Transcript glcw_5_07_hamelin_legall_ganymede_ppx

A PERMITTIVITY PROBE FOR THE GANYMEDE LANDER
1
1
Le Gallle style
, A.,des
V. sous-titres
Ciarletti du
, M.
Hamelin
Modifiez
masque
1
1,
R. Grard2
Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Université
Versailles Saint-Quentin (UVSQ), Paris, France, [email protected]
2 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)
Wenner, 1915; Grard, 1990
Mutual Impedance measurements in
the magnetospheric plasma with ESA
satellites GEOS1 and GEOS2
Storey, 1969
Heritage (2)
Planetary Permittivity Probes
PWA-HASI
HUYGENS
Hamelin et al., 2000
Seidensticker et al., 2004
PP-SESAME
PHILAE-ROSETTA
Pluto mole (DLR-ESTEC)
on Beagle Mars lander
The scientific approach: physical constraints through permittivity measurements
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
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
Permittivity measurements alone will not allow accurate identification of the cometary material. Synergy with
other experiments is crucial.
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
The Permittivity Probe in homogeneous media
In homogeneous media
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 Philae
Permittivity Probe instruments models
N electrodes
in medium
1
DAC
DATA
Analog electronics
and cables
Digital
electronics
2
3
ADC
N
Perfect
conversion
MODELS
Algorithms
N x N admittance
(sparse) matrix
N x N admittance matrix
(geometry, medium)
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
(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.
Re(V/V)
Re(V/V)
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
A Permittivity Probe for the Ganymede lander
Main requirement:
Insulating sections on legs
T
R
R
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
Discussion - Conclusion
A simple instrument that uses the lander geometry: no specific booms but insulationg sections
are needed.
Electronics: one card in the lander + 2 small preamplifiers close to the receiving electrodes
(optionnal) + triaxial cables.
In combination with other physical measurements, it
would allow to characterize the crust at the landing site
for a volume commensurate to the lander.
As a bonus: measurement of the electromagnetic activity
in passive mode with the two receivers… but preferred
location of the lander opposite to Jupiter.
L’enlèvement de Ganymède, François Chauveau
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 PWAHASI 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.