Northern-hemisphere Gullies on Mars – Distribution and Orientation from the Evaluation of Hrsc and Moc-na Data

Introduction: Gullies show morphologic features that indicate the presence of a flowing liquid during their forming process [1]. However, the current climate in the northern hemisphere of Mars does not allow the existence of substantial amounts of liquid water for a long time. Therefore, the exploration of particular young landforms, such as gullies is a valuable tool for understanding the most recent climatic history of Mars. In order to complement and complete an earlier survey conducted by Balme et al. (2006) [2] for the southern hemisphere, we evaluated HRSC [3] and MOC-NA [4] data covering the northern hemisphere for analysing geographical and latitudinal distribution, geological context and orientation of gullied slopes. Constraining such parameters helps to shed light upon the formation mechanisms of gullies on Mars, as preferred latitudinal regions or orientations strongly indicate the influence of insolation and/or climatic conditions. Generally, HRSC and MOC-NA are very different camera concepts. While the MOC-NA instrument provides a very high spatial resolution with low coverage of the planetary surface, HRSC has a wide coverage with a lower but sufficient resolution capable of detecting gullies if atmospheric conditions are favourable. In combination, these two datasets provide a balanced basis for our study on gullies in the northern hemisphere. MOC-NA results: Out of 21,042 MOC-NA images (0°-90°N) we detected 3195 gullies in 311 images. All these gullies are located between 30°N and 76.6°N (Fig.1), with a significant increase of frequency between 35°N and 55°N. The maximum of 4.1 gullies/1000 km2 is situated between 40°N-45°N. Most gullies found in this survey are located on the slopes of impact craters (n=2594, 81.2%). Gullies in graben structures and fretted terrains amount to approx. 11.7% (n=373). The remainig gullies are located at isolated knobs and hills (n= 228, 7.1%). Gullies positioned at crater walls show orientation trends towards northwest and southeast. However, the orientation of gullies changes with latitude (Fig.2). Between 30°N and 40°N 61.9% (n=635) of the gullies are situated on poleward facing slopes. Between 40°N and 50°N 69.4% (n=819) of all gullies are located on equator-facing slopes. Due to the small number of gullies at crater walls observed north of 60° (n=89, 3.4%) we binned all gullies north of 50°N into one group spanning a latitude band of 50°N-80°N with n=386 (14.9 %). 67.3% (n=261) of these gullies occur on slopes facing the equator.