Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..115l&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 Small cone-shaped features with summit pits can be found in several regions on Mars; mainly in Isidis Planitia; Elysium Planitia; Amazonis Planitia; Acidalia Planitia; in the Cydonia Region; in Cerberus Planum; the Phlegra Montes and on several volcanic flanks. They vary greatly in size and morphology and have been compared to terrestrial features of various origins; namely (1) cinder cones (e.g. [1]), (2) tuff cones or tuff rings (e.g. [2]), (3) rootless cones (pseudocraters) (e.g. [3], [4]), (4) pingos (e.g. [5], [6]) and (5) mud volcanoes (e.g. [7]). They are often found near volcanic centers and large lava fields or cluster in regions where the volatile content of the Martian regolith was/is supposedly high. This has led to the assumption that (ground-) water or ground ice was a trigger or driving force of cone formation. They could therefore, be an important indicator of the history of water on the planet. We have studied an area in western Elysium Planitia, bordering the Aeolis Planum plateau, which exhibits a large number of pitted cones, ridges and dome-like structures. Their distribution and morphology differs strongly from pitted cones elsewhere in Elysium Planitia, which have mainly been interpreted as hydrovolcanic rootless cones, and from other regions on Mars. Based on our observations, we present an alternative model for cone formation in the study area that might hint towards hydrothermal processes in the Aeolis Planum region and possibly young igneous activity. Aeolis Planum Cones The Aeolis Planum pitted cones (referred to as APCs from now on) cluster along the southern edges of the broad shallow valley that borders the Aeolis Planum Formation (APF) to the north. Cones along the northern edges of the valley are rare and can only be found in association with APF remnants where they strongly resemble the cones in the south. Along the southern border the cone coverage is almost continuous, describing a narrow band approximately 2 to 3 km wide. There are distinct morphological changes both within the band from north to south and along the band from east to west (Fig. 2). The cones are mostly circular but elongated, irregular forms are common. They are of varying size with basal diameters ranging from 20 to 200 meters, though most (single) cones have basal diameters below 100 meters. The heights of the cones are difficult to determine as their sizes are far below the resolution limits of either MOLA or HRSC stereo data, yet photoclinometric calculations have given approximate heights between ~ 10 up to several dozens of meters. Often the cones show hardly any elevation above the surroundings (e.g. Fig. 2c, e or f). Most of the APCs have steep convex flanks and large summit pits with diameters at least half as wide as their bases. The overall morphology of the cones changes from S to N with distance from the APF and from E to W along the edges of the APF. Toward the south, close to the strongly eroded borders of the APF, broad ridges and elongated domes are dominant. They form a narrow band approximately 2 km wide. The ridges and domes are a few dozen to several hundred meters long and between 10 to 50 meters wide and show numerous cracks and fissures. They are often topped by small cones, elongated pits and remnants of APF sediments. Further north follows a rather abrupt transition from the ridged area to more cone-dominated regions. Here single cones are prevalent with a more random distribution. Their number decreases rapidly with increasing distance from the APF and approximately 3 km off the southern edge of the APF no further cones are found. Hydrothermal venting on Mars? Morphology and stratigraphic relationships indicate that the cones are young and that they have, at least in places, developed inside the APF complex. APF remnants can be found covering the central pits of cones and APF units have been tilted and eroded by coneforming processes. Furthermore, cones are mainly found inside a narrow band 2-3 km wide along the APF-lava contact. A connection between APF-lava interaction and cone-forming processes is therefore likely. We propose that a combination of contact metamorphosis and associated hydrothermal venting comparable to hydrothermal vent complexes on Earth could have been the driving force of cone-formation in the study area based on the assumption of a high volatile content of the APF. The processes might then have proceeded as follows: Phase 1: The flooding of the study area by lava caused initial explosive reactions along the lava-APF-boundary forming clusters of pseudocraters. Pseudocraters are only visible towards the edges of the depression where the lava cover is thinnest. Towards the center the thick lava coverage prevented pseudocrater formation or quickly reburied forming cones. Phase 2: The heat of the cooling lava, which could be as thick as 500 m based on the diameters of flooded craters, causes contact metamorphosis and the mobilization of volatiles in the surrounding APF-sediments. Similar to hydrothermal vent complexes on Earth, this may have caused hydrofracturing of the sediments and the formation of sediment pipes and dikes that transport the volatiles to the surface. Pre-existing fissures would have served as additional pathways. At the surface rapid decompression causes phreatic explosions and the formation of small cones. Phase 3: Close to the lava-body mobilization of volatiles (e.g. by dehydratation of hydrated minerals, mobilization of ground- or pore ice or even juvenile waters and other volatiles from the lava itself) was strongest. In combination with lower sediment thickness and shorter pathways to the surface, phreatic explosion were more violent and conduits may have been repeatedly active. The lower atmospheric pressure and lower gravity on Mars would have further enhanced the explosive activity. While the lower gravity leads to a faster ascent of the volatile-sediment-phase, thereby preventing early degassing, the lower atmospheric pressure causes stronger decompression and expansion of gases. With increasing distance and increasing APF-thickness the surface manifestation of the processes weakens and phreatic explosive activity decreases. The cracked domes and elongated ridges may then be the surface expression of sediment pipes and dikes that have cooled and degassed before reaching the surface. The flow structures surrounding many cones and ridges could be interpreted in this context as fluidized sediment as lava would not have been discharged from the vents. This kind of sediment volcanism took place after the erosion of the APF and marks the end of the hydrothermal activity. Phase 4: Erosion of the APF, enhanced by the cone-forming processes themselves, later exhumed deeper parts of the vents and the brecciated sediment cores, leaving remnants of APF sediments in central pits and on top of cones, ridges and domes. References: [1] Plescia J. B. (1980) NASA Tech. Memo., 82385, 263-265. [2] Bridges J. C. et al. (2003) JGR, 180(E1), 5001, doi:10.1029/2001JE001820. [3] Fagents S. A. (2002) LPSC XXXIII, Abstract #1594. [4] Bruno B. C. (2004) JGR, 109, doi:1029/2004JE002273. [5] Theilig E. and Greeley R. (1979) J. Geophys. Res., 84, 7994-8010. [6] Page and Murray (2006) Icarus, 183, 46-54. [7] Skinner J. A. and Tanaka K. L. (2006) Icarus, 186, 41-59. [7] Watters T. R. et al. (2007) Sciencexpress, science. 1148112, 10.1126.
Lanz Julia K.
Saric Matthias B.
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