Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..652r&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
Introduction: The Isidis Basin is one of the largest impact basin on Mars, and is located at the lowland/highland boundary at 13°50'N, 87°00'E (Figure 1). Large parts of the Isidis floor are covered with thumbprint terrain [e.g., 1-4], consisting of curvilinear ridges of coalesced cones with central depressions. While thumbprint terrain can be observed in many areas of the northern lowlands [4,5], it is particularly well developed in the Isidis basin. We have mapped parts of the thumbprint terrain in Isidis Planitia using images of the High Resolution Stereo Camera (HRSC) (Fig. 1), augmented by MOLA, MOC, THEMIS, HIRISE and other data. Many different scenarios have been postulated for the formation of thumbprint terrain, and possible terrestrial analogues include for example pingos, moraines, shield volcanoes, pseudo craters, and eskers [e.g., 1,5]. Here, we focus on the geomorphologic and geomorphometric characterization of the thumbprint terrain in Isidis Planitia in order to address the following questions: Is there a relationship between height and length of the ridges and the cones? What is the distribution of the ridges and isolated cones with respect to the position within the basin? Are there any systematic differences between the ridges/cones with respect to different geological units? What process formed the isolated cones and the curvilinear ridges? Methods: We constructed a detailed map of the ridges/cones in order to retrieve information about their distribution (Fig. 2). Our map is based on HRSC orbits h2948_0000, h5180_0000, and h5162_0000 with a spatial resolution of 12.5 - 25 m/pixel [6] (Fig. 1). We classified the thumbprint terrain into six categories, based on varying length of the ridges. Category I includes individual isolated cones, category II consists of 2-3 coalesced cones, category III is characterized by ridges consisting of 4-6 coalesced cones, category IV contains ridges with 7- 10 cones, and category V includes ridges with more than 10 coalesced cones. In addition, we identified and mapped ridges without cones with central depressions, which we termed "no cone ridges" (NCR) (Fig. 2;3). For each category, we measured the height of at least 20 ridges/cones per orbit. For these measurements we used individual MOLA tracks crossing the features of interest. On the basis of these preliminary measurements, we performed an initial statistical analysis of thumbprint terrain in order to reveal possible correlations between the height and the length of the ridges/cones. Results: Our map indicates spatial variations in the distribution of the thumbprint terrain. For example, the number of ridges and cones of all categories is larger in the northern regions compared to the southern areas of the floor. In addition, ridges and cones are absent on the ejecta blankets of young rampart craters, which postdate the formation of the thumbprint terrain. If we compare our map with the geologic map of Greeley and Guest [7], we see that the thumbprint terrain occurs in areas mapped as the Hesperian Vastitas Borealis ridged member (Hvr) or the Amazonian smooth plains unit (Aps). While unit Aps has been interpreted to have diverse origins, including eolian deposition, unit Hvr has been interpreted as being volcanic, periglacial or erosional in origin [7]. Low mounds have been interpreted as volcanic spatter cones [7]. We observed differences in the roughness of the terrain between the ridges exposed within unit Hvr. The smoothness, the occurrence of flow fronts, and the lack of impact craters suggest that in some regions resurfacing of unit Hvr by a viscous liquid occurred. In regions that have not been resurfaced, we observed a rougher terrain with numerous small-scaled impact craters and a higher concentration of no-cone ridges (NCR) and individual hills without central depression (Fig. 3). The Hesperian to Noachian undivided unit HNu exposed at the transition to Syrtis Mayor is devoid of ridges and cones. In the geologic map of Tanaka et al. [4], the study area is mapped as Isidis Planitia unit (AIi), which is of early Amazonian age and of sedimentary origin. On the basis of mapping 8500 ridges/cones and 7000 NCRs in all three orbits we observed an anticorrelation between the number of ridges and their lengths. The majority of the investigated features are isolated cones (Cat. I: 43%) and relatively short ridges consisting of 2-3 coalesced cones (Cat. II: 35%). The number of ridges with 4-6 (Cat. III: 15%), 7-10 (Cat. IV: 5%), and more than 10 coalesced cones (Cat. V: 3%) are significantly smaller (Figure 3). We also measured the heights of the ridges within each category. Our measurements revealed that the median height is largest (33 m) for category III. This is similar to the heights of the longer ridges of categories IV (28 m) and V (32 m). The heights of individual cones (13 m) and small ridges with less than 4 cones (21 m) are typically smaller (Figure 4). NCR ridges/cones exhibit median heights of 15 m. These heights are comparable to the height of numerous terrestrial analogues, including pingos, volcanic cones, pseudocraters, and mud volcanoes. Conclusions: We have performed a detailed map of the distribution, lengths and heights of ridges on the floor of western Isidis Planitia. Based on this study we conclude: (1) thumbprint ridges consist of coalesced cones with central depressions and exhibit a large range of lengths and number of cones; (2) a larger number of ridges occurs in the northern parts of basin floor; (3) thumbprint ridges are on average less than 35 m high, but there are a few with heights up to more than 70 m; (4) the occurrence of central depressions associated with the ridges/cones in Isidis Planitia is indicative of either collapse (e.g., pingo) or an explosive origin (e.g., volcanic cones); (5) on the basis of the morphology and morphometry of the ridges and cones, that other origins, including various types of moraines, sand ridges, dunes, eskers, drumlins, kames, crevasse fill, beach ridges, berms, table mountains, mud volcanoes, and inverted topography appear less likely; (6) that the rough terrain in Hvr might represent older ridges/cones that have been eroded and/or covered with subsequently deposited material, hence appearing less prominent and without cones. References [1] Grizzaffi and Schultz, (1989), Icarus 77; [2] Hiesinger and Head, (2003) 6. International Conference on Mars; [3] Hiesinger and Head (2004) LPSC 1167; [4] Tanaka et al. (2005), USGS SIM 2888; [5] Pomerantz and Head, (2003) LPSC 34; [6] Jaumann and Neukum (2007), Planet. Space Sci. 55; [7] Greeley and Guest, (1987), USGS I-1802B;
Hiesinger Harald
Reiss David
Rohkamp D.
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