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Infrared Imaging Spectroscopy of Martian Clouds and Volatiles

as presented at the 27^th Annual Meeting of the Division for Planetary Sciences of the American Astronomical Society in Mauna Lani, Hawaii

D. R. Klassen (Univ. of Wyoming)
J. F. Bell (Cornell University)
R. R. Howell (Univ. of Wyoming)

The Data Set

The work here uses a subset of a much larger image set taken by Jim Bell at the NASA Infrared Telescope Facility using the new NSFCAM 256x256 InSb camera. (More information on this larger data set can be found in Jim Bell's presentation. Note: this is a one-way link so to get back here you will have to use the 'back' function of your Web browser.) The images were taken using the camera's circular variable filter (CVF) which has a spectral resolution of about 2%. The data for this work were taken on 1 FEB 95 at an L_s= 54° (northern spring on Mars).

Recent Hubble images have shown that during the northern spring season Mars has shown considerable cloud coverage, even at tropical latitudes. This at a time when the atmospheric temperature should be too high to condense the atmospheric volatiles.

Our spectral image set contains images at wavelengths that include the absorptions bands of both H₂O and CO₂ frosts. Using these images we will look for and track the clouds and ice cap. What makes this set superior to the Hubble images is that in the infrared we can discriminate between the two types of volatiles and thus indirectly be able to measure the temperature of the atmosphere.

Analysis

The images are reduced using standard infrared techniques. They are first linearized, then sky-subtracted to remove any dark-current, bias-offset and sky flux, and finally flat-fielded to equalize the gains of each pixel. Analysis then consists of creating Relative Band Depth (RBD) images. These images are created from the normalized in-band image ratioed to a normalized continuum image created from a linear interpolation of two continuum images on either side of the band. This is represented mathematically as:

The choices for the 1 and 2 continuum and in-band (b) wavelengths were:

These choices were made by applying the RBD formula to published ice spectra. Using these laboratory ice reflectance spectra, a line is fit between the points being used as the continuum wavelengths and the spectra are then divided by this line. The following plots show these band depth spectra:

In these graphs, the circled points are the continuum wavelengths and the lowest point is the in-band wavelength. As can be seen, these pure volatile band depths are easily visible, being on the order of a 50% absorption.

Results

The band depth images created are presented here in comparison to images created using Viking albedo and topography data.
Scan One:

Viking Albedo	Viking Topography
3.33µm RBD (CO₂ frost)	3.00µm RBD (H₂O frost)

For orientation, north is at top, the bright central region is Arabia, the dark region to the south east is Syrtis Major, the large dark region in the west is Acidalia and the bright region in the south (which is seen to be dark, or low, in the topographic image) is the basin Hellas.

The RBD images were normalized to the bright region Arabia. As a note, the dark/bright vertical/horizontal bar edge effects and the bright/dark limb edge effects are due to image registration and are not real effects. In the 3.33µm RBD not much is seen. Any patterns are only of the order of 2% above or below Arabia. This would be an indication of either no CO₂ frost or at least none relative to Arabia. It seems plausible that the springtime atmosphere of Mars is not cold enough to condense CO₂ into clouds. However, in the 3.00µm RBD image, one can easily see the topographically low basin of Hellas as well as the northern lowlands. These differ from Arabia by 20-30% and would indicate less absorption in these regions. This would then imply some kind of H₂O frost in the darker regions of the RBD, most likely in the form of clouds.

Scan Two:

Viking Albedo	Viking Topography
3.33µm RBD (CO₂ frost)	3.00µm RBD (H₂O frost)

Again, for orientation, north is at top. Since these images come from the second CVF scan, Mars has rotated considerably. The bright region Arabia is now near the eastern limb and the northern dark region Acidalia is near the center of the disk. In the topographic image the long dark streak in the south is Valles Marinaris and the Tharsis plateau is just over the western limb. Since Arabia has move toward the limb, to avoid any limb darkening confusion the bright region Chryse, just south of Acidalia is used as the normalizing region.

In the 3.33µm image the variations are again only on the order of 3-5% from Chryse. So this would indicate either a uniform CO₂ frost cloud coverage or none at any detectable levels. The latter is the more reasonable interpretation. However, in the 3.00µm RBD image patterns with variations on the order of 10% can be seen. These pattern also appear to follow the topography; one can easily see the two lobed structure near the western limb in both the RBD and the topographic maps. The darker areas in the RBD again indicate more absorption than the normalizing region and since these are over the higher topographic regions this would indicate high, cold H₂O frost clouds.

One interesting point is that the polar ice cap, which should still be very visible at this season (see the above Hubble images) is missing in both the CO₂ and the H₂O images. For the CO₂ images this has yet to be explained. However for the H₂O images this can be explained. It can be assumed that the ice at the polar region would be of a fairly coarse, perhaps blocky, nature. The wavelengths and continua points for the 3.00µm RBD map were chosen to best show fine H₂O frosts and coarser frosts are less visible with these wavelength choices by about a factor of two.

Future Work

Our interest lies in absolutely calibrating all the images in the spectral scans using the stellar image data gathered at the same time and then to make true band depth maps instead of the relative band depth maps presented here. We believe, based on this preliminary work, that it will be possible to locate and track the atmospheric frost absorptions throughout the 1995 opposition. It will be interesting to see if, during the early spring, the atmosphere was ever cold enough to support CO₂ clouds.