Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..327l&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
Introduction An extensive light-toned layered formation covers the plains surrounding Valles Marineris on Mars. It is particularly visible south of Ius Chasma and of Melas Chasma [1], southwest of Juventae Chasma [2,3], north of Tithonium Chasma and west of Ganges Chasma. Some deposits of this formation may be enriched in hydrated silicates such as hydrated glasses, chalcedony, opal or other hydrated Si-rich phases according to CRISM data [1]. From an analysis of HRSC, THEMIS, MOC, HiRISE, MOLA PEDR, OMEGA and CRISM data, we discuss the morphology, the composition, the age, the spatial extent and the emplacement processes of these layered deposits (LDs). Here we focus on two regions where the LDs are particularly spectacular: Ganges Chasma and Juventae Chasma. Regional map We have compiled a regional map of the LDs around Valles Marineris (orange in Fig. 1a). In some cases their spatial extent is unclear due to their being covered either by dark material or by dust that appears yellow on IRB color HiRISE images (Fig 1b). Dashed contours on Fig. 1a outline these poorly constrained boundaries, whereas plain contours correspond to regions where the stratigraphic contact between the LDs and the underlying basement is unambiguous. The light-toned LDs are located stratigraphically and topographically above the basaltic basement that constitutes the plains surrounding Valles Marineris. The total thickness of the LDs does not exceed a hundred meters on average. They consist of subparallel light-toned layers of various thicknesses that are apparently interbedded with darker beds (Fig. 1b). This difference in albedo can be due to variations in mineralogical composition, topographic slope, roughness, grain size or state of erosion of the different layers, or to partial covering of certain layers by a dark mantle. Ganges Chasma West of Ganges Chasma, the LDs rest topographically and stratigraphically above the Noachian plains that have been defined as the Npl2 unit [4] (Fig. 1c). Npl2 has been interpreted as thin interbedded lava flows and eolian deposits that partly bury underlying rocks [4]. Outcrops of bright material on CTX and night-time infrared THEMIS images are exposed a few tens of kilometres west of the LDs. These light-toned outcrops are located near the Hesperian outflow channel Allegheny Vallis [5] (Fig. 1c). They may correspond to exposures of the Npl2 unit or alternatively to a formation covering Npl2. The LDs are located stratigraphically above the outflow channel, so they are Hesperian in age or younger. LDs are characterized by a particularly low brightness temperature on night-time THEMIS images in comparison with the surrounding plains (Fig. 1d). It suggests that the LDs are composed of a soft material that is sensitive to erosion. This is consistent with the occurrence of remnant buttes, kilometres away from the largest outcrop. The buttes attest that the LDs were more extensive in the past and have been removed by erosion in many places. The thickness of the LDs ranges from 10 to 40 m on average. Their constitutive layers are subparallel to each other but they locally exhibit deformation features such as folds and fractures. Some layers display a polygonal texture. The polygons range from a few metres to about 10 m in diameter. This polygonal network may be due to thermal contraction, desiccation or sublimation [6]. Sinuous ridges, as wide as 60 m, are visible on the LDs (Fig. 1c). At night, they have a similar brightness temperature as the LDs and they are apparently composed of the same material. Their morphology suggests that they are eskers or inverted fluvial channels [7-9]. In that latter case, a fluvial origin could explain the deposition of the LDs. However, other origins such as lacustrine or glacial deposition, ash or dust air-fall and ash flows cannot be excluded. The analysis of CRISM data (FRT 8949) shows that some layers have spectral characteristics of hydrated materials. Ratio spectra display small absorption bands at ~1.39 μm and ~1.92 μm, a deep band centred near ~2.2-2.24 μm, and a drop between ~2.3 μm and ~2.5 μm. This may be consistent with SiOH-bearing phases [1]. Juventae Chasma Southwest of Juventae Chasma, the LDs rest stratigraphically above the Hesperian plains. Some layering is also visible in a depression west of Juventae Chasma [10]. At night, these LDs have a brightness temperature similar to those of Ganges Chasma and many remnant buttes are also visible. The total thickness of the LDs ranges from a few to ~ 60 metres. The outcrops are thinner to the north and to the south of the mapped region. The layers do not display deformation features, in contrast with those near Ganges Chasma. Polygonal cracks are visible on the LDs, on the underlying basement and on the dark overlying flows. In the latter case, the polygons may be due to the cooling of the lava. A branched network of sinuous ridges is embedded in the LDs (Fig. 1e). The ridges are 20 to 200 m wide and more than 10 km long [9]. They are wider to the north and they follow a northward regional slope of 0.2°, which is consistent with their flowing northwards [9]. Some sinuous valleys, carved in the basement, are also visible on the plains surrounding the LDs (e.g. HiRISE image PSP_005346_1755). These observations are consistent with the interpretation that the sinuous ridges and valleys were formed by aqueous flows [9]. Some layers have spectral properties similar to those of Ganges Chasma. Ratio spectra show absorption bands near 1.94 μm and 2.23 μm that are consistent with palygorskite and/or hydrated amorphous Al-rich silica [3]. This mineralogical composition is consistent with aqueous alteration of LDs during or after their emplacement but also with the transport and deposition in aqueous networks of previously altered material. Conclusion and perspectives The plains around Valles Marineris are overlain by layered deposits. These deposits consist of alternating light and dark flat-lying layers that have a total thickness smaller than 100 metres. They are composed of a soft material, which could be volcanic ash, dust, glacial till or lacustrine or fluvial sediments. The observation of sinuous ridges near Ganges and Juventae Chasma and of sinuous valleys in Juventae Chasma as well as the hydration of the deposits suggest that water played a role in the alteration, erosion, transport and deposition of the LDs. The comprehensive mapping of the LDs, requiring high spatial resolution data, is still in progress and other LDs probably exist elsewhere in the plains around Valles Marineris. The night-time infrared THEMIS mosaic at 250 m/pixel shows that many areas located near the canyon edges display similar low brightness temperatures as the LDs mapped so far. These areas correspond to materials that have physical properties (grain size, degree of induration, and proportion of rock / soil) that are similar to the LDs. They likely correspond to light-toned LDs as well. References [1] Milliken R. E. et al. (2008) LPSC XXXIX, Abstract #2025. [2] Lucchitta B. K. (2005) LPSC XXXVI, Abstract #1500. [3] Bishop J. L. et al. (2008) LPSC XXXIX, Abstract #2334. [4] Scott D. H. and Tanaka K. L. (1986) USGS. Misc. Inv. Series Map I-1802-A. [5] Coleman N. M. (2007) Icarus, 189, 344-361. [6] Mangold N. (2005) Icarus, 174, 336-359. [7] Williams R. M. E. and Edgett K. S. (2005) LPS XXXVI, Abstract #1099. [8] Williams R. M. E. (2007) LPS XXXVIII, Abstract #1821. [9] Mangold N. et al, JGR, in press. [10] Catling (2006) Icarus, 181, 26-51.
Bourgeois Olivier
Le Deit Laetitia
Le Mouélic Stéphane
Massé Marion
Mège Daniel
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