Our study area (latitude 2°N-66°S, longitude 8°W-208°W) covers a large portion of the southern hemisphere of Mars (see Fig. 1 and 2). The region is predominated by the Hellas impact basin some 2000 km wide and 9 km deep, possibly caused by an oblique impact with a trajectory from northwest (Leonard and Tanaka 1993, 2001; Tanaka and Leonard 1995). Another major impact basin, Isidis, is immediately to the north of our study area.
The region around Hellas impact basin is a geologically diverse area. The highland units and especially the western rim of the Hellas basin, Hellespontus Montes, represent the oldest material in the region (Tanaka and Leonard 1995). They include the wide old cratered highland west of Hellas (Noachis Terra) and the eastern highland region. The very shallowly sloped large volcanic edifices of Tyrrhena and Hadriaca Paterae east of Hellas, and Peneus and Amphitrites Paterae south of Hellas represent some of the most ancient recognizable post-impact Hellas volcanism on Mars (Plescia and Saunders 1979). These paterae with highly degraded appearance of the slopes portray evidence of pyroclastic eruptions (Tanaka and Scott 1987; Crown and Greeley 1993; Tanaka and Leonard 1995). They are surrounded by the low-lying volcanic plains of Hesperia and Malea Plana (Tanaka et al. 2002). In the north our study area is on the verge of the volcanic plains of Syrtis Major Planitia. Also Elysium Planitia, a major center of Martian volcanism, reaches the northeastern corner of our study area. The border of Elysium Planitia also marks the global dichotomy boundary.
The Hellas internal rim boundary displays interesting delta formations of several large outflow channels as well as shorelines resulted in standing water body or in an ancient ice-covered lake (Moore and Wilhelms 2001). Although channels implying fluvial activity can be observed on both sides of Hellas, particularly the low eastern plains region bears softened features adjacent to large channel formations. It has been suggested that the origin for the large outflow channels in the northeastern Hellas area was due to the late-stage effusive volcanism of Tyrrhena Patera (Leonard and Tanaka 2001). It may have triggered collapse and outflow erosion producing the valleys of Dao, Niger, and Harmakhis (Squyres et al. 1987), which are roughly radial to Hellas suggesting a possible connection to Hellas-centered tectonism, i.e., mainly fracturing (Crown et al. 1992). The eastern Amphitrites area has also a wide, channel-like formation connecting to the Hellas basin.
The youngest geological unit consists of the material transported by eolian activity and deposited mainly as dune fields within craters. This unit has distinctly concentrated to the western side of the basin. The absence of large dune fields in the eastern region is surprising because the whole Hellas region is proposed to provide dust storm materials (Martin and Zurek 1993).
The volcanic plains of Hesperia and Malea Plana bear a rich collection of wrinkle ridges similar to lunar mare ridges (e.g., Raitala 1988; Crown et al. 1992). Many of these are induced by the paterae, but the numerous ridges radial and concentric to Hellas imply tectonic control by the basin. Some of the ridges in Hesperia Planum continue to the adjacent highland areas (Raitala 1988). The wrinkle ridges have their origin mainly in the sinking of the Planum, which lead to compression and faulting. The trends of the ridges are probably reflecting older crustal fractures and other similar structures (Raitala 1988 and references therein).
The investigation of the various craters of the region and combining the crater distribution of morphologically classified craters (rampart, collapsed, craters modified by fluvial activity) (Aittola et al. 2002) with a geological analysis of the region has increased our knowledge of the geology and evolution of the region (Kostama et al. 2002). The already obtained crater distributions indicate that the Hellas area displays a versatile sample of Martian impact craters.
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