THE SCALE-HEIGHT OF DUST AROUND PAVONIS MONS FROM HRSC STEREO IMAGES

The Martian atmosphere contains large and variable amounts of dust and other aerosols. The High Resolution Stereo Camera (HRSC) onboard the European orbiter Mars Express is a powerful tool for studying how these aerosols are distributed. An essential parameter for such studies is the optical depth, which can often be estimated from contrast differences between HRSC stereo images with the so called ‘stereo method’. Software for this purpose has been developed at MPS in Lindau Germany. The method uses map-projected ortho-images and complementary data on the imaging geometry from photogrammetric software developed at DLR. During orbit 902 of Mars Express, HRSC observed the volcano Pavonis Mons. On the summit plateau and at its foot we selected 6 regions that span more than 10 km in altitude to study how optical depth depends on altitude. We found a scaleheight for the optical depth of 10.8 km +0.9/-0.8 km. This is equal to, or very close to, the expected local gas-scale-height. Introduction: The optical depth of the Martian atmosphere is almost completely determined by the amount of aerosols it contains, and is considerable since this amount is large. Most aerosols are particles of airborne dust. Knowledge of the amount of this dust, of its distribution through the atmosphere, and of its composition is important for understanding the Martian environment. First of all, since this airborne, or atmospheric, dust is quite important for shaping it; i.e., aerosols determine how much insolation reaches the surface and how much is absorbed in the atmosphere. Thus they have a big influence on the climate, the weather, the circulation patterns, and with that on Aeolian processes. Most aerosols, at least those in the lower atmosphere, are reddish dust agglomerates that can act as condensation kernels for vapors, invoking white hazes when they become covered with ice. Knowledge of the aerosols is also important for interpreting observations of Mars since they have a big impact on much of the remote sensing data; e.g., they diminish the contrast and spatial resolution of images and the light they scatter creates a strong and diffuse reddish illumination of the surface. Interpretation of (surface) images and spectra should consider such effects. The impact of the dust partly depends on where it resides in the atmosphere. For instance, is it homogeneously mixed in the air so that the dust has a scaleheight comparable to that of the air itself, or is there strong layering, for example close to the surface? This paper intends to contribute to a better understanding of such questions. It describes a stereo method analysis as conceived by [1] of Pavonis Mons. The images were taken with the High Resolution Stereo Camera (HRSC) onboard Mars Express (MEX). During the last decennia, the distribution of aerosols in Mars’ atmosphere, and more in particular their scale-height, have been investigated by several authors. [2] used limb scans from the Viking orbiters and observed discrete, optically thin, detached haze layers between 30 and 90 km elevation that may have consisted of water ice. Below about 50 km they observed a continuous, reddish haze. In the 30 to 45 km altitude range the scale height of the reddish haze was typically 5 to 7 km, while its color implies that it was mostly dust. The authors can not offer much useful information on the lowest 10 to 15 kilometers of the atmosphere, since these regions are optically thick when viewed from the limb. [3] analyzed the changing sky brightness during the Martian twilight as observed by the Viking landers. They concluded that the dust is exponentially distributed in the lowest 30 km, with a scale-height close to that of the atmosphere. The Pathfinder mission yielded new estimates. [4] used egress observations of Phobos. Their data best fit models with dust-scale-heights of 10 to 15 km, but they remark that a dust-scale-height that decreases with increasing altitude would provide a better fit. Since 2004, the stereo imagery of HRSC, and the Digital Elevation Models (DEMs) that are derived from these, offer a new way to measure the optical depth of the Martian atmosphere. By comparing optical depths above surfaces at varying altitudes, these can be estimated as a function of altitude. This technique has disadvantages: obviously, it can only measure the total optical depth from surface to space, thus can not resolve any vertical structure above a given location. Also, the method merely allows measuring dust-scale-heights between the lowest and the highest surface in the image. Moreover, since it depends on comparing (nearby) regions at various elevations it will be hampered by horizontal fluctuations in the concentration of aerosols. On the other hand, this offers an important advantage as well, in that it allows studying Seventh International Conference on Mars 3154.pdf