Optical depth and its scale-height in Valles Marineris from HRSC stereo images N.M. Hoekzema a, ⁎, M. Garcia-Comas a,b , O.J. Stenzel a , B. Grieger c , W.J. Markiewicz a , K. Gwinner d , H.U. Keller a a MPS, Max Planck Strasse 2, 37191 Katlenburg Lindau, Germany b Instituto de Astrofisica de Andalucia (CSIC), Apdo. 3004, 18080 Granada, Spain c ESAC, E-28691 Villanueva de la Cañada, Madrid, Spain d DLR, Rutherfordstr. 2, 12489 Berlin, Germany abstract article info Article history: Accepted 8 February 2010 Available online 6 March 2010 Keywords: stereo scale height optical depth Valles Marineris Mars We measured the optical depth of the Martian atmosphere as a function of altitude above two opposing scree walls of the Valles Marineris, from stereo images that were taken with the High Resolution Stereo Camera (HRSC) of Mars Express on June 3, 2004, during orbit 471. The optical depths were measured from contrast differences between the stereo images with the so called “stereo method”. For 7 regions in the northern wall of the Valles, we estimated the optical depth and found values between 1.0 and 1.6. These regions span more than 6 km in altitude and the results show a clear relation with altitude. A fit on these results yielded a scale- height for the optical depth of 14.0 km + 1.3/-1.1 km. The expected local pressure–scale height is smaller: 11.5–12.0 km. The difference is most likely explained by small (around 1.5%) offset errors in the intensity calibration of HRSC images. We also selected 9 regions in the opposing southern wall and from these we measured values of optical depth in the range 1.3–1.5. Our result suggests the presence of clouds above this part of the Valles because the optical depth appears almost independent of the surface altitude. Possibly these are banner clouds, forming at the edge of the canyon, that contain dust that is blown over the canyon by winds from the high plains to the South. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The optical depth of the Martian atmosphere in the visible is almost fully determined by the amount of aerosols it contains. Usually it is above 0.3, and regularly it is larger than 1.0. Knowledge of the amount of the aerosols, of their distribution through the atmosphere, and of their composition, is important for understanding the Martian environment. Aerosols determine how much insolation reaches the surface and how much is absorbed in the atmosphere, thus having a big influence on the climate, the weather, the circulation patterns, and on Aeolian processes. Most aerosols, at least those in the lower atmosphere, are particles of reddish dust that can act as condensation kernels for vapors, invoking white hazes when they become sufficiently covered with ice. Their ability to absorb volatiles and the reaction surface they offer for chemical processes make them important for the atmospheric chemistry. Knowledge of the aerosols is also important for interpreting observations of Mars since these 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 so that the Martian surface appears redder than it actually is. Interpretation of (surface) images and spectra should consider such effects. The impact of the airborne dust partly depends on where it resides in the atmosphere, and thus on its vertical distribution. Often it is more or less homogeneously mixed into the air so that the dust invokes an opacity scale-height (or: scale height of optical depth) that is comparable to the pressure scale-height of the atmosphere; this has been found and published by many authors and below we cite several. On the other hand, one may expect that topography and the weather can influence this. For example, what should one expect near big mountains and deep valleys that often create their own weather? This paper intends to contribute to a better understanding of such questions. It describes a stereo method analysis (Hoekzema et al., 2007; Hoekzema et al., in preparation) of two opposing walls of the Valles Marineris. The images were taken with the High Resolution Stereo Camera (HRSC) onboard Mars Express (MEX), the European remote sensing satellite that orbits Mars since December 2003. During the last decennia, several authors investigated the distribution of aerosols in Mars' atmosphere. More in particular, they estimated the scale height of optical depth. For example, Jaquin et al. (1986) used limb scans from the Viking orbiters. The authors observed discrete, optically thin, detached haze layers between 30 and 90 km altitude that may have consisted of ice. Below about 50 km they observed a continuous, reddish haze that extended all the way to Earth and Planetary Science Letters 294 (2010) 534–540 ⁎ Corresponding author. Max Planck Institute for Solar System Research, Max Planck Strasse 2, 37191 Katlenburg Lindau, Germany. Tel.: +49 5556 979 438; fax: +49 5556 979 141. E-mail address: hoekzema@linmpi.mpg.de (N.M. Hoekzema). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.02.009 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl