The effect of the internal structure of Mars on its seasonal loading deformations

Mars is continuously subjected to surface loading induced by seasonal mass changes in the atmosphere and ice caps due to the CO₂ sublimation and condensation process. It results in surface deformations and in time variations of gravity. Large wavelength annual and semi-annual variations of gravity (particularly zonal coefficients ΔJn) have been determined using present day geodetic satellite measurements. However loading deformations have been poorly studied for a planet like Mars. In this paper, we compute these deformations and their effect on spacecraft orbiting around Mars. Loading deformations of terrestrial planet are typically investigated assuming a spherical planet, radially symmetric. The mean radial structure of Mars is not well known. In particular the radius of the liquid or solid core remains not precisely determined. One may then wonder what is the effect of these uncertainties on loading deformations. Moreover, Mars presents a strong topography and probably large lateral variations of crustal thickness (relative to the Earth). The paper answer the questions of what is the effect of such lateral heterogeneities on surface deformations, and is the classical way to calculate loading deformation well adapted for a planet like Mars. In order to answer these questions we have investigated theoretically loading deformations of Mars-like planets. We first investigated classical load Love numbers. We show that for degrees inferior to 10, the load Love numbers mainly depend on the radius of the core and on its state, and that for degree greater than 10, they depend on the mean radius of mantle-crust interface. Using a General Circulation Model (GCM) of atmosphere and ice caps dynamics we show that loading vertical displacements have a 4-5 cm magnitude and present a North-South pattern with periodic transitions. Finally we investigated the effect of lateral variations of the crustal thickness on these loading deformations. We show that thickness heterogeneities perturb the deformations and the time variation of gravity at about 0.5%. However this perturbation on ΔJn is only about 1‰ due to main direct attraction of surface fluid layers. We conclude that lateral variations of crustal thickness are today negligible. However, observation of load Love numbers would bring information on the radial internal structure of the planet, particularly on the core radius. ΔJn study would permit to infer the load Love number , particularly for degree 2 and 3, knowing surface fluid layer dynamics. However load Love numbers are quite small (about 0.05), and despite the present good agreement between GCM and ΔJn observations, will only be estimated in the near future when a slightly better precision in observation and modeling will make it possible to infer these numbers. The investigation of load Love number , which are larger than numbers, would be particularly interesting. It would permit to study degree 1 contribution of atmosphere and ice caps dynamics, which is the most important component of surface fluid dynamics on Mars. Surface displacement measurements would be necessary on a few places near the pole regions, which may be possible in the future, with a project involving precise positioning of a lander on the surface of Mars.     

Synthesis of In‐House Produced Calibrated Silver Phosphate with a Large Range of Oxygen Isotope Compositions

The large range of stable oxygen isotope values of phosphate‐bearing minerals and dissolved phosphate of inorganic or organic origin requires the availability of in‐house produced calibrated silver phosphate of which isotopic ratios must closely bracket those of studied samples. We propose a simple protocol to synthesise Ag₃PO₄ in a wide range of oxygen isotope compositions based on the equilibrium isotopic fractionation factor and the kinetics and temperature of isotopic exchange in the phosphate–water system. Ag₃PO₄ crystals were obtained from KH₂PO₄ that was dissolved in water of known oxygen isotope composition. Isotopic exchange between dissolved phosphate and water took place at a desired and constant temperature into PYREX? tubes that were placed in a high precision oven for defined run‐times. Samples were withdrawn at desired times, quenched in cold water and precipitated as Ag₃PO₄. We provide a calculation sheet that computes the δ¹⁸O of precipitated Ag₃PO₄ as a function of time, temperature and δ¹⁸O of both reactants KH₂PO₄ and H₂O at t = 0. Predicted oxygen isotope compositions of synthesised silver phosphate range from ?7 to +31‰ VSMOW for a temperature range comprised between 110 and 130 °C and a range of water δ¹⁸O from ?20 to +15‰ VSMOW.     

Knowledge Recovery for Continental-Scale Mineral Exploration by Neural Networks

This study is concerned with understanding of the formation of ore deposits (precious and base metals) and contributes to the exploration and discovery of new occurrences using artificial neural networks. From the different digital data sets available in BRGM's GIS Andes (a comprehensive metallogenic continental-scale Geographic Information System) 25 attributes are identified as known factors or potential factors controlling the formation of gold deposits in the Andes Cordillera. Various multilayer perceptrons were applied to discriminate possible ore deposits from barren sites. Subsequently, because artificial neural networks can be used to construct a revised model for knowledge extraction, the optimal brain damage algorithm by LeCun was applied to order the 25 attributes by their relevance to the classification. The approach demonstrates how neural networks can be used efficiently in a practical problem of mineral exploration, where general domain knowledge alone is insufficient to satisfactorily model the potential controls on deposit formation using the available information in continent-scale information systems.     

