Color images of bright red dust deposits at the Mutch Memorial Station were acquired at variable incidence angles during sol 611 (subsolar longitude ~70¿, northern spring season). After removing effects due to atmospheric scattering and absorption, the data were used to estimate the independent variables in the Hapke (1986) photometric function. In blue, green, and red coordinates the vector representing the space radiance factor of the landing site extracted from Viking orbiter images acquired on sol 609 is separated by a Euclidean distance of only 0.022 units and an angle of only 1.5¿ from the vector estimated from the station data for the orbiter lighting and viewing geometries. This result implies that light reflected from dust exposures dominates the orbiter signal; multiplicative and additive atmospheric terms cancel one another and surface roughness is a second-order effect in the orbiter data. Dust radiance factors computed from station data are most like laboratory spectra for fine-grained Hawaiian palagonite and are indistinguishable from Earth-based spectra of classical bright areas. Color composites of orbiter images show that the dust is found immediately south of Acidalia Planitia and in association with topographic barriers such as craters and cliffs.

Examination of Viking infrared thermal mapper data shows that the dust deposits typically do not have distinctive thermal inertia signatures, implying that the deposits are optically thick (hundreds of micrometers) but thinner than the diurnal thermal skin depth (centimeters). Dark gray material with thermal inertia values (8--12¿10-3 s-1/2 K-1) indicative of sand grain sizes (0.5--1.5 mm) dominates the Acidalia Planitia lowlands and parts of Kasei Vallis. This material also occurs as dark streaks extending from craters in Xanthe Terra and Oxia Palus. Space radiance factors of dark gray material are similar to spectra of mafic rock mixed with a minor amount of palagonitelike material. Material that is darker than but just as red as the dust deposits occurs in Lunae Planum, Xanthe Terra, and Oxia Palus, areas of intermediate elevation. Multiple phase angle orbiter images suggest that the dark red exposures are a mixture of bright red and dark gray materials, with the dark red exposures being relatively rough at a subpixel scale as compared to exposures of the other two materials. Thus only two types of materials can be detected in the data covering the study area: palagonitelike dust and mafic rock fragments.

Thermal inertia values (4--8¿10-3 cal cm-2 s-1/2 K-1) for the dark red material are consistent with the presence of fine to medium sand size particles (0.10--0.5 mm); such material should be easily moved by winds. The lack of aeolian features implies that the dark red deposits are not composed of loose material. Rather, they are probably more eroded versions of blocky soil duricrust (cemented dust and rock fragments) exposed at the station. Both bright red dust (aeolian suspension load) and dark gray materials (saltation, traction loads) migrate over the dark red substrate. The overall distribution of the surficial units is controlled by topography at a variety of length scales. Lower with threshold friction velocities associated with higher atmospheric densities in lowlands keep dust from accumulating and duricrust from forming, while higher threshold velocities in highlands lead to net accumulation of dust. Local topographic obstacles (craters, walls, ridges) perturb wind flow and lead to local accumulation or erosion at a variety of elevations. In addition, the bright red dust ubiquitously found between exposures of dark gray and dark red materials may accumulate as the wind velocity gradient decrease at the transition from smoother dark gray exposures to rougher dark red exposures. The distribution of the materials must also be modulated by climatic variations induced by quasi-periodic oscillations in obliquity, eccentricity and spin axis direction, constraining these surficial deposits to be ≤1 Ma. ¿ American Geophysical Union 1989