Atmospheric Escape Processes and Planetary Atmospheric Evolution

The habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, chemical and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our Solar system: it is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H-dominated thermosphere for astrophysicists, or the Sun-like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth's atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so-called Xenon paradox

The Impact of Mesoscale Processes on the Atmospheric Circulation of Mars

The study of the modern martian atmosphere is (1) a key to the climate of Mars’s past; (2) useful for comparison with other terrestrial planets such as the Earth; and (3) can support hazard analysis and weather forecasting for future exploration and habitation of the planet. Recently, it was found that middle atmospheric downwelling near the south pole during southern winter is much more vigorous than predicted by most Mars general circulation models. This underestimate may be due to models erroneously representing the radiative forcings in the atmosphere due to aerosol and/or the mechanical forcings due to wave breaking. Errors of this kind would influence middle atmospheric dynamics and likely would result from incomplete understanding of lower atmospheric processes such as dust transport. Here, retrievals of vertical profiles of temperature, pressure, dust, and water ice from the Mars Climate Sounder (MCS) on Mars Reconnaissance Orbiter (MRO) are used to characterize the atmospheric circulation of Mars and its forcings. First, I consider the annual cycle of the thermal structure and aerosol distributions of the lower and middle atmosphere and investigate the degree of coupling between the lower and middle atmospheric mean meridional circulations. To evaluate the role of wave breaking, I look for local convective instabilities in the Martian middle atmosphere: a key indicator of saturating vertically propagating waves such as the gravity waves and the thermal tides, which are important sources of wave drag in the Earth’s mesosphere. I then characterize the vertical distribution of dust and its approximate radiative effects during northern spring and summer and show there is usually a maximum in dust mass mixing ratio at ~15—25 km above the tropics, which is not currently simulated by models. Next, I evaluate the relative importance of dust storm activity, pseudo-moist convection due to the solar heating of dust, orographic effects, and scavenging by water ice clouds in producing this maximum. Finally, I show that published models underestimate the thickness and altitude of water ice clouds in northern summer

THE IMPACT OF MESOSCALE PROCESSES ON THE ATMOSPHERIC CIRCULATION OF MARS

Maxima pars uatum, pater et iuuenes patre digni, decipimur specie recti. Breuis esse laboro, obscurus fio; sectantem leuia nerui deficiunt animique; professus grandia turget; serpit humi tutus nimium timidusque procellae; qui uariare cupit rem prodigialiter unam, delphinum siluis adpingit, fluctibus aprum. The vast majority of poets, both the laureate And the young ones some day laurelworthy, We all are deceived by the appearance of right. I strive to be succinct, yet I become obscure. My mind and nerves fail in the pursuit of eloquence. Turning epic, I merely might appear swollen. Or fearful of such storms I could creep along Safely upon well-trodden ground

A paleogeographic approach to aerosol prescription in simulations of deep time climate

Aerosols have important effects on the Earth's radiative balance and are normally included in simulations of present day climate. For simulations of present day or recent past climates, observational information can be used to constrain spatiotemporal variability in aerosol loading. For the deep past, aerosol changes are generally ignored. Here we describe how to use the Community Climate System Model version 4 (CCSM4), standard boundary conditions for a deep time climate simulation, and pre-industrial emissions information to generate a “paleogeographic” aerosol prescription. This prescription is then applied to a previously published simulation of the Late Permian (251 Ma) to evaluate the how the model climate is affected by the new aerosol prescription relative to the aerosol distribution originally imposed. The new aerosol prescription results in a broadly warmer and wetter climate with a somewhat stronger Pangaean monsoon. Using spatiotemporally varying and speciated aerosol is equivalent to reducing the optical depth of a uniform background aerosol with sulfate-like properties by ∼30–50%