Between 2009 and 2013, 16 ice-bearing permafrost cores were collected from 10 polygons along the floor of University Valley (McMurdo Dry Valleys of Antarctica) and were subsequently analysed in order to assess the geochemical properties of the valley’s icy soils and ground ice. Elemental analysis showed that icy soils located in the seasonally non-cryotic zone (NCZ) of the valley contained (on average) twice as much organic carbon (1.19 mg/g) as the ice cemented permafrost soils sampled in its perennially cryotic zone (PCZ). It also showed that nitrogen accumulation in the icy soils was a result of atmospheric fallout and chemical weathering of mineral soils. Isotopic analysis showed that the organic matter contained in the valley’s icy soils are mostly derived from the deposition and burial of cryptoendolithic communities living in the adjacent sandstone valley walls. Dissolved organic carbon (DOC) concentration measures indicated that soils containing the highest amounts of DOC were enriched in 13CDOC relatively to soils with low DOC concentrations. This indicated that microbial activity in soils was the highest during past super interglacial periods. A soil habitability index calculation from Stoker et al. (2010) was used to establish that soils located in the NCZ were more habitable than soils sampled in the PCZ and also presumably more habitable than soils at many Mars landing sites.

It is clear that a planet's ability to support life is intimately associated with its physical evolution, but many aspects of this link have not been resolved. For example, differing geologic histories have the potential to drive large-scale differences in the chemistry of planets’ waters, with unknown implications for habitability. In this thesis, I link the geochemical evolution of Mars to the habitability of its associated evaporitic or brine environments, which have been widespread throughout the planet's history. Their habitability is compared with the Earth system, where a chloride-dominated chemistry permits the microbial colonisation of brines with extremely low water availability. By assessing the physicochemical environments in martian brines, I present evidence that high ionic strength, driven to extremes on Mars by the ubiquitous occurrence of divalent ions, can influence habitability even if water availability is high. The importance of this parameter has been overlooked in terrestrial microbiology, likely due to the paucity of environments with high levels of di- or multivalent ions, and its possible mechanics and significance for defining habitat space on Earth and other planets are discussed. Additionally, cultivation techniques and next-generation DNA sequencing were used to identify organisms capable of growth in extreme Mars-relevant brines, which contrast with those typically found in NaCl-rich brines on the Earth. The isolation of a novel sulfate-tolerant Marinococcus strain, and its growth response to fluctuating martian brine compositions are reported. These results show that microbial growth kinetics are defined not merely by additive ion effects, but rather by bulk physicochemical conditions defined by complete ion assemblages. Changes to composition driven by evaporation or freezing can therefore push a brine into more biologically clement conditions by altering a brine’s physicochemical profile The data herein present a strong case that geochemical context is essential to understanding habitability in extreme saline environments. A new framework for predicting brine habitability is required, taking into account the geochemical history of the brine as well as the effects different ionic compositions exert on microorganisms. This work is a significant contribution across several fields, and emphasises the value of interdisciplinary science in answering questions of planetary habitability. Furthermore, this thesis provides a case study for exploring the impact of planetary-scale geochemical evolution on the ability of a planet to support life.

