[1] The Martian cryosphere is defined as that region of the crust where the temperature remains continuously below the freezing point of water. Previous estimates of its present thickness have ranged from ∼2.3-4.7 km at the equator to ∼6.5-12.5 km at the poles. Here we revisit these calculations, review some of the assumptions on which they were based, and investigate the effects of several parameters, not previously considered, on the cryosphere's thermal evolution and extent. These include astronomically driven climate change, the temperature-dependent thermal properties of an ice-rich crust, the potential presence of gas hydrate and perchlorate-saturated groundwater, and consideration of recent lower estimates of present-day global heat flow (which suggest a mean value roughly half that previously thought, effectively doubling the potential thickness of frozen ground). The implications of these findings for the continued survival of subpermafrost groundwater and its potential detection by the MARSIS radar sounder onboard Mars Express are then discussed. Although our estimates of the maximum potential thickness of the cryosphere have significantly increased, consideration of the likely range and spatial variability of crustal heat flow and thermal properties, in combination with the potential presence of potent freezing point depressing salts, may result in substantial local variations in cryosphere thickness. The locations that appear best suited for the detection of groundwater are those that combine low latitude (minimizing the thickness of frozen ground) and low elevation (minimizing the depth to a water table in hydrostatic equilibrium). Preliminary results from a MARSIS investigation of one such area are discussed.

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This book is dedicated to Dr. Keith A. Kvenvolden, a pioneer in the studies of gas hydrate and the broader issues of petroleum in the natural environment. Keith has been one of the most knowledgeable scientists in the field of gas hydrate geochemistry. Furthermore, he is a true gentleman who has encouraged others and has been a guiding force to his peers and younger scientists.

Re-examination of the Mona Complex (Anglesey, North Wales) has led to a radical re-interpretation of its stratigraphy, structure and geological history. The gneisses are regarded as representatives of an earlier continental basement, as Greenly (1919) originally proposed. In the northern part of Anglesey the Bedded Succession can be separated into 3 structural units. The South Stack Unit of Holy Island is overlain by the more highly deformed New Harbour Unit; the contact between these two units is interpreted as a thrust plane. A third unit, the Cemlyn Unit, outcropping in the NW, comprises the Church Bay Tuffs, Skerries and Gwna Groups of Greenly's succession, as well as greywackes and slates, correlated by Greenly with the New Harbour Group. However, the Cemlyn Unit is less deformed than the New Harbour Unit and their relationships may be unconformable. It is proposed that Greenly's Fydlyn Group does not form part of the Mona Complex but may be correlated with the Caradocian volcanics of Parys Mountain. No major structural discordance occurs between the Gwna Group and the overlying Ordovician rocks on the N coast. The time gap represented by this contact has also been reduced by recent fossil finds, which indicate that some members of the Mona Complex are of Cambrian Age. The implications of these discoveries are that there was no 'Irish Sea land mass' in the Anglesey area in the Lower Palaeozoic and that major structures in the complex previously attributed to a late Precambrian orogeny are of Caledonian age. Using the new structural and stratigraphic data, a synthesis of the sedimentary, tectonic and structural evolution of the complex is proposed, in the context of plate tectonics.

Abstract. The search for life on Mars has recently focused on its potential survival in deep (>2 km) subpermafrost aquifers where anaerobic bacteria, similar to those found in deep subsurface ecosystems on Earth, may have survived in an environment that has remained stable for billions of years. An anticipated by-product of this biological activity is methane. The detection of large deposits of methane gas and hydrate in the Martian cryosphere, or as emissions from deep fracture zones, would provide persuasive evidence of indigenous life and confirm the presence of a valuable in situ resource for use by future human explorers. IntroductionWith an average surface temperature of -200 K, a CO2 atmosphere with a surface pressure of-6 mbar, and a high incident flux of UVB, the present Martian surface environment is hostile to life as we know it. However, there is abundant The depth to which significant porosity, permeability, and water persist on Mars is unknown. However, in light of the considerable extent to which impacts, volcanism, tectonics, and the presence of abundant water have affected the evolution of its surface, it is likely that the gross physical and hydraulic properties of the Martian crust will closely resemble those found on Earth, appropriately scaled to reflect the gravitationally induced differences in lithostatic pressure at a given depth. This suggests that the porosity and permeability conditions 4165