Context. Olivine responds to space weathering in the fastest and most profound way, which results in significant space weathering spectral alteration effects (SWSAEs) on airless silicate bodies. Although Mg-rich olivine (Fa(10)) has been subjected to extensive studies, SWSAEs of Fe-rich (Fa# > 20) or Fa-dominant (Fa# > 50) olivine are still poorly understood.Aims. We aim to systematically characterize the space weathering effects and the associated spectral alterations of Fe-rich olivine on the surface of Phobos and the Moon.Methods. We conducted nanosecond pulsed laser irradiation experiments on a set of synthetic Fe-rich olivine (Fa(29), Fa(50), Fa(71), and Fa(100)) with energy levels simulated for Phobos and the Moon and analyzed the irradiated olivine for microscopic characteristics and near-infrared (NIR) and Raman spectroscopy.Results. Micron-level thick alteration layers are found in Fa(100) compared to those hundreds of nanometers thick in Fa(29), Fa(50), and Fa(71). With increasing irradiation energy levels and Fa# values, nanophase iron (np-Fe-0) particles increase in size but decrease in quantity. The np-Fe-0 formed via in situ decomposition are ubiquitously present, while those formed via vapor deposition are primarily found in Fa(29) but rarely in Fa# > 50. The size fraction of intermediate (10-40 nm) and large (40-60 nm) np-Fe-0 proportionally increases with Fa# values. The NIR spectra of weathered olivine show darkening over reddening in most cases, but Fa100 under the most irradiated condition shows brightening-reddening spectral effects. The Raman spectra of weathered olivine show a reduction in intensity without peak shifts.Conclusions. The Fa# values of olivine are a more critical factor in controlling the SWSAEs on Phobos than those on the Moon. If Phobos and Deimos contain substantial Fe-rich or Fa-dominant olivine, similar to Mars, thick alteration rims and large np-Fe-0 formed via space weathering may cause darkening-reddening and potentially brightening-reddening spectral effects on the Martian moons.

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