Capturing Evidence for Past Life in the Martian Loess

Vast loess deposits in Europe and the US accumulated after the last glacial maxima (<20 kyrs). In Argentina, however, they gradually accumulated from dried riverbeds that carried sediments from the Andes over the last 12 myrs [1]. These 300m-thick loessoid deposits resemble the accumulations of unconformable deposits found on Mars, some exceeding 3km in thickness. While short-lived as surface materials on the Earth (e.g., <12 myrs in Argentina), they remain exposed for billions of years on Mars (since the Late Noachian). Unlithified loessoid deposits represent a special target type affecting both crater excavation and melt generation. Craters as large as 20km in diameter may not reach the underlying "basement" (e.g., cratered highlands). Porous targets also result in greater amounts of impact melt derived from different levels [2, 3]. Moreover, melt breccias can be soft captured, buried, and trapped until re-exposed [4]. In Argentina, some folded vesicular glasses as old as 9.3 myrs contain flash-heated yet well-preserved biomatter (down to < 5 microns) including plant materials [5] and even partially vitrified cartilage fragments [6]. The entrained plant materials also contain organic relicts such as derivatives of chlorophyll. This biomatter becomes trapped as the melt is rolled and folded during excavation or emplacement. Exploratory impact experiments at the NASA Ames Vertical Gun Range simulated this process at a much smaller scale (5km/s at 45 deg impact angle). Fragments of wetted Pampas grass and tardigrades buried near the surface were entrained within small, twisted and folded glasses. Grass positioned uprange of the impact, however, survived intact within a projectile radius of the impact point. Consequently, the low impact speeds available in the experiments could be more than offset by an uprange location for a much higher speed impact. While plant material should not be expected, other primitive forms could be mixed within seams and folds within proximal impact melts. [1] Zarate, M. A. (2003), Quat. Sci. Revs. 22, p. 1987-2006; [2] Schultz, P. H. et al. (2004), EPSL, v. 219, 221-238; [3] Schultz, P. H. et al. (2006), MAPS, v. 41, 749-771; [4] Schultz and Mustard (2004), JGR 109, doi: 10.1029/ 2002JE002025; [5] Schultz et al. (2014), Geology, 42, 515-518; [6] Harris and Schultz (2007), LPSC 38, #2306.