How astronomers probe the Sun’s explosive past
The memory of a million years of the moon
This terrestrial limitation raises a question: Is solar activity during the Holocene special? To answer this, scientists must look to a completely different planetary body: the Moon.
“Lunar rock or any rock not protected by the atmosphere is a coarse spectrometer,” explains Ilya Usoskin from 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 the laboratory. Some of these cosmic rays are charged particles from the Sun; others (which tend to be more energetic) come from sources further away from the Milky Way, beyond our solar system.
The Apollo missions returned with many lunar rock samples – including a deep, 8 feet long (2.4 meters) drill core collected on Apollo 15. This nucleus is important because galactic cosmic rays, more abundant at high energies than solar particles, plant isotopes deep in rocks. In contrast, solar cosmic rays only leave their imprints in shallower rocks. The deep core of Apollo 15 allows scientists to understand the contributions of galactic cosmic rays, which means they can then distinguish the contributions of solar particles in the shallower layers to better understand the behavior of the Sun over time. .
Scientists can only extract information about an average particle bombardment over several million years. Nevertheless, the method provides valuable information. For example, isotope concentrations suggest that, on average, solar activity has remained relatively constant over the past millions of years. In addition, the number of super-eruptions deduced from 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 stop anytime soon.
Sometimes, however, we have to look to distant stars to learn more about the Sun’s distant past. “Other stars tell us how the Sun behaved over time,” says Veronig. For example, younger stars generally spin faster. And because a star’s rotation drives its magnetic dynamo, faster rotation produces stronger magnetic fields, leading to stronger flaring events. Scientists therefore believe that the Sun was much more active in its youth.
The activity of the young Sun may not be so relevant to us today, but it was very important to our early 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 pushes can help build complex molecules like RNA and DNA from simpler building blocks. But too many intense eruptions can strip entire atmospheres, rendering a planet uninhabitable.