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Astron. Astrophys. 362, 1072-1076 (2000)

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1. Introduction

Most of our knowledge about Mars geology and surface mineralogy has been gathered by spectroscopy. In the visible-near infrared, the reflected spectrum of classical bright and dark regions show a deep absorption in the 0.3 [FORMULA] 0.6 µm. Mars spectral behavior is dominated by Fe mineralogy; [FORMULA] is present in the dust and soil and poorly crystalline or nanophase ferric-bearing materials dominate the visible to near-IR spectral properties of Mars (Morris et al. 1985). More amorphous materials such as palagonite, have also been suggested as the surface materials. Crystalline ferric oxides such as hematite in a crystalline form have also been discovered on the basis of ground-based spectroscopic observations and spacecraft remote sensing measurements (Bell et al. 1992, Christensen et al. 1998). The albedo of classical surface markings is not constant over time probably due to seasonal and interannual variations of contrast between dark and bright regions (Slipher 1962, Baum 1974, Capen 1976, Martin et al. 1992, Bell et al. 1999). These effects are in general due to global dust storms and seasonal winds, which transport light-colored dust from some areas to others (Christensen 1988). A specific phenomenon linked to spectral properties of the soils is the contrast reversal, that is one region brighter than an other in the red light, appears darker at shorter wavelengths (Barth et al.1972, Thomas & Veverka 1986). Observations of contrast reversal have been reported by many investigators by means of ground-based observations (McCord 1969, Adams & McCord 1969, McCord & Westphal 1971, Thompson 1973). Regions involved are Syrtis Major, Arabia, Sinus Meridiani, Sinus Sabeus, Pandorae Fretum and Mare Serpentis. The analysis of Viking Orbiter images has demonstrated that contrast reversal can be a surface phenomenon associated to specific eolian features as intra-crater deposits and wind streaks originating from deposits (Thomas & Veverka 1986). However the instances of albedo reversal reported by ground based observers can be influenced by the scattering properties of the atmosphere at different viewing conditions. In particular, we point out the possible effect of water ice clouds on spectral remote sensing of Mars. In this paper we present the result of observations made by means of imaging spectroscopy instrumentation during the 1997 opposition.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000