The nucleus of 103P/Hartley 2, target of the EPOXI mission

103P/Hartley 2 was selected as the target comet for the Deep Impact extended mission, EPOXI, in October 2007. There have been no direct optical observations of the nucleus of this comet, as it has always been highly active when previously observed. We aimed to recover the comet near to aphelion, to a) confirm that it had not broken up and was in the predicted position, b) to provide astrometry and brightness information for mission planning, and c) to continue the characterisation of the nucleus. We observed the comet at heliocentric distances between 5.7 and 5.5 AU, using FORS2 at the VLT, at 4 epochs between May and July 2008. We performed VRI photometry on deep stacked images to look for activity and measure the absolute magnitude and therefore estimate the size of the nucleus. We recovered the comet near the expected position, with a magnitude of m_R = 23.74 \pm 0.06 at the first epoch. The comet had no visible coma, although comparison of the profile with a stellar one showed that there was faint activity, or possibly a contribution to the flux from the dust trail from previous activity. This activity appears to fade at further epochs, implying that this is a continuation of activity past aphelion from the previous apparition rather than an early start to activity before the next perihelion. Our data imply a nucleus radius of \le 1 km for an assumed 4% albedo; we estimate a ~6% albedo. We measure a colour of (V-R) = 0. 26 \pm 0.09.Comment: 5 pages, 4 figures, accepted for publication in A&

The properties of asteroid (2867) Steins from Spitzer observations and OSIRIS shape reconstruction

We report on the thermal properties and composition of asteroid (2867) Steins derived from an analysis of new Spitzer Space Telescope (SST) observations performed in March 2008, in addition to previously published SST observations performed in November 2005. We consider the three-dimensional shape model and photometric properties derived from OSIRIS images obtained during the flyby of the Rosetta spacecraft in September 2008, which we combine with a thermal model to properly interpret the observed SST thermal light curve and spectral energy distributions. We obtain a thermal inertia in the range 100\pm50 JK-1m-2s-1/2 and a beaming factor (roughness) in the range 0.7-1.0. We confirm that the infrared emissivity of Steins is consistent with an enstatite composition. The November 2005 SST thermal light curve is most reliably interpreted by assuming inhomogeneities in the thermal properties of the surface, with two different regions of slightly different roughness, as observed on other small bodies, such as the nucleus of comet 9P/Tempel 1. Our results emphasize that the shape model is important to an accurate determination of the thermal inertia and roughness. Finally, we present temperature maps of Steins, as seen by Rosetta during its flyby, and discuss the interpretation of the observations performed by the VIRTIS and MIRO instruments

Millimetre continuum observations of comet C/2009 P1 (Garradd)

Little is known about the physical properties of the nuclei of Oort cloud comets. Measuring the thermal emission of a nucleus is one of the few means for deriving its size and constraining some of its thermal properties. We attempted to measure the nucleus size of the Oort cloud comet C/2009 P1 (Garradd). We used the Plateau de Bure Interferometer to measure the millimetric thermal emission of this comet at 157 GHz (1.9 mm) and 266 GHz (1.1 mm). Whereas the observations at 266 GHz were not usable due to bad atmospheric conditions, we derived a 3-sigma upper limit on the comet continuum emission of 0.41 mJy at 157 GHz. Using a thermal model for a spherical nucleus with standard thermal parameters, we found an upper limit of 5.6 km for the radius. The dust contribution to our signal is estimated to be negligible. Given the water production rates measured for this comet and our upper limit, we estimated that Garradd was very active, with an active fraction of its nucleus larger than 50%.Comment: Accepted for publication in Astronomy & Astrophysics. 5 pages, 2 figure

Résurrection du passé à l’aide de modèles hétérogènes d’évolution des séquences protéiques

