The first billion years of the Sun's existence shaped the Solar System, determining which planets would become habitable, and which would not. For the first time, astronomers may have a handle on the extent of early solar activity using evidence preserved in our Moon's crust.
Solar outbursts are thought to have stripped Mars of most of its air, and changed the atmospheres of Earth and Venus. We're less certain how large and frequent those outbursts were, and how close the Solar System came to having two habitable planets, or none.
Dr Prabal Saxena of NASA's Goddard Space Flight Center noted young stars show a direct relationship between the strength of a star's flares and the rate at which it spins. So if we knew the Sun's initial rate of spin we would have a good idea how active it was. We know stars start out spinning fast and slow down, and Saxena thinks we can determine the Sun's early spin rate using an inner Solar System object that never had an atmosphere; our Moon.
"The reason the Moon ends up being a really useful calibrator and window into the past is that it has no annoying atmosphere and no plate tectonics resurfacing the crust," Saxena said in a statement. "So as a result, you can say, 'Hey, if solar particles or anything else hit it, the Moon's soil should show evidence of that.'"
Saxena reached this conclusion while investigating a different question: why the rocks the Apollo astronauts brought back have so much less sodium and potassium than the Earth's crust, despite having been formed from the same material.
In Astrophysical Journal Letters Saxena argues the loss of sodium and potassium is a result of high-energy particles from the Sun hitting the lunar surface and vaporizing the upper layers. A variety of factors, previously explored by Saxena's co-author Dr Rosemary Killen determine which elements escape, and which the Moon recaptures.
Modeling these, Saxena and Killen concluded the Sun in its early days was a low to mid-activity star. Had the Sun turned faster, and therefore released more energetic particles, the Moon would have even less sodium and potassium today.
Saxena and Killen acknowledge their findings are preliminary. Developing more detailed measures of this question is one of the reasons they are excited by proposals to return to the Moon in the next decade.
An unusually slowly rotating Sun would have allowed Mars' atmosphere to survive, and let Venus hang onto its hydrogen, and therefore water. For Earth, the Sun's activity was probably just right. It was sufficiently high to strip away the primordial haze, but low enough to allow our later atmosphere – released by volcanic activity – to survive once Earth's magnetic field provided partial protection.