Concepts and Approaches for Exploration 6125.pdf

DETECTION AND CHARACTERIZATION OF MARTIAN VOLATILE-RICH RESERVOIRS: THE NETLANDER APPROACH. F. Costard1, J.J. Berthelier2, and the GPR team, G. Musmann3, M. Menvielle2, and the MAGNET team, P. Lognonné4, D. Giardini5, B. Banerdt6, and the NL-SEIS team, A-M Harri7, F. Forget8, and the ATMIS team. 1UMR 8616, CNRS, Orsay, France, [email protected], 2C.E.T.P., UMR 8639, Saint-Maur, France, 3TUBS-IMG, Braunschweig, Germany, 4IPGP, Saint Maur, France, 5ETH, Zurich, Switzerland, 6JPL, Pasadena, USA, 7FMI, Helsinki, Finland, 8 LMD, Paris 6, Jussieu, France,

Introduction: Geological and theoretical modeling do them has its own advantages and limitations and the choice indicate that, most probably, a significant part of the vola- is mostly guided by the range of depths to be explored, the tiles present in the past is presently stocked within the needed spatial resolution and the horizontal extent to be Martian subsurface as ground ice, and as clay minerals covered. We therefore propose to fly on each of the 4 (water constitution). The detection of liquid water is of landers of the NETLANDER mission a set of geophysical in- prime interest and should have deep implications in the struments to explore the first few kilometers of the subsur- understanding of the Martian hydrological cycle and also in face with, as major objectives, the detection of possible exobiology. In the frame of the 2005 joint CNES-NASA water rich layers and the characterization of the main fea- mission to Mars, a set of 4 NETLANDERs developed by an tures of the geological structures of the superficial planetary European consortium is expected to be launched between crust. The study of the subsurface, including the ground ice, 2005 and 2007 [1]. The geophysical package of each will be actively performed with a geo-radar, and passively will include a geo-radar (GPR experiment), a with the magnetometer and seismometer. These geophysical (MAGNET experiment), a seismometer (SEIS experiment) instruments will probe the uppermost kilometers of Mars to and a meteorological package (ATMIS experiment). The search for signatures of ice reservoirs and possible transition NETLANDER mission offers a unique to explore to liquid water layers. Simultaneously geophysical studies simultaneously the subsurface as well as deeper layers of will give access to the main structural and geomorphological the planetary interior on 4 different landing sites. The com- features of the subsurface. plementary contributions of all these geophysical soundings The major goals of our experiment will thus be: onboard the NETLANDER stations are presented. - to determine the presence of ground ice layers, compare Water reservoirs: The presence of valley networks and their depth and thickness to the results inferred from the outflow channels [2] suggests that liquid water has been analysis of rampart craters and detect the possible exis- present on the surface throughout most of Martian history tence of liquid water layers at large depths [3, 4]. Morphological studies (e.g., rampart craters, perigla- - to determine the characteristics of the volatile rich layers cial features, terrain softening), theoretical modeling [5, 6] of sedimentary deposits and/or lava flow that will provide and analysis of SNC meteorites, both strongly suggest that a new into sedimentation processes and/or volcanic large amount of volatiles (H2O, CO2, chathtrates) might still episods. stored within the Martian megaregolith as a deep and global - to understand the association between the surface mor- underground ice [5, 7, 8, 9]. Theoretical estimates of the phology and the deep layers of ground ice ground ice thickness range from 3 to 7 km near the poles to Geo-radar investigations (GPR experiment): Due to between 1 and 3 km near the equator [5, 10]. Due to subli- its relative simplicity, the technique of HF ground pene- mation processes and to the porous nature of the megarego- trating radar appears as a unique tool for planetary explora- lith, the ground ice table is covered by a dry layer of 100 m tion. We plan to use a set of 3 monopole electric antennas to 1 km thick. It is expected that, under the ground ice, liq- angularly spaced by 120° and 3 magnetic receiving antennas uid water exist, at least at middle latitudes. The depth of the to determine the directions of the returning electromagnetic 0°C isotherm could be reduced by both pressure and solute waves and, therefore, obtain a 3D imaging of the subsur- effects [6]. face. Three monopole electric antennas powered by a 10 W transmitter are used to transmit electromagnetic waves at a central frequency of 2 MHz with linear as well as planar (circular or elliptical) polarization. Two electric and three magnetic components of the electromagnetic waves return- ing to the radar are measured. The subsequent signal analy- sis performed on these data will allow to retrieve the direc- tion of propagation of the returning waves and thus the di- rection of the reflectors while their distance is obtained from the propagation time of the waves [11]. The possibility to Fig. 1: The Martian ground-ice (from Squyres et al., 1992). operate with various polarization schemes provides a capa- bility of significant interest for the GPR instrument since it Geophysical studies of water reservoirs: On Earth, will allow to study in more details the backscattering prop- several techniques are available to explore the structure and erties of the reflectors in the subsurface. the nature of the subsurface; these are passive soundings A numerical simulation has been performed to assess such as magneto-telluric techniques, active electromagnetic the performances of the GPR instrument. The model subsur- soundings, and passive or active seismic methods. Each of face consisted in a number of layers with depth, thickness Concepts and Approaches for Mars Exploration 6125.pdf

