Sotar Synm Rntaidt. ^iL 33. Ne. 6,199Í, pp. 41S-4S2. Tmalaudinm AitnmaidditMi Kimli^ Hit 31. No. 6. Í99S, pp. 4SJ~SI}. Origbul Kuiiiai Ttn Cepyrífht O 1993 by Marthnko. Baiiitvsky, Hoffmaa\, Haniítii CooL Ntnhm. Geology of the Common Mouth of the Ares and Tiu Valles, Mars A. G. Marchenko*» A. T. BasUevsky*, R Hoftinann**, E. Hfluber**, A. C. Cook***, and G. Neukum** • Vemadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina ¡9, Moscow, ¡17975 Russia ** DLR Institute of Planetary Exploration, 12489 Berlin, Gerrriany *** Center cf Earth and Planetary Studies, National Air and Space Museum, ^¥askinglon. DC, USA Received June 23, 1998 Abstract•Since the Mars Paif^nder landing site is located at the mouth of the Ares and Tiu valles, this region attracts keen scientific interest. In order to better undcretand experinienta] data ffom this small area of the sur- face of Mars, which has been iavesti|ated by the rover, and the properties of the malciials occurring here, it is necessary to answer how, from where, and when this material was transponed. To answer these questions, we performed photogeological mapping and counted impact craterin the region. A photogeological analysis of 320 TV images of the studied area, thermal-inertia maps, and digital elevahon models were used in mapping. Our results, and those published by other scientists, allow us to distinguish several principal stages in the geological history of Üie mouth of the Ares and Tiu valles: the destruction stage of an ancient plateau and three stages of fluvial activity. Deposits transported by water flows to the Mars Pathfinder landing site presumably consist of fragments of the material of ancient Martian highlands (impact breccias and lavas), of younger sedimentary or volcano-sedimentary material of ridged plains, of impact-crater materials, and of eoUan products. The main constituents should be as old as approximaxely 4 Gyr (highland materials) and 3.5 Gyr (ridged-plain materials). It is very hkely that the fluvial reworking of this ancient material took place between 3.6 and 2.6 Gyr ago and, possibly, even lattr, between 2,3 and ) .4 Gyr ago. 1. INTRODUCTION lack of drainage basins related to outöow valleys exclude this mechanism for their formation. Glacier 1.1. Martian Outflow Valleys movements represent another process forming valley- In 1970, images obtained by Aionnej'9revealed val- shaped landfonns on the Eaith. Some researchers (Luc- leys on the Martian surface, which are similar in mor- chittaeio/., 1981; Lucchitta, 1982; Kai^cle/a/., 1995) phology to valleys and dry river channels on the Earth believe this prtwess was similarly responsible for the (McCauley et al., 1972). The similarity is, however, origin of the Martian outflow valleys. However, gla- incomplete; for example, many Martian valleys, in con- ciers of the Earth usually move along the fluvial (water- trast to their terrestrial counterparts, lack drainage formed) valleys, transforming them into glacial troughs basins. Several morphological varieties of Martian vaJ- (Hamblin, 1975). In addition, the growth, movement, leys, such as valley networks, outflow valleys, runoff and thawing (tHit not evaporation) of glaciers can pro- valleys, and ñttted valleys, are distinguished (Sharp ceed under terrestrial, but not under Martian atmo- and Malin, 1975; Carr. 1996). The Arcs and Tiu valles spheric conditions. studied in this work are of the outflow-vallQr class. According to some scientific ideas, a possible alter- About 15 Martian valleys of this class are known at native is the catastrophic (geologically instantaneous) present (Baker, 1982); they all arc large (up to several origin of outflow valleys, which was so quick that there thousand kilometers long) and almost completely lack- was no time for water evaporation. In this case, we ing in tributaries. In the recent geological epoch, water should assume an instantaneous release of a huge vol- on Mars' surface could exist only in the form of ice and ume of water. vapor, because the atmospheric pressure at the planet The outflow valleys could have been formed as a surfte is only 5-10 mb. The outflow valleys were consequence of catastrophic outflows of groimd water foimed 3,6-0.5 Gyr ago after a period of heavy nveieor- to the surface (Baker and Milton, 1974; Baker and itic bombardment, when atmospheric conditions were Kochel, 1978; Carr, 1979). There is evidence for the most likely similar to the cuiient state (Masursky et al., idea that the Martian lithosphère may include horizons 1977; Scott et al., 1979; Wise et al., 1979; Baker and saturated with ice or water (see, for instance, Kuz'min, Kochel. 1979). 