abstract: As the detection of planets become commonplace around our neighboring stars, scientists can now begin exploring their possible properties and habitability. Using statistical analysis I determine a true range of elemental compositions amongst local stars and how this variation could affect possible planetary systems. Through calculating and analyzing the variation in elemental abundances of nearby stars, the actual range in stellar abundances can be determined using statistical methods. This research emphasizes the diversity of stellar elemental abundances and how that could affect the environment from which planets form. An intrinsic variation has been found to exist for almost all of the elements studied by most abundance-finding groups. Specifically, this research determines abundances for a set of 458 F, G, and K stars from spectroscopic planet hunting surveys for 27 elements, including: C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, La, Ce, Nd, Eu, and Hf. Abundances of the elements in many known exosolar planet host stars are calculated for the purpose investigating new ways to visualize how stellar abundances could affect planetary systems, planetary formation, and mineralogy. I explore the Mg/Si and C/O ratios as well as place these abundances on ternary diagrams with Fe. Lastly, I emphasize the unusual stellar abundance of τ Ceti. τ Ceti is measured to have 5 planets of Super-Earth masses orbiting in near habitable zone distances. Spectroscopic analysis finds that the Mg/Si ratio is extremely high (~2) for this star, which could lead to alterations in planetary properties. τ Ceti's low metallicity and oxygen abundance account for a change in the location of the traditional habitable zone, which helps clarify a new definition of habitable planets. / Dissertation/Thesis / Ph.D. Astrophysics 2014

L’exploration spatiale effectuée par les missions orbitales et in situ a permis de mettre en évidence la grande diversité d’objets planétaires rencontrés dans le Système Solaire. Les propriétés de leurs surfaces et notamment leurs compositions sont d’excellents témoins des différents processus physiques endogènes et exogènes ayant façonné ces différents corps depuis leur formation jusqu’à aujourd’hui. Ma thèse a pour contexte l’exploration spatiale multi-échelle des surfaces de deux types d’objets que sont les astéroïdes de type-C et Mars. Les astéroïdes de type C sont des corps très primitifs et ainsi reconnus pour contraindre les premiers stades d’évolution du Système Solaire. Ils peuvent notamment apporter de nombreux indices concernant la présence de phases altérées et de matière organique lors des phases primordiales du Système Solaire. Quant à Mars, sa surface se révèle fascinante en particulier grâce à la grande variété des minéraux détectés par les différentes sondes spatiales. Ces minéraux sont traceurs de nombreux processus physiques qui ont prévalu à la surface permettant ainsi de mieux comprendre les interactions entre les différentes enveloppes que sont la structure interne, la surface, l’atmosphère et l’environnement spatial de Mars. Dans ce travail de thèse je me suis intéressée à l’étude de la minéralogie des surfaces obtenue par imagerie hyperspectrale, d’une part à l’échelle macroscopique avec l’instrument OMEGA/Mars Express dans le but de quantifier les abondances des minéraux traceurs des roches ignées sur Mars et, d’autre part, à l’échelle microscopique avec le microscope MicrOmega, en préparant les futures investigations de la mission Hayabusa-2 à destination de l’astéroïde de type-C Ryugu grâce à l’étalonnage de l’instrument et la caractérisation d’échantillons en laboratoire.Concernant Mars, les résultats majeurs sont les suivants. Un nouveau produit basé sur le jeu complet de données de l’instrument OMEGA dans le proche infra-rouge a été construit, combinant toutes les observations adaptées à l’étude globale des minéraux. Ce cube 3D global de réflectance de Mars a été utilisé pour produire de nouvelles cartes de détections et obtenir un niveau supplémentaire d’analyse en comparaison aux études passées. Un modèle de transfert radiatif a alors été appliqué à tous les spectres des zones présentant des signatures de minéraux mafiques dans le but de quantifier les abondances de ces minéraux traceurs de l’activité magmatique et volcanique. Les cartes globales de pyroxènes, olivine et plagioclase présentées dans cette thèse représentent les premières cartes de minéralogie modale de Mars à une résolution de ~1.5 km/px. Une méthode de classification a mis en lumière la présence de plusieurs classes minéralogiques variées révélant ainsi une hétérogénéité de la surface à différentes échelles. La composition chimique a ensuite été calculée et comparée avec les mesures orbitales et in situ.Dans le cadre de la mission Hayabusa-2, j’ai exploité les données