The molecular reconstruction and resurrection of ancestral proteins is the major issue tackled in this thesis manuscript. While fossil molecular data are almost nonexistent, phylogenetic methods allow to estimate what were the most likely ancestral protein sequences along a phylogenetic tree describing the relationships between extant sequences. With these ancestral sequences, several biological hypotheses can be tested, from the evolution of protein function to the inference of ancient environments in which the ancestors were adatapted. These probabilistic estimations of ancestral sequences depend on substitution models giving the different probabilities of substitution between all pairs of amino acids. Classicaly, substitution models assume in a simplistic way that the evolutionary process remains homogeneous (constant) among sites of the multiple sequence alignment or between lineages. During the last decade, several methodological improvements were realised, with the description of substitution models allowing to account for the heterogeneity of the process among sites and in time. During my thesis, I developed new heterogeneous substitution models in Maximum Likelihood that were proved to better fit the data than any other homogeneous or heterogeneous models. I also demonstrated their better performance regarding the accuracy of ancestral sequence reconstruction. With the use of these models to reconstruct or resurrect ancestral proteins, my coworkers and I showed the adapation to temperature is a major determinant of evolutionary rates in Archaea. Furthermore, we also deciphed the nature of the phylogenetic signal informing substitution models to infer a non-parsimonious scenario for the adaptation to temperature during early Life on Earth, with a non-hyperthermophilic last universal common ancestor living at lower temperatures than its two descendants. Finally, we showed that the use of heterogeneous models allow to improve the functionality of resurrected proteins, opening the way to a better understanding of evolutionary mechanisms acting on biological sequencesLa reconstruction et la résurrection moléculaire de protéines ancestrales est au coeur de cette thèse. Alors que les données moléculaires fossiles sont quasi inexistantes, il est possible d'estimer quelles étaient les séquences ancestrales les plus probables le long d'un arbre phylogénétique décrivant les relations de parentés entre séquences actuelles. Avoir accès à ces séquences ancestrales permet alors de tester de nombreuses hypothèses biologiques, de la fonction des protéines ancestrales à l'adaptation des organismes à leur environnement. Cependant, ces inférences probabilistes de séquences ancestrales sont dépendantes de modèles de substitution fournissant les probabilités de changements entre acides aminés. Ces dernières années ont vu le développement de nouveaux modèles de substitutions d'acides aminés, permettant de mieux prendre en compte les phénomènes biologiques agissant sur l'évolution des séquences protéiques. Classiquement, les modèles supposent que le processus évolutif est à la fois le même pour tous les sites d'un alignement protéique et qu'il est resté constant au cours du temps lors de l'évolution des lignées. On parle alors de modèle homogène en temps et en sites. Les modèles récents, dits hétérogènes, ont alors permis de lever ces contraintes en permettant aux sites et/ou aux lignées d'évoluer selon différents processus. Durant cette thèse, de nouveaux modèles hétérogènes en temps et sites ont été développés en Maximum de Vraisemblance. Il a notamment été montré qu'ils permettent d'améliorer considérablement l'ajustement aux données et donc de mieux prendre en compte les phénomènes régissant l'évolution des séquences protéiques afin d'estimer de meilleurs séquences ancestrales. A l'aide de ces modèles et de reconstruction ou résurrection de protéines ancestrales en laboratoire, il a été montré que l'adaptation à la température est un déterminant majeur de la variation des taux évolutifs entre lignées d'Archées. De même, en appliquant ces modèles hétérogènes le long de l'arbre universel du vivant, il a été possible de mieux comprendre la nature du signal évolutif informant de manière non-parcimonieuse un ancêtre universel vivant à plus basse température que ses deux descendants, à savoir les ancêtres bactériens et archéens. Enfin, il a été montré que l'utilisation de tels modèles pouvait permettre d'améliorer la fonctionnalité des protéines ancestrales ressuscitées en laboratoire, ouvrant la voie à une meilleure compréhension des mécanismes évolutifs agissant sur les séquences biologique

Activity of comets: Gas Transport in the Near-Surface Porous Layers of a Cometary Nucleus

