The young Sun's XUV-activity as a constraint for lower CO$_2$-limits in the Earth's Archean atmosphere

Abstract

Despite their importance for determining the evolution of the Earth's atmosphere and surface conditions, the evolutionary histories of the Earth's atmospheric CO 2 abundance during the Archean eon and the Sun's activity are poorly constrained. In this study, we apply a state-of-the-art physical model for the upper atmosphere of the Archean Earth to study the effects of different atmospheric CO 2 /N 2 mixing ratios and solar activity levels on the escape of the atmosphere to space. We find that unless CO 2 was a major constituent of the atmosphere during the Archean eon, enhanced heating of the thermosphere by the Sun's strong X-ray and ultraviolet radiation would have caused rapid escape to space. We derive lower limits on the atmospheric CO 2 abundance of approximately 40% at 3.8 billion years ago, which is likely enough to counteract the faint young Sun and keep the Earth from being completely frozen. Furthermore, our results indicate that the Sun was most likely born as a slow to moderate rotating young G-star to prevent rapid escape, putting essential constraints on the Sun's activity evolution

Time evolution of magnetic activity cycles in young suns: The curious case of kappa Ceti

“…Young Sun-like stars provide a unique window into our solar environment during the first few hundred million years. The young Sun κ Ceti is particularly interesting, as its moderate rotation rate suggests it falls in the rotational evolution track that our Sun could have taken in the past (Lammer et al 2020;Johnstone et al 2021). This star was investigated as part of the 'Sun in Time' project (Güdel et al 1997;Ribas et al 2005) which studied stellar magnetic activity in the X-ray and ultraviolet wavelength, and its implications for young exoplanetary atmospheres (Güdel 2007).…”