arXiv subjects

Daniel Cordier

Publications and source records attributed to Daniel Cordier.

15 recordsLinked to original sources

arXiv2024

Capillary processes in extraterrestrial contexts

The Earth is no longer the only known celestial body containing one or more liquid phases. The Cassini spacecraft has discovered seas of hydrocarbons at the surface of Titan, while a series of corroborating evidences argue in favour of the existence of an aqueous ocean beneath the icy crust of several moons. Capillarity embraces a family of physical processes occurring at the free surface of a liquid. These phenomena depend on the liquid properties and on the local planetary conditions. Capillarity may have important direct or indirect implications on the geoscientific and astrobiological points of view. In this paper, we discuss capillarity physics among solar system objects and expected consequences for planetary science.

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arXiv2021

Science goals and new mission concepts for future exploration of Titan's atmosphere geology and habitability: Titan POlar Scout/orbitEr and In situ lake lander and DrONe explorer (POSEIDON)

In response to ESA Voyage 2050 announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn largest moon Titan. Titan, a "world with two oceans", is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low-eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a "heavy" drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan northern latitudes with an orbiter and in situ element(s) would be highly complementary with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration of Titan equatorial regions in the mid-2030s.

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arXiv2021

Vertical compositional variations of liquid hydrocarbons in Titan's alkanofers

According to clues left by the Cassini mission, Titan, one of the two Solar System bodies with a hydrologic cycle, may harbor liquid hydrocarbon-based analogs of our terrestrial aquifers, referred to as "alkanofers". On the Earth, petroleum and natural gas reservoirs show a vertical gradient in chemical composition, established over geological timescales. In this work, we aim to investigate the conditions under which Titan's processes could lead to similar situations. We built numerical models including barodiffusion and thermodiffusion (Soret's effect) in N_2+CH_4+C_2H_6 liquid mixtures, which are relevant for Titan's possible alkanofers. Our main assumption is the existence of reservoirs of liquids trapped in a porous matrix with low permeability. Due to the small size of the molecule, nitrogen seems to be more sensitive to gravity than ethane, even if the latter has a slightly larger mass. This behavior, noticed for an isothermal crust, is reinforced by the presence of a geothermal gradient. Vertical composition gradients, formed over timescales of between a fraction of a mega-year to several tens of mega-years, are not influenced by molecular diffusion coefficients. We find that ethane does not accumulate at the bottom of the alkanofers under diffusion, leaving the question of why ethane is not observed on Titan's surface unresolved. If the alkanofer liquid was in contact with water-ice, we checked that N_2 did not, in general, impede the clathration of C_2H_6, except in some layers. Interestingly, we found that noble gases could easily accumulate at the bottom of an alkanofer.

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arXiv2021

Distribution and intensity of water ice signature in south Xanadu and Tui Regio

Titan's surface was revealed by Cassini's instruments, showing the presence of liquid hydrocarbons in lakes, and features like dry riverbed. In order to study the sediment transport in Titan's channels and to map distribution of the water-ice signature in these terrains, we use a radiative transfer model to retrieve the surface albedo, after we estimated VIMS error via an original method. We also establish a criteria related to the intensity of the water ice signature. The tuning of the radiative transfer model shows that the fractal dimension of Titan's aerosols is higher than previously thought, around 2.3 - 2.4. We find spots of increasing signal of water ice downstream, at the margins of Tui Regio, that could correspond to alluvial fans, deltas or crater rims. We also observe a very low water ice signal on Tui Regio, with a positive gradient between the central region and the boundary of the area, possibly due to the thickness variation of an evaporitic layer. The riverbeds show within the error bars a decreasing grain size from the top to the bottom of the channels.

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arXiv2020

The 3.4 μm absorption of the Titan's stratosphere: contribution of ethane, propane, butane and complex hydrogenated organics

The complex organic chemistry harbored by the atmosphere of Titan has been investigated in depth by Cassini observations. Among them, a series of solar occultations performed by the VIMS instrument throughout the 13 years of Cassini revealed a strong absorption centered at 3.4 $μ$m. Several molecules present in Titan's atmosphere create spectral features in that wavelength region, but their individual contributions are difficult to disentangle. In this work, we quantify the contribution of the various molecular species to the 3.4 $μ$m band using a radiative transfer model. Ethane and propane are a significant component of the band but they are not enough to fit the shape perfectly, then we need something else. Polycyclic Aromatic Hydrocarbons (PAHs) and more complex polyaromatic hydrocarbons like Hydrogenated Amorphous Carbons (HACs) are the most plausible candidates because they are rich in C-H bonds. PAHs signature have already been detected above ~900 km, and they are recognized as aerosols particles precursors. High similarities between individual spectra impede abundances determinations.

