arXiv subjects

Francois Leblanc

Publications and source records attributed to Francois Leblanc.

5 recordsLinked to original sources

arXiv2026

Atmospheric Escape Rates from Mars - If it Orbited an Old M-Dwarf Star

Atmospheric escape is an important process that influences the evolution of planetary atmospheres. A variety of physical mechanisms can contribute to escape from an atmosphere, including thermal escape, ion escape, photochemical escape, and sputtering. Here we estimate escape rates via each of these processes for a hypothetical Mars-like exoplanet orbiting Barnard's star (an old, inactive M dwarf star). We place the planet at an orbital distance that receives the same total stellar flux as it does in our solar system. We use the measured stellar extreme ultraviolet (EUV) spectrum and assumptions on the star's magnetic field to determine both the high-energy radiation and the stellar wind environment around the planet. This information is used to model the response of the planet's thermosphere, exosphere and magnetosphere using a variety of models that have been validated against solar system observations. We find overall escape rates that are dominated by thermal processes and elevated by 2-5 orders of magnitude relative to present-day Mars, suggesting that a Mars-like planet orbiting Barnard's star would not retain a significant atmosphere for more than 10's of millions of years. Recently reported planets around Barnard's star should also not have retained significant atmospheres. By extension, Mars-like planets orbiting any M dwarf near the 'Habitable Zone' should not retain atmospheres for extended periods of time.

astro-ph.EP↗

arXiv2026

Theoretical Ion Sputtering Yields from Loose Powders using a Multiscale Monte Carlo Approach

Ion sputtering from loose powders remains poorly understood despite its relevance to planetary science and industry. We developed a multiscale Monte Carlo model to simulate sputtering from powders, using a higher-fidelity approach for the target geometry compared to voxel-based methods. Simulating Kr+ ions impacting Cu powders and flat slabs, we show that sputtering from loose powders differs markedly from that of flat slabs or rough surfaces. The main differences are: (1) for incident angles a > 0 degree relative to the bulk normal, the escaping sputtering yield is dominated by backward-directed ejecta for all ion energies; (2) for a < 60 degrees, the yield peaks toward the ion-beam origin, similar to the opposition effect seen in optical observations of airless bodies; (3) the angular distribution peak is half or less than that of a flat slab; (4) as ion energy increases, no evolution occurs from primary to secondary knock-on sputtering in the ejecta angular distribution. We attribute these behaviors to the powders interconnected voids. Ions penetrate these voids and sputter underlying grains; the ejecta then preferentially escape toward the ion-beam origin, where shadowing is minimal. We derive two fitting functions: 1) relating the escaping sputtering yield of a powder to that of a flat surface, depending only on porosity, incident angle, mean local incidence angle, and the corresponding flat slab yield; 2) providing the double-differential angular distribution of the escaping ejecta for porosities > 0.49. These provide a potentially universal fitting function of the absolute doubly-differential escaping sputtering yield from loose powders.

physics.app-ph↗

arXiv2018

The origin and fate of O$_2$ in Europa's ice: an atmospheric perspective

The early prediction and subsequent detection of an O$_2$ atmosphere on Europa, coupled with the discovery that Europa has an ocean under its ice mantle, has made this moon a prime astrobiologic target, soon to be visited by the JUICE and Europa Clipper spacecraft. In spite of the considerable number of observational, modeling, and laboratory efforts, understanding the physics leading to the observed morphology of Europa's near surface O$_2$ atmosphere has been problematic. This is the case as the observed emissions depend on the local incident plasma ion flux, the local temperature and composition of the regolith, as well as on the near surface electron temperature and density. Here we rely heavily on earlier reviews briefly summarizing the observational, laboratory and simulation efforts. Although it is agreed that radiolysis of the surface ice by the incident Jovian plasma is the ultimate source of observed O$_2$, a recent, simple model of thermal desorption from a regolith permeated with O$_2$ has changed the usual paradigm. This suggests that the observed orbital dependence of the local source of the near-surface O$_2$ atmosphere is due to thermal release of O$_2$ likely trapped on the ice grains at dangling bonds with a smaller contribution due to direct sputtering. This could also impact our understanding of the suggestion that the radiolytic products in Europa's regolith might be a source of oxidants for its underground ocean.

astro-ph.EP↗

arXiv2018

On the orbital variability of Ganymede's atmosphere

Ganymede's atmosphere is produced by radiative interactions with its surface, sourced by the Sun and the Jovian plasma. The sputtered and thermally desorbed molecules are tracked in our Exospheric Global Model (EGM), a 3-D parallelized collisional model. This program was developed to reconstruct the formation of the upper atmosphere/exosphere of planetary bodies interacting with solar photon flux and magnetospheric and/or the solar wind plasmas. Here, we describe the spatial distribution of the H$_2$O and O$_2$ components of Ganymede's atmosphere, and their temporal variability along Ganymede's rotation around Jupiter. In particular, we show that Ganymede's O$_2$ atmosphere is characterized by time scales of the order of Ganymede's rotational period with Jupiter's gravity being a significant driver of the spatial distribution of the heaviest exospheric components. Both the sourcing and the Jovian gravity are needed to explain some of the characteristics of the observed aurora emissions. As an example, the O$_2$ exosphere should peak at the equator with systematic maximum at the dusk equator terminator. The sputtering rate of the H$_2$O exosphere should be maximum on the leading hemisphere because of the shape of the open/close field lines boundary and displays some significant variability with longitude.

astro-ph.EP↗

arXiv2018

Dusk Over Dawn O$_2$ Asymmetry in Europa's Near-Surface Atmosphere

The evolution of Europa's water-product exosphere over its 85-hour day, based on current models, has not been shown to exhibit any diurnal asymmetries. Here we simulate Europa's exosphere using a 3-D Monte Carlo routine including, for the first time, the role of Europa's rotation on the evolution of exospheric molecules throughout the orbit. We focus on O$_2$, sputtered by a trailing hemisphere source with a temperature-dependence under isotropic plasma conditions. We find that the O$_2$ component, while global, is not homogenous in Europa local time. Rather, the O$_2$ accumulates at the dusk hemisphere. When rotation is explicitly excluded, no diurnal asymmetries exist. We find that the assumed thermal-dependence on the O$_2$ source is critical for a diurnal asymmetry: the diurnal surface temperature profile is imprinted on to the near-surface O$_2$ atmosphere, due to the small hop times of the non-adsorbing O$_2$ effectively rotating with Europa. Altogether, our simulations conclude that the dusk-over-dawn asymmetry is driven by Europa's day-night O$_2$ cycle synchronized with Europa's orbital period based on our model assumptions on O$_2$ production and loss. This conclusion is in agreement with the recent understanding that a non-adsorbing, rotating O$_2$ source peaking at noon will naturally accumulate from dawn-to-dusk, should the O$_2$ lifetime be sufficiently long compared to the orbital period. Lastly we compare hemispherically-averaged dusk-over-dawn ratios to the recently observed oxygen emission data by the Hubble Space Telescope. We find that while the simulations are globally consistent with brighter oxygen emission at dusk than at dawn, the orbital evolution of the asymmetries in our simulations can be improved by ameliorating the O$_2$ source & loss rates, and possibly adsorption onto the regolith.

astro-ph.EP↗