Seventh International Conference on Mars 3293.pdf PROSPECTING FOR SUBSURFACE LIQUID WATER USING MAGNETOTELLURICS ON MARS. G. T. Delory1, R. E. Grimm2, T. Nielsen3, and W. M. Farrell4, 1Space Sciences Laboratory, University of California, Berkeley CA 94720 [email protected], 2Southwest Research Institute, 1050 Walnut St., Suite 400, Boulder CO 80403, 3QUASAR Federal Systems, 5764 Pacific Center Blvd., Suite 203 San Diego, CA 92121, NASA God- dard Space Flight Center, Mail Code 695, Greenbelt, MD 20771. Introduction: Detection of liquid water on Mars have become trapped in the frozen upper portion of the remains a key exploration objective, but geophysical crust, thus creating a cryosphere [8]. Depths to the methods currently in use or under consideration for base of the cryosphere vary between 2-6 km [9] de- subsurface exploration (radar and seismic, respec- pending on latitude; the discovery of the gullies [4] tively) are not optimal for water detection. Electro- indicates that the cryosphere may only be hundreds of magnetic (EM) sounding is sensitive to water with meters thick in some regions, enabling the emergence even modest dissolved solids, and brines are a near- of liquid water within relatively recent timescales de- ideal EM target. The magnetotelluric technique is a pending on the geothermal gradients present [10]. Al- passive, low-frequency EM sounding method that can ternatively, the gullies may be due to basal snowmelt be compactly implemented and is capable of the char- without the need for groundwater [11], or result from acterization of groundwater from depths ranging from dry flows of Aeolian material [12]. There is direct evi- 100m to 10 km on Mars [1]. With support from NASA dence for water-ice in the near subsurface, detected planetary and Mars instrument development programs, within the first meter by the neutron/gamma-ray spec- we have developed an autonomous magnetotelluric trometer on Mars Odyssey [13], and concentrated sensor platform and demonstrated it at several field- poleward of 60° latitudes [14], while initial results test sites in Idaho’s eastern Snake River Plain, the ter- from the Mars Advanced Radar for Subsurface and restrial type location for planetary plains-style basaltic Ionospheric Sounding (MARSIS) indicate the presence volcanism. Soundings over 1 km deep detected the of relatively pure ices to km depths, particularly in the contact between aqueously altered tuff and relatively northern polar layered deposits [15]. While measure- unaltered basalt, a proxy for the water table on Mars. ments of recent changes in the gully features may hint Here we discuss passive EM methodologies, our in- at active subsurface processes on Mars [16], direct strument development effort and field-test results, and detection of liquid water remains an elusive goal. Or- the use of low-mass, passive instrumentation to enable bital radar sounding investigations may not be able to deep subsurface exploration on future Mars scout or identify subsurface water without significant ambigui- network class missions. ties [17, 18]. Where radar methods are successful, fol- Science Background & Objectives: The ability low-up ground truth measurements will need to be to detect and characterize extraterrestrial water and ice performed in order to corroborate these results. has important implications for the understanding of Low frequency EM methods can play a crucial role planetary geological and climate histories, extinct or in the search for subsurface water on Mars, both as a extant life, and for the planning of future robotic and primary means of detection or as a ground truth meas- human exploration of the solar system. There is a sig- urement [1]. Where radar methods are successful, low- nificant amount of geologic and geomorphic evidence frequency EM methods can perform deep soundings at that water on Mars was abundant at some point in the high priority sites. More likely, the penetration depth past, forming valley networks, outflow channels and and conductivity discrimination of low-frequency possibly ocean basins [2-4]. The Mars Exploration methods may enable detection of subsurface water in Rovers have provided additional insights into the his- areas that render radar methods ineffective. In either tory of water on Mars, including indications of abun- case, the sensitivity and depth of penetration inherent dant groundwater in Meridiani Planum [5], while rocks in low-frequency EM exploration makes this tool a and soils in the Columbia Hills region appear to have compelling candidate method to identify subsurface undergone significant aqueous alteration [6]. While it liquid water from a landed platform on Mars or other is clear that Mars once had significant amounts of liq- targets of interest. Electromagnetics is also superior to uid water both in the crust and on the surface, the state, seismic methods for water detection. Seismic veloci- distribution, and overall inventory of present-day wa- ties are not as sensitive