284 research outputs found
Shatter cones : nature and genesis
No full textShatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism and has also been produced in the laboratory in several shock experiments. The occurrence of shatter cones is the only accepted meso-to macroscopic recognition criterion for impact structures. Shatter cones exhibit a number of geometric characteristics (orientation, apical angles, striation angles, sizes) that can be best described as varied, from case to case. Possible links between geometric properties with impact or crater parameters have remained controversial and the lack of understanding of the mechanism of formation of shatter cones does not offer a physical framework to discuss or understand them. A database of shatter cone occurrences has been produced for this introduction paper to the special issue of Meteoritics and Planetary Science on shatter cones. Distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometers in diameter, with a few, currently questionable exceptions. All pertinent hypotheses of formation are presented and discussed. Several may be discarded in light of the most recent observations. The branching fracture mechanism and the interference models proposed, respectively, by Sagy et al. (2002) and Baratoux and Melosh (2003) require further evaluation. New observations, experiments, or theoretical considerations presented in this special issue promise an important step forward, based on a renewed effort to resolve the enigmatic origin of these important features
Shatter cones : nature and genesis
No full textShatter cones are curved fractures decorated with divergent striations that are exclusively associated with impact metamorphism. The terminology "cone" was chosen from the observation of complete or near-complete, roundish, axisymmetric objects with a well-defined apex/apical area. It also imprecisely extends to the curved or subplanar fractures decorated with striations commonly observed at impact sites. The geometry of these objects is therefore highly variable but its significance was never addressed due to the lack of appropriate data. Here, we apply two methods to derive shape models of shatter cones. The first one is based on images acquired using a commercial camera and may be applied on large samples in the field. The second one uses an articulated arm equipped with a digital laser scanner and produces high-resolution and precision shape models of hand-sized samples. The analysis of 20 shape models of shatter cones from nine different impact sites indicates that the surface of shatter cones may be described by quadric surfaces and are generally consistent with hyperboloids, whereas occurrence of paraboloid objects cannot be ruled out. The surface characteristics are generally not consistent with the mathematical definition of a cone. The value of these shape models to discriminate between the different hypotheses of formation of shatter is still limited, as it remains to be resolved which type of surface pertains to which hypothesis. This requires theoretical developments, and experimental or numerical simulations of the propagation of tensile fractures associated with shock waves
Chemistry of the Surface of Mars
No full textConcentration of 11 major elements at the surface of Mars (SiO2, TiO2, Al2O3, FeO,MnO,MgO, CaO, Na2O, K2O, P2O5 and Cr23O3) following the approach presented in the following article :
Baratoux, D., H. Samuel, C. Michaut, M. J. Toplis, M. Monnereau,
M. Wieczorek, R. Garcia, and
K. Kurita (2014), Petrological constraints on the density of the Martian crust, J. Geophys. Res. Planets, 119, 1707-1727, doi:10.1002/2014JE004642
Shatter cones : nature and genesis
No full textAssociations between impact structures and meteorite occurrences are rare and restricted to very young structures. Meteorite fragments are often disrupted in the atmosphere, and in most cases, meteorite falls that have been decelerated by atmospheric drag do not form a crater. Furthermore, meteorites are rapidly weathered. In this context, the finding of shatter cones in Jurassic marly limestone in the same location as a recent (105 +/- 40 ka) iron meteorite fall near the village of Agoudal (High Atlas Mountains, Morocco) is enigmatic. The shatter cones are the only piece of evidence of a meteorite impact in the area. The overlap of a meteorite strewn field with the area of occurrence of shatter cones led previous researchers to consider that the meteorite fall was responsible for the formation of shatter cones in the context of formation of one or several small (<100 m) impact craters that had since been eroded. Shatter cones are generally