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GDS

HEGGY ET AL.: MARTIAN GEOELECTRICAL MODELS

volcanic context and is covered by an iron oxide-rich dust

layer, more probably constituted of altered basalts [Pinet

and Chevrel, 1990], hematite [Christensen et al., 2000],

maghemite and other ferromagnetic minerals [Hargraves et

al., 1977]. This dust material is overlaying volcanic layers

of fractured basalt and lava flows, with a geographically and

stratigraphically variable component of massive and inter-

stitial ice [Clifford, 1993; Clifford and Parker, 2001].

Deeper subsurface material could be mainly constituted of

fractured ground ice [Clifford and Johansen, 1982]. If we

assume this configuration to be representative of the Mar-

tian subsurface, then materials present in the first few

hundred of meters of the subsurface could significantly

attenuate the probing radar signal, due to electric and

magnetic losses, thus limiting the penetration depth to few

hundreds of meters at the 2 MHz frequency [Heggy et al.,

2001].

[6] Radar sounders should then operate at specific sites

where the geoelectrical context is locally less conductive

and where local geothermal conditions could lead to the

presence of liquid water at shallow depths [Clifford and

Parker, 2001]. In this paper, we present the geoelectrical

modeling of such favorable sites in order to define future

potential landing sites for the GPR experiment of the Net-

lander mission, and derive some criteria for optimal sound-

ing sites for future radar experiments. Numerical simulations

of the radar echo for the selected sites are presented and

discussed.

2. Geological Models

[7] Four geological models of Martian subsurface are

proposed in order to highlight the effect of several compo-

nents such as liquid water, magnetic minerals and sedimen-

tary deposits. The presence of fine grained or coarse

deposits of different petrology (especially with varying

porosity and permeability) may substantially affect the ice

Figure 1. Top: the Viking image of the Hadriarca Pateraa

content in the subsurface and the radar signatures. These

volcano (31 S, 267 W). The arrow on the image shows

models correspond to possible local stratigraphy on Mars

Amazonian fluvial features formed by interactions of lava

but large uncertainties exist about the composition and

with water. Bottom: the proposed geological profile for a

nature of the subsurface material. Examples are given to

shallow aquifer associated with local geothermalism that

illustrate each proposed model refers to locations on Mars

might exist for similar sites.

where the subsurface could correspond to the model, but

detailed thickness and composition of the layers are spec-

sphere Sounding (MARSIS) experiment onboard the Mars

ulative. These models do not take into account the regional

Express ESA orbiter will be the first instrument to perform a

variability of the selected geological unit. The possibility of

global vertical sounding at the 2 MHz frequency [Picardi et

finding shallow aquifers in the Martian near-surface is low

al., 1999]. It will be followed by the Ground-Penetrating

due to cold temperatures, and liquid water should not be

Radar experiment; the Netlander mission in 2007, which

present at less than 1 km according to realistic thermal

will land four autonomous geophysical stations at different

gradients [Clifford, 1993]. Nevertheless, we detail three

sites [Berthelier et al., 2000]. These two experiments will

examples where local residuals of subsurface water could

mainly focus on the deep water detection, while a third

be found at shallow depth. These locations would corre-

instrument focused on Shallow Radar sounding (SHARAD)

spond to regions of high geothermal flow (2.1), outflow

is planned for 2005 onboard the NASA Mars Reconnais-

channels (2.2), or ice-rich northern plains (2.3). The last

sance Orbiter (MRO) will operate at a higher frequency

case (2.4) does not consider liquid water but sediments

around 20 MHz, in order to detect probable water Layers at

formed by desiccation of an ancient lake.

shallow depth [Beaty et al., 2001].

2.1. Shallow Aquifers Associated With Local

[5] The performances of all of these radar systems are

Geothermal Anomalies

strongly dependent on the petrology and mineralogy of the

[8] Large geothermal gradients may occur within or near

Martian subsurface [Olhoeft, 1998; Heggy et al., 2001],

areas of recent volcanic activity. For example, the Hadriarca

which define the electrical behavior of each geological layer

Patera volcano (31°S, 267°W) shows Amazonian fluvial

of the sounded sites. Most of the Martian surface presents a