Characterization of lunar space weathering by MoonSWA project

Airless planetary bodies are continuously exposed to extreme space conditions. Micrometeorite bombardment, diurnal thermal cycling, cosmic rays and solar wind ion implantation - collectively known as space weathering [1] - gradually and permanently alter the chemical and physical properties of their surfaces. The lunar surface is characterized by a variable amount of mafic minerals, particularly pyroxenes, and olivine. The Moon near-infrared reflectance spectra of specific areas, such as the basaltic maria, or small portion of highland’s craters, often around wall and central peak or peak ring, typically exhibit two prominent absorption bands near 1 µm and 2 µm, attributed to the presence of pyroxenes in lunar rocks. The exact position and profile of these absorption features depend on the relative concentrations of Fe²⁺, Ca²⁺, and Mg²⁺ within the M1 and M2 crystallographic sites of pyroxene, as well as in associated olivine, glassy materials, plagioclase or some opaque phases [2].

Space weathering was first identified on the Moon through analyses of samples returned by NASA’s Apollo and the Sovietic Luna missions. On the Moon, the space weathering effects manifests in three key spectral changes: albedo decrease (surface darkening), an increase in spectral slope (reddening), and a reduction of the depth of the absorption bands [3]. These spectral alterations are primarily caused by the formation of nanophase metallic iron (npFe⁰) particles, which occur within agglutinates and along the rims of individual soil grains [1, 4]. The visual effects of this process are evident in the fading of bright ray systems around young craters, which gradually darken and blend into the mature lunar regolith.

The darkening effects of space weathering on the Moon are evident in the bright ray systems surrounding young, fresh craters, which gradually fade and vanish as the surface material weathers over time. Additionally, the pattern and intensity of proton bombardment on the lunar surface are influenced by the Moon’s synchronous rotation and its interaction with Earth’s magnetosphere. These factors lead to an asymmetric distribution of solar wind exposure, resulting in non-uniform space weathering effects across the surface [5].

In this context, the Moon Space Weathering Analysis (MoonSWA) project, selected in the framework of the “Bando Ricerca Fondamentale INAF 2023” has the goal of investigating the effects of space weathering on the lunar surface in the visible and near-infrared spectral range by considering:

• Spectral analysis of selected regions using publicly available multi- and hyperspectral datasets and high-level data products;
  
  
• Irradiation experiments on lunar analogs (e. g. lunar meteorites) to emulate the space weathering conditions (see [6]);
  
  
• Integrated analysis combining remote sensing data with laboratory-irradiated samples.

This work also supports the ESA/ASI LUMIO mission, aimed at observing, quantifying, and characterizing meteoroid impacts on the lunar far side of the Moon through remote sensing of impact-generated luminous flashes. 

References

• C.M. Pieters and S.K. Noble, 2016. JGR. Doi: 10.1002/2016JE005128.
• R. G. Burns, 1993. Mineralogical Applications of Crystal Field Theory, Cambridge University Press.
• C.M. Pieters et al., 1993. JGR. Doi: 10.1029/93JE02467.
• S. Noble et al., 2005, MAPS. Doi: 10.1111/j.1945-5100.2005.tb00390.x.
• E. Kallio et al., 2019. PSS. Doi: 10.1016/j.pss.2018.07.013.
• Rubino et al., 2025, EPSC-DPS.

Acknowledgements:  This project is funded by  “Bando Ricerca Fondamentale INAF 2023” - MoonSWA mini-grant (PI: F. Zambon). Lunar meteorite samples are provided by the “MELODY” project selected in 2020 in the framework of the PRIN INAF 2019 (RIC) call (PI: F. Tosi). This work is in support of the ESA/ASI LUMIO mission.