Comets, often referred to as cosmic time capsules, serve as invaluable repositories of information from the nascent phases of our solar system. Varying significantly in size, with nuclei ranging from a few kilometers to tens of kilometers in diameter, these celestial bodies are complex, porous aggregates of organic molecules, silicate particles, and entrapped volatile gases. Their orbits, which can be categorized into the Main Belt, the Kuiper Belt, and the Oort Cloud, offer distinct insights into their origins and the early conditions of the solar system. Understanding the physical processes occurring within these nuclei is critical, particularly in the context of comet outbursts—sudden increases in brightness accompanied by the release of gas and dust. These outbursts are the consequence of intricate internal mechanisms triggered when the comet approaches the Sun, leading to the sublimation of ice and subsequent gas production. Existing theories attribute outbursts to a buildup of internal stress, often facilitated by thermodynamic factors, such as temperature and pressure gradients, or mechanical factors, such as changes in angular momentum. However, one of the least understood aspects of these celestial bodies is the interaction of heat energy with their porous structure. This study aims to shed light on this very phenomenon, focusing on how heat energy from the Sun penetrates the surface of the comet and diffuses into its sub-layers, subsequently impacting phase transitions, gas production, and ultimately, the formation of outbursts. To accomplish this, we employ a multidisciplinary approach that combines thermodynamics, heat transfer equations, and computational modeling. We introduce a novel pore network model based on percolation theory to simulate the behavior of gas within the comet’s porous structure, allowing us to probe the intricate dynamics of gas movement and pressure build-up. Our work is validated against observational data, specifically from the European Space Agency’s Rosetta mission to Comet 67P/Churyumov-Gerasimenko. Our models have yielded preliminary results that emphasize the role of the formation of a first cluster in the porous network as a critical point for outburst occurrence. Particularly for comets approaching the perihelion position, the internal pressure and temperature dynamics become increasingly complex, and our findings contribute to a nuanced understanding of these dynamics. These insights not only advance our understanding of the comet nucleus but also offer a robust theoretical framework for investigating similar phenomena in other celestial bodies.

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On may 1, 1996, Ulysses spacecraft crossed the ion tail of comet Hyakutake, revealing an ion tail length of more than 3 times the Sun-Earth distance. The signatures of an ion tail, especially the ion tail far from the nucleus, are not well explored and many question marks remain. This report summarizes previous observations of spacecraft - ion tail crossings and what signatures that can be expected, as well as signatures of other known solar wind structures. A data analysis is made of possible ion tail encounters from Rosetta spacecraft measurements, Ulysses spacecraft measurements and Earth-orbiting spacecraft measurements. A search from Venus Express data to detect ion tails of sungrazing comets is presented.

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It is generally assumed that the temperature in the inner part of the primordial nebula was too high to form and agglomerate ice bodies. Yet, water is present in most of the terrestrial planets of our solar system. To investigate the reason for this we concentrate on comets as a potential water deliverer to the inner planets. The data for this work is taken from 3D simulations of comet trajectories inside a gas-dust disk with embedded planet embryos. We have developed a MATLAB code that could read data files from these simulations containing orbital elements of the comets and planetary embryos. Comets experience inward migration due to friction with the remains of the disk. We study comets of four different sizes.      We find that smaller comets migrate faster and therefore spend less time in the inner solar system. Smaller comets are numerous and are therefore destroyed in greater numbers than the larger comets. Larger comets on the other hand slow their migration and spending much more time among inner planets they are more massive and produce more massive collisions with the inner planets than the smaller comets. Therefore are larger comets a more promising source of water delivery to the inner planets. In Studying individual collisions we observe that a typical interaction involves regular close encounters with the same embryo during periods of 100000 years or more before the two orbits intersect. This also tells us that with a large number of comets expected at this stage of solar system evolution the collective effect of comets on low-mass planetary embryos may be significant.

Thesis (Ph. D.)--University of Chicago, Dept. of Astronomy and Astrophysics, March 2000. / Includes bibliographical references. Also available on the Internet.