not reported in association with subkilometer-diameter impact craters. Here, we present new field observations and an analysis of the distribution and characteristics of shatter cones, breccia, and meteorites in the Agoudal area. Evidence for local deformation not related to the structural High Atlas tectonics has been observed, such as a vertical to overturned stratum trending N150-N160. New outcrops with exposures of shatter cones are reported and extend the previously known area of occurrence. The area of in situ shatter cones (similar to 0.15 km(2)) and the strewn field of meteorites are distinct, although they show some overlap. The alleged impact breccia is revealed as calcrete formations. No evidence for a genetic relationship between the shatter cones and the meteorites can be inferred from field observations. The extent of the area where in situ shatter cones and macrodeformation not corresponding to Atlas tectonic deformation are observed suggest that the original diameter of an impact structure could have been between at least 1-3 km. For typical erosion rates in the Atlas region (similar to 0.08 cm yr(-1)), the period of time required for the erosion of such a structure (1.25-3.75 Ma) is much larger than the age of the meteorite fall. This line of reasoning excludes a genetic link between the shatter cones and the meteorite fall and indicates that the observed shatter cones belong to an ancient impact structure that has been almost entirely eroded
Asymmetric thermal evolution of the Moon
No full text[1] The Moon possesses a clear dichotomy in geological processes between the nearside and farside hemispheres. The most pronounced expressions of this dichotomy are the strong concentration of radioactive heat sources on the nearside in a region known as the Procellarum KREEP Terrane (PKT) and the mare basaltic lava flows that erupted in or adjacent to this terrane. We model the thermochemical evolution of the Moon using a 3-D spherical thermochemical convection code in order to assess the consequences of a layer enriched in heat sources below the PKT on the Moon's global evolution. We find that in addition to localizing most of the melt production on the nearside, such an enriched concentration of heat sources in the PKT crust has an influence down to the core-mantle boundary and leaves a present-day temperature anomaly within the nearside mantle. Moderate gravitational and topographic anomalies that are predicted in the PKT, but not observed, may be masked either by crustal thinning or gravitational anomalies from dense material in the underlying mantle. Our models also predict crystallization of an inner core for sulfur concentrations less than 6 wt %
Maké sopky na Marsu: obrazová analýza, numerické modelování a srovnání s pozemskými analogy
No full textSmall-scale volcanoes represent diverse group of landforms which vary in morphology, morphometry, and mechanisms of their formation. They are the most common volcanic form on Earth, and their existence and basic characteristics were also predicted for Mars. Availability of high-resolution image data now allows to search, identify and interpret such small volcanic features on the martian surface. This thesis extends our knowledge about the small-scale volcanoes with the following objectives: (a) to document the existence of martian analogues to some of the terrestrial volcanoes, in particular scoria cones, tuff cones, tuff rings and lava domes; (b) to establish their morphological and morphometrical parameters; and (c) to examine the effect of environmental factors, which differ on Earth and Mars, on the mechanisms of formation of the scoria cones. Interpretation of remote sensing images and digital elevation models reveals that scoria cones, tuff rings and cones, and lava domes exist on different parts of the martian surface and, in some cases, far away from previously well-known volcanic provinces. Scoria cones have been identified in the volcanic field Ulysses Colles situated within the Tharsis volcanic province; tuff cones and tuff rings have been found in the Nephenthes/Amenthes region at the...Malé sopky představují různorodou skupinu povrchových těles, které se od sebe liší morfologií, morfometrií, ale i mechanismem svého vzniku. Na Zemi představují tyto malé sopky nejrozšířenější druh sopečných těles a jejich existence byla předpovězena i pro Mars. Dostupnost snímků ve vysokém rozlišení nyní umožňuje tyto malé sopky hledat, identifikovat a interpretovat jejich přítomnost na povrchu této planety. Tato dizertace se právě na tato malá sopečná tělesa zaměřuje a to ve snaze a) zdokumentovat existenci některých druhů malých sopek na povrchu