No abstract

The. ce.c.c•ncl largest volcanic province on Mar• lies in the E!?ium region I•ike the larger Tharsis province, Elysium is marked by a topographic rise and a broad free air gravity anomaly and also exhibits a complex assortment of tectonic and volcanic features. We test the hypothesis that the tectonic features in the Elysium region are the product of stresses produced by loading of the Martian lithosphere. We consider loading at three different scales: local loading by individual volcanoes, regional loading of the lithosphere from above or below, and quasi-global loading by Tharsis. A comparison of flexural stresses with lithospheric strength and with the inferred maximum depth of faulting confirms that concentric graben around Elysium Mons can be explained as resulting from local flexure of an elastic lithosphere about 50 km thick in response to the volcano load. Volcanic loading on a regional scale, however, leads to predicted stresses inconsistent with all observed tectonic features, suggesting that loading by widespread emplacement of thick plains deposits was not an important factor in the tectonic evolution of the Elysium region. A number of linear extensional features oriented generally NW-SE may have been the result of flexural uplift of the lithosphere on the scale of the Elysium rise. The global stress field associated with the support of the Tharsis rise appears to have influenced the development of many of the tectonic features in the Elysium region, including Cerberus Rupes and the systems of ridges,in eastern and western Elysium. The comparisons of stress models for Elysium with the preserved tectonic features support a succession of stress fields operating at different times in the region. While the order in which those stress fields operated cannot be determined from present geological observations, thermal and mechanical arguments favor the hypothesis that any flexural uplift of the lithosphere by a mantle thermal anomaly preceded or occurred contemporaneously with emplacement of the largest volcanic loads. 11,377

Geological mapping of Elysium Planitia has led to the recognition of five major surface units, in addition to the three volcanic constructs Elysium Mans, Hecates Tholus, and AIbor ThoIus. These units are interpreted to be both volcanic and sedimentary or erosional in origin. The volcano Elysium Mons is seen to have dominated constructional activity within the whole region, erupting lava flows which extend up to 600 km from the summit. A major vent system, covering an area in excess of 75 000 km*, is identified within the Elysium Fossae area. Forty-one sinuous channels are visible within Elysium Planitia; these channels are thought to be analogous to lunar sinuous rilIes and their formation in this region of Mars is attributed to unusually high regional topographic slopes (up to -1.7"). Numerous circumferential graben are centered upon Elysium Mons. These graben, located at radial distances of 175, 205-225, and 330 km from the summit, evidently postdated the emplacement of the Elysium Mons lava flows but pre-dated the eruption of extensive flood lavas to the west of the volcano. A great diversity of channel types is observed within Elysium Fossae. The occurrences of streamlined islands and multiple floor-levels within some channels suggests a fluvial origin. Conversely, the sinuosity and enlarged source craters of other channels suggests a volcanic origin. Impact crater morphology, the occurrence of chaotic terrain, probable pyroclastic deposits upon Hecates Tholus and fluvial channels all suggest extensive volcano-ground ice interactions within this area.

Calculations have been performed to quantify the cost and delivered mass advantages of aerocapture at all destinations in the Solar System with significant atmospheres. A total of eleven representative missions were defined for the eight possible destinations and complete launch-to-orbit insertion architectures constructed. Direct comparisons were made between aerocapture and competing orbit insertion techniques based on state-of-the-art and advanced chemical propulsion, solar electric propulsion, and aerobraking. The results show that three of the missions cannot be done without aerocapture: Neptune elliptical orbits, Saturn circular orbits, and Jupiter circular orbits. Aerocapture was found to substantially reduce the cost per unit mass delivered into orbit for five other missions based on a heavy launch vehicle: Venus circular orbits (55% reduction in $/kg costs), Venus elliptical orbits (43% reduction); Mars circular orbits (13% reduction), Titan circular orbits (75% reduction), and Uranus circular orbits (69% reduction). These results were found to be relatively insensitive to 30% increases in both the estimated aerocapture system mass and system cost, suggesting that even modestly performing aerocapture systems will yield substantial mission benefits. Two other missions consisting of spacecraft in high eccentricity elliptical orbits at Mars and Jupiter were not shown to be improved by aerocapture. The last mission in the set consisting of an aeroassisted orbit transfer at Earth showed that aerocapture offered a 32% $/kg reduction compared to chemical propulsion, but that aerobraking offered even better performance. Nevertheless, the problems of repeated passes through the Van Allen radiation belts are likely to preclude Earth aerobraking for most applications.