Pre-perihelion evolution of the NiI/FeI abundance ratio in the coma of the interstellar comet 3I/ATLAS: From extreme to normal
- Details
- Published on 30 January 2026
Vol. 706
10. Planets, planetary systems, and small bodies
Pre-perihelion evolution of the NiI/FeI abundance ratio in the coma of the interstellar comet 3I/ATLAS: From extreme to normal
Discovered on July 1, 2025, and with prediscovery observations in June, 3I/Atlas is the third confirmed interstellar object, after asteroid 1I/ʻOumuamua and comet 2I/Borisov. Revealed to be a remarkably active comet immediately after discovery, 3I/Atlas has since been the target of countless photometric and spectroscopic observations from the ground and space, with, for example, JWST data indicating a CO2-dominated and icy-grain-rich coma at 3.3 au from the Sun. Optical/near UV spectroscopy, which mostly samples radicals and atoms, further indicated that 3I/Atlas is a C2-depleted comet, but rich in Ni I, detected as far as 3.88 au pre-perihelion. Based on VLT/UVES and X-SHOOTER data, Hutsemékers et al. here report the first detection of Fe I in 3I/Atlas, as well as the monitoring of the NiI and FeI production rates along its inbound orbit. Compared to Solar-System comets and to 2I/Borisov, 3I/Atlas features an exceptionally high (NiI+FeI) production rate. Most remarkably, the NiI/FeI ratio was initially measured to be very large (~20 at 2.64 au) but became consistent with that found for other C2-depleted comets at distances below 2 au. Ni and Fe in comets have been proposed to originate from the sublimation of carbonyls Ni(CO)4 and Fe(CO)5 and their attendant photodissociation. In this picture, the lower sublimation temperature of Ni(CO)4 versus Fe(CO)5 would explain the preferentially high Ni/Fe ratio at large distances. The scenario could not have been confirmed in Solar-System comets but appears here supported by the 3I/Atlas data. Soon-to-be-expected post-perihelion observations should help clarify the trend with the heliocentric distance, possibly ultimately revealing the intrinsic Ni/Fe ratio in this interstellar visitor.