 THE SEARCH FOR LIQUID WATER & MODERN-DAY HABITATS IN THE MARTIAN SUBSURFACE. V. Stamenković1, R. E. Grimm2, M. S. Burgin1, N. Barba1, R. Manthena1, K. Carpenter1, D. Wright1, S. Krieger1, D. Arumugam1, R. Beauchamp1, B. Wilcox1, C. Edwards1, 1NASA Jet Propulsion Laboratory, California Institute Technology, 2Southwest Research Institute.   Introduction: The "holy grail" of planet exploration is the quest for life. This quest has been framed around "follow the (liquid) water", with the implication that seeking liquid water is tantamount to seeking habitable environments. For Mars, pure liquid water is generally only thermodynamically stable in the deep subsurface, at depths of kilometers [1, 2]. Recent geodynamic models that consider both temporal and the 3D spatial variations in heat flow suggest a cryosphere depth of 2-6 km in the tropics and 11-20 km at the poles [2]. The possibility of such underground water has regained momentum since the announcement of a possible subsurface lake beneath the South Polar Layered Deposits on Mars with MARSIS [3] and trace gas measurements of methane and oxygen that could be possibly compatible with processes that include liquid groundwater [4, 5].  However, due to attenuation, orbiting radar such as MARSIS have difficulties to detect groundwater beneath a depth of a few hundred meters away from the poles. As estimates of the average groundwater table are generally far beyond a depth of 1 km, it is possible that Martian groundwater might be much more widespread but has so far not just remained undetected, but was rather undetectable.  The potential for subsurface groundwater existing today is further supported by the measured deuteriumto-hydrogen ratio (D/H), which indicates that the total water loss since the Hesperian has only been about 60 m (interquartile range 30-120: [6]). Considering that from geologic evidence, ancient Mars up to the late Hesperian (~3 Ga) possessed a 0.5-1 km-thick global equivalent layer (GEL) of H2O [7], and taking into account modern-day heat flow estimates [8] suggest that liquid groundwater likely exists globally on Mars today. Thus, a new opportunity to sound for liquid groundwater on Mars awaits. Here, we will present (i) how to use transient electromagnetic techniques (TEM) to sense liquid groundwater in the Martian subsurface, and (ii) discuss the capabilities of the TH2OR (Transient H2O Reconnaissance) TEM instrument and implications for potential missions in the 2020s. This instrument is currently being developed at JPL with the goal of detecting deep groundwater at depths of kilometers with modest payload power and mass requirements.  Methods: Our TEM approach is different from radar soundings as it is not sensitive to the dielectric constant of the Martian subsurface but rather to its electric conductivity. TEM exploits the fact that slightly-saline water has an electric conductivity orders-of-magnitude greater than dry rock [9]. On Mars, we expect highly-saline waters due to the much longer residence time of groundwater than on the geologically and hydrologically much more active Earth.  Moreover, due to the lower frequencies that are being used (~Hz-kHz for TEM versus ~MHz-GHz for radar) TEM allows much deeper penetration down to aquifer depths of ~1-10 km.  We prefer an active TEM approach as natural Martian E/B fields are not well constrained and first results from the InSight magnetometer suggest that the naturally occurring frequencies are not ideal to sound for water at depths of kilometers [10].  The TEM approach requires a transmitter and receiver loop to be placed on the surface on Mars (see Figure 1, one single loop structure can take both functions). Induction in the loop creates primary eddy currents in the Martian subsurface which in turn create secondary eddy fields in bulk electric conductors such as liquid groundwater. The signal from such aquifers can then be detected with receiver loops on the Martian surface.  The measured time-dependent signal does not only determine the depth, but also the thickness and the salinity of liquid subsurface water body [9-11]. Salinity, on the other hand (ideally in combination with Figure 1: Illustration of the principle behind EM sounding on the example of active sounders going back to Faraday's induction law: a transmitter creates a temporally variable magnetic field, which induces in saline groundwater eddy currents, which re-induce measurable currents in the receiver loop on the surface.     