This study addressed the problem of water vapor production and distribution in comet P/Halley, based upon interpretation of observational data obtained during the recent 1985-86 apparition. The data was acquired using the Lunar and Planetary Laboratory charge-coupled device (CCD) on the 154-cm Catalina telescope of the University of Arizona Observatories. Our data acquisition system was employed in two modes. The long-slit (∼200") spectroscopy mode covered the wavelength range 5200-10400 Å at a spectral resolution ∼14 Å. The narrow band filter imaging mode allowed two-dimensional mapping of selected cometary emission features, as well as the continuum, with a field of view of roughly 10 arc-min. Both neutral and ionized (H₂O⁺) water species were studied, with emphasis on the ion distribution. This involved comparing long-slit spatial profiles obtained ∼UT 1986 March 05.5, as well as cuts across filter images (∼ March 06.5) centered on the H₂O⁺ 0,8,0-band emission, with the Vega-1 spacecraft in situ ion density measurements (∼ March 06.3). Our March 05 spectroscopic data revealed a central dip, of order 30% relative to the profile peak, in H₂O⁺ column density in the inner coma (inside ∼ 2 x 10⁴ km from the nucleus), which filled in farther tailward. Similarly the BD - 3 plasma detector aboard Vega-1 measured a decrease in local ion density, of roughly 60% at the closest approach distance (∼ 9000 km sunward of the nucleus), relative to the inbound maximum density at R ≃ 12000 km from the nucleus. These results suggest a bimodal flow of ions out of the coma and/or an extended region over which the H₂0 molecules were being ionized. Our imaging data showed that, while the falloff in ion density was relatively rapid sunward of the nucleus, it was much more gradual in the anti-solar direction. This is due to the solar wind sweeping ions from the head of the comet into the plasma tail, whose width was of order 10⁵ km in the inner coma, diverging slowly and breaking up into a ray pattern farther tailward. The distribution of neutral water was mapped out using the [O I] λ6300 emission as diagnostic probe. In contrast to the ions, the H₂0 molecules were mainly confined to the inner few x 10⁴ km of the coma, and exhibited a much more symmetrical distribution. Integration of the [O I] slit profiles, assuming azimuthal symmetry, allowed calculation of the H₂0 production rate, which ranged from ∼ 10²⁸ molecules s⁻¹, when Halley was at a distance r≳ 2 AU from the sun, to a value of ∼ 1.5 x 10³⁰ molecules s⁻¹ for 1986 March 05 (r ≃ 0.78 AU). Using the latter production rate, and assuming a 100/1 production ratio of H₂0/ H₂O⁺, a spatially-averaged, tailward flow speed of ions out of the inner coma, < v⁺ > ≃ 16 km s⁻¹, was derived by integrating our March 05 H₂O⁺ profile, for which the slit was oriented across the coma, just tailward of the nucleus.

Les substances génotoxiques ont la capacité d'altérer l'intégrité de l'ADN, support biochimique de l'hérédité et présentent un risque mutagène et tumorigène avéré pour de nombreuses espèces. Dans le milieu aquatique, les invertébrés sont d'une importance capitale pour le bon fonctionnement de l'écosystème, mais très peu d'outils sont disponibles pour évaluer la génotoxicité chez ces espèces. Pourtant, les nouvelles obligations réglementaires telles que la Directive Cadre Européenne acceptée en 2000, vont de pair avec le développement d'outils sensibles, fiables et précoces à même de proposer un diagnostic robuste de la qualité biologique des écosystèmes. Dans ce contexte, ce travail de thèse a porté sur le développement, la validation, l'interprétation et l'application au milieu naturel d'un biomarqueur de génotoxicité chez une espèce sentinelle du milieu dulçaquicole, le gammarus fossarum. Ces travaux sont à notre connaissance la première évaluation de la génotoxicité chez un crustacé d'eau douce d'intérêt écologique, par le test Comet. L'aspect novateur de ce travail repose sur la mise en évidence d'une relation entre endommagement de l'ADN des spermatozoïdes et succès reproducteur. En s'appuyant sur un protocole standard, nous avons établi une valeur de référence tenant compte de la variabilité spatio-temporelle de la réponse du biomarqueur, ainsi qu'un seuil de dommages à l'ADN au delà duquel des effets significatifs sur la descendance sont observés. Suite aux résultats obtenus sur le terrain par des méthodes d'encagement, l'intérêt d'utiliser ce biomarqueur à vocation écologique dans un cadre d'évaluation du risque environnemental est confirmé / Genotoxicants can induce deleterious changes in DNA, the biochemical carrier of inheritance. However, aquatic invertebrates such as crustaceans still lack genotoxicity biomarkers although these species play an essential role in ecosystems. The European Water Directive Policy (2000) is aimed at a holistic approach to towards integrated water protection, in order to improve the status of aquatic ecosystems. To this end, one approach is to develop and validate biomarkers in ecologically relevant species. In this perspective, the objectives of the present work were to develop, to validate, to study the ecological significance of a genotoxicity biomarker in the freshwater species gammarus fossarum and finally to assess it in the field. This is to our knowledge the first genotoxicity assessment in an ecologically relevant freshwater crustacean, using the comet assay. The novelty of our approach was to evaluate the consequences of the sperm DNA damage on reproduction, a crucial biological function. Taking into account biotic and abiotic factors, a robust reference value in gammarus fossarum spermatozoa DNA damage has been established. Then, a relationship bridging the biomarker response measured in spermatozoa and its consequences on reproduction has been shown enabling the establishment of a DNA damage threshold level leading to significant consequences for the reproduction and possibly subsequent population dynamics. Our work clearly demonstrated that DNZ damage in gammarus fossarum spermatozoa is an accurate and relevant biomarker for environmental risk assessment

Dietary factors have been implicated in the risk of head and neck squamous cell carcinoma (HNSCC). Much interest has been placed upon the effects of suspected chemopreventative agents found in fruit and vegetables associated with low HNSCC risk. However, studies investigating specific uptake of these agents have failed to show positive results. The possibility exists that single chemopreventative agents fail to provide the same beneficial effect as the various compounds found in a fruits and vegetables is examined. Curatively treated head and neck squamous cell carcinoma patients ingested Juice Plus, a F&V extract supplement containing multiple chemopreventative agents, for 12 weeks. Lymphocyte samples of participants were collect pre- and post- treatment and examined in pairs. Despite the study currently still blinded, surrogate endpoint biomarkers were evaluated to observer any modification between pre- and post treatment samples. Although a paired t-test showed no significant difference between pre- and post treatment samples on surrogate endpoint biomarkers, there is a significant difference in population distribution between treatment times signifying a modification of the surrogate endpoint biomarkers. The exact nature of this difference is pending due to the blinded status of the study.