to low saturation as EM and ter continues to be heavily debated. While significant furthermore these velocity variations are confined to quantities of water may have been lost to space pressures corresponding to depths less than a few through impacts or solar wind erosion [7], it may also kilometers on Mars. Seventh International Conference on Mars 3293.pdf Low Frequency EM Sounding: EM waves used signals are recorded as an indicator of subsurface con- in low-frequency soundings are in the diffusive re- ductivity. On Earth, some of the deepest non-invasive gime, in which the dominant parameter controlling soundings have been obtained by monitoring the myr- signal behavior is the finite conductivity of the subsur- iad of naturally generated EM wave sources present face. A passive sounding method includes the mag- throughout the terrestrial environment. These include netotelluric (MT) technique, in which the electric and ULF emissions produced by Solar wind- magnetic amplitudes of long-period waves from natu- magnetosphere interactions above the ionosphere, and ral planetary EM sources are monitored near the sur- lightning-related sources within the terrestrial atmos- face in order to determine the subsurface electrical phere. Together these sources provide a near contin- impedance as a function of wave skin depth [19]. uum (with occasional nulls) in the 0.001 Hz – 20 kHz Since the conductivity of even mildly saline liquid band that can be used as a source for MT soundings. water is several or more orders of magnitude greater The essential physics behind the MT technique re- than the surrounding medium, its presence signifi- lies on the fact that the surface and subsurface are not cantly modifies the subsurface resistivity. Using this perfect conductors, and will generally possess a finite methodology, both the depth and thickness of multiple electrical resistivity depending on composition, tem- aquifers can be determined depending on the availabil- perature, and water content of subsurface materials ity of naturally produced low frequency EM waves and (typically between ~1 to 1000s of ohm-m in value.) In the conductivity contrast between the liquid water and MT soundings the central quantity of interest is the the surrounding media. The response of ice to low- apparent impedance, given by: frequency EM waves is essentially the same as radar 1 E 2 and becomes difficult to detect, unless massively seg- ρ = x (1) 5 f B 2 regated [1]. Thus low-frequency methods may be ideal y for the detection of liquid water in permafrost-rich where f is the wave frequency (Hz), Ex is the magni- mediums such as the Martian cryosphere. tude of the horizontal component of the electric field The focus of our instrument development effort has (in μV/m), and By (in nT) is the magnitude of the hori- been to demonstrate a system capable of performing zontal magnetic field perpendicular to Ex, each meas- the MT technique on Mars, in which the horizontal ured at the surface [19]. The quantity ρ, given in ohm- components of naturally occurring electromagnetic m, represents the apparent electrical resistivity of the STOWED subsurface at a given frequency f. The dissipation rep- Efield Sensors resented by ρ, caused by the flow of direct currents Wireless (stowed) Transceiver through the resistive media of the subsurface, has a frequency dependent penetration distance given by the wave skin depth: 1 ρ Li-Ion cells & δ = ~ 500 m (2) data processing πμσf f Because the depth of penetration of EM waves is fre- quency dependent, the quantity ρ can then also be es- Power, data, Magnetic Field testing & arming I/F Sensor (stowed) timated as a function of depth. The result of the sound- ing is then a depth-conductivity profile, yielding the EX1 EY1 depth and thickness of subsurface conductors. Instrument Development Effort: Among the m tools necessary to perform MT soundings are low- 5 1 1 0 - 0 -1 5 1 m frequency electric and magnetic field sensors capable BX of being deployed from a lander or rover such that -2m horizontal components of the fields can be measured BY ~1 free of structural or electrical interference. Under Magnetic Field 10 -15 Sensor m EX2 Planetary Instrument Definition and Development Pro- m 5 1 - 0 gram (PIDDP) funding, we developed small electric 1 EY2 field sensors capable of measuring horizontal fields with light contact on a variety of surfaces. This work Efield Sensors was extended into the Mars Instrument Development Figure 1 Deployable EM sounder system design. Program (MIDP), where these sensors were combined Central station is ~30 cm wide and < 8 kg. Seventh International Conference on Mars 3293.pdf typical upper sample rates of ~50 kHz used in practice. The depth penetration of this system varies between 1- 4 km depending on the subsurface properties. Deployable EM sounding station. A central support package was developed to perform data acquisition and processing tasks, provide power, and a wireless telemetry system (Figure 1 and Figure 2).