“What does it mean when a comet carries more carbon dioxide than water?” This question is now at the heart of the most unprecedented observation campaign in history in response to the passage of an interstellar object. Comet 3I/ATLAS is just the third confirmed interstellar object observed so far and has yielded a volatile composition that has never been witnessed in our own solar neighborhood.
While the infrared spectroscopy carried out by the James Webb Space Telescope has been the most precise chemical analysis yet conducted, the information was gathered with the help of the JWST’s Near Infrared Spectrograph’s integral field unit employing the rovibrational emission bands for CO₂, CO, and H₂O, as well as dust and ice absorption features. It was seen that the coma was dominated by CO₂ with a ratio of CO₂: H₂O = 7.6 ± 0.3. This is 4.5σ away from the trend for solar system comets at the same heliocentric distance. This was measured with the rate of 1.70 × 10²⁷ s⁻¹ for CO₂ and 2.23 × 10²⁶ s⁻¹ for the rate of H₂O emission. This ratio is extremely high and could indicate either the CO₂ nature of the nucleus or the reduced sublimation rate due to the albedo or low thermal conductivity of the nucleus.
Ultraviolet data from NASA’s Europa Clipper, equipped with its Ultraviolet Spectrograph (Europa-UVS), brought additional specificity. With its distinct sunward perspective, which was available when Earth and Mars based platforms were sunlit and therefore invisible, Europa-UVS was able to observe elementary transitions involving the splitting of water molecules into hydrogen and oxygen and analyze dust and plasma tails from “behind” in order to determine their three dimensional morphology. Such a sunward perspective was employed to map non uniform patterns of outgassing, where the distinct distributions of CO₂, CO, and H₂O are associated with their respective sublimation temperatures.
As 3I/ATLAS went by the Sun within the range of ground telescopes, the Parker Solar Probe, of NASA, provided visual observation. With the WISPR imager on the Parker Solar Probe, images were taken ~ 10 times a day between October 18 and November 5, 2025, to record the shape of the tail of the comet when passing by the Sun at its perihelion. Since the observation was done near the Sun, the orientation of the spacecraft was highly controlled, coupled with sophisticated image analysis techniques to reduce glare due to the coronal light.
Complementary high energy observations were provided by ESA’s XMM Newton and Japan/NASA’s XRISM X-ray observatories. The XRISM satellite’s Xtend telescope showed X-rays out to 400,000 kilometers from the nucleus, characteristic of collisions between solar wind ions and neutral gases in the coma. Spectroscopic analysis showed the presence of carbon, nitrogen, and oxygen, which are hard to spot in other wavelengths. The EPIC pn camera on XMM Newton confirmed the X-ray emission and showed gradients of where the solar wind and coma interacted most strongly.
The value of such data sets is immense in the engineering and plasma physics communities. The interaction area generated by solar wind and comet interactions can serve as a meaningful laboratory for mass loading processes, where plasma flows and the topology of the magnetic fields are altered by ionized volatiles. Multi fluid models and more general MHD models, improved by data gathered by earlier missions like Giotto’s flyby of comets, are ready for use in 3I/ATLAS missions related to the generation of bow shocks, draping of tails, and turbulence at the boundary condition of the interstellar medium composition. The high CO₂ concentrations will pose a different ionization balance, potentially impacting the population of pickup ions compared to water filled comets.
Its chemical uniqueness is also oscillating in harmony with research on protoplanetary cloud chemistry. The process of CO2 production via reactions of CO and OH on dust grains is a competing process to the formation of H2O, and simulations have suggested that either UV or cosmic ray irradiation could cause it to favor the production of CO2 either exterior to the location of the CO2 ice line or above it. In both these cases, the original thick disk, low metallicity star like precursor to 3I/ATLAS would be sufficient, and these ratios of volatiles could be considered a fossil record of unusual, distant planetary formation site(s) elsewhere within our galaxy.
For amateur sky watchers, 3I/ATLAS is currently a faint but easily accessible target, standing at a brightened 12.8 magnitudes, which is currently situated close to the constellation Leo. However, due to its strongly hyperbolic path, there is no doubt that it is on a temporary visit, but all together, it is apparent that JWST, along with the Europa Clipper spacecraft, Parker Solar Probe, XRISM, and XMM-Newton, in the aggregated capacity of these respective research groups, highlight just how contemporary spacecraft can be leveraged together to capture a holistic cross section perspective of a temporary visitor to our world on every level, before it travels off into interstellar space.