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arXiv2019

The Physical Origin of the Venus Low Atmosphere Chemical Gradient

Venus shares many similarities with the Earth, but concomitantly, some of its features are extremely original. This is especially true for its atmosphere, where high pressures and temperatures are found at the ground level. In these conditions, carbon dioxide, the main component of Venus' atmosphere, is a supercritical fluid. The analysis of VeGa-2 probe data has revealed the high instability of the region located in the last few kilometers above the ground level. Recent works have suggested an explanation based on the existence of a vertical gradient of molecular nitrogen abundances, around 5 ppm per meter. Our goal was then to identify which physical processes could lead to the establishment of this intriguing nitrogen gradient, in the deep atmosphere of Venus. Using an appropriate equation of state for the binary mixture CO2-N2 under supercritical conditions, and also molecular dynamics simulations, we have investigated the separation processes of N2 and CO2 in the Venusian context. Our results show that molecular diffusion is strongly inefficient, and potential phase separation is an unlikely mechanism. We have compared the quantity of CO2 required to form the proposed gradient with what could be released by a diffuse degassing from a low volcanic activity. The needed fluxes of CO2 are not so different from what can be measured over some terrestrial volcanic systems, suggesting a similar effect at work on Venus.

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arXiv2019

Science goals and mission concepts for a future orbital and in situ exploration of Titan

In this white paper, we present a cross-section of important scientific questions that remain partially or completely unanswered, ranging from Titan exosphere to the deep interior, and we detail which instrumentation and mission scenarios should be used to answer them. Our intention is to formulate the science goals for the next generation of planetary missions to Titan in order to prepare the future exploration of the moon. The ESA L-class mission concept that we propose is composed of a Titan orbiter and at least an in situ element (lake lander and/or drone(s)).

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arXiv2019

The floatability of aerosols and waves damping on Titan's seas

Titan, Saturn's largest moon, has a dense atmosphere, together with lakes and seas of liquid hydrocarbons. These liquid bodies, which are in polar regions and up to several hundred kilometres in diameter, generally have smooth surfaces despite evidence of near-surface winds. Photochemically generated organic aerosols form a haze that can settle and potentially interact with the liquid surface. Here we investigate the floatability of these aerosols on Titan's seas and their potential to dampen waves. We find that the majority of aerosols are denser than the liquid hydrocarbons, but that some could have liquid-repelling properties. From calculation of the capillary forces, we propose that these 'liquidophobic' aerosols could float and form a persistent film on Titan's seas. We numerically model the wave damping efficiency of such a film under the conditions on Titan, demonstrating that even a film one molecule thick may inhibit formation of waves larger than a few centimetres in wavelength. We conclude that the presence of a floating film of aerosols deposited on Titan's lakes and seas could explain the remarkable smoothness of their surfaces.

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arXiv2017

Bubble streams in Titan's seas as product of liquid N2-CH4-C2H6 cryogenic mixture

Titan, Saturn's largest moon, is the only extraterrestrial body known to support stable liquid on its surface, in the form of seas and lakes that dot the polar regions. Many indications suggest that the liquid should be composed of a mixture of N2, CH4 and C2H6. Recent RADAR observations of Titan's large sea, called ``Ligeia Mare'', have shown unexplained and ephemeral bright features, possibly due to rising bubbles. Here we report that our numerical model, when combined with experimental data found in the literature, shows that Ligeia Mare's bed is a favourable place for nitrogen exsolution. This process could produce centimeter-sized and radar-detectable bubbles.

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arXiv2015

Dissolution on Titan and on Earth: Towards the age of Titan's karstic landscapes

Titan's polar surface is dotted with hundreds of lacustrine depressions. Based on the hypothesis that they are karstic in origin, we aim at determining the efficiency of surface dissolution as a landshaping process on Titan, in a comparative planetology perspective with the Earth as reference. Our approach is based on the calculation of solutional denudation rates and allow inference of formation timescales for topographic depressions developed by chemical erosion on both planetary bodies. The model depends on the solubility of solids in liquids, the density of solids and liquids, and the average annual net rainfall rates. We compute and compare the denudation rates of pure solid organics in liquid hydrocarbons and of minerals in liquid water over Titan and Earth timescales. We then investigate the denudation rates of a superficial organic layer in liquid methane over one Titan year. At this timescale, such a layer on Titan would behave like salts or carbonates on Earth depending on its composition, which means that dissolution processes would likely occur but would be 30 times slower on Titan compared to the Earth due to the seasonality of precipitation. Assuming an average depth of 100 m for Titan's lacustrine depressions, these could have developed in a few tens of millions of years at polar latitudes higher than 70° N and S, and a few hundreds of million years at lower polar latitudes. The ages determined are consistent with the youth of the surface (<1 Gyr) and the repartition of dissolution-related landforms on Titan.

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arXiv2013

On the chemical composition of Titan's dry lakebed evaporites

Titan, the main satellite of Saturn, has an active cycle of methane in its troposphere. Among other evidence for a mechanism of evaporation at work on the ground, dry lakebeds have been discovered. Recent Cassini infrared observations of these empty lakes have revealed a surface composition poor in water ice compared to that of the surrounding terrains --- suggesting the existence of organic evaporites deposits. The chemical composition of these possible evaporites is unknown. In this paper, we study evaporite composition using a model that treats both organic solids dissolution and solvent evaporation. Our results suggest the possibility of large abundances of butane and acetylene in the lake evaporites. However, due to uncertainties of the employed theory, these determinations have to be confirmed by laboratory experiments.

