Deep Basalt Aquifers in Orcus Patera, Elysium Basin Mars: Perspectives for Exobiology

Lunar and Planetary Science XXIX 1010.pdf

DEEP BASALT AQUIFERS IN ORCUS PATERA, ELYSIUM BASIN MARS: PERSPECTIVES FOR EXOBIOLOGY. E. A. Grin and N. A. Cabrol, NASA Ames Research Center, Space Science Division, Center for Mars Exploration, MS 245-3. Moffett Field, CA 94035-1000 ( [email protected]).

Introduction: Direct indicators of shorelines, Considering the localization of Orcus Patera in the spillways, and terraces allowed to determine the extent Elysium paleolake, its altimetry, and the magmatic of the Elysium Paleolake between the contour-lines origin of this caldera, we propose the existence of a 1000 and 500 m below the Martian datum [1]. The paleolake in Orcus Patera generated (a) by juvenile Elysium Paleolake is bordered north by Orcus Patera water from magma during the Noachian period, and (b) (14°N/181°W), which lies west of the Tartarus Montes by intermittent influx of the Elysium Basin from Hes- and Tartarus Colles (see figure 1). The Orcus Patera perian to Amazonian. Results are encouraging to con- displays an ellipse-shaped collapsed caldera of 360-km sider this site as a potential high-energy source envi- long and 100-km wide. Viking topographic data show ronment for microbial communities. that the bottom of the caldera is located at 2500 below Orcus Patera: The outward walls of Orcus Patera the Martian datum, and surrounded by a steep-walled are circumscribed by a 50-km wide lava field mapped rampart, which crest is located at about 0 m elevation. as Noachian material [1]. The structure of Orcus Patera represents the record of material erupted from a magmatic reservoir. The caldera is enclosed by steep inner walls (25% measured from topographic data), values which could be in agreement with the presence of a deep magmatic reservoir, as suggested by the typology of Crumpler et.al., [2]. The depth of the caldera might be due to the collapse of the magma reservoir, and the release of gases accompanying the magma thermal evolution [3]. Origins of water for the paleolake(s): The water that generated a paleolake in Orcus Patera may have come from two origins: (1) Juvenile water: Plescia and Crips [3] estimated a magma H 2O content by weight between 0.5% and 1.5% using for the first value a comparison with terrestrial basalt, and for the second values from a Martian meteorite. The amount of H2O can be estimated by the volume of erupted lava, and the lava content of the caldera. In this study, we adopt a water content of 1% [4]. The total volume of magma that has been contained in the caldera, and the volume of lava contained in the observed lava field Figure 1: Orcus Patera in Elysium Basin. The arrow is about 110 x 106km3, that gives a total volume of 1.10 shows a possible spillway for the water from the Elysium x 10 6km3 of water. The juvenile water expelled by the paleolake (the squared area corresponds to figure 2 below). overpressure within the magma chamber charged with desolved water-vapor may have moved into the crust. The decrease in overburden pressure led to bubble formation. The ascent of these bubbles generated a pressurization of the magma, which was sufficient to fracture the overlaying magma layer, (2) Water from Elysium paleolake . During the Amazonian, the rise of the Elysium paleolake level generated an overspilling that supplied the caldera with water. The southern por- tion of the crest shows a deep gap 12-km wide at -1500 m elevation, locating the gap between 500 to 1000 m below the assumed water of Elysium paleolake, thus facilitating the influx of Elysium paleolake water into Orcus Patera. Bathymetric calculations give a floor area of 25,500 km2 at -2000 m elevation, and a water volume of 42,000 km3, with a lake-level at -1500 m [1]. A substantial amount of water may have perco- lated through the fractured lava, and part of the volume may have overspilled the northern crest of Orcus Lunar and Planetary Science XXIX 1010.pdf

DEEP BASALT AQUIFERS IN ORCUS PATERA MARS: E. A. Grin and N. A. Cabrol

Patera [1] to debouch in the Tartarus Montes region. bubbles interconnected with exsolved gas bubbles con- Deep subsurface basaltic aqueous environment: We stitute a potential geothermal environment for geo- envision the formation of a subsurface aqueous envi- chemical energy production from basalt and water that ronment in basaltic rocks at the contact of the two wa- does not require excessive temperatures. This process ter-source origins, possibly the percolating surface lake can start at +20°C. water, and more likely the juvenile water [5]. Similarly to terrestrial calderas, Orcus Patera might be surrounded by ring-fractures caused by the collapse of the magma chamber that followed the release of gases. These ring-fractures may have been covered later by sedimentation in the caldera (lacustrine, aeolian, and volcanic), and by mass wasting (figure 3).

Figure 4: Vesiculation of juvenile water

Conclusion: Similar types of environments have Figure 3 : Stratigraphic model of Orcus Patera been shown on Earth as potential energy sources for microbial metabolism [6], and could have provided The detumescence of the magma in the caldera, and deep aqueous basaltic niches for possible Martian mi- the vesiculation of the juvenile water may have oper- croorganisms, even geologically recently. During the ated simultaneously [6]. Comparatively to terrestrial Amazonian, combination of volcanism and water ac- melts, Martian iron-rich melts are denser. This greater tivity still existed on Mars. Moreover, this type of po- density implies greater effusion rates (eight-times ter- tential niches open ways for investigation of possible restrial values), and larger fissuration widths (two- oases of extinct or extant life, not only on paleolakes, times terrestrial ones) [7]. With increasing vesiculation and surface hydrothermalism spring areas, but also all of magma, the bubbles interact with one-another be- large systems of fossae, which combine hydrologic and cause there are of similar pressure. They make a volcanic activities, and which provide an energy magma froth at the contact with the caldera surface, source, and an underground shelter to prevent surface and on the walls of the fractures [8]. In the saturated UV bombardment . magma, froth, where the volume ratio of gases-to- liquid is about 4:1, the bubbles form a huge surface References: [1] Scott and Chapman 1995. USGS Map I- area of interconnected spaces. Bubbles near the caldera 2397, [2] Crumpler et al., 1994. LPSC 25th, [3] Plescia and surface disrupt the magma, and fragmentation takes Crips 1992. LPSC 23th, [4] Greeley 1987. Science 236, [5] place, which moves downward through the magma Cave 1993. JGR 98-E6, [6] Steven and McKinley 1995. Sci- column. (figure 4).On Earth, the bubbles are likely to ence 270, [7] Bertka 1992. LPSC 23th, [8] Sparks 1978. grow between 1 and 50 mm in diameter due to the dif- J.Volc.Geo.Res. 3., [9] Francis 1995. Volcanoes , Oxford ference between the magma surface tension, and the U.Press. bubble supersaturation pressure [9]. The Martian low- pressure at surface level is likely to accelerate the ex- Acknowledgment: this study is financially supported by pansion of the bubbles, and increase their final diame- the National Research Council (NRC) grant for the second ter and number, creating more voids in the magma. author. The strong magma froth with enclosed juvenile water