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

TL;DR

This study models the Archean Earth's upper atmosphere to constrain early CO2 levels and solar activity, revealing that sufficient CO2 was necessary to prevent atmospheric loss and suggesting the young Sun was likely a slow to moderate rotator.

Contribution

It introduces a state-of-the-art atmospheric model to constrain Archean CO2 levels and solar activity, providing new lower limits and insights into early Earth's atmosphere and Sun's evolution.

Findings

Lower CO2 limit of ~40% at 3.8 billion years ago.

Enhanced solar XUV radiation would cause rapid atmospheric escape without sufficient CO2.

The young Sun was likely a slow to moderate rotator to prevent rapid atmospheric loss.

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 throughout the solar system's history. In case that there were yet unknown cooling mechanisms present in the Archean atmosphere, this could reduce our CO$_2$ stability limits, and it would allow a more active Sun.