d’étalonnage du microscope hyperspectral proche infra-rouge MicrOmega développé à l’IAS. La réduction et l’analyse des données a permis la construction de la fonction de transfert 4D (position sur le champ de vue, longueur d’onde et température d’opération) de l’instrument dans la gamme complète des valeurs de ces paramètres fonctionnels. Les performances instrumentales concernant l’aspect détection ont été également validées. Cet étalonnage a aussi mis en évidence l’importance de bien définir les opérations en amont de façon à maximiser le rapport signal sur bruit en fonction des paramètres fonctionnels et ainsi d’interpréter au mieux les données scientifiques qui seront acquises une fois au sol de l’astéroïde. Enfin, j’ai également participé à différentes campagnes de mesures d’échantillons naturels avec MicrOmega révélant la capacité de cet instrument à caractériser des échantillons à l’échelle microscopique (10s µm/px). / Space exploration carried out through orbital and in situ missions enables us to highlight the great diversity of objects found in the Solar System. The properties of planetary surfaces and especially their compositions are excellent witnesses of the various endogenous and exogenous physical processes that have shaped these different bodies from their formation to the present day. My thesis is based on the multi-scale spatial exploration of the surfaces of two types of objects, the C-type asteroids and Mars. C-type asteroids are very primitive bodies and are thus recognized as excellent candidates to constraint the early stages of evolution of the Solar System. In particular, they should provide numerous insights concerning the presence of altered phases and organic matter during the primordial phases of the Solar System. As for Mars, its surface is fascinating especially thanks to the wide variety of minerals detected by various space probes. These minerals are tracers of many physical processes that have prevailed on the surface, allowing us to better understand the interactions between the different envelopes that are the internal structure, the surface, the atmosphere and the space environment of Mars. In the work I carried out during my thesis, I focused on the study of the surface mineralogy obtained by hyperspectral imagery, at a macroscopic scale with the OMEGA/Mars Express instrument for quantifying the abundance of minerals tracing the magmatic and volcanic activities on Mars and, at the microscopic scale, with the microscope MicrOmega, focusing on the calibration of the instrument and the characterization of samples in laboratory, within the framework of the future investigations of the Hayabusa-2 mission to the C-type asteroid Ryugu.Regarding Mars, the major results are the following. A new product based on the entire OMEGA instrument dataset acquired in the near infrared has been constructed, combining all the observations adapted to the study of mineral distribution at the global scale. This global 3D reflectance cube of Mars was used to produce new mineral maps providing a further step of analysis compared to past studies. A radiative transfer model was then applied to all spectra presenting mafic mineral signatures in order to quantify the abundances of minerals tracing the magmatic and volcanic activities. The global maps of pyroxenes, olivine and plagioclase presented in this thesis represent the first maps of modal mineralogy of Mars at a resolution of ~ 1.5 km/px. A classification method has highlighted the presence of several distinct mineralogical classes revealing a certain heterogeneity of the surface at different scales. The chemical composition was then calculated and compared with the orbital and in situ measurements.As part of the preparation of the Hayabusa-2 mission, I exploited the calibration data of the hyperspectral imaging microscope MicrOmega. The data reduction and analysis allowed us to derive the 4D transfer function (position on the field of view, wavelength and operating temperature) of the instrument in the full range of values of its functional parameters. The instrumental performances regarding to the identification were also validated: detection of absorption bands of the order of 1% in reflectance with an accuracy of 5 nm on the position of the band and a quantification of the overall reflectance level of the order of 20%. This calibration also highlighted the fact to carefully prepare in advance the operations so as to maximize the signal-to-noise ratio as a function of the functional and environmental parameters and, thus to interpret as much as possible the scientific data that will be acquired once on the asteroid surface. Finally, I also participated in various campaigns of measurements of natural samples with MicrOmega revealing the ability of this instrument to characterize samples at a microscopic scale (10s μm/px).

