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Farah, M. G. Leakey, E. Mbua, M. Muungu, S. N. Muteti, and the staff of the NMK Palaeontology Division for support. The analysis and manuscript benefited from discussions with S. Anto ´n, F. Spoor, and B. Wood. Supporting Online Material www.sciencemag.org/cgi/content/full/305/5680/75/ DC1 SOM Text Tables S1 and S2 References 8 March 2004; accepted 25 May 2004 Evidence for Precipitation on Mars from Dendritic Valleys in the Valles Marineris Area Nicolas Mangold, 1* Cathy Quantin, 2 Ve ´ronique Ansan, 1 Christophe Delacourt, 2 Pascal Allemand 2 Dendritic valleys on the plateau and canyons of the Valles Marineris region were identified from Thermal Emission Imaging System (THEMIS) images taken by Mars Odyssey. The geomorphic characteristics of these valleys, especially their high degree of branching, favor formation by atmospheric precipitation. The presence of inner channels and the maturity of the branched networks indicate sustained fluid flows over geologically long periods of time. These fluvial landforms occur within the Late Hesperian units (about 2.9 to 3.4 billion years old), when Mars was thought to have been cold. Our results suggest a period of warmer conditions conducive to hydrological activity. The formation of valley networks on Mars has been the subject of considerable scientific debate (1–4 ). Valleys were attributed to flu- vial erosion implying a warm and wet climate on early Mars (5 ), possibly conducive to biological activity (6 ). Several recent obser- vations argue in favor of such conditions on early Mars (7, 8), but valley networks could also have formed by water-lubricated debris flows (4 ), hydrothermal activity (9, 10), or groundwater sapping (11–13) due to geother- mal activity (14 ), and these hypotheses do not require conditions warmer than the cur- rent cold climate. Here, we describe terrestrial-like dendritic valleys identified using THEMIS images (15 ) of the Mars Odyssey mission. These valleys are located in the Valles Marineris region (Fig. 1) on the plateau west of Echus Chasma (Fig. 2) and on the inner plateau west of Melas Chasma (Fig. 3). These landforms occur within Late Hesperian units (16), about 2.9 to 3.4 billion years old (17 ); they are thus unexpectedly younger than the Noachian (16) period, which is considered to be the poten- tial primitive warm period (4 ). The plateau west of Echus Chasma (0°N, 81°W ) is covered by densely branched val- leys that are frequently sinuous and extend over tens of kilometers (Fig. 2). Infrared (IR) images taken at night by THEMIS (Fig. 2A) show mainly intrinsic thermal properties of the ground (15 ). Valleys buried under loose material are outlined by variations of the thermal signal and resemble terrestrial deserts where sand covers the floor of dry valleys (fig. S1). On drainage basin G, the main valley is larger upstream than downstream close to the mouth, implying a thicker man- tling at this location. With the exception of this place, most valleys have widths increas- ing from their sources to their mouth, as seen in terrestrial valleys. An IR image taken dur- ing the day by THEMIS shows that these dendritic valleys have their heads scattered at random points on the plateau (Fig. 2C). Hills 1 Laboratoire IDES, UMR CNRS and Universite ´ Paris- Sud, Orsay Campus, 91405 Orsay, France. 2 Labora- toire des Sciences de la Terre, UMR CNRS UCB Lyon 1 and ENS Lyon, La Doua Campus, 69622 Villeurbanne, France. *To whom correspondence should be addressed. E- mail: mangold@geol.u-psud.fr Fig. 1. MOLA shaded re- lief map of West Valles Marineris and the neigh- boring canyons. White squares indicate valley networks observed over Hesperian geologic units on THEMIS images of Figs. 2 and 3. R EPORTS 2 JULY 2004 VOL 305 SCIENCE www.sciencemag.org 78