  While the Earth and Moon are generally similar in composition, a notable
difference between the two is the apparent depletion in moderately volatile
elements in lunar samples. This is often attributed to the formation process of
the Moon and demonstrates the importance of these elements as evolutionary
tracers. Here we show that paleo space weather may have driven the loss of a
significant portion of moderate volatiles, such as sodium and potassium from
the surface of the Moon. The remaining sodium and potassium in the regolith is
dependent on the primordial rotation state of the Sun. Notably, given the joint
constraints shown in the observed degree of depletion of sodium and potassium
in lunar samples and the evolution of activity of solar analogues over time,
the Sun is highly likely to have been a slow rotator. Since the young Sun's
activity was important in affecting the evolution of planetary surfaces,
atmospheres, and habitability in the early Solar System, this is an important
constraint on the solar activity environment at that time. Finally, since solar
activity was strongest in the first billion years of the Solar System, when the
Moon was most heavily bombarded by impactors, evolution of the Sun's activity
may also be recorded in lunar crust and would be an important well-preserved
and relatively accessible record of past Solar System processes.
