We present multi-epoch optical spectroscopy of the interstellar comet 3I/ATLAS obtained between December 2025 and January 2026 (heliocentric distances 1.8-3.3 au), yielding post-perihelion production rates and mixing ratios for CN, C3, C2, CH, and gaseous metals (Fe I and Ni I). Our results show that the coma is less depleted in C2 after perihelion than before, indicative of subsurface activation or compositional heterogeneity. The outgassing profiles reveal a pronounced perihelion asymmetry: CN and metal production rates decline more gradually outbound than inbound, consistent with the reported behavior of H2O and implying a change in the comet's activity pattern across perihelion. Despite being metal-rich relative to its H2O content, 3I follows the metal-CO correlation observed in comets of diverse origins, suggesting that gaseous metal release is more closely linked to a CO-bearing volatile reservoir than to H2O, potentially in the form of metal carbonyls. In addition, the significant residual in the [O I] λ6300 emission cannot be explained by H2O, CO2, and CO alone, indicating a contribution from additional oxygen-bearing parents.
@article{arxiv.2603.07718,
title = {Post-perihelion Coma Composition of the Interstellar Comet 3I/ATLAS from Optical Spectroscopy},
author = {Ruining Zhao and Xiliang Zhang and Bin Yang and Xiangyu Fan and Shu Wang and Yang Huang and Jifeng Liu},
journal= {arXiv preprint arXiv:2603.07718},
year = {2026}
}