information on local geothermal gradients either through models or future measurements), then allows us to indirectly estimate the potential chemical composition of the liquid groundwater. This ability to characterize the water chemistry to some degree is a particularly important feature of TEM sounding, which other methods generally lack.  Results: We find that in comparison to landed ground penetrating radar, seismology, and surface nuclear resonance methods, TEM is by far the technique that—with modest investments in mass, power, and volume—can sound for liquid water to the largest depths.  On the Earth, the common rule of thumb for TEM is that for single-turn loops the sounding depth is approximately the diameter of the loop, as terrestrial crustal conductivities are generally around ~10-2 S/m [9]. For Mars, and expected overburden conductivities of ~10-7 S/m, we find the penetration depth to be up to an order of magnitude larger. The maximal sounding depth on Mars is even further enhanced in comparison to the Earth thanks to lower coil resistivities and noise levels on Mars due to lower surface and noise temperatures.  We find that depths of many kilometers can be reached with TH2OR configurations consisting of a ~100 m diameter loop, low transmit power (<10 W), low weight (1-7 kg) and small integration time (~1 h). Increases in system power, integration time and number of turns could reduce the loop size considerably if need be. In addition to its inherent simplicity, such an instrument allows significant implementation flexibility by the choice of loop diameter, number of loop turns, transmit power, antenna wire gauge, and integration time while facilitating modest payload power and mass.  Implication for Future Potential Missions: Loop deployment is simple, can be performed, depending on size, pneumatically or with projectiles,  or with small Puffer-like robots (~1 kg weight) and ties into mission concepts ranging from heritage-based Discovery class missions and New Frontiers missions to high-g, kgmass, small spacecraft lander options, as with the JPLdeveloped SHIELD hard lander concept (see Figure 2). The SHIELD-concept based TH2OR approach would consist of three SHIELD landers with a total target price of $250 M (A-D & launch & cruise stage). For the different mission concepts, 1-4 landing sites would be feasible (one for a Discovery-type mission, three with the SHIELD concept, and four if we combined both surface deliver methods, likely below a New Frontiers budget). Potential landing site(s) selection would be based on regions with potential recent water discharge (e.g., Kasei, Athabasca), low altitude (to test current global water table hypothesis), and geophysical groundwater modeling. Even non-existence of groundwater at landing site(s) would help to estimate the global groundwater inventory and its history.  Summary: TH2OR would allow us to finally answer the question whether there is still today liquid water in the Martian subsurface, with potentially profound implications for the existence of and search for extant life on Mars [12] — and this at modest mass, power and cost.  References: [1] Clifford, S. et al., (2010), JGR, 115 (E7). [2] Stamenković, V. et al., (2019), LPSC Abstracts 2019, LPI Contrib. No. 2132. [3] Orosei, R. et al., (2018), Science, 361 (6401), 490-493. [4] Webster, C. et al., (2018), Science, 360, 1093-1096. [5] Trainer, M. et al., (2019), JGR Planets, 124 (11), 30-00-3024. [6] Grimm, R. E. et al., 2017, JGR, 122 (1). [7] Carr, M. H., (1987), Nature, 326, 30-35. [8] Plesa, A.-C. et al., (2016), JGR Planets, 121 (12), 2386-2403. [9] Grimm, R. E., (2002), JGR Planets, 107. [10] Grimm, R. E., AGU Fall Meeting Abstracts 2019, P44B-01. [11] Stamenković et al. (2019), Nature Astronomy, 3, 116-120. [12] Stamenković et al. (2018), Nature Geoscience, 11 (12), 905-909.   Acknowledgments: This work was performed in part at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. ©2020, California Institute of Technology. Figure 2: Artist's impression for two delivery methods for TH2OR using (i) a low-cost Small Spacecraft lander like SHIELD or (ii) using a heritage-based Phoenix lander as a Discovery-Class mission. Credit NASA/JPL by Corby Waste. 