DETECTION OF VOLATILE RESERVOIRS ON MARS: THE NETLANDER APPROACH: F. Costard et al.

and physical properties representative of the expected aver- liquid water below the permafrost, and to get relevant esti- age Martian subsurface. Results are consistent with the mates of its depth and integrated conductivity. simpler calculation which were performed to design the Seismological investigations (SEIS experiment): A last radar and show that a liquid water interface should be de- piece of information will be done with the seismometer. The tectable at depths of approximately 2.5 km. During night- output used will be those of the BRB Short period 3 axis time we foresee to operate the GPR instrument in a passive seismometer, which will record seismic signals in the band -9 2 1/2 mode in order to receive the waves emitted by the MARSIS 0.05-50 He with a resolution bettor than 5 10 ms /Hz radar on-board the orbiter. [15]. The body wave will then be the subject of site effect (see Horwarth et al., 1980 for the use of this method on the Moon) and the analysis does not need the location of the seismic source, which will allow the use of seismic signals released by the regional quakes. Two parameters will be searched: the first will be the position of the subsurface discontinuities, the latter being smoothed by the magnetic inversion, mostly sensitive to the resistivity jumps. The second will be the seismic high frequency attenuation, which is very sensitive to the water content at depth below the 0°C isotherm. Interaction regolith/atmosphere (ATMIS experi- ment): Netlander also include a meteorological package (ATMIS) Fig. 1: baseline configuration of the geophysical soundings. which will provide some observations of interest to study the climate water cycle. The local relative humidity during Electromagnetic soundings (MAGNET experiment): warm hours will be monitor day after day from the four The magnetometer experiment consists of a net of identical landers by the Humicap sensors. Clouds and condensat will ultra small low noise and light triaxial vector fluxgate mag- be observed netometer sensor, with a resolution of 0.025 nT. The mag- by the Optical Depth Sensor (ODS) and the camera. Such netometer sensor is placed at the surface of Mars, outside observations should allow us to constrain the diurnal and the lander, by means of a light deployable boom [12]. The seasonal exchange between the regolith and the atmosphere attitude of the vector components of each triaxial fluxgate (adsorption and condensation) as well as the vertical distri- sensors will be known with an absolute accuracy of few bution of the water vapor above the Lander. tenths of a degree in both vertical and horizontal directions. A complementary approach: The results of electro- The impedance of the internal structure will be deduced magnetic and ground penetrating radar ground ice investi- from the ratio of the vertical component of the magnetic gations are clearly complementary, and combining their field and the horizontal gradients of its horizontal compo- results would greatly improve our knowledge of the perma- nent. With simultaneous recordings from three stations or frost structure at the NETLANDER landing sites. On one more available, the impedance will be estimated from the hand, radar measurements are likely to provide quite clear frequency-wave vector spectrum of the electromagnetic field information on the thickness of the uppermost very resistive using a high-resolution method developed by Pinçon et al., layer of the ground ice. On the other hand, electromagnetic [13] soundings will provide information on the depth of the con- Below 1-2 km, the determination of the mean resistivity ductive layer associated with liquid water, if any. Besides, profile will be done with the magnetometer. This will allow electromagnetic soundings will provide information on the the determination of the thickness of the resistive ground thickness and conductivity of this layer. ice, and will provide information about the presence (or References: [1] Harri A-M et al. Adv. Space Res., 23, absence) of liquid water under the ground ice. As the resis- No 11. [2] Baker, V.R., (1982) The channels of Mars. 198 tivity of the permafrost is very high, the presence of liquid pp., Univ. of Texas Press. [3] Masursky et al., 1977. J. water at the bottom of the ground ice will then correspond to Geophys. Res. 82, 4016-4038. [4] Carr M.H., 1986. Icarus a decrease of the resistivity by two or more orders of mag- 68, 187-216. [5] Squyres S.W. et al. (1992) in Mars Book nitude. Simulations have been made with models of resis- Univ. of Arizona Press. [6] Clifford, S.M. (1993) JGR 98, tivity distribution within the Martian ground ice. The model E6. 10973-11016. [7] Rossbacher, L.A. and S. Judson, resistivity profiles have been extrapolated from laboratory (1981) Icarus, 45, 25-38. [8] Kuzmin, R. et al. (1989) Solar measurements on water saturated porous rocks for tem- System Res. 22, 121-133. [9] Costard, F. (1989). Earth, perature ranging from – 25°C to + 5°C [14]. The measure- Moon and Planets. 45: 265-290. [10] Fanale F.P., et al. ments have been made on a sandstone sample of 18% po- (1986) Icarus, 67, 1-18. [11] Berthelier J.J. et al. Planet. rosity saturated with salted water of conductivity 1 S/m. Space Sci., 2000, in press. [12] Menvielle M. et al., PSS, Two profiles corresponding to typical high- and low-latitude 2000, in press. [13] Pinçon J.L. et al. PSS, 2000, in press. situations have been considered. The obtained results make [14] Guichet, (1998), Etude des propriétés Géophysiques du clear that knowing the apparent resistivities within a preci- pergélisol martien, Mémoire de DEA, IPGP. [15] Lognonne sion of about 16% for frequencies ranging from 10 to 0.001 Ph. et al., PSS, 2000, in press Hertz allows to evidence a conductive layer associated to