The search for life beyond Earth is the driving force behind several future missions to Mars. An essential task in the lead-up to these missions is a critical assessment of the habitability for, and feasibility of, life. However, little research has been conducted on this issue, and our understanding of the plausibility for life on Mars remains unconstrained. Owing to the anoxic and iron-rich nature of Mars, microbial iron reduction (MIR) represents a compelling candidate metabolism to operate in the Martian subsurface, past and present. The objectives of this thesis are to address the feasibility of MIR on Mars by i) better defining the habitability of MIR on Earth, and ii) assessing the range and availability of organic electron donors in the subsurface of Earth and Mars. Samples collected from Mars-relevant environments on Earth were used to initiate MIR enrichment cultures at 4°C, 15°C and 30°C. Results indicate MIR is widespread in riverbed and subglacial sediments but not sediments from desert or recent volcanic plains. The iron-reducing microorganisms in subglacial enrichments are at least psychrotolerant and in some cases psychrophilc. Culture-independent methods highlighted the changes in diversity between temperature conditions for subglacial sediments, and indicated that members of the prolific MIR Geobacteraceae family are common. The genera Geobacter and Desulfosporosinus are responsible for MIR in the majority of enrichments. Long-term anoxia and the availability of redox constituents are the major factors controlling MIR in these environments. A MIR enrichment culture was unable to use shales and kerogens as the sole source of electron donors for MIR, despite the presence of known electron donors. Furthermore, MIR was inhibited by the presence of certain kerogens. The causes of inhibition are unknown, and are likely to be a combination of chemical and physical factors. Experiments were conducted to assess the ability of three pure strains and a MIR enrichment to use non-proteinogenic amino acids common to carbonaceous meteorites as electron donors for MIR. Results demonstrate that γ-aminobutyric acid served as an electron donor for the enrichment culture, but no other amino acids supported MIR by this or other iron-reducing cultures. The D-form of chiral amino acids was found to exert a strong inhibitory effect, which decreased in line with concentration. Theoretical calculations using published meteoritic accretion rates onto the surface of Mars indicate that the build up inhibitory amino acids may place important constrains on habitability over geologic time scales. Contamination of a pure strain of Geobacter metallireducens with a strain of Clostridium revealed a syntrophic relationship between these microorganisms. Anaerobic heterotrophs are likely to play an important role in maintaining an available supply of electron donors for MIR and similar chemoorganic metabolisms operating in the subsurface. This research indicates that MIR remains a feasible metabolism to operate on Mars providing a readily available redox couple is present. However, given the observed inhibition in the presence of bulk carbonaceous material and certain amino acids found in meteorites, the use of extraterrestrial carbonaceous material in the Martian subsurface for microbial iron reduction is questionable, and should be the focus of future research.

Understanding the physical properties as well as the potential for organic material in the Martian near-surface environment can give us a glimpse into the history of the site with regards to water, soil formation processes, as well as the conditions necessary for life. This work is done to support the interpretation of data from the Phoenix Mars Lander as well as other past and future landed missions. The Antarctic Dry Valleys are a hyper-arid cold polar desert that is the most Mars-like place on Earth. Soils from two different soil and climate regimes are analyzed to determine their physical properties such as mineralogy, particle size, shape, color, and specific surface area. These data are used to describe the sample locations in Antarctica and infer properties of Martian soils by comparison to Antarctic sites. I find that the particle size distribution can be used to determine the water history of the site and that the behavior of soluble species in the soil can also be used to trace the movement of water through the soil and could be instructive in understanding how soil organic material is processed by the environment. Continuing with the theme of soil organic matter, we revisit the Viking conclusions with regards to organics on Mars and look at the Phoenix data on the same subject. First, we assume that Mars receives organic material from meteoritic infall. These organics will be processed by chemical oxidants as well as UV light down to 200 nm. Chemical oxidation is predicted to produce molecules such as mellitic acid, which could preserve up to 10% of the original organic mass. Using mellitic acid and other similar organic molecules, we irradiate these molecules with Mars-like ultraviolet light, analyzing the gases that come off as irradiation takes place. We find that organic molecules can survive Mars-like UV conditions as layers of UV-resistant organics build up, shielding the remaining organic material. Additionally, the gas products of irradiation depend on the composition of the original organic molecule, implying that even irradiated molecules will carry some information about the composition of the original molecule. Finally, we take this irradiated organic/soil stimulant mixture and analyze it via pyrolysis, similar to the Viking GC/MS and TEGA instruments that are the only instruments operated on Mars capable of detecting organics. We find that the pyrolysis of mellitic acid (and other similar) molecules primarily produces inorganic fragments but that the reduced carbon fragments released depend on the composition of the original organic. However, the introduction of perchlorate, discovered on Mars by the Phoenix Lander, complicates the issue by creating the conditions for molecular oxidation. The high-oxygen content and high pyrolysis temperatures lead to organic combustion during thermal analysis, meaning that, regardless of the initial composition, most soil organics will be oxidized to CO₂ during the detection process. By assuming that organic material was oxidized to CO₂ in the Phoenix and Viking samples. We show that this assumption gives organic concentrations consistent with meteoritic accumulation rates. This finding reopens the possibility for organic molecules in the near-surface environment at the Viking and Phoenix landing sites.

