HRSC/MEX ANALYSIS of VALLEY NETWORKS on ECHUS CHASMA PLATEAU and in AEOLIS REGION Ph

Lunar and Planetary Science XXXVI (2005) 1340.pdf

HRSC/MEX ANALYSIS OF VALLEY NETWORKS ON ECHUS CHASMA PLATEAU AND IN AEOLIS REGION Ph. Masson1, V. Ansan1 , N. Mangold1, C. Quantin2, G. Neukum3 and the HRSC co-Investigator team. Lab. IDES-CNRS, bât. 509, CNRS and Université Paris-Sud, 91405 ORSAY, France, 2Lab. des Sciences de la Terre, UCB et ENS Lyon, France, 3 DLR, Berlin, Germany, Contact: [email protected], [email protected]

Introduction: : Since visible images have been acquired by Viking orbiter in 1976, valley networks have Valley networks in Echus region: been mainly identified in the heavily cratered uplands This region is located at the North of Valles Marineris. dated Noachian (>3.5 Gyr). Valley networks on Mars On the plateau at the west of Echus Chasma canyon have been the subject of considerable debates about (278-281°E, 0-5°N), valley networks show similar flu- their formation processes since three decades [1-5]. vial dendritic pattern visible on the Earth (Fig. 2). They have been first attributed to fluvial processes These valleys were identified from their difference of under conditions 3.5 Gyr ago that were warmer than thermal properties on THEMIS images [10]. HRSC the present cold climate. However, many other hy- provides the possibility to look in detail to their mor- pothesis like geothermal activity, subglacial melting or phology and geometry with the spatial resolution at sapping processes have been proposed because of the ~20 m/pixel (Fig. 2). Valleys in Echus area are thus not low organisation. The Mars Express High Resolution restricted to the locations where they have been ob- Stereo Camera (HRSC) data give a wide range of tools served through THEMIS images [10]. They extend to study fluvial valleys on Mars both using high reso- over more than 200 km along Echus Chasma western lution images and DTM extracted from stereoscopic plateau. On the colored HRSC image (Fig. 2), the val- images [6]. We determined the organisation of valleys ley networks appear in yellow in contrast to brown in relationships with regional topography and structural substratum because of the dust filling by wind (some geology. We analysed quantitatively the valley mor- dunes are visible on HRSC images). This landscape is phology and the valley network morphometry in order similar to that observed in the Sahara where valley to determine process(es) which is(are) at the origin of networks formed during wet Pleistocene are now par- valleys. We focussed our study on two locations: Echus tially buried by sand dunes. These valley networks dis- Chasma plateau and Aeolis region. play a mature dendritic pattern with high bifurcation ratio [11, 12] ranging from 2 to 5. The drainage den- Valley networks in Aeolis region: sity, i.e. the ratio between the total length of valleys This region is characterized by densely cratered ter- and the area of the watershed [11], allows to evaluate rains dated of Noachian[7, 8]. The terrains between the level of maturity. The highest drainage system large craters are incised by valley networks which reaches a density of 1.4 km-1 mapped at 13 m/pixel Ma’adim valley is the most known. In february 2004, resolution. This is the second highest drainage density HRSC acquired some images with a spatial resolution on Mars after Alba Patera volcano and it is similar to ranging from 10 m to 40 m in 5 stereo bands and 4 terrestrial values of valleys on Hawaiian volcanoes color channels in Aeolis region. These images show [13]. Only few valleys are detected on the 200 m/pixel that the densely cratered terrain has been modified by resolution HRSC DTM. This shows that they are shal- numerous valley networks. For example, the orbite 228 low pristine landforms even if they could have been nadir image centered at 157.355°E and 30.576°S (Fig. originally deeper and filled later by sand. They can be 1) displays a set of valley networks with a mature or- dated to the Late Hesperian epoch from chronological ganisation (dendritic pattern with high bifurcation ra- relationships. All of the characteristics of the dendritic tio) similar to that of terrestrial valley network in tem- valleys are similar to terrestrial features of surface run- perate climatic zones. These valley networks have off due to atmospheric precipitations. large drainage basins and high density drainage. The dendritic pattern of these valley networks and their Conclusion: Valley networks both observed in Echus morphometric parameters indicate that they formed by and Aeolis regions display dendritic shape and mor- geologic processes related to precipitations. In addi- phometric characteristics of terrestrial fluvial networks. tion, it seems that they are relatively young derived They may have formed by precipitations during a wet- from the frequency of fresh craters, and the fluvial ac- ter and warmer climate than the present-day climate on tivity would have ceased during the Hesperian [9]. The Mars surface. DTMs generated from stereo images give quantitative information about the morphometry of valleys : shal- low and large, flooding on low tilted area. Lunar and Planetary Science XXXVI (2005) 1340.pdf

Fig. 2: HRSC color image (orbit 97) of West Echus Chasma plateau. Numerous dendritic valley networks incised the Hesperian terrains.

Figure 1. Extract of HRSC image (orbit 228) located in Aeolis region. Numerous dendritic valley networks incised Noachian densely cratered terrains.

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