Martian spectral reflectivity properties from Mariner 7 observations (1971)

An analysis of data taken by the Mariner 6 and Mariner 7 spacecraft has been made in order to extend the knowledge of spectral reflectivity differences on the surface of Mars. The data were collected with the Mariner wide-angle television camera which utilizes a vidicon image tube as sensor. The data consist of a series of pictures taken through sequenced color filters with passbands in the visible region of the electromagnetic spectrum (0.43 - 0.65 microns). The Martian regions photographed lie in the longitude range 300° to 30° (East Longitude) and include the prominent dark areas Meridiani Sinus, Sabaeus Sinus, and Margeritifer Sinus, as well as the light 'desert' areas Moab, Thymiamata and Deucalionis Regio.

It was necessary to remove harmonic noise from the television images in order to detect and measure significant spectral reflectivity differences in the equatorial regions of Mars. The resolution element near the center of the planetary disc was 30 km. by 200 km. The measurements represent an order of magnitude improvement in linear spatial resolution over previous measurements made with earth based telescopes and comparable broad band spectral resolution. No reliable published measurements exist for these regions of Mars but the new measurements showed a general agreement with the previous low resolution measurements of other parts of the planet.

The colors of Martian features are very sensitive to the geometry of illumination and viewing. However, color differences attributable to geographic variations in the reflecting properties of the surface and/or atmosphere are recognized in an area near the center of the martian disc. The reflectivity ratio red/green for these areas is strongly correlated with the normal albedo but varies inversely as the reflectivity ratio blue/green. The spectral resolution of the system for measurements within the dark area Meridiani Sinus is limited by aliasing effects with small albedo features.

The dependence of color on local geometry is investigated on the assumption that all the martian features in an area extending from 20°N to 30°S and from 310°E to 30°E have the same correlation between color and normal albedo when this is measured under standard illumination and viewing conditions. Models of a wavelength dependent photometric function and of a uniform atmospheric scattering layer are both consistent with the data.

Atmospheric scattering alone can account for the local geometry effect but if the scattering layer is geographically uniform and independent of Martian local time the atmospheric optical depth is three times that predicted from ultraviolet reflectivity measurements.

Near the center of the martian disc, three distinct reflectivity groups (D, L1 and L2) are recognized indicating the existence of abrupt transitions in reflectivity and extensive homogeneous areas with little variation in intrinsic reflectivity. These areas are identified and the boundaries between them studied using high resolution photography. The boundary between the areas with reflectivities D and L1, corresponds to a change in surface reflectivity. The boundary between the areas with reflectivities L1 and L2 corresponds to the edge of a cloud or haze layer. The reflectance properties of this layer are comparable to those which have been previously measured from limb photographs.

Color-color plots and color-reflectivity plots for Martian surface areas show some similarities with the moon but significant differences. Martian light and dark areas cannot be explained as simply different polymers of carbon suboxide. It may be possible to match the reflectivity characteristics of both light and dark areas with oxidized basalt but if this is the case there must be a change in particle size as well as composition.