1,721,107 research outputs found
Simulation of Europa's water plume and structures related to energetic activities on solar system bodies from satellite images
Get PDFThis PhD thesis focuses on the analysis of different high energetic processes that affect the surface of planets, satellites and minor bodies as well as modify their surrounding environment. Specifically, this work concerns three main topics: (i) the simulation and analysis of one of the most geological energetic process, i.e. impact cratering; (ii) the investigation of the fragmentation processes that could have generated boulders on comet Churyumov-Gerasimenko 67P; (iii) the analysis of a possible transient plume originating from cryovolcanic events on Europa, the Jovian icy satellite, combined with an accurate characterisation of its exospheric background.
The first topic addresses the investigation of the impact formation process through numerical modelling. Shocks code represent the most feasible method for studying impact craters, as they can simulate a large span of conditions beyond the reach of experiments (e.g., velocity, size). The iSALE hydrocode was used to simulate two different impact structures located on Mercury and Mars. On Mercury, the simulation allows to determine the genesis of a particular landform, i.e. a steep-sided cone with associated pyroclastic deposits, which was revealed by images acquired by MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. On Mars, the simulation of the Firsoff crater in Arabia Terra permits a better understanding the subsequent geologic processes that led to crater post modification, defining which rheological structure is more likely in that region. In both cases, we conclude that the numerical modelling of impact process is a powerful tool to improve the comprehension of the Solar System.
The second topic of the thesis has been developed after the Rosetta mission got inserted around the comet Churyumov-Gerasimenko 67P. We investigated the surface of comet Churyumov-Gerasimenko 67P focusing on the possible energetic events that lead to the formation of boulders; i.e. blocks that are ubiquitous on the surface of the comet. Different energetic formation processes were invoked to explain the presence of boulders, such as sublimation, fragmentation, outbursts and gravitational falls. Using images acquired by OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) and CIVA (Comet Infrared and Visible Analyser) cameras on board the spacecraft and the lander respectively, a quantitative analysis of different-size boulders has been performed in order to understand if the same energetic formation processes occur equally on different scales on the comet (m, cm and mm). Specifically, by means of different resolution images, we obtained several size-frequency distribution for: (i) boulders larger than 7 m, (ii) boulders larger than 1 m from higher resolution images used to analyse the Abydos site, the location where Philae is supposed to be, and (iii) pebbles (mm-scale structures) visible on CIVA images.
The third topic is the icy satellite Europa in view of the future ESA/JUICE mission and because of our involvement in JANUS (Jovis, Amorum ac Natorum Undique Scrutator) visible camera. The presence of a subsurface ocean is a primary topic on Europa, in addition the recent discovery of a transient plume at the south pole by HST observations has raised many questions regarding the interaction between the subsurface/surface and the outer environment of Europa in terms of active processes affecting the icy surface. In this context, a possible plume deposit originating from cryovolcanic events was simulated to understand its detectability by JANUS camera during the Europa flyby phase of the JUICE mission. In addition, since the study of transient plumes has as a mandatory prerequisite an accurate characterisation of the exospheric background, a detailed study of the loss rates of Europa's tenuous atmosphere was performed. In particular, loss rates for electron impact dissociation and ionization processes, for charge-exchange (considering plasma torus, pick up and ionosphere ions) and for photo processes (for both cases of quite and active Sun) were calculated
Micrometeoroids flux on the Moon
Get PDFContext. The Moon has a tenuous exosphere consisting of atoms that are ejected from the surface by energetic processes, including hypervelocity micrometeoritic impacts, photon-stimulated desorption by UV radiation, and ion sputtering.
Aims: We calculate the vapor and neutral Na production rates on the Moon caused by impacts of meteoroids in the radius range of 5-100 μm. We considered a previously published dynamical model to compute the flux of meteoroids at the heliocentric distance of the Moon.
Methods: The orbital evolution of dust particles of different sizes is computed with an N-body numerical code. It includes the effects of Poynting-Robertson drag, solar wind drag, and planetary perturbations. The vapor production rate and the number of neutral atoms released in the exosphere of the Moon are computed with a well-established formulation.
Results: The result shows that the neutral Na production rate computed following our model is higher than previous estimates. This difference can be due to the dynamical evolution model that we used to compute the flux and also to the mean velocity, which is 15.3 km s-1 instead of 12.75 km s-1 as reported in literature.
Conclusions: Until now, the micrometeoritic impacts have been considered a negligible source for the release of neutral sodium atoms into the exosphere compared to other mechanisms, but according to our calculations, the contribution may be 8% of the photo-stimulated desorption at the subsolar point, becoming similar in the dawn and dusk regions and dominant on the night side
Asteroidal and cometary dust flux in the inner solar system
Get PDFContext. Meteoroids impacting terrestrial planets at high speed may have different effects. On bodies without atmospheres, such as the Moon and Mercury, they form impact craters and contribute to the gardening process through which the surface material is constantly mixed. The interaction of high-speed meteoroids with the atmosphere of Venus, the Earth, and Mars, may lead to the deposition in the ionosphere of species such as neutral Mg or Fe and their ionized atoms, caused by ablation processes during the entry.
Aims: In this work we estimate and compare the flux and impact speeds onto the planets of the inner solar system by numerically integrating the orbital evolution of putative dust particles of asteroidal and cometary origin.
Methods: The trajectories of dust particles of different sizes are computed with a numerical code that accounts for the gravitational forces due to all planets, the Poynting-Robertson drag and the solar wind drag. The flux of dust grains on each planet is estimated by calibrating the outcome of our model with the flux on the Earth reported previously.
Results: We obtain new estimates of the flux and impact velocities for both asteroidal and cometary dust particles on Venus and Mars. For Venus we find that cometary grains enter the planet atmosphere at higher speeds, possibly contributing to the upper layers, while asteroidal grains would be relevant for the lower layers, possibly leading to a compositional gradient. This effect is also present for Mars, but it is less marked. We also find that analytical predictions, not taking radiative forces into account, of both flux and average impact speed are reliable for Mars but fail for Venus because of the complex dynamical evolution of grains in the inner solar system.
Conclusions: Our results on the velocity distributions and fluxes of micrometeoroids on the terrestrial planets can be used to put stringent contraints on models that estimate either the superficial material mixing that is due to meteoroid impacts or the formation of ionospheric layers for planets with an atmosphere
Abundance and size-frequency distribution of boulders in Linné crater's ejecta (Moon)
Get PDFThis paper presents the abundances and the size-frequency distributions (SFD) of the ejected boulders surrounding the Linnè crater, located on the Moon's Mare Serenitatis basin. By means of Lunar Reconnaissance Orbiter Camera high-resolution images we prepare a context geological map of the Linné crater as well as we identify 12000 boulders ≥4.4 m, with a maximum measured size of 30.8 m. The cumulative number of boulders per km2 is fitted with a power-law curve with index −4.03 + 0.09/-0.10. By studying the radial ejecta abundances, we find that the largest ones are located within the first 2 km from the crater's centre, while few tens of boulders with sizes 8 m are detectable above 5 km from the crater's rim. We find that the Linné proximal ejecta blanket is slightly asymmetrical, as indicated in the geological map too, showing a density increase in the NE-SW direction. This may be the result of an oblique impact emplacement of the original impactor, or it may be explained with a perpendicular impact in the Mare Serenitatis location, but on a surface with lunar basalts with different local mechanical properties. By exploiting our boulders size density as a function of the distance from the crater's centre, we derive a possible regolith thickness at the Linnè impact of ∼4.75 m, supporting similar values based on Earth-based radar and optical data in the Mare Serenitatis basin
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
