Abstract The Moon and Mercury are airless bodies, thus they are directly exposed to the ambient plasma (ions and electrons), to photons mostly from the Sun from infrared range all the way to X-rays, and to meteoroid fluxes. Direct exposure to these exogenic sources has important consequences for the formation and evolution of planetary surfaces, including altering their chemical makeup and optical properties, and generating neutral gas exosphere. The formation of a thin atmosphere, more specifically a surface bound exosphere, the relevant physical processes for the particle release, particle loss, and the drivers behind these processes are discussed in this review.
We show that the measured present day atmospheric
Ar,Ne isotope ratios and elemental K/U ratios
measured for Venus and Earth can be reproduced by
a combination of EUV -driven hydrodynamic
hydrogen escape and impacts happening during
accretion. We find that both protoplanets formed
within the solar nebula and accreted large enough
masses able to capture thin hydrogen envelopes,
which were then lost within a few 10s of million
years after disk dispersal. We further show that early
Venus was surrounded by a denser primordial
hydrogen- dominated atmosphere compared to a less
massive proto-Earth that accreted its final mass by
pre-fractionated dry impactors and about two percent
carbonaceous chondrites after the thin primordial
hydrogen envelope was lost. Our results agree with
hafnium-wolfram isotope chronometric evidence that
favors a fast accretion scenario of the Earth with a
late Moon-forming impact. We conclude with a
discussion on the implications of these findings in
relation to planetary evolution of terrestrial
exoplanets and their potential habitability.
<p>Without aerobic life, the simultaneous presence of N<sub>2</sub> and O<sub>2</sub> in the Earth's atmosphere, as on any other planet, would be chemically incompatible over geologic timescales. The existence of an N<sub>2</sub>-O<sub>2</sub>-dominated atmosphere on an exoplanet would, hence, not only constitute a strong biosignature of aerobic life. It would also have to meet certain astro- and geophysical conditions to originate, evolve and to sustain.</p> <p>Our definition of Eta-Earth (&#951;<sub>Earth</sub>), therefore, builds on the concept of a so-called Earth-like Habitat (EH), i.e., a planet within the complex habitable zone for life, at which N<sub>2</sub> and O<sub>2</sub> are simultaneously present as the dominant species while CO<sub>2</sub> only comprises a minor constituent in its atmosphere. By our present scientific knowledge, certain criteria must be fulfilled to allow the existence of such an Earth-like atmosphere. These can be subsumed within a new probabilistic formula for estimating a maximum number of EHs which we will present within this talk. Some of these criteria, such as the initial mass function, the bolometric luminosity and XUV flux evolution of a star, or the distribution of rocky exoplanets within the habitable zones of different stellar spectral types, are already rather well studied and can be tested through further observations. Other important criteria, like the prevalence of working carbon-silicate and nitrogen cycles, or the origin of life are by now poorly, or entirely un-constrained. Further factors, like the presence of a large moon or the importance of an intrinsic magnetic field, are not only poorly constrained but its importance for the evolution and stability of an Earth-like Habitat are even debated. While our new formula for estimating the maximum number of EHs can in principle incorporate all these factors as well as unknowns, we by now must restrict ourselves to the ones that are either well understood or can at least be tested soon. Based on our current knowledge, this approach only allows us to probabilistically estimate a maximum number of exoplanets on which an Earth-like Habitat can in principle evolve. The real number of EHs might, therefore, be significantly lower than our current best estimate but additional criteria should be verifiable in near future by upcoming ground- and space-based instrumentation such as PLATO, the E-ELT, or by the kinds of the proposed space-based observatory LUVOIR.</p> <p>By considering all the factors that are presently scientifically quantifiable to at least some extent, we will present our current best estimate for the maximum number of EHs that might exist within the galaxy. If we redefine &#951;<sub>Earth</sub>, the mean number per star of rocky planets within the habitable zone, to only account for the mean number of EHs per star, &#951;<sub>EH</sub>, we end up with a number much smaller than current best estimates for &#951;<sub>Earth</sub>. It is, therefore, scientifically not justified to presume the astrobiological Copernican assumption that all potential habitats inside a habitable zone for complex life will evolve similar to Earth.</p>
We apply a 1D upper atmosphere model to study thermal escape of nitrogen over Titan's history. Significant thermal escape should have occurred very early for solar EUV fluxes 100 to 400 times higher than today with escape rates as high as $\approx 1.5\times 10^{28}$ s$^{-1}$ and $\approx 4.5\times 10^{29}$ s$^{-1}$, respectively, while today it is $\approx 7.5\times 10^{17}$ s$^{-1}$. Depending on whether the Sun originated as a slow, moderate or fast rotator, thermal escape was the dominant escape process for the first 100 to 1000 Myr after the formation of the solar system. If Titan's atmosphere originated that early, it could have lost between $\approx 0.5 - 16$ times its present atmospheric mass depending on the Sun's rotational evolution. We also investigated the mass-balance parameter space for an outgassing of Titan's nitrogen through decomposition of NH$_3$-ices in its deep interior. Our study indicates that, if Titan's atmosphere originated at the beginning, it could have only survived until today if the Sun was a slow rotator. In other cases, the escape would have been too strong for the degassed nitrogen to survive until present-day, implying later outgassing or an additional nitrogen source. An endogenic origin of Titan's nitrogen partially through NH$_3$-ices is consistent with its initial fractionation of $^{14}$N/$^{15}$N $\approx$ 166 - 172, or lower if photochemical removal was relevant for longer than the last $\approx$ 1,000 Myr. Since this ratio is slightly above the ratio of cometary ammonia, some of Titan's nitrogen might have originated from refractory organics.
