Icelandic pseudocraters as analogs to some volcanic cones on Mars

Pseudocraters are rootless vents formed by the interaction of lava flows wi th surface or near-surface water. This interaction can produce mild explosi ons and the accumulation of scoria and spatter into small constructs. Pseud ocraters in several localities in Iceland were examined in the field and co mpared to similar appearing features observed on Mars. The Icelandic pseudo crater cones in this study range in size from 6 to 70 m in diameter, have s ummit craters which range from 2 to 28 m in diameter (many cones lack crate rs entirely), and have flanks that are either concave-up or convex-up. The size and spacing of Icelandic pseudocraters might be a function of the avai lability of water, in which larger, closely spaced features result from eff icient lava-water interaction, as suggested by the environments in which th e features formed. Possible Martian pseudocrater cones in Amazonis Planitia range in diameter from 30 to 180 m and have craters 12 to 80 m in diameter . A numerical model for volcanic explosions was adapted to study the format ion of pseudocraters under terrestrial and Martian conditions. The results suggest that explosions forming Martian cones require significantly less wa ter (calculated masses are less by a factor of 4 to 16) than those forming Icelandic pseudocraters, despite their larger sizes. This is attributed to the low gravity and atmospheric pressure in the Mars environment and is con sistent with the likely lower abundance of water, which might be present as interstitial ice at shallow depths in the regolith. Locations of potential pseudocraters on Mars at latitudes as low as similar to8 degreesN, imply t he presence of crustal ice stores at the time of their formation.