All eyes in the astronomical community are currently focused on 3I/ATLAS, our most recent interstellar visitor discovered in early July. With its relatively short observational window within our solar system and its impending perihelion in October, significant observational efforts are being directed toward this fascinating object. Among the numerous telescopes scrutinizing 3I/ATLAS is the James Webb Space Telescope (JWST), which has made groundbreaking discoveries about the comet's coma, the region surrounding its nucleus.
A recent paper published on the pre-print server arXiv, led by Martin Cordiner from NASA's Goddard Space Flight Center and the Catholic University of America, reveals that the coma of 3I/ATLAS is unlike any other previously observed. The primary component of its coma is carbon dioxide (CO2), which is significant as it comprises a staggering ratio of 8:1 when compared to water, marking the highest ratio ever recorded in a comet. This value is six standard deviations above the typical measurements found in other comets.
In addition to carbon dioxide, the coma of 3I/ATLAS is also composed of water, carbon monoxide (CO), and carbonyl sulfide. Interestingly, the carbon monoxide to water ratio is consistent with previous observations, standing at 1.4, which indicates a familiar pattern in comet composition. The JWST utilized its NIRSpec infrared camera to capture these chemical signatures on August 6th, when the comet was located 3.32 AU from the sun.
Further observations provide additional insights into the coma's structure. The JWST detected water and dust dispersed throughout the coma, with a notable increase in dust concentration on the side facing the sun. This phenomenon is typical, as higher outgassing occurs on the sunlit side due to increased heating. Moreover, the study revealed that the ratios of carbon isotopes, specifically Carbon-12 and Carbon-13, closely resemble those found on Earth. This similarity suggests that the material forming 3I/ATLAS was created in an environment with comparable carbon species.
Several theories could explain the unusual carbon dioxide to water ratio observed in 3I/ATLAS. One possibility is that the object originated in a host star system characterized by high levels of ultraviolet radiation. Another explanation posits that 3I/ATLAS was formed beyond the CO2 ice line, where carbon dioxide ice is more prevalent than water. Additionally, the way heat from the sun interacts with the comet's nucleus may play a role; if the nucleus is more resistant to heating, carbon dioxide—which has a lower melting point than water—would sublimate first. This could account for the observed lopsided ratio, despite the presence of ample water stored within the nucleus, ready to be released as the comet approaches the sun.
This is only the third confirmed interstellar visitor we have observed, with the first, 'Oumuamua, lacking the brightness necessary to capture its coma's spectra. The second interstellar visitor, 2I/Borisov, serves as our only point of comparison for coma spectra among interstellar comets. Interestingly, 2I/Borisov exhibited a higher carbon monoxide to water ratio than 3I/ATLAS, suggesting that each interstellar visitor offers unique insights into the composition of comets.
As we continue to study 3I/ATLAS, more research papers are anticipated, including additional findings from the JWST. With only a few weeks remaining before the comet passes too close to the sun to be visible, astronomers are eager to gather as much data as possible. Once it reaches perihelion in early October, visibility will be hindered, though there remains a chance that some probes near Mars may capture glimpses of it during this period. Ultimately, when 3I/ATLAS becomes visible again in December, it will be on its exit trajectory from our solar system, likely having shed much of the material it contained.
