Concepts and Approaches for Mars Exploration 6013.pdf

“FOLLOWING THE WATER” ON MARS: WHERE IS IT, HOW MUCH IS THERE, AND HOW CAN WE ACCESS IT? N. G. Barlow, Dept. Physics, University of Central Florida, Orlando, FL 32816 [email protected].

Introduction: Analysis of Mariner, Viking, Mars suggesting the presence of ground water. However, Pathfinder, and Mars Global Surveyor (MGS) data these features are localized in specific regions of the have revealed that water has played an important role planet and do not provide information about the global in the evolution of Mars. Although the planet is cold distribution of subsurface H2O. and dry today, increasing evidence points to warmer The most wide-spread indicators of subsurface and wetter episodes in the past, perhaps to be repeated volatiles are the impact crater ejecta morphologies. in the future. Although the evidence from the valley The layered appearance of the majority of fresh ejecta network and outflow channels has been recognized morphologies has been interpreted to result from im- since Mariner 9, it has been the analysis of Viking and pact into subsurface volatiles [7, 8, 9, 10]. Barlow and MGS data which have revealed such features as prob- Bradley [8] and Barlow [11] argue that variations in able paleolacustrine deposits [1], possible paleo- the types of layered ejecta morphologies result from shorelines [2], and abundant evidence for large impact into target material with varying concentra- standing bodies of water in the northern plains [3, 4]. tions and/or physical states of H2O. Costard [9] and Much of the water which appears to have existed Kuzmin [10] have measured the regional changes in on the planet is likely still there. The challenge still onset diameters for craters with specific ejecta mor- facing us is to identify where those water/ice reser- phologies, resulting in maps of the depths to the vola- voirs are located, determining how much water is tile-rich layers. Those studies have found that ice is available in these reservoirs, and devising strategies located <200 m below the surface at high latitudes by which future astronauts to Mars can obtain it for (generally poleward of about 30° latitude) and at their use. depths of about 200 to 500 m in the equatorial region. Where Is The Water and How Much is There?: Koroshetz and Barlow [12] have used a similar tech- Three main sources of H2O are recognized for present- nique to identify a near-surface ice reservoir (<200 m) day Mars. The thin Martian atmosphere is typically in the Solis Planum region, south of Valles Marineris saturated with water vapor, although it comprises only (Figure 1). Unfortunately, the amount of H2O con- 0.03% of the atmosphere. The amount varies season- tained in these reservoirs is very uncertain since such ally between about 1 and 2 x 1015 g, equivalent to a estimates depend on a number of parameters (i.e., the 3 total volume of about 1 to 2 km of H2O [5]. The volatile-to-rock ratio needed to produce the layered white clouds and fog are visible manifestations of the morphologies, porosity, etc.) which are poorly con- atmospheric water vapor [6]. strained at the present time. The remnant north polar cap is believed to be Terrain softening is another indicator that ice is composed primarily of H2O ice, based mainly on near- present in the substrate, particularly at high latitudes IR spectral observations [5]. Models suggest that the [13]. Terrain softening is the rounding of sharp edges

cap contains >20 pr µm of water mixed with CO2 ice. (such as crater rims) and shallowing of crater floors How much water is contained in the south polar cap is due to creep-related relaxation of ice-rich terrains. more problematic since CO2 frost usually covers the The geologic observations of small onset diameters for cap even at the height of summer. The layered depos- layered ejecta morphology craters and abundant ter- its surrounding the north and south poles also likely rain softening at high latitudes on Mars is consistent contain vast stores of CO2-H2O mixtures. with the geothermal models for the distribution of The advantages of looking for water in the atmos- near-surface water and ice [14, 15]. phere and polar regions is that these regions are fairly Study of impact crater ejecta morphologies and accessible. The disadvantage is that these reservoirs terrain softened features provide constraints on the likely account for only a small amount of the total distribution of subsurface water and ice, but are only water available on the planet. indirect methods for determining the locations of these The largest store of water on Mars is probably in reservoirs. Since H2O has played a major role in the the near-surface region. There is abundant geologic history of the planet and will be of great importance to evidence that vast stores of H2O are contained in the future human explorers and settlers of Mars, direct martian substrate. The outflow channels and valley measurements are needed to verify the locations of networks are the most obvious of the geologic features these subsurface H2O reservoirs and obtain better con- Concepts and Approaches for Mars Exploration 6013.pdf

IDENTIFYING AND ACCESSING WATER ON MARS: N. G. Barlow

straints on the amount of water/ice available. Sound- different reservoirs, and how can the water stored in ing radar, such as the MARSIS experiment on the these reservoirs best be accessed. Mars Express mission [16], can provide important References: [1] Cabrol N. A. and E. A. Grin information from orbit. Among the types of experi- (1999), Icarus, 142, 160-172. [2] Parker T. J. et al. ments which can provide needed data about subsurface (1993), JGR, 98, 11061-11078. [3] Baker V. R. et al. volatile reservoirs from surface sites are soil electrical (1991), Nature, 352, 589-594. [4] Head J. W. et al. conductivity experiments, seismic profiling, and of (1998), GRL, 25, 4401-4404. [5] Jakosky B. M. and course drilling. R. M. Haberle (1992), in Mars, Univ. AZ Press, 969- How Can We Access the Water?: Water is a 1016. [6] Smith P. H. et al. (1997), Science, 278, particularly valuable resource for human exploration 1758-1765. [7]Carr M. H. et al. (1977), JGR, 82, and settlement missions to Mars. Research is ongoing 4055-4065. [8] Barlow N. G. and T. L. Bradley with regard to how to extract useable water from the (1990), Icarus, 87, 156-179. [9] Costard F. M. atmosphere. Determining how pure the water in the (1989), Earth Moon Planet., 45, 265-290. [10] Kuz- polar ice caps might be and how to transport the water min R. O. et al. (1988), Solar System Res., 22, 121- over the probable large distances between the caps and 133. [11] Barlow N. G. (1994), JGR, 99, 10927- human habitats are issues which still need to be ad- 10935. [12] Koroshetz J. and N. G. Barlow (1998), dressed. Once the direct experiments outlined above LPSC XXIX, Abstract #1390. [13] Squyres S. W. and provide information on the depth and concentrations M. H. Carr (1986), Science, 231, 249-252. [14] Fa- of water/ice in the Martian substrate, methods to ex- nale F. P. (1976), Icarus, 28, 179-202. [15] Clifford tract this water can be discussed in more detail than is S. M. (1993), JGR, 98, 10973-11016. [16] Plaut J. J. currently possible. The primary focus of the upcom- (1999), LPSC XXX, Abstract # 1136. ing Mars missions should be to address these ques- tions of where the water is, how much is located in the

Figure 1. Map outlining the area (in black) in the Solis Planum region south of Valles Marineris where Koroshetz and Barlow [12] have identified a near-surface ice reservoir, based on the small onset diameters for single layer ejecta morphology craters. The reservoir lies between 20°S to 30°S, 50°W to 90°W. Depth to the reservoir is estimated at 200 m or less.