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Geologic Map of the Hellas Region of Mars

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Geologic Map of the Hellas Region of Mars U.S. DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP I–2694 U.S. GEOLOGICAL SURVEY GEOLOGIC MAP OF THE HELLAS REGION OF MARS By Gregory J. Leonard and Kenneth L. Tanaka INTRODUCTION Available Viking images of the Hellas region vary This geologic map of the Hellas region focuses greatly in several aspects, which has complicated the on the stratigraphic, structural, and erosional histories task of producing a consistent photogeologic map. Best associated with the largest well-preserved impact basin available image resolution ranges from about 30 to 300 on Mars. Along with the uplifted rim and huge, partly m/pixel from place to place. Many images contain haze infilled inner basin (Hellas Planitia) of the Hellas basin caused by dust clouds, and contrast and shading vary impact structure, the map region includes areas of ancient among images because of dramatic seasonal changes in highland terrain, broad volcanic edifices and deposits, surface albedo, opposing sun azimuths, and solar incli- and extensive channels. Geologic activity recorded in the nation. Enhancement of selected images on a computer- region spans all major epochs of martian chronology, display system has greatly improved our ability to observe from the early formation of the impact basin to ongoing key geologic relations in several areas. resurfacing caused by eolian activity. Determination of the geologic history of the region The Hellas region, whose name refers to the clas- includes reconstruction of the origin and sequence of for- sical term for Greece, has been known from telescopic mation, deformation, and modification of geologic units observations as a prominent bright feature on the sur- constituting (1) the impact-basin rim and surrounding face of Mars for more than a century (see Blunck, 1982). highlands, (2) volcanic and channel assemblages on the More recently, spacecraft imaging has greatly improved northeast and south sides of the basin, (3) interior basin our visual perception of Mars and made possible its geo- deposits, and (4) slope and surficial materials throughout logic interpretation. Here, our mapping at 1:5,000,000 the map area. Various surface modifications are attrib- scale is based on images obtained by the Viking Orbiters, uted to volcanic, fluvial, eolian, mass-wasting, and pos- which produced higher quality images than their prede- sibly glacial and periglacial processes. Structures include cessor, Mariner 9. Previous geologic maps of the region basin faults (mostly inferred), wrinkle ridges occurring include those of the 1:5,000,000-scale global series based mainly in volcanic terrains and interior plains, volcanic on Mariner 9 images (Potter, 1976; Peterson, 1977; King, collapse craters, and impact craters. Our interpretations 1978); the 1:15,000,000-scale global series based on in some cases rely on previous work, but in many signifi- Viking images (Greeley and Guest, 1987; Tanaka and cant cases we have offered new interpretations that we Scott, 1987); and detailed 1:500,000-scale maps of Tyr- believe are more consistent with the observations docu- rhena Patera (Gregg and others, 1998), Dao, Harmakhis, mented by our mapping. Our primary intent for this map- and Reull Valles (Price, 1998; Mest and Crown, in press), ping has been to elucidate the history of emplacement Hadriaca Patera (D.A. Crown and R. Greeley, map in and modification of Hellas Planitia materials, which form preparation), and western Hellas Planitia (J.M. Moore the basis for analysis of their relevance to the global cli- and D.E. Wilhelms, map in preparation). mate history of Mars (for example, Tanaka and Leonard, We incorporated some of the previous work, but our 1995). map differs markedly in the identification and organiza- tion of map units. For example, we divide the Hellas HISTORICAL OBSERVATIONS assemblage of Greeley and Guest (1987) into the Hellas The immense size of the sharp albedo changes Planitia and Hellas rim assemblages and change the way observed for Hellas have mystified observers for well units within these groupings are identified and mapped over a hundred years (Blunck, 1982). Nineteenth-century (table 1). The new classification scheme includes broad, and earlier observers believed this region (designated geographically related categories and local, geologically on older maps as “Lockyer Land”) to be a raised land- and geomorphically related subgroups. Because of our mass or an island adjacent to the “Hourglass Sea” (Syrtis mapping at larger scale, many of our map units were Major). Giovanni Schiaparelli mapped two large canals, incorporated within larger units of the global-scale map- Peneus and Alpheus, in the form of an “X” pattern ping (see table 1). within Hellas Planitia (Schiaparelli, 1877), yet today we 1 find no features that correlate with this observation. Per- and