Muon production in the lunar regolith: Opportunities for muon imaging in the Moon

Muography studies density differences within a medium using muons. They are elementary particles generated by primary cosmic rays as they collide with the matter. On Earth, muons are produced at ca. 15-25 km altitude in the upper atmosphere and penetrate down to ca. 1 km depth in the bedrock (with ever-decreasing numbers by increasing depth due to attenuation). Muons provide a powerful local probe to investigate density variations in any material they pass through (e.g., soils, rock, buildings, magma, or even the atmosphere itself).

Although muography has so far only been applied on Earth, several extra-terrestrial applications have recently been proposed. Many of them focus on possible lunar applications. However, first, we need to understand how muons are formed on the Moon.

As the Moon has no atmosphere the primary cosmic radiation hits the surface unobstructed. Muon production can thus be expected to occur within the lunar regolith, i.e., the ca. 5-10 m thick lunar "soil" layer. Regolith consists of crushed rock dust and shards (bulk density ca. 1.5 g/cm3 with rock fragments, e.g., lunar anorthosite 2.7 g/cm3 [1]).

We simulated lunar muon production using silica (SiO2, density 2.65 g/cm3) as it is easy to construct in a simulation. Silica is a common constituent in silicate minerals, which are abundant also on the Moon, although free quartz itself is rare there. It is also more realistic than water, which we used earlier for testing and developing the simulations' routines and methods [2]. Simulated primary cosmic-ray particles were protons with two energies: 1 PeV and 3 PeV. Protons were chosen since they dominate up to the knee region and are the most relevant primary particles for these studies. The incoming proton zenith angle was selected to be uniform and limited to 75 degrees. Simulations were performed by the Fluka simulation package using the CSC (IT Center For Science Ltd., Finland) supercomputer.

Our preliminary results suggest that about 50% of the muons are generated in the topmost 125 cm. About 90% of the muons are generated in the range of 275 cm. Interestingly, this depth is almost independent of the primary-particle energy. Hence, if these quartz-based simulations are taken as a simplified model for lunar muon production, all muons are generated within just some metres of material.

Consequently, lunar muography should not only work, but it should work for small targets quite close to the surface. Muography could be applied, e.g., to identify H2O ice sources at elevated locations (e.g., crater walls, central peaks, hills, and cliffs), investigate the structural integrity of lunar lava tubes (which are often suggested as possible human habitation sites), and monitoring structural weaknesses of lava tubes or artificial in-situ constructs.

[1] C. Meyer, 2003. The Lunar Petrographic Educational Thin Section Set. https://www-curator.jsc.nasa.gov/education/lpetss/index.cfm.

[2] T. Enqvist, 2021. Exploration of Lunar In Situ Resources Can Be Conducted by Applying Density-Sensitive Cosmic-Ray-Based Geophysical Muon Imaging Method Called Muography. ST.040. SEG 100 Conference.