45th Lunar and Planetary Science Conference (2014) 1027.pdf

ALCOVES AS PROTECTED SITES FOR MARTIAN LIFE: R. A. De Hon, Department of Geography, Texas State University, San Marcus, Texas 78133. [email protected]

Introduction: The operational paradigm percolating through the overhanging for the current search for life (past or roof provides nourishment for natural present) on is to follow the water hanging gardens. In the southwest [1]. Add to this—sites such as caves, United States alcoves are common sites lava tunnels, or natural grottos that of early pueblo development. Springs at provide shelter from an inhospitable the rear of the alcove provide water for atmospheric environment. Terrestrial colonizing biota. alcoves associated with spring sapping Potential on Mars: Requirements for and plunge-pool development furnish terrestrial-like alcoves on Mars include analogs for Martian alcoves and grottos steep escarpment; layered permeable that might serve as protected sites that and impermeable materials, and could have harbored early life forms on groundwater discharge. Stratified Mars. materials and water-rich environment of Terrestrial Alcoves: Alcoves—arched the past provides ample opportunity for recesses in steep escarpments—are alcove development on Mars. Theater- found in a variety of terrestrial headed valleys on Mars such as Nanedi environments. Examples include Vallis [6, 7], , Nirgal Vallis spring-sapping alcoves in Cliff House and Bahram Vallis [8] are cut into Sandstone of Mesa Verde [2]; deeply resistant rock—presumed to be lavas— entrenched, theater-headed valleys in underlain by softer rock. Whether Navajo Sandstone [3]; plunge pool carved exclusively by spring-sapping [3] alcoves in limestone such as Alcove or a combination of sapping and Springs of northeastern Kansas [4]; and overland flow [9], it is apparent that grottos in basalt of Hawaii and the discharge in the channels was Snake River [5]. sufficiently large to remove debris as the Terrestrial alcoves are associated channels eroded headward [7, 9]. with cliff-forming, permeable rock layers Formation of overhanging rock shelters underlain by more easily erodible in sapping channels; fretted channels; materials or permeable soluble rock canyons; and in crater walls is highly underlain by impermeable shale. probable (Fig. 1). Undercutting and removal of soluble or Search for Life: The past existence of friable rock by groundwater sapping or an ocean, lakes, streams, and springs plunge pool development leads to [10] offer the possibility that life could spalling of the overlying rock to form an develop on Mars. The natural shielding arched alcove. This process is effective provided by ocean waters, caves, lava in both arid and humid environments. tunnels [11], or alcoves increases the Grottos are found at the heads of possibility of survival of early biota by canyons and in major bends in steep- providing a water-rich environment, sided sapping channels [2]. The protected from harsh radiation and recessed shelf with an overhanging severe dust storms. Further, such rock-roof provides a shelter in which natural shelters might provide a biota are protected from the harsher potential basecamp for long term aspects of the environment. Water exploration. 45th Lunar and Planetary Science Conference (2014) 1027.pdf

References: [1] Shirley, DL. and D. J. 2nd Workshop of Mars Valley Networks, McCleese (1996) 34th Aerospace Mtg Moab, UT, Smithsonian Inst., 27-30. [7] and Exhibit, Reno. [2] Harris, A.G. and De Hon, R.A., and P.A. Washington others (2004) in Geology of National (2000) LPSC XXXI, Abs. 1147. [8] De Parks. 6th ed. Dubuque, Iowa: Hon, R.A. and P.A. Washington (1999) Kendall/Hunt Pub. Co., 91-102. [3] Laity, LPSC XXX Abs,1928. [9] Lamb M.P. J.E. and M.C. Malin (1985) Geol. Soc. and others (2006) Journ. Geopys. Rev. Amer. Bull. 96, 203-217. [4] Shoewe, 111, E0 70002,18p. [10] Parker, T.J. W.H. (1953) in Kansas Academy of and others (1989) Icarus, 82 111–145. Science, Transactions 56, 146. [5] [11] Cushing, G.E. and others (2007) Stearns, H.T. (1936) J. Geol., 44, 429 Research Letters, v. 34. 450. [6] Harber, E. and others, 2008,

Figure 1. Channel near exhibiting multiple layered materials, steep slopes, and many potential sites of alcove development, (NASA HiRise image 033685).