A PERMITTIVITY PROBE FOR THE GANYMEDE LANDER

1. A PERMITTIVITY PROBE FOR THE GANYMEDE LANDER • Le Gall1, A., V. Ciarletti1, M. Hamelin1, R. Grard2 1 Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Université Versailles Saint-Quentin (UVSQ), Paris, France, alice.legall@latmos.ipsl.fr 2 RSSD, ESA-ESTEC, Noordwijk, Netherland

2. Plan • History / Heritage • Scientific goals for measuring complex permittivity • Measurement techniques • The case of Ganymede • Discussion V century BC; Musée du Louvre

3. A wide diversity of materials Biological materials Granular heterogeneous materials Crystals / Amorphous The Permittivity of various materials (dielectric constant & conductivity) is deduced from impedance measurements Mutual Impedance measurements (between emitting and receiving dipoles) are preferred because they minimize the effect of electric inhomogeneities around the electrodes.

4. Heritage (1) MutualImpedancemeasurements in the magnetospheric plasma with ESA satellites GEOS1 and GEOS2 Resistivitymeasurements for oil prospection in the early 20th century. (From A.G. Schlumberger ‘The Schlumberger Adventure’, ARCO, N.Y.,1982) Storey, 1969 Wenner, 1915; Grard, 1990

5. Heritage (2)Planetary Permittivity Probes PWA-HASI HUYGENS Hamelin et al., 2000 Seidensticker et al., 2004 Pluto mole (DLR-ESTEC) on Beagle Mars lander PP-SESAME PHILAE-ROSETTA

6. The scientific approach: physical constraints through permittivity measurements The surface MutualImpedance Probe • Integratedmeasurements in the close subsurface (depth ~1m) • ELF-VLF frequency range (sensitivityto polar molecules) • Permittivitydepends on porosityand composition • Conductivitycanextend in severalorders of magnitude The subsurface Mutual Impedance Probe (not considered here) • Heritage from the Beagle mole (failed to land on Mars) • Could be considered, integrated with other instruments. Joint studies Permittivitymeasurementsalonewill not allowaccurate identification of the cometarymaterial. Synergywithotherexperimentsis crucial.

7. The case of water ice The MI isvery sensitive to the ice content of the regolith but alsodependent of porosity, granular structure, temperature, dielectric and conductivity of otherconstituents. AtGanymede surface typicaltemperatures, the dielectric constant of iceis ~3. MI measurementsprovidespecificconstraints for characterizing the regolith

8. The Permittivity Probe in homogeneous media In homogeneous media Model: pin point electrodes; no booms; no wires • Simple. Good for satellites withelectrodesheld by long wires • Perfectcurrentgenerator and perfectvoltmeter

9. Huygens and PhilaePermittivity Probe instruments models N electrodes in medium 1 DAC 2 DATA Analog electronics and cables Digital electronics 3 ADC N Perfect conversion MODELS N x N admittance (sparse) matrix N x N admittance matrix (geometry, medium) Algorithms

10. Derivation of the ground permittivity (the case of Huygens on Titan) Procedure • Model of electronics + Model of electrodes in medium (εr, σ)  theoretical value of potential (normalizedwith respect to the full vacuum value) • Draw the abacus of constantRe(ε) & Im(ε) intheRe(V/V0) - Im(V/V0) plane. • Report experimentalvalues in the abacusresultingε value. Herethe Huygens probe issupposed to beat a few cm abovethe ground, withoutany contact of the body and electrodeswith the interface plane. Symbolsrepresentexperimental data and corresponding values of ε canbededuced. When electrodes have no contact with the interface the abacus calculation is much simpler, because it is reduced to a full vacuum model and a mirror interface model. Im(V/V0) Re(V/V) Re(V/V) PWA data are stillunderanalysis, takingintoaccount the uncertaingeometry of the Probe and electrodearrayafter semi-hard landing. Calculationsare performed for eachgeometry and each value of ε

11. A Permittivity Probe for the Ganymede lander Main requirement: Insulating sections on legs T R R The rectangulararray of electrodesiswellsuited for PP

12. Contribution to the characterization of the terrain and geology at the landing site • Constraints for the physical and chemicalproperties of the uppericycrust. • Age • Ice content • Porosity • Rocky fraction • Impurities (conductivity)

13. Discussion - Conclusion A simple instrument that uses the landergeometry: no specific booms but insulationg sections are needed. Electronics: one card in the lander + 2 smallpreamplifiers close to the receivingelectrodes (optionnal) + triaxial cables. In combinationwithotherphysicalmeasurements, itwouldallow to characterize the crustat the landing site for a volume commensurate to the lander. As a bonus: measurement of the electromagneticactivity in passive mode with the tworeceivers… but preferred location of the lander opposite to Jupiter. L’enlèvement de Ganymède, François Chauveau

14. References Grard, R., 1990a. A quadrupolar array for measuring the complex permittivity of the ground—application to Earth prospection and planetary exploration. Meas. Sci. Technol. 1, 295-301. Grard, R., 1990b. A quadrupolar system for measuring in situ the complex permittivity of materials—application to penetrators and landers for planetary exploration. Meas. Sci. Technol. 1, 801-806. Hamelin, M. et al., 2000, Surface and sub-surface electrical measurement of titan with the PWA-HASI experiment on HUYGENS, Advances in Space Research, Volume 26, Issue 10, Pages 1697-1704 Storey, L.R.O., Aubry, M.P., Meyer, P., 1969. A quadrupole probe for the study of ionospheric resonances. In: Thomas, J.O., Landmark, B.J. (Eds.), Plasma Waves in Space and in the Laboratory. Edinburgh University Press, pp. 303-332. Seidensticker, K. et al., The Rosetta lander experiment SESAME and the new target comet 67P/ Churiumov-Gerasimenko, in The new Rosetta targets, 297-307, Klüver Acad. Publications. Wenner, F., 1915. A method of measuring the Earth resistivity. U.S. Bur. Stand. Bull. Sci. Pap. 25 (12), 469-478.