Evidence of Possible Spectral Variability in the Patroclus-Menoetius Binary System

TL;DR

This study tests the long-standing hypothesis of spectral variability across the Patroclus–Menoetius Trojan binary by obtaining two visible spectra with the GMOS instrument at near-opposite rotational phases and using a consistent solar analog for calibration. After careful data reduction and cross-checks, the authors measure slopes of $2.51\%/100\,\mathrm{nm}$ and $8.13\%/100\,\mathrm{nm}$, with an empirical systematic floor of $0.7\%/100\,\mathrm{nm}$, and find the difference to be significant at $5.7\sigma$ despite potential biases. They explore two scenarios to explain the variability: (A) a substantial inter-component color difference, which is unlikely given prior constraints and formation models, or (B) localized surface inhomogeneities that rotate into view, potentially generated by impacts or subsurface exposure in the ant-MenoitiUS hemisphere. The results, while tentative, have important implications for the surface evolution of Trojan binaries and for planning the Lucy 2033 flyby, underscoring the need for follow-up, space-based, or slitless spectroscopy and high-cadence observations during mutual events to map surface heterogeneity.

Abstract

We present new visible-wavelength spectroscopic observations of the Patroclus-Menoetius binary system in the Jupiter Trojan population. Motivated by previously published spectra from different instruments that showed evidence of significant longitudinal variability, we obtained two spectra spanning 440-680 nm at near-opposite rotational phases with the Gemini Multi-Object Spectrograph on the Gemini South telescope during the late 2024 apparition. The same solar analog was used for both observations to remove one source of inconsistency. We measured spectral slopes of 2.51% $\pm$ 0.05%/100 nm and 8.13% $\pm$ 0.05%/100 nm at the two different rotational phases. The first of these measurements was serendipitously obtained during an occultation of Menoetius by Patroclus. Although the statistical significance of the spectral slope discrepancy persists even after considering possible systematic errors stemming from differences in slit position angles and air masses between the asteroid and solar analog exposures, we consider this report of variability to be tentative. We briefly explore several scenarios that could explain the measured spectral slope variability. Additional follow-up observations are necessary to definitively confirm and characterize any inhomogeneities across the surface, which will have major implications for the 2033 flyby of Patroclus-Menoetius by the Lucy spacecraft.

Figures (2)

• Figure 1: Viewing geometries at the start times of the GMOS observations, centered in the stationary frame of Patroclus. The sky-projected disks of Patroclus and Menoetius are approximated by circles with radii of 49 and 45 km, respectively. The hatched circles indicate the shadow of Patroclus or Menoetius. The direction of Menoetius's instantaneous relative motion is indicated by the black arrows. The "N" and "E" directions denote increasing decl. and R.A., respectively. The rotational poles of the two tidally locked components point downward in these plots.
• Figure 2: Reflectance spectra of the PM binary from the two visits (blue = 2024 September 2, magenta = 2024 October 4). The best-fit spectral slopes are overplotted in green and yellow, respectively. Both spectra are normalized to unity at 550 nm. The uncertainties are inflated so that the reduced $\chi^{2}$ statistic of the linear fit is 1.