La recherche de nouvelles formes de vie est une quête passionnante mais quidemande avant tout de comprendre l’origine de l’apparition d’une forme de vie.La seule planète qui abrite la vie à notre connaissance est la Terre. Comprendrepourquoi les autres planètes de notre système solaire n’en abrite pas ou plus estune étude nécessaire pour pouvoir mieux cibler nos cherches de nouvelles vies dansles autres systèmes stellaires. L’objectif de cette thèse est d’apporter des premierséléments de réponse à cette question. Nous nous sommes principalement concentréssur la comparaison d’évolution thermique entre Mars, la Terre et Vénus vers lafin de leur accrétion lors du refroidissement de leur océan de magma. L’évolutionthermique d’océans de magma produits par collision avec des impacteurs géantslors de l’accrétion est supposée dépendre de la composition et de la structure del’atmosphère à travers l’effet de serre du CO2 et H2O relâché par le magma durantsa cristallisation. Afin de contraindre les différentes échelles de temps de refroidissementdu système, nous avons développé un modèle 1-D de convection paramétréd’un océan de magma couplé avec un modèle atmosphérique 1-D radiatif-convectif.Nous avons conduit une étude paramétrique et décris l’influence de plusieurs variablestelles que le contenu initial en volatils, la profondeur initiale de l’océan demagma ou encore la distance planète-soleil. Nos résultats suggèrent que la présenced’une atmosphère de vapeur retarde la fin de la phase d’océan de magma d’environ1 Ma. De plus, nous observons également que la vapeur d’eau condense en un océanaprès 0.1, 1.5 et 10 Ma respectivement pour Mars, la Terre et Vénus. Ce tempsserait virtuellement infini pour une planète de la taille de la Terre située à moins de0.66 ua du soleil. Au regard de ces résultats, nous remarquons que pour la Terre etMars, les échelles de temps de formation d’un océan d’eau sont plus courtes que lagamme de temps entre chaque impacts majeurs. Ceci impliquerait que des océansd’eau successifs peuvent s’être développés durant l’accrétion. En revanche, Vénus,du fait de sa grande proximité avec le seuil de distance au soleil (0.66 ua), pourraitavoir maintenu sa phase d’océan de magma plus longtemps durant l’accrétion.Par la suite, la prise en compte de l’échappement hydrodynamique nous a permisde constater que ce phénomène a très peu d’incidence sur le réservoir global d’eaud’une planète durant la phase d’océan de magma. Cependant, on observe qu’aprèsla condensation de la vapeur d’eau, l’échappement devient de plus en plus efficaceet le réservoir d’eau fini par être totalement évaporé peu de temps avant la fin de lasolidification du manteau. Enfin, nous avons commencé à étudier l’influence d’autresgros impacts durant le refroidissement de l’océan de magma. Les premiers résultatsmontrent que dans le cas de Mars et la Terre, la durée de leur phase d’océan demagma est plus courte que la gamme de temps entre chaque impact majeur. Il en résulte que ces planètes ont dû connaitre une alternance entre phase d’océan demagma et phase d’océan d’eau. Ce phénomène n’a en revanche pas dû avoir lieusur Vénus. En effet, la durée de sa phase d’océan de magma est plus longue que lagamme de temps entre chaque impact majeur. C’est pourquoi, la phase d’océan demagma sur Vénus a dû se prolonger durant toute la phase d’impacts et qu’aucunocéan d’eau n’a pu se former avant la fin de cette période. / The research of new life forms is an exciting quest but requires understandingthe origin of the appearance of a form of life. The only planet that houses life as weknow is the Earth. Understand why the other planets in our solar system do nothouse it, is needed to better target our looking for new lives in other star systems.The objective of this thesis is to provide preliminary answers to this question.We mainly focused on the comparison between thermal evolution of Mars, Earthand Venus to the end of their accretion during their cooling magma ocean. Thethermal evolution of magma oceans produced by collision with giant impactorsduring accretion is expected to depend on the composition and structure of theatmosphere through the greenhouse effect of CO2 and H2O released by the magmaduring its crystallization. In order to constrain the various cooling timescales ofthe system, we developed a 1-D parameterized convection model of a magma oceancoupled with a 1-D radiative-convective model of the atmosphere. We conducted aparametric study and described the influence of several variables such as the initialvolatile inventories, the initial depth of the magma ocean and planet-sun distance.Our results suggest that the presence of a steam atmosphere delays the end ofthe magma ocean phase by about 1 Myr. In addition, we also observe that thewater vapor condenses to an ocean after 0.1, 1.5 and 10 Myr respectively for Mars,Earth and Venus. This time would be virtually infinite for an Earth-sized planetlocated at less 0.66 UA from the sun. In view of these results, we note that for theEarth and Mars, the