Third Conference on Early Mars (2012) 7060.pdf DID MARS EVER HAVE A LIVELY UNDERGROUND SCENE? Joseph. R. Michalski, Natural History Mu- seum, London, UK and Planetary Science Institute, Tucson, AZ, USA. [email protected] Introduction: Prokaryotes comprise more than are investigating environments that might never have 50% of the Earth’s organic carbon, and the amount of been inhabited on a planet that is very much habitable. prokaryote biomass in the deep subsurface is 10-15 Spectroscopic results over the last 5-10 years have times the combined mass of prokaryotes that inhabit revealed significant diversity, abundance, and distribu- the oceans and terrestrial surface combined [1]. We do tion of alteration minerals that formed from aqueous not know when the first life occurred on Earth, but the processes on ancient Mars (recently summarized by first evidence is found in some of the oldest preserved Ehlmann et al. [6]). The mineralogy and context of rocks dating to 3.5 or, as much as 3.8 Ga [2]. While the these altered deposits indicates that deep hydrothermal concept of a “tree of life” breaks down in the Archean processes have operated on Mars, and might have per- [3], it seems likely that the most primitive ancestors of sisted from the Noachian into the Hesperian or later. In all life on Earth correspond to thermophile this work, I consider the implications of recent results chemoautotrophs. Perhaps these are the only life forms for the habitability of the subsurface, the occurrence of that survived intense heat flow during the Late Heavy groundwater, and the possibility to access materials Bombardment or perhaps they actually represent the representing subsurface biological processes. first life forms, which may have developed to take ad- Results: I carried out a survey of deep craters with vantage of existing chemical gradients within the crust the intention of evaluating morphologic and minera- [3-4]. logical evidence for groundwater upwelling [7] in the The environments on early Mars and Earth may deepest basins on Mars – where groundwater would have been similar (Figure 1). But, Mars cooled quickly have been most likely to emerge. A survey of deep, and since the Noachian, the surface has been cold, hy- ancient craters in the northern hemisphere shows that perarid, oxidizing, and probably inhospitable to life. most of the basins do not show any evidence for On Earth, life never developed photosynthesis until ~3 groundwater upwelling. But, several craters in the Ga (which corresponds to the end of the Hesperian) northwest Arabia Terra region do show such evidence, [2], and probably didn’t colonize the land surface until which could indicate that a regional event occurred in the Proterozoic (Amazonian) [5]. It is entirely possible this area. The null results provide a way to constrain that even if life did form on Mars, it never colonized the minimum depth below the surface of a saturated the surface. Ongoing efforts to characterize the habita- groundwater zone, and craters that do show evidence bility of Mars by studying sediments that formed at the for upwelling allow for constrains on the slope and surface might produce misleading results because they absolute elevation of a past groundwater surface. Figure 1: A concept diagram comparing the early histories of the Earth and Mars, with major events in the bio- logical history of the Earth compared to epochs of alteration on Mars. Third Conference on Early Mars (2012) 7060.pdf I propose a model of the subsurface geology of regard to the habitability of ancient Mars. The types of Mars which includes 4 zones involving groundwater geologic processes that allowed life for form or survive [8]: 1) a surface cryosphere contain acidic ice deposits on the early Earth were also occurring on early Mars. within which, sulfates might form and below which, In order to truly characterize the habitability of Mars, it clays may have formed [9]; 2) a shallow (1-2 km will ultimately be necessary to focus on the geology of depth) unsaturated zone through which transient melt- the subsurface. Subsurface prokaryotes are not simply water from surface ice (or episodic rain) could have extremophiles that could survive in the deep crust of passed, leaching the most mobile cations from basaltic Earth or Mars. In fact, we are the extremophiles living materials and remerging after short traverse distances at the surface looking at the largest category of simple to deposit chloride salts; 3) a deep unsaturated zone life forms known, which occur at depth. containing disseminated clays and other hydrous sili- cates, through which strong brines may have passed References: [1] Whitman, W. B. D. C. Coleman and rarely or never reemerged at the surface; and 4) a (1998), and W. J. Wiebe, PNAS, 95, 6578-6583. [2] very deep (>4-6 km) saturated zone with dense brines Rothschild, L. J. and R. L. Mancinelli (2001), Nature, in limited pore space associated with highly altered 409. [3] Martin, W. F. (2011), Biology Direct, 6:36 [4] crust. The combined total of all of these reservoirs Parkes, R. J. et al. (2011), Geology, 39 (3), 219-22 [5] could constitute a significant amount of Mars’ global Knauth, L. P. and M. J. Kennedy (2009), Nature, 460, water budget, in subsurface fluid and structural water 728-732. [6] Ehlmann, B. E. et al. (2011), Nature, 479, in minerals. 53-60. [7] Andrews-Hanna, J. et al. (2010), JGR, 115. The subsurface was hydrothermally active early in [8] Clifford, S. M et al. (2010), JGR, 115, E07001. [9] Martian history [6]. Serpentinization in particular Niles, P. B. and J. Michalski, Nature, 2(3), 215-220. would have been an important process to consider with escape sublimation and evaporative loss zone 1% porosity 35% T-gradient acidic ice 1 a paleocryosphere b clay formation c dilute water 1 magmatism weak brine 2 d 2 e 20 e p ( o e C/Km ancient gradient a r l 3 GEL 300 m y ? ) s e q u modern cryosphere + brine intermediate Ca-Mg-brines e s 10 3 GEL 200 m t r (sticky water?) a t io o C/Km recent gradient n depth (km) 5 GEL 100 m deep hydrothermal !uid altered crust 4 hydrothermal locally elevated T-gradient? 10 -100 0 100 200 T (oC) Figure 2: A conceptual diagram of the distribution and context of groundwater on Mars. At the left, a model of porosity as a func- tion of depth (after [8]) shows that the deep crust could contain a significant amount of pore water. Two estimates of thermal gra- dients on Noachian Mars and in recent time show that the base of the cryosphere would have grown to significant depth since the Noachian. But earlier, hydrothermal processes could have occurred in the deep crust (zone 4) or possible in the unsaturated zone (zone 3). In zone 2, weak cryobrines could have traversed the crust only weakly altering basaltic material and becoming enriched in Na and K that would have been deposited as chlorides upon emergence during upwelling events. Sulfates correspond to ice- driven weathering processes in zone 1. .