The gas transport through non-volatile random porous media is investigated numerically. We extend our previous research of the transport of molecules inside the uppermost layer of a cometary surface (Skorov and Rickmann, 1995; Skorov et al. 2001). We assess the validity of the simplified capillary model and its assumptions to simulate the gas flux trough the porous dust mantle as it has been applied in cometary physics. A new microphysical computational model for molecular transport in random porous media formed by packed spheres is presented. The main transport characteristics such as the mean free path distribution and the permeability are calculated for a wide range of model parameters and compared with those obtained by more idealized models. The focus in this comparison is on limitations inherent in the capillary model. Finally a practical way is suggested to adjust the algebraic Clausing formula taking into consideration the nonlinear dependence of permeability on layer porosity. The retrieved dependence allows us to accurately calculate the permeability of layers whose thickness and porosity vary in the range of values expected for the near-surface regions of a cometary nucleus.Comment: 25 pages, 9 figure

Reconstructed Ancestral Enzymes Impose a Fitness Cost upon Modern Bacteria Despite Exhibiting Favourable Biochemical Properties

Ancestral sequence reconstruction has been widely used to study historical enzyme evolution, both from biochemical and cellular perspectives. Two properties of reconstructed ancestral proteins/enzymes are commonly reported—high thermostability and high catalytic activity—compared with their contemporaries. Increased protein stability is associated with lower aggregation rates, higher soluble protein abundance and a greater capacity to evolve, and therefore, these proteins could be considered “superior” to their contemporary counterparts. In this study, we investigate the relationship between the favourable in vitro biochemical properties of reconstructed ancestral enzymes and the organismal fitness they confer in vivo. We have previously reconstructed several ancestors of the enzyme LeuB, which is essential for leucine biosynthesis. Our initial fitness experiments revealed that overexpression of ANC4, a reconstructed LeuB that exhibits high stability and activity, was only able to partially rescue the growth of a ΔleuB strain, and that a strain complemented with this enzyme was outcompeted by strains carrying one of its descendants. When we expanded our study to include five reconstructed LeuBs and one contemporary, we found that neither in vitro protein stability nor the catalytic rate was correlated with fitness. Instead, fitness showed a strong, negative correlation with estimated evolutionary age (based on phylogenetic relationships). Our findings suggest that, for reconstructed ancestral enzymes, superior in vitro properties do not translate into organismal fitness in vivo. The molecular basis of the relationship between fitness and the inferred age of ancestral LeuB enzymes is unknown, but may be related to the reconstruction process. We also hypothesise that the ancestral enzymes may be incompatible with the other, contemporary enzymes of the metabolic network.France. Agence nationale de la recherch

Earth-based detection of the millimetric thermal emission of the nucleus of comet 8P/Tuttle

Little is known about the physical properties of cometary nuclei. Apart from space mission targets, measuring the thermal emission of a nucleus is one of the few means to derive its size, independently of its albedo, and to constrain some of its thermal properties. This emission is difficult to detect from Earth but space telescopes (Infrared Space Observatory, Spitzer Space Telescope, Herschel Space Observatory) allow reliable measurements in the infrared and the sub-millimetre domains. We aim at better characterizing the thermal properties of the nucleus of comet 8P/Tuttle using multi-wavelentgh space- and ground-based observations, in the visible, infrared, and millimetre range. We used the Plateau de Bure Interferometer to measure the millimetre thermal emission of comet 8P/Tuttle at 240 GHz (1.25 mm) and analysed the observations with the shape model derived from Hubble Space Telescope observations and the nucleus size derived from Spitzer Space Telescope observations. We report on the first detection of the millimetre thermal emission of a cometary nucleus since comet C/1995 O1 Hale-Bopp in 1997. Using the two contact spheres shape model derived from Hubble Space Telescope observations, we constrained the thermal properties of the nucleus. Our millimetre observations are best match with: i) a thermal inertia lower than ~10 J K-1 m-2 s-1/2, ii) an emissivity lower than 0.8, indicating a non-negligible contribution of the colder sub-surface layers to the outcoming millimetre flux.Comment: 7 pages. Accepted for publication in Astronomy & Astrophysic

