“I have never seen such a strong CO2 peak in a comet spectrum”. That signal came as an interstellar object tore through the inner solar system at a speed of about 250,000 kilometers per hour and was fast enough to cause an average comet to appear small. Interstellar comet 3I/ATLAS was not lingering. It provided a small viewing perspective, an intense solar background, and a scarcity of opportunities to quantify chemistry that was constructed around another star.
The closest glance was when Mars based assets moved away out of their normal targets. The Mars Reconnaissance Orbiter swiveled the HiRISE, which was designed to decode the surface finer details of Mars, to a dim, moving object. At 19 miles per pixel the comet appeared as a small, pixelated sphere: not the nucleus, but a brilliant coma of dust and ice grains thrown into space. That patch of signal is important as it limits the size of the nucleus and the concentration of debris around it, which dictate the effect of sunlight and solar wind on the visible form of the coma.
MAVEN introduced another type of leverage. Between September 27, 2025, and more than 10 days later, on September 27, 2025, its Imaging Ultraviolet Spectrograph photographed the comet across several ultraviolet bands and then extended it to higher-resolution images which are intended to isolate hydrogen, one of the most obvious of the water tracers being torn apart by sunlight. These ultraviolet measurements also provided a maximum value on the deuterium to hydrogen ratio in the comet, an ancient indicator of where a body of ice was formed and how it developed prior to approaching the Sun. “The detections we are seeing are significant, and we have only scraped the surface of our analysis”, the principal investigator of MAVEN.
Nonetheless, the most significant chemistry demanded an observatory beyond the atmosphere of the earth. James Webb space telescope measured a coma with approximately eight times the amount of CO2 than the amount of water vapor using its infrared spectrograph. The air in the atmosphere of the earth absorbs major wavelengths falling around the CO2 band, thus Webb was the only tool capable of making the measurement clean. This counter-intuitive ratio is such a powerful compositional fingerprint in a discipline where water usually prevails in comet activity around the Sun, than a gentle adaptation to known archetypes.
Scientific literature independent analysis has placed that ratio as being unusually extreme in those comets observed at similar distances, with a CO2/H2O mixing ratio of 7.6 +- 0.3, and characterized as much higher than the trend observed in most of the solar system comets. The same work substantiates the open trajectory of the visitor, stating an inbound orbital eccentricity 6.144 + 0.016 and a heliocentric velocity of about 57.95 + 0.05 km/s, which would be the case of an object that came out interstellar space rather than reentry on an extended solar orbit.
When the comet passed Mars, Juice spacecraft, a vehicle maintained by ESA, took a shot of a navigation-camera, showing that the comet was in a different stage of the journey. The Juice team chose to only downlink a portion of an image early and had to say, when the clearly seen comet was surrounded by indications of action, to their surprise. The same update recounts a suggestion of two tails, isolating electrically charged gas, dust, the observational point that the look of a comet is a chart of activities that interact, rather than one line of gas.
There was no single campaign that “solve” 3I/ATLAS. It created a record of multi-wavelengths-ultraviolet limits on water dissolution, the infrared signs of carbon-rich outgassing and imaging which grounds models of dust production-assembled fast enough to snare an object which will not recur.
