What shall we do with a comet that is seen in a telescope, but is like no comet made in the neighbourhood of the Sun? The interstellar traveler titled 3I/ATLAS has made that question a practical problem to spectators: how to read a transient, quick object when the “rules of thumb” of composition which are generally applied to comets are not applicable. Having few interstellar objects ever verified, each data set is an appendix and a stress test to comet science.
One of the major components of that stress test came in the form of infrared spectroscopy. According to the researchers with the help of the Near-Infrared Spectrograph of the James Webb Space Telescope, the coma was dominated by carbon-dioxide with nearly eight times more carbon-dioxide than water vapor. Water vapor is generally the primary contributor of activity in most of the comets of the solar system when a nucleus becomes heated, and CO2 is a secondary factor in the activity. In case of 3I/ATLAS, the pendulum swung the other way. The measurement of that one compels a re-examination of a popular assumption: that the processes of brightness and outgassing around the inner solar system are, a priori, water-led processes. The implication does not just do chemical bookkeeping; it alters the way scientists deduce the mass loss, the size of a nucleus based on coma behavior and makes comparisons between objects which did not share the same environment of solar system formation.
Intimate viewing geometry was able to assist, as well the 3I/ATLAS came within the field of view of spacecraft at Mars. One of the nearest views that NASA expected to get with Mars Reconnaissance Orbiter was made with the ultraviolet spectrograph of MAVEN, which revealed the extended hydrogen and hydroxyl emissions related to the chemistry of water in the coma. The HiRISE images, with an average pixel size of about 30 kilometers, observed the object as a small bright coma information that narrowed the possibilities of size and properties of particles in the nucleus, and not merely created a more detailed image. According to Shane Byrne, “observations of interstellar objects are uncommon enough to know something new each time.”
The reason why these measurements, so heavily pressed upon, are so indirect is not entirely uncovered: and there is a tendency to infer comet composition indirectly, and sometimes falsely. Based on a new interpretation of in situ measurements of Rosetta at comet 67P/Churyumov-Gerasimenko, dust in the coma was found to have the potential to bias the deuterium-to-hydrogen ratio (D/H) upwards towards the nucleus, and that the gas best mixed farther away provided a more terrestrial value. In that, dust grains serve as chemical confounders ice on dust is being sublimated, and locally alters what an instrument believes the comet to be. The lesson is generalized: a coma is not a glass jar of suppository. It is an active, spatially-organised reactor which is driven by grain physics, solar heating and time-dependent outgassing.
It is against such a background that 3I/ATLAS would not be just a curiosity. A CO 2 rich coma seen in the infrared, and ultraviolet imaging of hydrogen bearing species show that “comet” is an observational category, but not a compositional one. The engineering fact, or what the instruments can measure, at what wavelengths, and at what distance, is now brought nearer to the heart of interpretation. As sample collection of an object in interstellar space is still beyond reach, the science is being driven to be more cautious about cross-calibration of remote spectra, vantage points of spacecraft, and models where dust is more of an active component than a clutter component.
