The galactic year, cosmic rays and extraterrestrial spinels

A new empirical approach will be developed to reconstruct from Earth´s geological record the changes in the flux of energetic cosmic rays in the Solar System as it moves through the Milky Way. The Sun orbits the centre of the galaxy once every ∼250 Myr and may cross spiral arms and the galactic midplane.

Because of a higher density of supernovae in the spiral arms and other effects the flux of galactic cosmic rays may vary as a function of these positional changes and leave an imprint on micrometeoroids in space, but not on Earth because of atmospheric shielding.

Recently the applicant has recovered many thousands of chrome-spinel grains from micrometeorites that fell on Earth´s sea floors during eighteen "time windows" of the Phanerozoic Eon.

State-of-the-art microanalyses of the grains have been used to reconstruct for the first time the types of meteorites that fell on Earth in deep time, providing information about events, such as the formation of major asteroid families.

Here the research will be taken in a novel direction and the recovered spinel grains will be analysed for cosmic-ray tracks and noble-gas isotopes formed in the grains in collisions with cosmic rays.

The latest developments in synchrotron-light X-ray techniques at nanometre resolution and ultra-sensitive noble-gas mass spectrometry will be used.

This will provide the first empirical data on the relation of cosmic-ray flux versus spiral-arm passages and climatic and biotic events on Earth.