MARS SUBSURFACE WATER: OUTFLOW CHANNELS AND THARSIS RECHARGE K. P. Harrison1 and R. E. Grimm1, 1Southwest Research Institute, 1050 Walnut St., Ste 400, Boulder, CO 80302, harri- [email protected], [email protected].

Introduction: Large fluvial outflow channels stantaneous discharge at the OCs five times lower than (OCs) in Chryse Planitia, Mars, suggest an episodi- that of the Tharsis recharge model. Scaling TH model cally active regional to global Hesperian hydrologic permeability to reasonable upper limits yields sedi- cycle. The importance of OCs in this cycle is indicated ment concentrations on the order of the lowest pub- by the large volumes of water they are thought to have lished estimates (~10-4 by volume [6]) or alternatively, carried (about 106 km3 [1]). These volumes suggest allows the total sediment load to be removed from the that, to some extent, constraints on Hesperian hydro- OCs in only 1 Myr, assuming maximum sediment logic dynamics cannot be imposed without a compre- concentration. hensive model of OC formation. Such a model must be Support: Our quantitative demonstration of the based on observations (which suggest the basic mecha- strengths of Tharsis recharge, together with other ad- nism of cryosphere disruption and groundwater dis- vantages such as elevation [7], enhanced permeability charge) and on observationally-informed numerical along fractures, orographic precipitation, and elevated simulations (which should provide constraints on dy- geotherms, are supported by observations. These in- namics, including discharge rates, relevant time clude high elevation OC features in Syria Planum and scales, and changes in groundwater supply and stor- on the rims of Valles Marineris Chasmata. Mars Od- age). yssey GRS observations at high latitudes are thought We suggest here that low-latitude Hesperian ice- to indicate ice-rich subsurface deposits. Their high ice sheets recharged the Tharsis aquifer, providing suffi- concentration may require an atmospheric, rather than cient groundwater for outflow channel formation. The subsurface, source [8]. However, some climate models ice sheets developed during periods of high planetary do not predict suffi- obliquity when net deposition of volatiles occurred at ciently strong transport low latitudes [2, 3]. Regionally elevated crustal heat of polar volatiles to the due to Tharsis magmatic processes allowed the ice high latitude deposits, sheets to melt at their bases, producing aquifer infiltra- and a low latitude tion. source may be more Model: Our previous modeling efforts used a Lam- likely. Such a source bert equal-area projection of the Martian topography would accumulate dur- cast in a Cartesian finite-difference grid [4]. Although ing periods of high the orientation of the projection minimized the spatial obliquity, and would distortion of key model features, no systematic com- later provide volatiles pensation for distortion was made. It was also difficult to higher latitudes dur- to avoid the adverse affects of necessarily unrealistic ing periods of low boundary conditions. obliquity [9]. We have since modified the MODFLOW-2000 Figure 1. Orthographic projection of hydraulic head code to simulate global, spherical thin shell geometry. at 500 Myr in Tharsis (top) Each simulation adopts one of two initial hydraulic and South Polar (bottom) head configurations: 1) fully saturated aquifer [4], or recharge models, draped 2) uniform groundwater table at the elevation of the over shaded topography. Head varies from -5 km lowest outflow channel source. We ran 4 nominal (purple) to 6 km (red). simulations (Table 1) spanning permutations of the 2 Black and white regions initial configurations and 2 recharge zones: Tharsis correspond to recharge (TH) and South Pole (SP). The 10 modeled OC zones and OC sources, respectively. sources are permitted to discharge groundwater simul- References: [1] Carr M. H. (1996) Water on Mars. [2] taneously and continuously. Typical model time scales Jakosky B. M. and Carr M. H. (1985) Nature, 315, 559-561. (10s to 100s Myr) represent a conservative upper [3] Mischna M. A. et al. (2003) JGR, 108, DOI bound: the duration of actual OC discharge events may 10.1029/2003JE002051. [4] Harrison K. P. and Grimm R. E. have been shorter (although possibly still up to 1000s (2004), GRL, 31, DOI 10.1029/2004GL020502. [5] Klein- yr, [5]), but were also likely repeated episodically, per- hans M. G. (2005) JGR, 110, DOI 10.1029/2005JE002521. haps over much longer time scales. [6] de Hon R. A. et al. (2003) LPSC XXXIV, Abstract #1178. Results: Snapshots of hydraulic head (Figure 1) [7] Coleman N. M. et al. (2003) 6th Int. Conf. Mars, Ab- reveal relatively high gradients imposed by TH re- stract #3071. [8] Head J. W. et al. (2003) Nature, 426, 797- charge. SP recharge is comparatively weak, with in- 802. [9] Levrard B. et al. (2004) Nature, 431, 1072-1075.