arXiv subjects

Vincent Hue

Publications and source records attributed to Vincent Hue.

22 records · Page 2Linked to original sources

arXiv2020

The deep composition of Uranus and Neptune from in situ exploration and thermochemical modeling

The distant ice giants of the Solar System, Uranus and Neptune, have only been visited by one space mission, Voyager 2. The current knowledge on their composition remains very limited despite some recent advances. A better characterization of their composition is however essential to constrain their formation and evolution, as a significant fraction of their mass is made of heavy elements, contrary to the gas giants Jupiter and Saturn. An in situ probe like Galileo would provide us with invaluable direct ground-truth composition measurements. However, some of the condensibles will remain out of the grasp of a shallow probe. While additional constraints could be obtained from a complementary orbiter, thermochemistry and diffusion modeling can further help us to increase the science return of an in situ probe.

astro-ph.EP↗

arXiv2018

Seasonal stratospheric photochemistry on Uranus and Neptune

A time-variable 1D photochemical model is used to study the distribution of stratospheric hydrocarbons as a function of altitude, latitude, and season on Uranus and Neptune. The results for Neptune indicate that in the absence of stratospheric circulation or other meridional transport processes, the hydrocarbon abundances exhibit strong seasonal and meridional variations in the upper stratosphere, but that these variations become increasingly damped with depth due to increasing dynamical and chemical time scales. At high altitudes, hydrocarbon mixing ratios are typically largest where the solar insolation is the greatest, leading to strong hemispheric dichotomies between the summer-to-fall hemisphere and winter-to-spring hemisphere. At mbar pressures and deeper, slower chemistry and diffusion lead to latitude variations that become more symmetric about the equator. On Uranus, the stagnant, poorly mixed stratosphere confines methane and its photochemical products to higher pressures, where chemistry and diffusion time scales remain large. Seasonal variations in hydrocarbons are therefore predicted to be more muted on Uranus, despite the planet's very large obliquity. Radiative-transfer simulations demonstrate that latitude variations in hydrocarbons on both planets are potentially observable with future JWST mid-infrared spectral imaging. Our seasonal model predictions for Neptune compare well with retrieved C2H2 and C2H6 abundances from spatially resolved ground-based observations (no such observations currently exist for Uranus), suggesting that stratospheric circulation -- which was not included in these models -- may have little influence on the large-scale meridional hydrocarbon distributions on Neptune, unlike the situation on Jupiter and Saturn.

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arXiv2018

Photochemistry, mixing and transport in Jupiter's stratosphere constrained by Cassini

We aim at constraining the diffusive and advective transport processes in Jupiter's stratosphere. The IR spectrum recorded by Cassini/CIRS during the Jupiter flyby contains the fingerprints of several atmospheric compounds and allows probing its atmospheric composition. C2H2 and C2H6, the main compounds produced by methane photochemistry, were retrieved as a function of latitude at certain pressure levels. CIRS observations suggest a different meridional distribution for these two species, difficult to reconcile with their photochemical histories, which are thought to be coupled to the methane photolysis. While the C2H2 abundance decreases with latitude, C2H6 becomes more abundant at high latitudes. A new 2D (latitude-altitude) seasonal photochemical model is developed to study whether the addition of stratospheric transport processes, such as meridional diffusion and advection, can explain the latitudinal behavior of C2H2 and C2H6. C2H2 observations are fairly well reproduced without meridional diffusion. Adding meridional diffusion to the model provides an improved agreement with the C2H6 observations by flattening its meridional distribution, but degrades the fit to the C2H2 distribution. Meridional diffusion alone cannot produce the observed increase with latitude of C2H6. When adding advective transport between 30 mbar and 0.01 mbar, with upwelling winds at the equator and downwelling winds at high latitudes, we can reproduce the C2H6 abundance increase with latitude. However, the fit to the C2H2 distribution is degraded. The strength of the advective winds needed to reproduce the C2H6 abundances is very sensitive to the value of the meridional eddy diffusion coefficient. The coupled fate of C2H2 and C2H6 suggests that an additional process is missing in the model. Ion-neutral chemistry was not accounted for in this work and might be a good candidate to solve this issue.

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arXiv2015

2D photochemical modeling of Saturn's stratosphere. Part I: Seasonal variation of atmospheric composition without meridional transport

Saturn's axial tilt of 26.7° produces seasons in a similar way as on Earth. Both the stratospheric temperature and composition are affected by this latitudinally varying insolation along Saturn's orbital path. A new time dependent 2D photochemical model is presented to study the seasonal evolution of Saturn's stratospheric composition. This study focuses on the impact of the seasonally variable thermal field on the main stratospheric C2 hydrocarbon chemistry (C2H2 and C2H6) using a realistic radiative climate model. Meridional mixing and advective processes are implemented in the model but turned off in the present study for the sake of simplicity. The results are compared to a simple study case where a latitudinally and temporally steady thermal field is assumed. Our simulations suggest that, when the seasonally variable thermal field is accounted for, the downward diffusion of the seasonally produced hydrocarbons is faster due to the seasonal compression of the atmospheric column during winter. This effect increases with increasing latitudes which experience the most important thermal changes in the course of the seasons. The seasonal variability of C2H2 and C2H6 therefore persists at higher-pressure levels with a seasonally-variable thermal field. Cassini limb-observations of C2H2 and C2H6 (Guerlet et al., 2009) are reasonably well-reproduced from the equator to 40° in both hemispheres

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