The solar wind propagates through the solar system at extremely high speed. Wherever particles of the solar wind strike rocks or grains of dust they leave behind fine tracks. By using precision measuring instruments it is possible to detect for example isotopes of neon that have penetrated into the dust grains or rocks as part of the solar wind. This reveals that the isotopic composition of the neon below the surface changes continuously. Whereas an isotope ratio corresponding approximately to that of the solar wind is found in the uppermost 20 to 30 nanometres, a distinctly different composition is measured in the deeper layers. Among other reasons, this is due to the fact that high-energy galactic radiation (= non-solar cosmic radiation) interacts with the crystals of the moon samples, leading to the formation of neon isotopes with an anomalous composition.n mit exotischer Zusammensetzung führt.

An unexpected distribution

The theory commonly held until now attempted to explain the distribution of the isotopes of neon in the moon samples as a three-component mixture, but the isotopic ratio does not change continuously. The ratio of neon-20 to neon-22 decreases continuously in the uppermost layers. However, this trend comes to a standstill beyond a certain depth. It appeared that additional solar neon with a markedly heavier composition accumulates as well as the neon produced in the crystals by the galactic radiation. Therefore it was assumed in the past that there must be another source that introduces heavy neon isotopes into the rock grains and that the observed pattern could be evidence of a greater level of solar activity in earlier epochs. A group of researchers from ETH Zurich led by Rainer Wieler of the Institute of Isotope Geochemistry and Mineral Resources(1) has now disproved this interpretation in the latest issue of the scientific journal Science. (2) Together with scientists from the University of Bern and American researchers, the team analysed neon isotopes that were captured on a metallic glass disk in the context of NASA’s Genesis Mission. The probe was launched in 2001, orbited the sun for more than two years and returned to earth in a crash-landing in 2004.

The heavier the deeper

The researchers have now found that the distribution of solar neon isotopes in the Genesis samples is similar to that in the moon samples. Ansgar Grimberg, first author of the study and a doctoral student at the Institute for Isotope Geochemistry, explains that “The activity of the sun was accurately monitored during the Genesis mission. If the knowledge from the Genesis study is now applied to the moon samples, we can exclude the explanation that the heavy isotopes entered the moon samples because of a higher level of solar activity.” On the contrary, he says this involves a mechanical phenomenon: “Because of their higher kinetic energy, the heavy neon isotopes of the solar wind penetrate more deeply into the dust grains. This leads to fractionation of the isotopes below the uppermost layer. Only at greater depths does the neon signature of the galactic radiation outweigh that of the solar wind.