1980). The morphology of head areas of outflow val- Large river valleys on the Earth evolve over millions leys is indicative of plentiful sinkholes. Several pro- of years, mainly under the control of atmospheric pre- cesses could provoke water outflows on the surface. cipitation. Environments at the surface of Mars and the According to a hypothesis by Carr (1979), this was a 0038-0946/98/32O&0425$20.0O O 1998 MAHK Hayu/lnteiperiodica Publishing 426 MARCHENKO et al. pressure increase in water-bearing horizons due to 1.3. Data Used and Investigation Methods freezing. Some scientists (McCauley et ai, 1972; Masursky et ai, 1977) assume water discharge to be a Geological mapping was performed using the pho- result of the thawing of pennafrost under the influence togeological analysis of TV images and some other of gcothermai heating. Others believe that outflow val- data (Marchenko, 1996; Marchenko etal., 1996, 1997, leys and their deposits were formed by the movement 1998). We used 320 images in the visible spectrum of a substance rich in solid material, resembling mud range obtained by the Viking Orbiter, thermal-inertia flows or laharï (Nunrunedal and Prior, 1981; Tanaka, and rock-abundance maps compiled on the basis of the 1988, 1997). thermal-mapping data obtained by this spacecraft Features similar to the Martian outflow valleys aie well (Christcnsen. 1986; Christensen and Kieffer, 1989; known on the Earth (Milton, 1973; McCauley tí al., Edgett and Christensen, 1997), along with a color 1972). They are relatcid in origin to catastrophic water mosaic of the images shot by this spacecraft through release ftom glacier-dammed lakes (Bretz et at., 1956; various photo-filters. The resolution of the Wang Bretz, 1969; Butvilovskii, 1993; Rudoi, 1995) and to images is 36-821 m per pixel. The resolution of the ice thawing in response to volcanic eruptions under gla- themial-mapping data used in this smdy is 30 km. In ciers (e.g.. Thorarinsson. 1957). These geomorpholog- addition, a semiautomatic analysis of stereoscopic ical features are rare terrestrial counterparts of the Mar- images (Thomhill et al., 1993) was used to construct tian outflow valleys, although the water-discharge val- topographic maps of several areas and to evaluate the ues in the last case should be 10-100 times greater altitudes of some relief features. When the distin- (Baker and Milton. 1974; Baker, 1982). guished age (stratigraphie) units of the surface were marked by a statistically significant number of impact craters, their calculated density was used to check the J.2. What is Stimulating Interest in the Mouth age interpretation. The impact-crater count was per- of the Ares and Tiu Valles? formed on the basis of 55 images. Whatever the formation mechanism of outflow val- In our studies, we refer to the global and regional leys•in particular, the Ares and Tiu•was {cata- stratign^hy of Mars elaborated recently by other strophic fluvial, related to mudflows, or glacial), it researchers. Scon and Carr (1978) were first to recog- obviously might involve the transport of the materials nize three major stratigraphic-age units of this planet: derived from geologically different regions of Mars cut Noachian (ancient), Hesperian (middle), and Amazo- by these valleys to the mouth of the flows. The Ares and nian (young). They also evaluated the crater densities Tiu valles begin in regions with a chaotic morphology characteristic of these units. The absolute age of the of the ground-collapse type, then they run across an ancient, heavily cratered highland region, and finally units is a point of controversy. Neukum and Wise open to the lower-laying Chryse Planitia, where they (1976) defined the ages of the Noachian-Hesperian and merge (Fig. la). Each of these valleys is about 1500 km the Hesperian-Anuzonian boundaries as 3.8 and 3.6 Gyr, long and 25-100 km wide. Thus, they are as long as the whereas Hartmann et al, (1981) suggest that they are largest rivers of the Earth, e.g., the Amazon River, but 3,5 and 1.8 Gyr old, respectively. are notably wider. Moreover, processes unrelated to the A map by Scott and Tanaka ( 1986), comprising the valley origin proper could also operate in their common region under discussion, was used as a basis for our mouth, being responsible for the influx of various mate- more detailed mapping. This map depicts more detailed rials, e.g., for sedimentation in temporary lakes and sea stratigraphie subdivisions than that of Scott and Carr basins (Parker et al., 1989, 1993; Baker et al., 1991; (1978), and each of the units is characterized by its Scottí/fl/., 1991;Craddockrto/.. 1997), eolian accu- impact-ciater density. In our study, we distinguished mulation (Kuzmin and Greeley, 1995a, 1995b), subse- several new stratigraphie units missing in the map by quent lava eruptions (Robinson et al.. 1996). and later mudflows (Jons, 1984), Scott and Tanaka (1986) and suggested a new interpre- tation to some others recognized before. Most detailed The high probability of detecting diverse geological among the geological-geomorphologic maps compris- materials in the Ares/Tiu mouth was the main reason ing the studied region is that of Rotto and Tanaka for choosing this area for a detailed study with a niKxl- (1995), although the stratigraphie and morphologic erate-sized rover brought by Mars Pathfinder units of valley floors and eolian deposits, especially (Golombek et aL, 1995, 1997b).