calibrated data, appears to be a systematic error perhaps having to do with NSFCAM itself, and as yet no correction has been devised.

Band-Depth Maps

In previous, relatively calibrated work [1,6] absorption features at 3.33 and 3.4µm were found in Syrtis Major. These bands lie close to features seen by Sinton [13] which he attributed to C-H absorption. Two of these original absorptions were later explained as due to telluric HDO and D₂O [14,15] leaving the feature at about 3.4µm unexplained.

Band depth maps created at 3.33 and 3.4µm relative to a local continuum between 3.2 and 3.5µm using images calibrated to the disk average, showed that the features are localized to northern Syrtis Major and are enhanced over the disk average by about 10% [1,6].

In this work we have recreated these band depth maps using the radiance factor images for the original date studied (950204) and the two other dates where Syrtis Major is visible (950201 and 950314). The band depth value is defined by the equation [16]:

The absorption features show clearly in the band depth maps from 950204 with a band depth value of about 0.1 (i.e. a 10% feature). In the 950201 images Syrtis Major is very close to the planet limb but does appear as a 5-10% feature. On 950314 Syrtis Major is well centered on the image disk. In these band depth maps the region has a band depth of about 5-10%, although it does not appear to stand out from the rest of the disk as well as the previous observations.

The absorption features appear to lie close to C-H absorption features. The fact that the band depth of the region appears constant relative to atmospheric pathlength would seem to indicate that the features are due to a ground source. However, the changing band depth of regions around Syrtis tend to cloud the issue and more work is needed to properly interpret these features, especially in light of the spectral "wiggle" noted above.

	950201-3	950204-3
3.262µm Image
3.20-3.33-3.51
3.20-3.40-3.51

	950201-1	950314-1
3.270µm Image
3.20-3.33-3.51
3.20-3.40-3.51

Conclusions

We have shown that these data can be absolutely calibrated and are consistent with previous absolutely calibrated data. These radiance images can be used with radiative transfer models to model the amount of volatile and dust clouds in the Martian atmosphere and to continue our search for CO₂ clouds near the Martian poles.

Our data also show that there may be a systematic problem with the NSFCAM in the 3.2 to 3.4µm region when observing extended objects. A correction for this effect must be devised in order to properly investigate the absorption features at 3.33 and 3.4µm seen in band depth maps both here and in the relatively calibrated data.

References

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[2] Klassen, D. R. et al. (1995) BAAS, 27, 1061.

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[4] Bell III, J. F. et al. (1996b) BAAS, 28, 1063.

[5] Klassen, D. R. (1997) Ph.D. Dissertation, Univ. Wyoming.

[6] Klassen, D. R. et al. (1998) LPSC XXIX, abstract no. 1658.

[7] Bell III, J. F. et al. (1994) Icarus, 111, 106.

[8] Roush, T. L. et al. (1992) Icarus, 99, 42.

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[11] Gezeri, D. Y., et al. (1993) Catalog of Infrared Observations, NASA Public Reference 1294.

[12] Strecker, D. W., et al. (1979) Astr. Phys. Supp., 41, 501.

[13] Sinton, W. (1959) Science, 130, 1234.

[14] Shirk, J. S. et al. (1965) Science, 147, 48.

[15] Rea, D. G., et al. (1965) Science, 147, 1286.

[16] Bell III, J. F. and Crisp, D. (1993) Icarus, 104,2.

For more information mail to: klassen@rowan.edu