Lava tubes and channels in the lunar Marius Hills

The Marius Hills region, a volcanic plateau in Oceanus Procellarum, contains numerous rilles, rille-like structures, and chains of elongate craters. Most of these structures characteristically: (1) originate on or near irregular shaped craters associated with features previously interpreted as volcanic domes, (2) trend downslope onto Plateau Plains, (3) generally taper in width and become shallower, (4) are often discountinuous, (5) occupy the center, or apparent crest of a broad ridge, (6) may contain cut-off branches and distributary structures, and (7) may have local reversals in longitudinal slope. Structures having these characteristics are interpreted to be lava channels or partly collapsed lava tubes. Terrestrial lava tubes form exclusively, and commonly, in fluid basalt flows. Recent evidence indicates that viscosities of lunar mare basalt lava flows were conducive for lava tube formation.Terrestrial analogs are offered for structures described in the Marius Hills. The analogs are comparable in qualitative and quantitative geomorphic aspects, excluding that of width. Scaling consideration of lunar reduced gravity accounts for increased width of the lunar structures. Linear and curvilinear rilles trending along equal elevations are interpreted to result from fracturing or faulting.     

Terrestrial analogs to lunar dimple (drainage) craters

In addition to modes of formation offered by previous investigators, terrestrial drainage craters formed over lava tubes in Oregon are presented as analogs to lunar drainage craters. Craters associated with lava tubes result from: (1) drainage of surface material through roof fractures; (2) plastic collapse of the partially cooled lava tube roof; and (3) drainage of surface material into roof collapses. The former two categories result in shallow, often elongate craters; the latter category forms classic dimple shaped craters. Elongate dimple craters formed over volcanic fissures in southern Idaho are also discussed and presented in support of one mode of formation proposed by previous investigators for lunar drainage craters.Movement of surface material toward the orifice is initiated on Earth largely by wind and water; in lunar conditions, initial movement is attributed to micro- and macro-meteoritic bombardment, seismic disturbances generated by internal and external processes, and by thermal creep.Nat. Res. Council Postdoctoral Resident Research Associate.     

Microdunes and other aeolian bedforms on Venus: Wind tunnel simulations

Previous studies confirm that sand can be entrained at the wind velocities recorded on Venus. Present results describe bedforms produced in the Venus Wind Tunnel (VWT) simulating the average Venusian environment. Even at the low wind speeds measured on Venus, dunelike structures form in fine-grained quartz sands (particles 50–200 μm in diameter). The dunelike structures, referred to as microdunes, are considered to be true dunes analogous to those on Earth because they have (1) slip faces, (2) a lack of particle-size sorting, (3) a low ratio of saltation path length to dune length, and (4) internal cross-bedding. The microdunes typically produced in the VWT are 9 cm long and 0.75 cm high. It is proposed that there may be fields of microdunes on Venus that are capable of very fast rates of migration and that they may grow into features much larger than those observed in the VWT. However, neither dunes nor other types of features develop above a wind speed of ～ 1.5 m/sec; at this wind speed, the bed is flat and featureless. Thus, it is predicted that relatively short periods of higher winds may destroy microdunes and other small bedforms which could account for the sparsity of definitive aeolian features observed in Venera images. Some apparent cross-bedding observed in Venera images, however, could represent preserved aeolian structures.