The lunar surface as a recorder of astrophysical processes

Abstract

The lunar surface has been exposed to the space environment for billions of years and during this time has accumulated records of a wide range of astrophysical phenomena. These include solar wind particles and the cosmogenic products of solar particle events which preserve a record of the past evolution of the Sun, and cosmogenic nuclides produced by high-energy galactic cosmic rays which potentially record the galactic environment of the Solar System through time. The lunar surface may also have accreted material from the local interstellar medium, including supernova ejecta and material from interstellar clouds encountered by the Solar System in the past. Owing to the Moon's relatively low level of geological activity, absence of an atmosphere, and, for much of its history, lack of a magnetic field, the lunar surface is ideally suited to collect these astronomical records. Moreover, the Moon exhibits geological processes able to bury and thus both preserve and 'time-stamp' these records, although gaining access to them is likely to require a significant scientific infrastructure on the lunar surface. This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades'.

Keywords: cosmic rays; lunar exploration; lunar regolith; moon; solar wind; supernovae.

Figure 1.

(a) Mare basalt stratigraphy in Mare Serenitatis exposed in the wall of the 16 km diameter impact crater Bessel (21.8° N, 17.9°  E; LROC image M135073175R/NASA/GSFC/ASU). (b) Similar layering exposed in walls of a collapse pit in Mare Ingenii on the farside (36.0° S, 166.1° E LROC image M184810930 L/NASA/GSFC/ASU). (c) Metre-scale basalt layers exposed in the wall of Hadley Rille (26.1° N, 3.6° E) photographed by Apollo 15 astronaut David Scott using a 500 mm focal length lens; the outcrop is about 1300 m from the camera (NASA image AS15-89-12104).

Figure 2.

(a) A prominent lava flow (arrows) on the surface of Mare Imbrium (31.5° N, 338.0° E; Kaguya Terrain Camera/JAXA). Based on CSFD measurements (see electronic supplementary material), we obtain an absolute model age of ${3.03}_{-0.17}^{+0.12}\hspace{0.17em}\text{Gyr}$ for this lava flow, and an age of ${3.30}_{-0.05}^{+0.04}\hspace{0.17em}\text{Gyr}$ for the older lava flows over which it has flowed, so we expect an approximately 3 Gyr old palaeoregolith to be trapped between the two (see text). (b) Schematic illustration of the trapping of a palaeoregolith layer by an overlying lava flow; a paleoregolith such as this would be expected to contain solar wind, GCR, and other astrophysical records that were implanted during its time on the surface, meanwhile a new regolith develops on the younger lava flow and captures more recent astrophysical records. (Online version in colour.)

Figure 3.

(a) Dark pyroclastic materials surrounding a presumed volcanic vent in the Schrödinger Basin on the lunar farside (75.3° S, 139.2° E; LROC image/NASA/GSFC/ASU). (b) Schematic illustration of a pyroclastic eruption covering a pre-existing regolith to preserve a palaeoregolith underneath (adapted with thanks from LPI CLSE Higher Education Resources). (Online version in colour.)

Figure 4.

(a) An unnamed fresh 185 m diameter impact crater in Mare Nubium (20.9° S, 350.3° E; LROC image M183588912R: NASA/GSFC/ASU); note the prominent ejecta blanket. (b) Schematic illustration of a crater ejecta blanket covering a pre-existing regolith to preserve a an underlying palaeoregolith.

Figure 5.

Artist's concept of astronauts supervising a lunar drilling system. Such a capability would permit access to the sub-surface, for example, to extract palaeoregolith samples containing ancient solar wind and GCR records, and is an example of how astrophysics, among other sciences, could benefit from establishing a human-tended research infrastructure on the Moon (image credit: NASA). (Online version in colour.)