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arXiv2011

Removal of Titan's Atmospheric Noble Gases by their Sequestration in Surface Clathrates

A striking feature of the atmosphere of Titan is that no heavy noble gases other than argon were detected by the Gas Chromatograph Mass Spectrometer (GCMS) aboard the Huygens probe during its descent to Titan's surface in January 2005. Here we provide an explanation of the mysterious absence or rarity of these noble gases in Titan's atmosphere: the thermodynamic conditions prevailing at the surface-atmosphere interface of the satellite allow the formation of multiple guest clathrates that preferentially store some species, including all heavy noble gases, over others. The clean water ice needed for formation of these clathrates could be delivered by successive episodes of cryovolcanic lavas that have been hypothesized to regularly cover the surface of Titan. The formation of clathrates in the porous lavas and their propensity for trapping Ar, Kr and Xe would progressively remove these species from the atmosphere of Titan over its history. In some circumstances, a global clathrate crust with an average thickness not exceeding a few meters could be sufficient on Titan for a complete removal of the heavy noble gases from the atmosphere.

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arXiv2009

A primordial origin for the atmospheric methane of Saturn's moon Titan

The origin of Titan's atmospheric methane is a key issue for understanding the origin of the Saturnian satellite system. It has been proposed that serpentinization reactions in Titan's interior could lead to the formation of the observed methane. Meanwhile, alternative scenarios suggest that methane was incorporated in Titan's planetesimals before its formation. Here, we point out that serpentinization reactions in Titan's interior are not able to reproduce the deuterium over hydrogen (D/H) ratio observed at present in methane in its atmosphere, and would require a maximum D/H ratio in Titan's water ice 30% lower than the value likely acquired by the satellite during its formation, based on Cassini observations at Enceladus. Alternatively, production of methane in Titan's interior via radiolytic reactions with water can be envisaged but the associated production rates remain uncertain. On the other hand, a mechanism that easily explains the presence of large amounts of methane trapped in Titan in a way consistent with its measured atmospheric D/H ratio is its direct capture in the satellite's planetesimals at the time of their formation in the solar nebula. In this case, the mass of methane trapped in Titan's interior can be up to 1,300 times the current mass of atmospheric methane.

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arXiv2008

Clathration of Volatiles in the Solar Nebula and Implications for the Origin of Titan's atmosphere

We describe a scenario of Titan's formation matching the constraints imposed by its current atmospheric composition. Assuming that the abundances of all elements, including oxygen, are solar in the outer nebula, we show that the icy planetesimals were agglomerated in the feeding zone of Saturn from a mixture of clathrates with multiple guest species, so-called stochiometric hydrates such as ammonia hydrate, and pure condensates. We also use a statistical thermodynamic approach to constrain the composition of multiple guest clathrates formed in the solar nebula. We then infer that krypton and xenon, that are expected to condense in the 20-30 K temperature range in the solar nebula, are trapped in clathrates at higher temperatures than 50 K. Once formed, these ices either were accreted by Saturn or remained embedded in its surrounding subnebula until they found their way into the regular satellites growing around Saturn. In order to explain the carbon monoxide and primordial argon deficiencies of Titan's atmosphere, we suggest that the satellite was formed from icy planetesimals initially produced in the solar nebula and that were partially devolatilized at a temperature not exceeding 50 K during their migration within Saturn's subnebula. The observed deficiencies of Titan's atmosphere in krypton and xenon could result from other processes that may have occurred both prior or after the completion of Titan. Thus, krypton and xenon may have been sequestrated in the form of XH3+ complexes in the solar nebula gas phase, causing the formation of noble gas-poor planetesimals ultimately accreted by Titan. Alternatively, krypton and xenon may have also been trapped efficiently in clathrates located on the satellite's surface or in its atmospheric haze.

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arXiv2006

A Large Stellar Evolution Database for Population Synthesis Studies. III. Inclusion of the full Asymptotic Giant Branch phase and Web tools for stellar population analyses

Stellar evolution tracks and isochrones are key inputs for a wide range of astrophysical studies; in particular, they are essential to the interpretation of photometric and spectroscopic observations of resolved and unresolved stellar populations. We have made available to the astrophysical community a large, homogenous database of up-to-date stellar tracks and isochrones, and a set of programs useful in population synthesis studies. In this paper we first summarize the main properties of our stellar model database (BaSTI) already introduced in Pietrinferni et al. (2004) and Pietrinferni et al. (2006). We then discuss an important update of the database, i.e., the extension of all stellar models and isochrones until the end of the thermal pulses along the Asymptotic Giant Branch. This extension of the library is particularly relevant for stellar population analyses in the near-infrared, or longer wavelengths, where the contribution to the integrated photometric properties by cool and bright Asymptotic Giant Branch stars is significant. A few comparisons with empirical data are also presentend and briefly discussed. We then present three web-tools that allow an interactive access to the database, and make possible to compute user-specified evolutionary tracks, isochrones, stellar luminosity functions, plus synthetic Color-Magnitude-Diagrams and integrated magnitudes for arbitrary Star Formation Histories. All these web tools are available at the BaSTI database official site: http://www.oa-teramo.inaf.it/BASTI.

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