timescales of the water ocean formation are shorter than timegaps between major impacts. This would imply that successive water oceans mayhave developed during accretion. However, Venus, due to its close proximity to thethreshold distance from the sun (0.66 AU), could have maintained its magma oceanphase longer during accretion. Thereafter, taking into account the hydrodynamicescape permitted us to see that this phenomenon has very little influence on theoverall water tank of a planet during the magma ocean phase. However, we canobserve that after the condensation of the water vapor, the hydrodynamic escapebecomes more efficient and the water tank be completely evaporated shortly beforethe end of the mantle solidification. Finally, we began to study the influence ofother major impacts during the cooling of the magma ocean. The first results showthat in the case of Mars and Earth, the duration of their magma ocean phase isshorter than time gaps between major impacts. In consequently, these planets hadto know an alternation between a phase magma ocean and a ocean water phase. Thisphenomenon does not, however, have taken place on Venus. Indeed, the durationof its magma ocean phase is longer than the time gaps between major impacts.Therefore, the magma ocean phase on Venus had to extend throughout the phaseimpacts and no ocean water has been formed before the end of this period.

O principal objetivo do trabalho foi estimar a possibilidade de sobrevivência de micro-organismos extremófilos na superfície de exoplanetas conhecidos, assim como na superfície de seus eventuais satélites naturais. Foi utilizado um modelo que simula a atmosfera terrestre primordial, composta principalmente por nitrogênio, água e dióxido de carbono. E em se tratando de extremófilos, esses cálculos não foram limitados à Zona Habitável dos sistemas planetários, pois esse conceito foi estendido para uma região mais ampla, a Zona Extremófila, onde a vida pode existir. Extremófilos são micro-organismos terrestres que vivem sob condições extremas de temperatura, nível de radiação, umidade, pressão, salinidade, pH, etc. . Eles são candidatos naturais para habitarem meios ditos extraterrestres onde tais condições são eventualmente encontradas. Alguns exemplos desses ambientes em nosso sistema solar são: Marte, Titã (satélite de Saturno) e Europa (satélite de Júpiter). Há algumas centenas de planetas orbitando outras estrelas (exoplanetas) e a maioria deles são gigantes gasosos, em particular Hot Jupiters. A temperatura superficial de um planeta depende fortemente de seu albedo, de sua distância orbital, de condições geodinâmicas intrínsecas, além do tipo espectral de sua estrela hospedeira. A estimativa dessa temperatura foi obtida considerando o ciclo silicato-carbono e um balanço de energia global, que contribuiram para se obter estimativas da pressão parcial atmosférica devido ao dióxido de carbono e da temperatura média, na superfície dos planetas e/ou de seus satélites hipotéticos. Os eventuais satélites naturais de planetas gigantes podem abrigar vida e essa possibilidade foi testada através da análise das condições de estabilidade orbital desses corpos celestes. Os resultados deste trabalho deverão fornecer subsídios para a hipótese da panspermia. / The main objective of this study is to estimate the chance of survival of microorganisms (extremophiles) on the surface of known exoplanets, as well as on the surface of its potential natural satellites. We used a model that simulates the primordial atmosphere composed by, primarily, nitrogen, water and carbon dioxide. And when it comes to extremophiles, these calculations were not limited to the Habitable Zone of planetary systems, since this concept was extended to a wider region, the Extremophile Zone, where life can exist. Extremophiles are terrestrial microorganisms living under extreme conditions of temperature, light level, humidity, pressure, salinity, pH, etc ... They are natural candidates for living in habitats considered extraterrestrials where such conditions are encountered eventually. Examples of such environments in our solar system are: Mars, Titan (moon of Saturn) and Europe (satellite of Jupiter). There are hundreds of planets orbiting other stars (exoplanets) and most of them are gas giants, particularly Hot Jupiters. The surface temperature of a planet/moon depends heavily on its albedo, its orbital distance, of geodynamic conditions intrinsic, in addition to the spectral type of their host star. The estimate of this temperature was obtained considering the carbon-silicate cycle and a global energy balance, which contributed to obtain estimates of the partial pressure due to atmospheric CO2 and the average temperature on the surface of planets and/or their hypothetical satellites. Natural satellites of giant planets may harbor life, and this possibility was tested by analyzing the conditions of orbital stability of these heavenly bodies. The results of this study should provide support for the hypothesis of panspermia.