Marsu, konkrétně sypaných kuželů, tufových kuželů, tufových prstenců a lávových dómů; b) určit jejich základní morfologické a morfometrické parametry a c) prozkoumat vliv rozdílného prostředí panujícího na povrchu Marsu a Země na příkladu mechanismu vzniku a vývoje sypaných kuželů. Interpretace satelitních snímků a topografických dat odhalila, že se na povrchu Marsu vyskytují sypané kužele, tufové kužele, tufové prstence a lávové dómy - v některých případech ležících daleko od dobře známých sopečných provincií. Sypané kužele byly popsány v rámci sopečného pole Ulysses Colles ležícího v Tharsis; tufové kužele a tufové prstence byly objeveny v oblasti Nephenthes/Amenthes, která se rozkládá na jižním okraji prastaré impaktní pánve Utopia. Dále byly popsány v...Institute of Petrology and Structural GeologyÚstav petrologie a strukturní geologieFaculty of SciencePřírodovědecká fakult
Magmatic intrusions and deglaciation at mid-latitude in the northern plains of Mars
No full textInternational audienceOn Earth, igneous rocks are generally affected by magmatic differentiation and the volume of intrusive bodies is considered to be 5-10 times larger than the volume of volcanic rocks. In contrast, in the case of Mars, the occurrence of olivine-phyric basalts at Gusev crater (McSween, H.Y. [2006]. J. Geophys. Res. (Planets) 111(E10), E02S10), the primitive nature of at least four picritic shergottites (Grott, M. [2013]. Planet. Space Sci. 174, 49-111) and the chemical composition of major volcanic provinces (Baratoux, D., Toplis, M.J., Monnereau, M., Gasnault, O. [2011]. Nature 472, 338-341) suggest that primary melts of the mantle may have commonly reached the surface. These observations, together with sparse morphologic evidence of intrusions in comparison with other terrestrial planets, raise the issue of the conditions for the formation and occurrence of sub-surface zones of magma storage on Mars. In this respect, a series of two types of young (Amazonian) domes located at mid-latitudes in western Arcadia Planitia, East of Phlegra Montes, previously interpreted as surface expressions of intrusive bodies (Farrand, W.H., Lane, M.D., Edwards, B.R., Yingst, R.A. [2011]. Icarus 211, 139-156), are examined here. A fraction of the domes consist of upraised plain material, whereas others, distinguished by a massive core and a large aspect ratio, could be partially exhumed intrusions or extrusions of viscous differentiated magma. The latter interpretation is however not compatible with the mafic compositions inferred from orbital visible and near infrared spectroscopic observations. The thicknesses and radii of both types of domes appear to be related through a power-law relationship with exponents close to 1 or 5/4. These exponent values are not compatible with extrusive morphologies but are characteristic of intrusions magma below an elastic layer (Michaut, C. [2011]. J. Geophys. Res. 10.1029/2010JB008108). We conclude that all types of domes are intrusive bodies. The large aspect ratios of some of these features may be reconciled with a mafic composition and a shallow emplacement (a few hundred meters) as well as a relatively large injection rate. Magma emplacement below an ice-rich horizon that was subsequently removed offers a plausible scenario for their partial exhumation. This scenario is supported by independent signs of ice-removal and deflation events in the northern plains. The formation of these intrusions appear therefore to be related to an unusual context of magma ascent below a thick and low-density ice-rich horizon
Overturn and evolution of a crystallized magma ocean: a numerical parameter study for Mars
No full text[1] Early in the history of terrestrial planets, the fractional crystallization of a magma ocean can lead to a mantle stratification characterized by a progressive enrichment in heavy elements from the core-mantle boundary to the surface. Such configuration is gravitationally unstable; it causes mantle overturn and the formation of a stable chemical layering. Using simulations of thermo-chemical convection, we analyzed the consequences of overturn and subsequent layering on mantle dynamics assuming Mars' scaling parameters. We found that the time needed to achieve chemical homogenization via convective mixing scales exponentially with the buoyancy-ratio inline image, which measures the relative importance of chemical to thermal buoyancy. In addition, when using a strongly temperature-dependent viscosity, the formation of a stagnant-lid prevents the uppermost crystallized layers from sinking into the mantle. In order to obtain their subduction an yielding mechanism must be invoked.