Spitzer Space Telescope Observations of the Nucleus of Comet 103P/Hartley 2

We have used the Spitzer Space Telescope InfraRed Spectrograph (IRS) 22-μm peakup array to observe thermal emission from the nucleus and trail of comet 103P/Hartley 2, the target of NASA’s Deep Impact Extended Investigation (DIXI). The comet was observed on UT 2008 August 12 and 13, while 5.5 AU from the Sun. We obtained two 200 frame sets of photometric imaging over a 2.7 hr period. To within the errors of the measurement, we find no detection of any temporal variation between the two images. The comet showed extended emission beyond a point source in the form of a faint trail directed along the comet’s antivelocity vector. After modeling and removing the trail emission, a NEATM model for the nuclear emission with beaming parameter of 0.95 ± 0.20 indicates a small effective radius for the nucleus of 0.57 ± 0.08 km and low geometric albedo 0.028 ± 0.009 (1σ). With this nucleus size and a water production rate of 3 × 10^(28) molecules s^(-1) at perihelion, we estimate that ~100% of the surface area is actively emitting volatile material at perihelion. Reports of emission activity out to ~5 AU support our finding of a highly active nuclear surface. Compared to Deep Impact’s first target, comet 9P/Tempel 1, Hartley 2’s nucleus is one-fifth as wide (and about one-hundredth the mass) while producing a similar amount of outgassing at perihelion with about 13 times the active surface fraction. Unlike Tempel 1, comet Hartley 2 should be highly susceptible to jet driven spin-up torques, and so could be rotating at a much higher frequency. Since the amplitude of nongravitational forces are surprisingly similar for both comets, close to the ensemble average for ecliptic comets, we conclude that comet Hartley 2 must have a much more isotropic pattern of time-averaged outgassing from its nuclear surface. Barring a catastrophic breakup or major fragmentation event, the comet should be able to survive up to another 100 apparitions (~700 yr) at its current rate of mass loss

Water Ice and Dust in the Innermost Coma of Comet 103P/Hartley 2

On November 4th, 2010, the Deep Impact eXtended Investigation (DIXI) successfully encountered comet 103P/Hartley 2, when it was at a heliocentric distance of 1.06 AU. Spatially resolved near-IR spectra of comet Hartley 2 were acquired in the 1.05-4.83 micron wavelength range using the HRI-IR spectrometer. We present spectral maps of the inner ~10 kilometers of the coma collected 7 minutes and 23 minutes after closest approach. The extracted reflectance spectra include well-defined absorption bands near 1.5, 2.0, and 3.0 micron consistent in position, bandwidth, and shape with the presence of water ice grains. Using Hapke's radiative transfer model, we characterize the type of mixing (areal vs. intimate), relative abundance, grain size, and spatial distribution of water ice and refractories. Our modeling suggests that the dust, which dominates the innermost coma of Hartley 2 and is at a temperature of 300K, is thermally and physically decoupled from the fine-grained water ice particles, which are on the order of 1 micron in size. The strong correlation between the water ice, dust, and CO2 spatial distribution supports the concept that CO2 gas drags the water ice and dust grains from the nucleus. Once in the coma, the water ice begins subliming while the dust is in a constant outflow. The derived water ice scale-length is compatible with the lifetimes expected for 1-micron pure water ice grains at 1 AU, if velocities are near 0.5 m/s. Such velocities, about three order of magnitudes lower than the expansion velocities expected for isolated 1-micron water ice particles [Hanner, 1981; Whipple, 1951], suggest that the observed water ice grains are likely aggregates.Comment: 51 pages, 12 figures, accepted for publication in Icaru