Multiple lines of evidence suggest that RNA performed all of the biological functions in the first life forms on earth. These functions included cleavage, ligation, polymerization, recognition, binding, and replication. In order to perform these functions, populations of RNA molecules with unevolved sequences must have been able to fold into compact three dimensional shapes, in unregulated environments, and without the help of proteins. Folding into compact tertiary structures is difficult because of the high charge density of RNA. Consequently, the ranges of temperature, salinity, pH, and pressure that allow RNA to fold into functional shapes is very restricted. We use thermodynamic arguments and Brownian dynamics simulations to compute the range of these environmental parameters that will allow RNA to fold. This is a non-trivial calculation due to the formation of an ion atmosphere around RNA that reduces its electric field. The results can be used to clarify the environments in which the transition to life is possible. Our preliminary calculations suggest that environments with low temperatures ($0-50^\circ C$) and high salt concentrations (greater than 100mM) are the most favorable for unassisted RNA folding and thus the transition to RNA-based life. Applications of our results include determining the environments on early earth where life formed, assesing the habitability of Europa, Titan, and (using modeled parameters) extrasolar planets. / text

In a multi-planet system, gravitational interactions cause orbital eccentricity variations. For non-resonant systems, classical secular theory reveals that the eccentricities are vector sums of contributions from several eigenmodes. Examination of the eigenvectors often reveals subsets of planets that interact especially strongly as dynamical groups. Perturbations from other sources, such as tides, are shared among the planets through the secular interactions. If one planet's eccentricity is tidally damped, all the eigenmodes damp so as to leave a signature on their amplitudes. Therefore, if one desires to include some a priori tidal damping in an orbital fit, solutions should not assume the current eccentricity of that planet to be low, but rather for the eigenmodes that damp quickly to have low amplitude. The tidally perturbed planet may retain a substantial eccentricity, because some eigenmodes will be longer-lived. The secular eigenmodes, including relative damping rates, have been calculated for all 72 non-resonant extra-solar systems with adequate data. Tides also affect evolution of planets' semi-major axes, which is coupled with eccentricity evolution. A planet that, alone, would be quickly circularized so as to not experience much semi-major axis migration, could rapidly be forced into the star in the presence of an outer planet. Also, though such an inner planet may now be gone, the eccentricity of the outer planet could have been damped due to tides that acted on the inner planet. Any inferences about the primordial orbits of observed planets must consider these effects. For systems where the inner planet has not yet reached the star, the planets' eccentricities can be constrained for any particular assumed tidal dissipation factor Q', e.g. for the KOI-543 system, if the inner planet is rocky, the eccentricities must be<0.001. The habitable zone around low-mass stars is close to the star, precisely where tides are important. Low-mass stars are very long lived, and can be very old currently. A habitable planet likely needs tectonics for cycles that regulate the atmosphere, but a planet's internal heat will decay over long timescales. However, an outer planet could maintain the inner planet's eccentricity, allowing tidal heating to maintain long-term habitability. Secular interactions, coupled with tidal effects, may be critical for planetary habitability.