Lunar and Planetary Science XXXII (2001) 2003.pdf

TOPOGRAPHY AND AEOLIAN FEATURES: DUNES AND STREAKS COMPARED WITH GLOBAL AND MESO SCALE WIND PREDICTIONS. R. Greeley1, S.C.R. Rafkin2, Haberle, R.M. 3, R.O. Kuzmin4, 1Ariz. State Univ., Dept. of Geological Sciences, Box 871404, Tempe, AZ, 85287-1404, @asu.edu, 2Dept. of Meteorology, San Jose State Univ., San Jose, CA, 95192-0104, 3NASA Ames Research Center, MS 245-3, Moffett Field, CA, 94035-1000, 4Vernadsky Institute, Russian Academy of Sciences, Kosygin St. 19, Moscow 117975, GSP-1, Russia.

Introduction. Duneforms and certain variable plain region containing a channel (Nirgal Vallis) was features [1] are generally considered to represent the studied, but only with the GCM predictions. orientation of the prevailing strong winds at the time crater. This impact structure is about 160 km of their formation. Previous studies [2] compared the in diameter, centered at 14.3 S, 184.5 W, as recently orientations of bright and dark wind streaks with near reviewed and mapped by Kuzmin et al. [5]. GCM runs surface winds predicted by the NASA-Ames general predict strongest winds toward the southeast circulation model (GCM; 3). The GCM can be run to during southern hemisphere summer, with winds the predict the surface shear stress (and, hence, the particle- rest of the year generally oriented toward the north. moving potential of the winds) as a function of Meso scale runs initiated at Ls 143 degrees set within season, global-scale topography, and other factors. the GCM show that the topography of the crater and There is general agreement between the orientation of the time of day are strong influences on the predicted the bright streaks (which are more stable than dark wind patterns. Winds are channeled by Ma’adim Vallis streaks) and the strongest predicted winds, but poor northward into the crater, consistent with the regional correlation with the dark streaks. This was interpreted wind pattern for this time of year. Nighttime winds to result from the influence of local topography on the general flow north across the crater floor until mid formation of dark streaks, which are recognized as morning, at which time winds begin to flow outward being more ephemeral [2]. A newly developed in all directions from the central floor zone up the atmospheric model [4] enables local topography to be walls of the crater where they interact with the regional taken into account in predicting near-surface winds, flow pattern. Flow is reversed (i.e. to the south) in the which might be important in the formation of some Ma’adim channel where it converges with the aeolian features. The study reported here involves the northward regional flow and is redirected westward. By assessment of local topography on both the late afternoon, extremely complex patterns are atmospheric model predictions and the orientations of produced by the interaction of the local topographic windblown features. winds and the regional flow. By about midnight, the Approach. The primary data for our study include uniform northward flow is re-established. Mars Observer Camera (MOC) and Viking Orbiter Initial analysis of aeolian features shows: 1) crater- (VO) images, Mars Orbiter Laser Altimeter (MOLA) associated dark wind streaks on the west-central floor topography, and Thermal Emission Spectrometer of Gusev trending eastward which correlate with the (TES) data. MOC and VO images were used for pre-dawn winds flowing down the crater walls on the analyzing wind streaks, dunes, and other aeolian west side of Gusev; 2) prominent dark streaks on features; Ls values were noted to determine the seasons the northwest part of the floor of Gusev, oriented when the images were taken. MOLA and TES data toward the northwest, visible on MOC frame 10-03814 were used as input for topography, thermal inertia and (Ls 271.5 degrees); albedo in the meso-scale atmospheric model [4]. For 3) barchanoid duneforms are found on MOC frame various study areas, we have run the GCM for the full MO3-06211 (Ls 165 degrees) within dissected Martian year to derive wind surface shear stress. We segments of the Ma’adim “delta” deposits at the mouth then “binned” the results into 8 equal periods of time of the channel where it enters Gusev. These duneforms for the year (to assess seasonal changes), from which have slightly-developed “horns” which would indicate the maximum 5% values were compiled as prevailing winds toward the southeast, consistent with representing the times when maximum aeolian activity the meso-scale modeled winds for mid-morning. It is would occur. These results were then plotted on base notable that the influence of the daily wind dynamics maps of each area. The meso-scale model was initiated on the orientation of aeolian features within Gusev with output from the GCM; it has a grid spacing Crater is rather considerable. typically of 12 km, uses MOLA 1/16 degree data (with Nirgal Vallis. This west-to-east trending channel is interpolation to match the grid), provides wind centered at ~28.5 S, 24 W, southwest of Margaritifer directions and strengths close to the surface, and can be Sinus. GCM runs show that the strongest winds occur output for any time of day as wind vector plots over in the southern hemisphere late spring and summer and the study area. The results from both models were then blow toward the south. The rest of the year, prevailing compared with images of dunes, streaks, and other winds are toward the north northwest and correlate wind-related surface features. This approach was used with crater-associated bright wind streaks. The plains to study a large crater basin (Gusev); in addition, a north and south of Nirgal show a “fabric” of faint Lunar and Planetary Science XXXII (2001) 2003.pdf

ridges and grooves of unknown origin trending governs flow patterns which change with time of day. northeast-southwest which do not correlate with any of These effects, coupled with seasonal changes in wind the winds predicted by the GCM. In contrast to the patterns, lead to complex aeolian features. An streaks and “fabric” on the plains, dunes within the understanding of the interaction of the atmosphere with Nirgal Vallis have crests, which are orthogonal to the the surface of Mars must take all of these factors into axis of the channel, suggesting that they are transverse account. As additional MGS data become available, it bedforms whose orientations are governed by will be possible to develop models to explain this channelized wind flow through Nirgal. interaction. Discussion. Topography has a substantial effect on References. [1] , C. et al. (1972) Icarus, 17, the formation and orientation of aeolian features. The 346. [2] Greeley, R. et al. (1993) JGR, 98, 3183. [3] effect is at least two-fold: 1) topography “channels” the Haberle, R. et al. (1993) JGR, 98, 3093. [4] Rafkin, S. direction of the wind flow and, in some cases, causes et al. (2001) Icarus (submitted). [5] Kuzmin et al. flow acceleration and 2) differential heating as a (2000) USGS I-2666. function of elevation, shadowing, and other influences