The Sun and stars as the primary energy input in planetary atmospheres

Abstract

Proper characterization of the host star to a planet is a key element to the understanding of its overall properties. The star has a direct impact through the modification of the structure and evolution of the planet atmosphere by being the overwhelmingly larger source of energy. The star plays a central role in shaping the structure, evolution, and even determining the mere existence of planetary atmospheres. The vast majority of the stellar flux is well understood thanks to the impressive progress made in the modeling of stellar atmospheres. At short wavelengths (X-rays to UV), however, the information is scarcer since the stellar emission does not originate in the photosphere but in the chromospheric and coronal regions, which are much less understood. The same can be said about particle emissions, with a strong impact on planetary atmospheres, because a detailed description of the time-evolution of stellar wind is still lacking. Here we review our current understanding of the flux and particle emissions of the Sun and low-mass stars and briefly address their impact in the context of planetary atmospheres.

The Mars Atmosphere and Volatile Evolution (MAVEN) Mission

“…The solar EUV, the solar-wind, and solar energetic particle fluxes were greater and more intense early in Martian history. The variability of these properties through time is estimated based on telescopic observations of solar-type stars (Ribas 2010). If we observe sufficiently large excursions from average behavior, and the upperatmospheric response to them, we can use this to inform extrapolations to early conditions.…”

Modeling of ice pinnacle formation on Callisto

“…Assuming instantaneous equilibrium with incident sunlight and unit emissivity (a reasonable assumption given the 16.8 day duration of one Callistoan day), the thermal energy contribution due to insolation E sub ( i , A , D ) is expressed in terms of a solar surface temperature, T s , due to incident solar radiation that is obtained from where σ is the Stefan‐Boltzmann radiation constant in W m −2 K −4 , A is the bolometric albedo, K is the solar constant at 1 AU in W m −2 , D is the distance from the Sun in AU (5.2 for Callisto), and i is the local solar incidence angle (0° means that the Sun is directly overhead) [ Spencer , ]. The solar constant has increased by almost 30% of its present value of 1361 W m −2 over the age of the solar system [ Ribas , ], and we have accounted for this in the model by setting it to 980 W m −2 at the beginning of our simulations and increasing it linearly over the 4.5 Gyr duration of the simulation to reach 1361 W m −2 by the end. The bolometric albedo of the surface will depend on whether it is ice, regolith, or exposed bedrock.…”

Solar Models with Dynamic Screening and Early Mass Loss Tested by Helioseismic, Astrophysical, and Planetary Constraints

“…It is possible that higher mass-loss rates seen in pre-main sequence stars extend into the early main sequence, and reasonable that the mass-loss rate decreases rather quickly with time on the main sequence (e.g., power law or exponentially as used by e.g., Wood et al, 2002Wood et al, , 2005bRibas, 2010;Guzik, Willson, and Brunish, 1987). Therefore, the Sun could have begun its mainsequence evolution at higher mass and luminosity (as investigated by e.g., Guzik, Willson, and Brunish, 1987;Turck-Chieze, Daeppen, and Casse, 1988;Sackmann and Boothroyd, 2003;Minton and Malhotra, 2007;Turck-Chièze, Piau, and Couvidat, 2011;Weiss and Heners, 2013).…”