TERRESTRIAL ALCOVES AS ANALOGS FOR SHELTERED SITES FOR MARTIAN BIOTA

Terrestrial alcoves are associated with cliff-forming, permeable rock layers underlain by more easily erodible materials. Alcoves are found in a variety of terrestrial environments including the spring-sapping alcoves in Cliff House Sandstone of Mesa Verde; deeply entrenched, theater-headed valleys in Navajo Sandstone; plunge pool alcoves in limestone at Hamilton Pool of central Texas and Alcove Springs of northeastern Kansas; and grottos in basalt of Kauai, Hawaii and the Snake River. Undercutting and removal of support—by groundwater sapping or plunge pool development—leads to spalling of the overlying rock to form an arched alcove. This process is effective in both arid and humid environments. The shelf with overhanging rock provides a sheltered environment in which biota is protected from the harsher aspects of the environment.

Formation of overhanging rock shelters in sapping channels and on canyons walls on Mars is highly probable. Theater-headed valleys such as Nanedi Vallis, Kasei Vallis, Nirgal Valles and Bahram Vallis are cut into resistant layers underlain by softer rock. Alcoves are found at the heads of channels and in major bends of steep-sided channels. Whether carved exclusively by spring-sapping or a combination of sapping and overland flow, it is apparent that discharge in channels was sufficient to remove debris as the channels advanced headward. Providing a water-rich environment protected from harsh radiation and severe dust storms, rock alcoves fed by springs would offer shielded habitats for early life-forms on Mars.