1,721,144 research outputs found
Traces of life on Mars are likely to be very small and very challenging to find
No full textThe volcanic materials at the surface of Mars provide an ideal potential habitat for chemolithotrophic microorganisms that obtain their energy from chemical reactions at the surfaces of minerals and their carbon from inorganic sources in the environment. If life ever appeared on Mars, hydrated environments in the Noachian period could have hosted such life forms. Other types of microorganisms that could possibly have lived at the surface during the Noachian include heterotrophic microorganisms that obtain their carbon source form organic carbon (either the degraded remains of pre-existing organisms or abiogenic sources). The rapid degradation in the environmental conditions at the surface of Mars imply an initially frozen and then dry surface that is unfavourable to the development of more sophisticated microorganisms, such as photosynthesisers. Whereas the latter produce readily recognisable macroscopic to microscopic structures, such as stromatolites and microbial mats, the former do not and their traces are very subtle and challenging to reveal. Fossilised chemolithotrophic microorganisms in littoral volcanic sands from the Early Archaean epoch (the Kitty’s Gap Chert in the Pilbara of Australia, c. 3.45 Ga) represent ideal analogues for eventual Noachian life in martian shallow water sediments. Surficial materials from this period, e.g. sediments and igneous rocks,
have been largely silicified because of high seawater silica saturation and extensive hydrothermal flushing of the environments and the Kitty’s Gap sediments as well as their microorganisms have been silicified. Investigations of the morphological and geochemical biosignatures of the early Archaean microfossils provide valuable information about the methods necessary to identify the fossils. The bulk carbon contents of the Kitty’s Gap Chert is < 0.01% and carbon isotope signatures range from – 25.9 to – 27.8 % . The microfossils are small, dividing coccoids 0.4-0.8 m in diameter that occur in colonial associations of many hundreds of individuals, the colonies reaching sizes of several tens of m in diameter. Because these sediments have been silicified, specific sample preparation involving delicate acid etching is necessary to reveal the microfossils. Although there may be other modes of preservation of microorganisms on Mars, the recent revelations of amorphous hydrothermal silica suggest that Noachian volcanic lithologies may have been silicified, as in the Early Archaean. Future in situ martian missions, such as MSL and ExoMars, carry microscopes but they will not be able to reveal the presence of individual microfossils like the chemolithotrophs of the Kitty’s Gap Chert and probably not even the colonies given the sample preparation necessary (the resolution of the MSL microscope is 12m and that of ExoMars is 4 m). However, Raman spectroscopy could identify carbonaceous phases and GC-MS could provide details of the organic composition, even at low bulk C concentrations. On the other hand, organic molecules in the Early Archaean cherts are very degraded and have generally lost their biogenic specificity. It may therefore not be possible to unambiguously identify martian biosignatures using in situ instrumentation and returning samples to Earth for detailed analysis in a terrestrial laboratory will probably be necessary
Ferruginous biolaminations within the pre-Hirnantian (Late Ordovician) of the Carnic Alps, Austria
Full text linkWell preserved laminated structures occur within the Upper Ordovician of the Cellon section in the Carnic Alps (Austria), a world-famous reference section for Silurian conodont biostratigraphy. Microfacies from the Upper Ordovician Uqua Formation were characterised by using optical and scanning electron microscopy (SEM), an environmental scanning electron microscopy coupled with microanalyses (SEM/ESEM-EDX) and a confocal laser Raman microscopy. Ferruginous laminated structures overgrowing specific skeletal fragments occur in the lower part of the studied unit in the form of finely red-to greenish coatings composed of chamosite and goethite alternating with calcite bands. Laminae have arborescent to dendrolitic morphologies. Such morphologies suggest a biomediated genesis and the existence of a potential microbial factory acting in a nearby location from which coated material was later redeposited. These ferruginous coatings around organisms are not documented within the latest Ordovician Plöcken Formation at Cellon or in the coeval Wolayer Formation elsewhere.Well preserved laminated structures occur within the Upper Ordovician of the Cellon section in the Carnic Alps (Austria), a world-famous reference section for Silurian conodont biostratigraphy. Microfacies from the Upper Ordovician Uqua Formation were characterised by using optical and scanning electron microscopy (SEM), an environmental scanning electron microscopy coupled with microanalyses (SEM/ESEM-EDX) and a confocal laser Raman microscopy. Ferruginous laminated structures overgrowing specific skeletal fragments occur in the lower part of the studied unit in the form of finely red-to greenish coatings composed of chamosite and goethite alternating with calcite bands. Laminae have arborescent to dendrolitic morphologies. Such morphologies suggest a biomediated genesis and the existence of a potential microbial factory acting in a nearby location from which coated material was later redeposited. These ferruginous coatings around organisms are not documented within the latest Ordovician Plöcken Formation at Cellon or in the coeval Wolayer Formation elsewhere
ANCIENT BIOSIGNATURES IN ROCKS AND THEIR RELEVANCE IN THE SEARCH FOR EXTRATERRESTRIAL LIFE
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Astrobiological implications of microbes in basaltic pillow lava crusts: a case study from recent(bio-)alteration rims, Coral Patch Seamount, Atlantic Ocean
No full textAstrobiological implications of microbes in basaltic pillow lava crusts (Atlantic Ocean)
No full textVariations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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