Observations of the northern seasonal polar cap on Mars II: HiRISE photometric analysis of evolution of northern polar dunes in spring

We present an overview of our analyses of HiRISE observations of spring evolution of selected dune areas of the north polar erg. The north polar erg is covered annually by seasonal volatile ice layer, a mixture of CO2 and H2O with mineral dust contamination. In spring, this layer sublimes creating visually enigmatic phenomena, e.g. dark and bright fan-shaped deposits, dark–bright–dark bandings, dark down-slope streaks, and seasonal polygonal cracks. Similar phenomena in southern polar areas are believed to be related to the specific process of solid-state greenhouse effect. In the north, it is currently unclear if the solid-state greenhouse effect is able to explain all the observed phenomena especially because the increased influence of H2O on the time scales of this process has not yet been quantified. HiRISE observations of our selected locations show that the ground exhibits a temporal behaviour similar to the one observed in the southern polar areas: a brightening phase starting close to the spring equinox with a subsequent darkening towards summer solstice. The resolution of HiRISE enabled us to study dunes and substrate individually and even distinguish between different developments on windward and slip face sides of single dunes. Differences in the seasonal evolution between steep slip faces and flatter substrate and windward sides of dunes have been identified and compared to CRISM data of CO2 and H2O distributions on dunes. We also observe small scale dark blotches that appear in early observations and tend to sustain a low reflectivity throughout the spring. These blotches can be regarded as the analogue of dark fan deposits in southern polar areas, leading us to the conclusion that both martian polar areas follow similar spring evolutions.

Mapping material distribution in a heterogeneous sand tank by image analysis

Water flow and solute transport through soils are strongly influenced by the spatial arrangement of soil materials with different hydraulic and chemical properties. Knowing the specific or statistical arrangement of these materials is considered as a key toward improved predictions of solute transport. Our aim was to obtain two-dimensional material maps from photographs of exposed profiles. We developed a segmentation and classification procedure and applied it to the images of a very heterogeneous sand tank, which was used for a series of flow and transport experiments. The segmentation was based on thresholds of soil color, estimated from local median gray values, and of soil texture, estimated from local coefficients of variation of gray values. Important steps were the correction of inhomogeneous illumination and reflection, and the incorporation of prior knowledge in filters used to extract the image features and to smooth the results morphologically. We could check and confirm the success of our mapping by comparing the estimated with the designed sand distribution in the tank. The resulting material map was used later as input to model flow and transport through the sand tank. Similar segmentation procedures may be applied to any high-density raster data, including photographs or spectral scans of field profiles.

Combined effects of heterogeneity, anisotropy, and saturation on steady state flow and transport: A laboratory sand tank experiment

Field soils show rather different spreading behavior at different water saturations, frequently caused by layering of the soil material. We performed tracer experiments in a laboratory sand tank. Such experiments complement and help comprehension of field investigations. We estimated, by image analysis, the first two moments of small plumes traveling through a two-dimensional, heterogeneous medium with strongly anisotropic correlation structure. Three steady state regimes were analyzed. Two main conclusions were drawn. First, low saturation led to very large heterogeneity and to strong preferential flow. Thus the description of the flow paths and the prediction of the solute arrival times require, in this case, more accurate knowledge about the topological structure. Second, saturation-dependent macroscopic anisotropy is an essential element of transport in unsaturated media. For this reason, small structural soil features should be properly upscaled to give appropriate effective soil parameters to be input in transport models.

La figure de la sorcière dans la littérature du XIXe siècle (Jules Michelet, George Sand, Eliza Orzeszkowa)

The nineteenth century uncovered and analysed the tragic episodes of witch-hunting and ‘witch’ trials common in Renaissance Europe. Fascinating not only to historians, this subject also inspired men of letters who popularized the image of the witch as an old, ugly and evil person, who thus deserved her lot. Jules Michelet’s La sorcière of 1862 takes a very different approach. Simultaneously a literary and historical work, the book proved scandalous as it rehabilitated the figure of the witch, shedding favourable light on her image: it was the witch who was able to save a last spark of humanity in moments of despair; it was she who acted as comforter and healer to the people. In the context of nineteenth-century literature, certain works by female authors that focused on ‘witches,’ stand out. Whilst certain male authors (Michelet included) presented the witch as a figure from the past, who had finally perished in the 17th century, texts such as George Sand’s La petite Fadette (1848) or Eliza Orzeszkowa’s Dziurdziowie (1885), suggest that the end of witch trials did not imply an end to accusations, persecutions, and even executions of ‘witches’ – and, that in terms of culture, witchcraft or sorcery had not disappeared from the societies they knew.

Le personnage de la femme-artiste chez l'écrivain polonaise Narcyza Żmichowska (1819-1876) : Helusia héritière de Corinne de Mme de Staël et de Consuelo de George Sand

Si les hommes bénéficient d’emblée de mythes (Pygmalion, Orphée) qui légitiment et motivent leur créativité, il faut attendre Corinne (1807) de Mme Staël et Consuelo (1842) de George Sand pour que le mythe de la femme créatrice trouve enfin, dans la littérature européenne, à se concrétiser et à se développer sous la forme de Künstlerromane féminins. Corinne la poétesse et Consuelo la cantatrice sont devenues des exemples littéraires non seulement pour les femmes victoriennes, comme le montre par exemple le livre de Linda Lewis intitulé Germaine de Staël, George Sand and the Victorian Woman Artist, mais également pour les femmes polonaises issues des classes favorisées - qui lisaient d’ailleurs couramment en français. En effet, même si elles ont été officiellement décriées par la critique conservatrice polonaise pour leur « immoralité », George Sand et Mme de Staël ont été lues, bien lues et même beaucoup lues par leurs contemporaines polonaises - comme le témoigne la correspondance de ces dernières, dans laquelle elles ont moins de peine à se livrer. Le thème de la femme artiste déchirée entre la carrière publique et la vie privée, tel qu’il est représenté dans Corinne et dans Consuelo, a en particulier attiré toute leur attention et suscité chez les femmes écrivains le désir d’apporter de nouvelles manières de résoudre ce conflit. Dans cet article, le Künstlerinroman polonais Książka Pamiątek (Livre des souvenirs, 1846) de Narcyza Żmichowska fait l’objet d’une analyse comparative détaillée avec les deux Künstlerinromane français dont par ailleurs il se réclame.