Candidate sites that meet these criteria are described. These dikes either stall within the body of the volcano as intrusions or, if they grow vertically by a great enough distance, erupt at the surface to feed lava flows or explosive eruptions.
Intrusions outnumber surface eruptions by a factor of several to one.
Once the products of any one eruption have cooled, the pore spaces between ash particles and the vesicles within lava flows form natural locations for the near-surface accumulation of water ice and solid carbon dioxide. Where successive dikes are intruded near to one another in a rift zone, part of the region between any two dikes can stay at a temperature such that H2O is a liquid for a much longer time provided that new dikes arrive on a time scale comparable with t 1. However, if these dikes are confined to a rift zone, the time interval between events is less. Estimates of the depth to the base of the cryosphere based on the current geothermal heat flow range from about 2 km near the equator to about 5 km near the poles. Thus, the vertical extent of the zone within which H2O could be present as liquid water could have been extended by at least 200 to 400 m, depending on the latitude.