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[1] Recent analyses of Mars Global Surveyor and Mars Odyssey data sets provide new insights into the geologic evolution of the eastern Hellas region of Mars, in particular, the role of volatiles. Here, we present results of our recent work and integrate these with previous studies by various investigators to provide a synthesis of the history of volatile-driven activity of the region. We utilize high-resolution images from the Mars Orbiter Camera and Thermal Emission Imaging System combined with Mars Orbiter Laser Altimeter digital elevation models and profiles to examine fluvial systems that dissect the circum-Hellas highlands, to characterize stages in the development of the Dao, Niger, Harmakhis, and Reull Valles canyon systems, and to evaluate evidence for ancient lakes in Hellas Planitia. The occurrence of valley networks, dissected highland crater rims, and crater interior deposits such as layered plateaus suggests widespread ancient degradation of the circum-Hellas highlands. Canyon development, which represents subsequent more localized activity, may have included an early fluvial phase followed by the collapse and sapping dominated stages that, along with recent wall erosion and floor resurfacing, produced the currently observed morphologies. The prominent role of collapse and sapping along the east rim of Hellas, along with the presence of numerous channels extending toward the basin and sequences of finely layered deposits along the basin rim, suggests a volatile-rich substrate across a broad depositional shelf. The east rim of the basin was an accumulation zone for atmospheric volatiles and/or the edge of volatile-rich deposits associated with the basin floor. This evidence combined with topographic data and cratered terrain preservation around the basin is consistent with a lacustrine period or periods in early Martian history. The style, magnitude, and spatial extent of volatile-driven activity in eastern Hellas have varied considerably with time, and these variations may represent a transition from a water-to an ice-dominated surface environment.Citation: Crown, D. A., L. F. Bleamaster III, and S. C. Mest (2005), Styles and timing of volatile-driven activity in the eastern Hellas region of Mars,

[1] The Ix-Chel, Kuanja, and Vir-ava Chasmata (IKVC) are a 7000-km-long collection of discontinuous topographic troughs and deformation suites. Collectively, they account for approximately 12.5% of the $55,000 linear km of chasmata on Venus; however, morphologically the IKVC differs significantly from the majority (75%) of Venusian chasmata because it lacks corona features. Coronae represent notable volumes of material both erupted onto the surface and intruded into the crust. Together with chasmata, coronae may also be important indicators of Venus's interior convective processes as well as significant contributors to Venus's heat loss. Although IKVC lacks coronae, detailed geologic and structural mapping along its length highlights previously unrecognized structural and volcanic relationships indicative of local uplift, collapse, and volcanism we call pseudocoronae. In light of this mapping, previous corona formation models (theoretical and numerical) are evaluated, and an amended hypothesis (diapir stagnation) is presented for pseudocorona formation. The diapir stagnation model proposes that variations of the crustal thickness perturb corona formation and thus affect their resultant surface morphology. Chasmata are accompanied by corona development when forming in regions of relatively thin, or normal (plains-type), crust (<20 km) but lack corona features when influenced by crustal heterogeneities (mechanical and chemical) at depth associated with thick crustal plateau roots (35-80 km (e.g., Ovda, Thetis, and Phoebe Regios)). Beneath crustal plateaus, diapiric rise is inhibited by (1) the presence of a low-density crustal keel that creates a deep-seated density trap and (2) a compositiondependent mechanically weak crust-mantle boundary layer. Diapir stagnation can be characterized or accompanied by minimal broad doming of the surface, formation of radial and concentric extension structures, lateral dike propagation, and collapse, all of which may be accompanied by limited volume surface eruptions of the lowest density melt phases. Through time, the crust-mantle boundary layer may repeatedly lift and subside through conductive heating and viscous flow, which may allow more voluminous melt migration and eruption; however, true coronae fail to develop. INDEX TERMS: 6295

[1] The nature and significance of collapse processes in Capri, Eos, and Ganges Chasmata remain poorly understood. Using Ganges Chasma as a type locality, these chasmata are interpreted to be the result of clustering and assimilation of multiple chaotic terrains, which primarily formed by localized depressurization-induced or thermally-triggered dissociation of buried gas clathrate hydrates and explosive eruption of gas-saturated ground water. Such crustal destabilization could have been triggered by (1) deep fracture propagation from the Martian surface, (2) magmatic intrusions and associated heating and inflation-induced terrain fracturing, and/or (3) climatic thaw and thinning/weakening of the permafrost over the clathrate and gas-rich groundwater zones. Volume increases associated with release of gases contributed to the expulsion of groundwater and fluidized sediments at the surface, thereby carving the higher outflow channels peripheral to the chasmata and the lower outflow channel floors of the chasmata and outflow channels.