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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.fr2 RSSD, ESA-ESTEC, Noordwijk, Netherland

Plan

•History / Heritage•Scientific goals for measuring complex permittivity•Measurement techniques•The case of Ganymede•Discussion

V century BC; Musée du Louvre

A wide diversity of materials

Crystals / Amorphous

Granular heterogeneous materials

Biological materials

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.

Heritage (1)

Resistivity measurements for oil prospection in the early 20th century.(From A.G. Schlumberger ‘The Schlumberger Adventure’, ARCO, N.Y.,1982)

Mutual Impedance measurements in the magnetospheric plasma with ESA satellites GEOS1 and GEOS2

Storey, 1969Wenner, 1915; Grard, 1990

Heritage (2)Planetary Permittivity Probes

PWA-HASIHUYGENS

PP-SESAMEPHILAE-ROSETTA

Pluto mole (DLR-ESTEC)on Beagle Mars lander

Hamelin et al., 2000

Seidensticker et al., 2004

The surface Mutual Impedance Probe• Integrated measurements in the close subsurface (depth ~1m)

• ELF-VLF frequency range (sensitivity to polar molecules)

• Permittivity depends on porosity and composition

•Conductivity can extend in several orders of magnitude

Joint studiesPermittivity measurements alone will not allow accurate identification of the cometary material. Synergy with other experiments is crucial.

The scientific approach: physical constraints through permittivity measurements

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.

The case of water ice

The MI is very sensitive to the ice content of the regolith but also dependent of porosity, granular structure, temperature, dielectric and conductivity of other constituents. At Ganymede surface typical temperatures, the dielectric constant of ice is ~3.MI measurements provide specific constraints for characterizing the regolith

a

Receiver

Currentsource

T1

T2

R1

R2

Vr

It

The Permittivity Probe in homogeneous media

In homogeneous media

)cos( oor AA

)sin( oooAA

Model: pin point electrodes; no booms; no wires Simple. Good for satellites with electrodes held by long wires Perfect current generator and perfect voltmeter

Huygens and PhilaePermittivity Probe instruments models

1

23

N

Analog electronics and cables

Digital electronics

DAC

ADC

DATA

N electrodes in medium

N x N admittance matrix

(geometry, medium)

N x N admittance (sparse) matrixAlgorithms

Perfect conversion

MODELS

Derivation of the ground permittivity (the case of Huygens on Titan)

Re(V/V) Re(V/V)

Im(V

/V0)

Procedure

• Model of electronics + Model of electrodes in medium (εr, σ) theoretical value of potential (normalized with respect to the full vacuum value)

• Draw the abacus of constant Re(ε) & Im(ε) in the Re(V/V0) - Im(V/V0) plane.

• Report experimental values in the abacus resulting ε value.

Here the Huygens probe is supposed to be at a few cm above the ground, without any contact of the body and electrodes with the interface plane. Symbols represent experimental data and corresponding values of ε can be deduced.

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.

PWA data are still under analysis, taking into account the uncertain geometry of the Probe and electrode array after semi-hard landing. Calculations are performed for each geometry and each value of ε

Main requirement:Insulating sections on legs

A Permittivity Probe for the Ganymede lander

RR

T

The rectangular array of electrodes is well suited for PP

Contribution to the characterization of the terrain and geology at the landing site

• Ice content • Porosity• Rocky fraction• Impurities (conductivity)

• Constraints for the physical and chemical properties of the upper icy crust.

• Age

L’enlèvement de Ganymède, François Chauveau

Discussion - Conclusion

A simple instrument that uses the lander geometry: no specific booms but insulationg sections are needed.

In combination with other physical measurements, it would allow to characterize the crust at the landing site for a volume commensurate to the lander.

Electronics: one card in the lander + 2 small preamplifiers close to the receiving electrodes (optionnal) + triaxial cables.

As a bonus: measurement of the electromagnetic activity in passive mode with the two receivers… but preferred location of the lander opposite to Jupiter.

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.