Assessing the potential of lunar geology as a window into galactic history

Our Solar System has orbited the Milky Way galaxy approximately twenty times since it formed 4.6 billion years ago, and during this time it will have been exposed to a wide range of galactic environments (Figure a).  Reconstructing this history would provide astronomically valuable information on the structure and evolution of this galaxy. In addition, because galactic events may influence life on Earth, knowledge of the galactic environment through time is of interest in assessing the past habitability of our own planet.

Artist’s rendition of the structure of the Milky Way Galaxy (NASA/ESO/ R.Hurt/Wikimedia Commons). The Sun orbits the Galaxy once every approximately 200 million years, passing through a wide range of galactic environments; its current position is shown.  Close-up of the lunar soil (‘regolith’) with astronaut’s boot for scale; the uppermost metre of the regolith is an efficient collector of cosmogenic nuclei produced by cosmic rays emitted by astrophysical processes in the Galaxy (NASA).

Our aim is to study how the flux of charged particles known as galactic cosmic rays (GCRs) impacting the lunar surface has changed with time, and to determine if this can be correlated with the Solar System’s past locations in our galaxy. The GCR flux is ultimately controlled by astrophysical phenomena (such as the occurrence of supernova explosions and the density of interstellar clouds surrounding our Solar System). The Earth does not retain a good GCR record which could be used for this purpose because its atmosphere and magnetic field greatly attenuate the primary cosmic ray flux, and because Earth’s surface rocks are generally younger than the astronomical events we wish to date. On the other hand, the ancient surface of the Moon (Figure b), which is completely lacking in an atmosphere and magnetic field, is potentially an ideal recorder of GCR records relevant to reconstructing the past galactic environment of our Solar System. 

Cosmic rays strike the lunar surface unhindered, where they induce the formation of a range of so-called cosmogenic isotopes (such as 3He, 21Ne and 38Ar) in lunar rocks and soils. We will study the concentrations of these isotopes in a range of lunar materials of different ages (including both Apollo samples and lunar meteorites) in order to determine what they may reveal about the history of our Solar System’s passage through this galaxy. In addition, our work will help identify locations on the lunar surface where future space missions may be able to collect samples that will reveal details of the galactic environment not covered by the existing lunar sample collections. 

Over the years a number of authors, myself included, have speculated about using the lunar geological record to study the galactic environment of our Solar System. However, as far as we are aware, no detailed study using actual lunar samples has yet been attempted. The award of this grant now gives us an opportunity to properly assess the value of lunar geology for galactic astronomy, and perhaps help lay the foundations for a new scientific field at the boundary of astronomy and planetary science.

Professor Ian Crawford
Birkbeck, University of London