Detectability of the passage of the heliosphere through an interstellar cloud with cosmogenic nuclides in lunar soil

¹ Sodankylä Geophysical Observatory, University of Oulu, 90014 Oulu, Finland
² Space Physics and Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
³ Centre for Space Research, North-West University, 2520 Potchefstroom, South Africa

^⋆ Corresponding authors; stepan.poluianov@oulu.fi; n.eugene.engelbrecht@gmail.com

Received: 21 September 2024
Accepted: 27 November 2024

Abstract

Context. As the Sun traverses interstellar space it may encounter interstellar molecular clouds (IMCs) characterised by higher particle densities than in the ambient interstellar medium. These occurrences have, for example, been proposed to explain the increase of ⁶⁰Fe measured in sea sediments. Magnetohydrodynamic (MHD) simulations show that such IMC crossings effectively shrink the heliosphere, thereby reducing its ability to modulate the incident spectrum of galactic cosmic rays (GCRs). Therefore, the hallmark of such encounters in the past may be increased GCR intensities, which can be detected via analyses of cosmogenic nuclides in lunar regolith samples.

Aims. The present study proposes a method for testing whether such IMC crossings have indeed occurred in the past, by analysing the rates at which the long-lived cosmogenic nuclide ²⁶Al (lifetime 1.0 Myr) is formed in lunar soil samples.

Methods. Cosmogenic nuclide production rates at varying depths in lunar soil are related to a corresponding GCR modulation potential, which in turn is related to a corresponding modulation boundary, and hence interstellar density, via a scaling relation based on published MHD simulation results.

Results. A lower limit for the detectability of past heliospheric crossings of IMCs is presented, governed by the amount of time spent in such a cloud: shorter passages may be undetectable, but longer passages would be clearly observable. However, we find no evidence of the Solar System encountering a cold, dense cloud.

Conclusions. Lunar cosmogenic nuclides represent a powerful tool whereby the past modulation history of the heliosphere can be revealed over timescales of millions of years, which in turn can provide invaluable insights as to the past interstellar environment encountered by the Sun. However, techniques such as the one proposed here will benefit greatly from new, higher-precision analyses of existing lunar samples.