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HEGGY ET AL.: MARTIAN GEOELECTRICAL MODELS

2000b]. The composition of layers is still unknown and a

lot of speculations propose different compositions depend-

ing on whether the deposits have an aeolian, volcanic,

fluvial or lacustrine origin. Interesting is a model contain-

ing evaporites that could correspond to layers formed by

the drying of a stagnant lake like Sebkhas in terrestrial

desert. Such conditions could correspond to the layers

observed in closed depressions of Valles Marineris such

as Melas Chasma (9°S, 77.5°W) (cf. MOC image M08-

04367 in the top of Figure 4), or to layers inside craters

like Gale or Henry. The layering proposed for such kind of

geological setting is described in the bottom of Figure 4.

The layer of mudstone is under erosion at the present time

and could correspond to the eroded layers as shown by the

arrow in the corresponding MOC image in the upper part

of Figure 4. It is chosen to be mainly composed of

kaolinite. This mudstone layer of 30 m thickness is

underlain in the model by 30 m of gypsum (CaSO4,

2H2O) and 30 m of aragonite (CaCO3) lying over a

basaltic basement. The chosen composition and thickness

of the layers is one configuration among many different

possible, but could likely correspond to Sebkha like

deposits.

3. Electromagnetic Characterization and

Geoelectrical Modeling

[12] Once the geological models are set and well defined,

we investigated representative laboratory samples, in terms

of mineralogy and porosity, for each layer of the above-

discussed models. In our analogy, the electromagnetic

properties of each layer of a geological profile are reduced

to the electromagnetic characterization of the representative

laboratory sample. Samples are compositionally homoge-

neous, with different porosities, temperatures and a varying

amounts of iron oxide-rich minerals (hematite, maghemite,

magnetite) for samples representing volcanic layers. It must

be kept in mind that this is a simple approach that does not

reproduce the heterogeneous composition of rocks and

their complex porosity. However, as we are mainly inter-

ested in the permittivity of the samples to build geoelec-

trical models to evaluate losses in wave propagation,

Figure 3. Top: the traverse across crater (30.3 N, 251.3 W)

homogenous mixtures of minerals and ice are relevant.

(MOC image M12-00506). Bottom: the corresponding

For the permittivity measurements, we used two capacitive

geological model.

cells. The first one characterizes powder materials ( poros-

ity ranging from 30 to 50%) and the second one measure

to 80 m) derived from MOLA profiles along some ejecta

pellets of compacted powder (porosity ranging from 15 to

deposits [Barlow et al., 2000]. The materials are interpreted

30%) or machined from a rock sample. For the perme-

to be impact-brecciated rocks at least for the upper layers.

ability measurements, we used a self-magnetic cell. More

The porosity can be high and include a mixing of large

details concerning the measurement procedure and samples

amounts of substrate material into the ejecta deposit

preparation have been described in earlier paper [Heggy et

[Melosh, 1989] In the proposed model; these ejecta deposits

al., 2001]. Each layer analog is described in terms of the

overlay different layers of sediments and basaltic materials.

real and imaginary part of its dielectric constant (e = e0 À

These sedimentary deposits and, in particular locations,

ie00), its conductivity s in S/m and its relative magnetic

volcanic flows or deeper layers, may already contain ice.

permeability m (in this work we only considered the real

The lower limit of these layers is believed to be in the range

part of the magnetic permeability, as mineral mixtures used

of a few hundred of meters to 1 or 2 kilometers. As indicated

to simulate the subsurface layers in the four models are not

above this is supposed to be an average situation, but current

highly magnetic). It is important to note that the choice of

conditions may be very different from site to site.

analog materials to construct our samples is a first order

2.4. Layered Deposits

approximation to illustrate the variation in the sounding

[11] Layered deposits have been found in many regions

radar performances in various possible Martian geoelectri-

with the Mars Global Surveyor camera [Malin and Edgett,

cal configurations.