How astronomers probe the Sun’s explosive past

The Moon’s million-year memory

That earthly limitation raises a question: Is the solar activity in the Holocene special? To answer that, scientists have to turn to a completely different planetary body: the Moon.

“Lunar rock or any other rock unprotected by atmosphere is a rough spectrometer,” says Ilya Usoskin of the University of Oulu in Finland. When a cosmic ray hits a rock, it induces a nuclear reaction and creates isotopes, which can be analyzed in a lab. Some of these cosmic rays are charged particles from the Sun; others (which tend to be higher in energy) come from sources farther out in the Milky Way, beyond our solar system.

The Apollo missions returned with many lunar rock samples — including a deep, 8-foot-long (2.4 meters) drill core collected on Apollo 15. This core is significant because galactic cosmic rays, which are more abundant at high energies than solar particles, plant isotopes deep into the rocks. In contrast, solar cosmic rays leave their imprints only in shallower rocks. The Apollo 15 deep core allows scientists to understand the contributions of galactic cosmic rays — meaning they can then single out the contributions of solar particles in the shallower layers to better understand the Sun’s behavior over time.

Scientists can only extract information about a bombardment of particles averaged over several million years. Nevertheless, the method provides valuable insights. For example, the isotope concentrations suggest that, on average, solar activity has remained relatively constant over the past several million years. Moreover, the number of superflares inferred from the lunar rocks agrees well with the observed number of events marked by isotope deposits in tree rings.

In other words, we shouldn’t expect the Sun’s activity to tail off anytime soon.

Solar adolescence

Sometimes, however, we need to gaze toward distant stars to learn about the Sun’s distant past. “Other stars tell us how the Sun behaved in time,” says Veronig. For example, younger stars typically rotate faster. And because the rotation of a star drives its magnetic dynamo, a faster rotation produces stronger magnetic fields, leading to stronger flaring events. Scientists thus believe that the Sun was much more active in its younger days.

The young Sun’s activity may not be so relevant for us today, but it was very important for our earliest, prehistoric predecessors. “The history of the Sun is linked to the history of the planets because strong flares and CMEs interact with the planets,” says Veronig. For example, having a few flares may help build up complex molecules like RNA and DNA from simpler building blocks. But too many intense flares can strip entire atmospheres, rendering a planet uninhabitable.

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