[2] In the context of Mars' evolution, our results suggest that complete chemical mixing is unlikely to take place within time-scales comparable with the planet's age. Magma ocean freezing could be thus responsible for the long-term preservation of compositional heterogeneities as required by meteoritic evidence. The lack of a surface highly enriched in incompatible elements and of a high-density lid is difficult to reconcile with a stagnant-lid regime operating throughout Mars' history. An episode of surface mobilization induced by compositional overturn can resolve this difficulty provided that inline image is large enough. Too large buoyancy ratios, however, tend to suppress convective heat transport, rendering it problematic to explain the late volcanic history of Mars
Observation from the ground and analysis of emissions associated with meteoroides falling on the moon
No full textLes météoroïdes, issus de petits corps du système solaire produisent des phénomènes lumineux (flashs d'impact) lorsqu'ils percutent le sol de la Lune. Ces événements lumineux transitoires sont observables depuis le sol en utilisant des télescopes de taille moyenne (typiquement > 200 mm de diamètre) à l'aide des caméras CCD rapides de haute sensibilité. Dans cette thèse, nous décrivons la configuration instrumentale, la procédure d'observation et d'analyse qui a été implémentée à l'Observatoire Universitaire de l'Oukaimden pour l'observation régulière des flashs lunaires. Les premiers impacts lunaires observés et confirmés depuis un observatoire situé en Afrique et dans le monde arabe sont analysés dans ce manuscrit. Nous discutons les caractéristiques de cinq flashs et les paramètres physiques attribués aux impacteurs associés. Nous présentons une première estimation du flux d'impact à l'issue de cette phase de surveillance des impacts lunaires depuis nos observatoires. Nous présentons également le développement et le test d'une stratégie pour déterminer avec précision les coordonnées des impacts observés depuis la Terre. Cette précision de séléno-localisation pourra être utilisée au profit de futures missions sismologiques qui utiliseront les impacts météoritiques pour explorer l'intérieur lunaire.Meteoroids, coming from small bodies of the solar system, produce luminous phenomena (impact flashes) when they strike the lunar surface. These transient light events are observable from the ground using medium-sized telescopes (typically> 200 mm in diameter) by using high-speed CCD cameras. In this thesis, we describe the instrumental configuration, observation and analysis procedure that was implemented at the Oukaimden Observatory for regular observation of lunar flashes. The first lunar impacts observed and confirmed from an observatory located in Africa and the Arab world are analyzed in this manuscript. We discuss the characteristics of five flashes and the physical parameters attributed to the associated impactors. We present an initial estimate of the impact flux from this first phase of monitoring lunar impacts from our observatories. We also present the development and testing of a strategy to accurately determine the coordinates of impacts observed from Earth. This precision of seleno-localization can be used for future seismological missions that will use meteoroid impacts to explore the lunar interior
Small-scale volcanoes on Mars: image analysis, numerical modeling and comparison with terrestrial analogs
No full textSmall-scale volcanoes represent diverse group of landforms which vary in morphology, morphometry, and mechanisms of their formation. They are the most common volcanic form on Earth, and their existence and basic characteristics were also predicted for Mars. Availability of high-resolution image data now allows to search, identify and interpret such small volcanic features on the martian surface. This thesis extends our knowledge about the small-scale volcanoes with the following objectives: (a) to document the existence of martian analogues to some of the terrestrial volcanoes, in particular scoria cones, tuff cones, tuff rings and lava domes; (b) to establish their morphological and morphometrical parameters; and (c) to examine the effect of environmental factors, which differ on Earth and Mars, on the mechanisms of formation of the scoria cones. Interpretation of remote sensing images and digital elevation models reveals that scoria cones, tuff rings and cones, and lava domes exist on different parts of the martian surface and, in some cases, far away from previously well-known volcanic provinces. Scoria cones have been identified in the volcanic field Ulysses Colles situated within the Tharsis volcanic province; tuff cones and tuff rings have been found in the Nephenthes/Amenthes region at the..
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