Phase reddening on near-Earth asteroids: Implications for mineralogical analysis, space weathering and taxonomic classification

TL;DR

This study investigates how phase reddening affects asteroid spectra, revealing its impact on mineralogical analysis, space weathering interpretation, and potential classification ambiguities, through observations of near-Earth asteroids and laboratory meteorite spectra.

Contribution

It provides the first comprehensive analysis of phase reddening effects on asteroid and meteorite spectra across visible and near-infrared wavelengths, highlighting implications for mineralogy and taxonomy.

Findings

Spectral slope increases with phase angle, similar to space weathering effects.

Band depths increase with phase angle, but do not significantly alter mineralogical analysis.

Phase reddening can cause spectral changes equivalent to millions of years of space weathering.

Abstract

Phase reddening is an effect that produces an increase of the spectral slope and variations in the strength of the absorption bands as the phase angle increases. In order to understand its effect on spectroscopic observations of asteroids, we have analyzed the visible and near-infrared spectra (0.45-2.5 \mu m) of 12 near-Earth asteroids observed at different phase angles. All these asteroids are classified as either S-complex or Q-type asteroids. In addition, we have acquired laboratory spectra of three different types of ordinary chondrites at phase angles ranging from 13\degree to 120\degree. We have found that both asteroid and meteorite spectra show an increase in band depths with increasing phase angle. The spectral slope of the ordinary chondrites spectra shows a significant increase with increasing phase angle for g > 30\degree. Variations in band centers and band area ratio (BAR) values were also found, however they seems to have no significant impact on the mineralogical analysis. Our study showed that the increase in spectral slope caused by phase reddening is comparable to certain degree of space weathering. In particular, an increase in phase angle in the range of 30\degree to 120\degree will produce a reddening of the reflectance spectra equivalent to exposure times of ~ 0.1x10^6 to 1.3x10^6 years at about 1 AU from the Sun. Furthermore, we found that under some circumstances phase reddening could lead to an ambiguous taxonomic classification of asteroids.