Mass, energy, and momentum capture from stellar winds by magnetized and unmagnetized planets: implications for atmospheric erosion and habitability

Abstract

The extent to which a magnetosphere protects its planetary atmosphere from stellar wind ablation depends upon how well it prevents energy and momentum exchange with the atmosphere and how well it traps otherwise escaping plasma. We focus on the former, and provide a formalism for estimating approximate upper limits on mass, energy, and momentum capture, and use them to constrain loss rates. Our approach quantifies a competition between the local deflection of incoming plasma by a planetary magnetic field and the increase in area for solar wind mass capture provided by this magnetosphere when the wind and planetary fields incur magnetic reconnection. Solar wind capture can be larger for a magnetized planet versus an unmagnetized planet with small ionosphere, even if the rate of energy transfer is less. We find this to be the case throughout Earth's history since the lunar forming impact-the likely start of the geodynamo. Focusing of incoming charged particles near the magnetic poles can increase the local energy flux for a strong enough planetary field and weak enough wind, and we find that this may be the case for Earth's future. The future protective role of Earth's magnetic field would then depend on its ability to trap outgoing plasma. This competition between increased collection area and reduced inflow speed from a magnetosphere is likely essential in determining the net protective role of planetary fields and its importance in sustaining habitability, and can help explain the current ion loss rates from Mars, Venus and Earth.

Colloquium: Physical constraints for the evolution of life on exoplanets

“…II.B. However, this paradigm has been challenged by recent theoretical studies, which indicate that the atmospheric escape rate does not always decline with an increase in the magnetic field strength (Dong et al, 2018c;Blackman and Tarduno, 2018;Gunell et al, 2018;Lingam, 2019). The basic reason can be understood qualitatively as follows.…”

Role of stellar physics in regulating the critical steps for life

“…Lastly, we observe that (8) is applicable to unmagnetized planets. In contrast, if one considers strongly magnetized planets, the atmospheric escape rate could decrease by a factor of (Dong et al 2017b), but the converse is also possible (Blackman and Tarduno 2018; Sakai et al 2018). In general, the escape rate is anticipated to be a non-monotonic function of the magnetic field (Gunell et al 2018; Lingam and Loeb 2018e).…”

Different types of star-planet interactions

“…This is, for example, what is believed to have happened to Mars, which does not have an intrinsic magnetic field, therefore lacking a protective 'umbrella' against the solar wind. Recently, there has been a debate in the literature whether this is indeed the case, with some authors arguing that magnetic fields do not affect atmospheric escape, with Mars and Earth being counter examples of unmagnetised and magnetised planets, respectively, with similar outflow rates (Strangeway et al 2010, see also Blackman & Tarduno 2018;Egan et al 2019). Although escape in solar system planets can help us understand exoplanet evaporation (or atmospheric survival), the different, and often very extreme, architectures of exoplanetary systems compared to the solar system does not necessary guarantee that the same evaporation mechanisms taking place in the solar system would operate (or be as strong) in the exoplanets knows to date.…”