Comparison of volcanic features of Elysium (Mars) and Tibesti (Earth). Age of Martian channels. Nature and origin of intercrater plains on Mars

摘要

This thesis consists of three separate parts, each addressing a specific aspect of the surface morphology of Mars.In Part 1, a comparison of large volcanic features is made using spacecraft imaging data. The Elysium volcanic province on Mars and the Tibesti volcanic province in Chad, Africa are studied using Mariner 9, Earth Resources Technology Satellite and Apollo photography. Elysium Mons on Mars and Emi Koussi on Earth show remarkable similarities in summit caldera and flank morphologies. Each has a large central caldera approximately 12 km in diameter and from 500 to 1000 m deep which contain numerous craters and large, irregular pits. Channel-like features which head at the calderas and taper downslope show evidence of collapse and possible lava erosion. Elysium Mons rises some 14 ? 1.5 km above its base and the summit is about 17 km above the 6.1 mbar mean martian pressure surface. Cratering data indicate most of the apparent cratering is endogenic in origin. The subdued, hummocky terrain on the flanks is distinctly different from the slopes of the younger Tharsis Ridge volcanoes, showing little if any sign of recent material flow.The lack of aqueous erosional forms on Elysium Mons argues strongly against recent ([approx.]10[superscript 5] to 10[superscript 6] year) pluvial episodes. The forms and associations of features throughout the Elysium region suggest central volcanism started earlier in Elysium than in Tharsis, and that the source of the Elysium volcanics is chemically evolved, with evidence of silicic magma. Finally, the data are consistent with the view that the martian crust has been stable and essentially motionless for an extended period of martian geologic time.Part 2 attempts to determine the age relationship between the large, sinuous channels on Mars and the terrains in which the channels are found. Crater counts, plain and mantle superposition, and geographic and geologic associations suggest that the channels are extremely old, are spatially and temporarily related to the ancient cratered terrain and are genetically related to the processes of fretting and chaos formation. There appears to be no evidence for recent channel activity.Part 3 presents the results of an investigation of the morphologic characteristics of the plains which separate the craters in the heavily cratered regions of Mars. These intercrater plains appear to be composed of stratified consolidated and unconsolidated materials, probably loose debris blankets and volcanic flows. The topmost layer of the plains unit varies from location to location. An older, cratered surface may be partly exposed where the kilometers-thick plains unit is locally incised and eroded. The association of chaotic terrain, fretted terrain and major channels with the plains suggests that the volatile(s) presumed to be necessary to produce these erosional landforms may have been present among the plains materials. It is speculated that the unconsolidated material is impact-generated debris and eolian deposits, suggesting an early atmosphere conducive to material transport and possibly flowing liquids.