Selective feeding of the oystercrassostrea gigas fed on a natural microphytobenthos assemblage

In estuarine ecosystems, microphytobenthos resuspended by tidal currents often represents a large part of the food supply available to bivalves which feed selectively. This study investigated the feeding behavior of the Pacific oysterCrassostrea gigas (Thunberg) relative to a natural microphytobenthic assemblage and the effect on the structure of this assemblage. Oysters were fed only benthic microalgae collected on the intertidal mud flats of Bourgneuf Bay (France) at a suspended particulate matter concentration above the threshold of pseudofeces production. All species in the assemblage were endemic diatoms characteritic of tidal mud flat environments. Four dominant ones, which were all solitary cells with spear-shaped frustules [Navicula ammophila (Grunow),Navicula rostellata (Kützing),Plagiotropis lepidoptera (Kuntze), andStaurophora amphioxys (Mann)] represented more than 95% of the 16 species involved. Analysis of feeding processes showed that the retention of the four main diatoms was not significantly different, but that two species, the smallest (N. ammophila [22×4 μm]) and the largest (P. lepidoptera [60×15 μm]), were preferentially ingested. The study of post-ingestive selection revealed that these two species were also preferentially digeted, i.e., preferentially directed into the digestive diverticulum, when they passed through the gut ofC. gigas. Cell size and shape did not appear to account for pre-ingestive and post-ingestive selection. The composition of the assemblage was significantly modified by oyster filtration, although the retention rates of the four main species were not significantly different. The composition of microalgae in pseudofeces and feces as a result of pre-ingestive and post-ingestive selection differed from that in seawater.     

Huge landslide blocks in the growth of piton de la fournaise, La réunion, and Kilauea volcano, Hawaii

Piton de la Fournaise, on the island of La Réunion, and Kilauea volcano, on the island of Hawaii, are active, basaltic shield volcanoes growing on the flanks of much larger shield volcanoes in intraplate tectonic environments. Past studies have shown that the average rate of magma production and the chemistry of lavas are quite similar for both volcanoes. We propose a structural similarity — specifically, that periodic displacement of parts of the shields as huge landslide blocks is a common mode of growth. In each instance, the unstable blocks are within a rift-zone-bounded, unbuttressed flank of the shield. At Kilauea, well-documented landslide blocks form relatively surficial parts of a much larger rift-zone-bounded block; scarps of the Hilina fault system mark the headwalls of the active blocks. At Fournaise, Hilina-like slump blocks are also present along the unbuttressed east coast of the volcano. In addition, however, the existence of a set of faults nested around the present caldera and northeast and southeast rift zones suggests that past chapters in the history of Fournaise included the slumping of entire rift-zone-bounded blocks themselves. These nested faults become younger to the east southeast and apparently record one of the effects of a migration of the focus of volcanism in that direction. Repeated dilation along the present set of northeast and southeast rift zones, most recently exemplified by an eruption in 1977, suggests that the past history of rift-zone-bounded slumping will eventually be repeated. The record provided by the succession of slump blocks on Fournaise is apparently at a relatively detailed part of a migration of magmatic focus that has advanced at least 30 km to the east-southeast from neighboring Piton des Neiges, an extinct Pliocene to Pleistocene volcano.     

Multiple jobholding farmers in agricultural policy

GeoJournal - The attitudes of public authorities as regards multiple jobholding farmers are various, and sometimes opposite. A study of multiple jobholding in farming, conducted by OECD for...     

Comparison of the anisotropic behaviour of undeformed sandstones under dry and saturated conditions

This article presents a systematic analysis of the anisotropic behaviours of the Bentheim and Rothbach sandstones using ultrasonic P-wave velocity, electrical conductivity and magnetic susceptibility measurements. For each sandstone, the data were obtained from three core samples drilled perpendicularly to each other and tested in dry- and water-saturated conditions. For acoustic and magnetic investigations, the same statistical analysis was applied in order to present the data on comparable stereoplots. Surprisingly, the Bentheim sandstone which appeared homogeneous at macroscopic scale showed a stronger elastic and electrical anisotropy than the Rothbach sandstone in which cross-laminations were clearly identified, as confirmed by a sedimentary magnetic fabric. A discussion on the velocity contrasts between dry and saturated samples led us to consider two different origins for the observed anisotropies. First, by comparing electrical and acoustic properties in the Bentheim sandstone, we conclude that the nature of the anisotropic behaviour is linked to the anisotropy of pore shape: the inclusion model developed by Kachanov (Kachanov, M., 1993. Elastic solids with many cracks and related problems. Advances in Applied Mechanics, vol. 30. Academic Press, Boston, MA, pp. 259–445) accounts for our observations if one considers that the pore space is made of parallel flat pores with moderate pore aspect ratio. Second, acoustic, electrical and magnetic properties indicate that the observed anisotropy in the Rothbach sandstone can be attributed to the matrix, and more specifically to cementation: we modified the Dvorkin and Nur (Geophysics 61 (5) (1996) 1363) model of cemented granular media by introducing a spatially variable contact length, and the model suggests that a very small variability of cemented contact length is enough to account for the observed P-wave velocity anisotropy. We emphasise the fact that combining several kinds of measurements is of great help in capturing the nature of the anisotropic behaviour of porous rocks.