broadest enclosed depression on the planet (about 9 cival Lowell also mapped numerous canals in the region km of relief; 2,000 km across) in the southern cratered (Lowell, 1895). During some oppositions the region terrain. Heavily cratered terrain ranges from –1 km eleva- appeared brilliantly white when viewed near the edge of tion within the west rim of the basin to as much as 5 the martian disc, fooling some into believing it was some km in highland areas surrounding the basin. Individual kind of polar cap. We now know this effect to be caused massifs of the basin rim stand as high as 5 km above by high albedo CO2 fog that seasonally condenses into surrounding terrain and are more common and extensive lower Hellas Planitia. on the southeast side of the basin (Leonard and Tanaka, Sending spacecraft to the red planet significantly 1993). The west side of Hellas basin is ringed by Helles- increased our understanding of Mars and the Hellas pontus Montes, a system of mountains marked by high region. The Mariner 4, 6, and 7 flybys combined to image scarps circumferential to the basin. about 20 percent of the planetʼs surface, including the The basin floor, Hellas Planitia, can be divided into Hellas region at fairly low resolutions (Moore, 1977; two parts: (1) an annular ring about 1,500 km in diam- Snyder and Moroz, 1992). However, surface features of eter and 200–500 km wide made up of plains marked by the Hellas region were concealed by thick dust and haze. ridges (including Zea Dorsa) at elevations of –5 to –1 Thus, throughout the early years of the spacecraft era, the km, and (2) an inner, rugged zone about 1,000 km across Hellas region continued to be a mystery of the martian consisting of mesas, ridges, scarps, depressions, and hills surface. Researchers speculated about large deserts and (including Coronae Scopulus and Alpheus Colles); ele- vigorous, perhaps ongoing, resurfacing, a view that is not vation of this zone is mostly –5 to –4 km, but parts lie as entirely untrue. In fact, the Hellas region became one of low as –5 km. three type areas (featureless plains) first differentiated by Paterae interpreted to be volcanic shields and circu- the Mariner data (Snyder and Moroz, 1992). The Mariner lar calderas form local topographic highs (having 1 to 2 7 spacecraft encounter in 1969 finally determined Hellas km of relief) on the basin rim. On the northeast rim are to be a large basin and not a raised plateau as previously Hadriaca and Tyrrhena Paterae, and on the south rim are believed. Amphitrites and Peneus Paterae. Narrow and broad chan- The Mariner 9 orbiter in 1971 imaged the region in nels dissect the flanks of these features. In turn, the pat- somewhat greater detail than previous spacecraft. How- erae are surrounded by high plains (Hesperia and Malea ever, dust and haze continued to obscure the basin floor Plana) that also are locally marked by channels oriented several months after the great global dust storm of 1971 downslope. These channels include Axius Valles, which had subsided, and only a few frames from the Mariner 9 dissect the entire south rim of Hellas basin. Extensive extended mission show part of the floor in some detail. wrinkle ridges also cross the paterae and surrounding The two Viking Orbiters, which visited just a few years plains along varying trends. later, showed that Hellas was not a featureless plain as The large Niger and Dao Valles originate as 1- to previously thought; rather it is a broad, deep basin having 2-km-deep broad depressions along the southeast flank a rugged, complex floor—now called Hellas Planitia— of Hadriaca Patera. Apparently related to these valles and large, ancient volcanoes and channels along the rim. are isolated depressions, Ausonia and Peraea Cavi, just The Viking Orbiter images finally allowed detailed geo- northeast of Hadriaca (lat 30° S., long 264.5° W.). The logic studies of the region to come to fruition. large valles appear to head from these depressions. South- One final distinction held by the Hellas region is east of Niger and Dao Valles is another similar channel that it is the site of the first manmade object to reach the system—Reull and Harmakhis Valles. Reull Vallis origi- surface of Mars. In late November 1971, the Soviet space- nates in southern Hesperia Planum (northeast of the map craft Mars 2 jettisoned a small capsule that hit the red area) and traverses rugged Promethei Terra; Harmakhis planet at about lat 44.2° S., long 313.2° W. (Moore, 1977), begins at a 1.5-km-deep depression near the termination which is along the western rim of the basin near Helles- of Reull Vallis amid the peaks of Centauri Montes. Lower pontus Montes. Apparently the capsule was intended to parts of Niger, Dao, and Harmakhis Valles form braided be a hard lander and carried only a pennant depicting the channel systems incised less than 1 km into the Hellas coat of arms of the Soviet Union; no scientific data were basin rim.
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