“We just cracked open the door to a whole new world of chemistry that we never had access to before,” said Thomas Puzia of Pontifical Catholic University of Chile, looking back on the incredible discoveries made on comet 3I/ATLAS. Discovered just on July 1, 2020, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) at Chile’s El Teatro National del Arte (National Theatre), this interstellar object is already one of a kind with its highly eccentric orbit of 6.13 the record for any passerby to our solar system and its speed of 68 km/s, which is far too fast to ever remain tied to our Sun’s gravity. Its orbit points to a distance traced back to the thick disk of our Milky Way galaxy. This population of stars is billions of years older than our own solar system, with ages potentially above 7.6 billion years up to 11 billion years.
The rarity of an object like this has a lot to do not only with its origin but also its chemical and physical characteristics. Data collected using the James Webb Space Telescope showed a coma composition that was very high in carbon dioxide, a ratio of CO2/H2O of 7.6 ± 0.3, 18 times higher than normal for comets at a similar heliocentric distance. This makes 3I/ATLAS just one of two objects known, C/2016 R2 being the other, which are ‘enriched in CO2’ to an unusual degree. The JWST also identified very high levels of methanol, nearly four times more than in local comets, alongside Hydrogen Cyanide, Nickel, and Cyanide gas. Methanol evaporation rates of 40 kg/s accounted for 8% of total vapor emission, compared to 2% in similar objects within our solar system.
The morphologies of the coma, determined using the NIRSpec IFU on the JWST, were uneven in terms of outgasing: the CO₂ and CO emissions were more symmetric, while that of water vapor was more strongly sunward, following the differences in sublimation temperatures: Tsub(H₂O) > Tsub(CO₂) > Tsub(CO). The micrometer-scale icy grains were responsible for broad absorptions around 3.0 μm, implying the presence of both crystalline and amorphous ices. Production rate studies indicated active surfaces of 3.1 km² for CO₂ and 2.2 km² for H₂O, with CO₂ likely responsible for the ejective dust process, even at 3.3 AU.
Orbital dynamics further corroborated activity dominated by volatiles. Non-gravity forces acting on acceleration, produced using ephemerides from NASA Jet Propulsion Laboratory’s HORIZONS, indicated a statistically significant radial and transversely anisotropic signal indicative of asymmetric activity. Thermophysical modeling confirmed that activity dominated by CO could explain acceleration for nucleus radii in 0.5 to 3 km, and reasonable activity extents and collimation coefficients (η = 2 to 3). NH3 and CH4 were subdominant volatiles, and CO2’s low vapor pressure made its activity negligible over this particular orbital distance. These phenomena place 3I/ATLAS in the realm of actively volatilating interstellar comets, like 2I/Borisov, and drastically different from 1I/’Oumuamua, which remained an enigma and did not expel its volatiles.
The detection of a distance object such as this would be a job for world survey telescopes. The early detection of ATLAS would be aided by rapid follow-up observations worldwide. However, the future Vera C. Rubin Observatory would represent a revolution in interstellar object detection. The Legacy Survey of Space and Time mission would be designed to discover between 5 and 50 interstellar objects over the coming decade. This would be a survey of the entire visible sky at a sensitivity level sufficient to make it an “ISO hunter” in every way at a scale. The interstellar objects are small and could easily be confused with stars in the background. It takes movement to distinguish them.
The possible chemical residues of interstellar processing in 3I/ATLAS might be the result of exposure to cosmic ray-induced chemistry or UV-induced chemistry over its billion-year lifetime. Analogues with similar chemistry have been observed to produce CO _{2} through the CO+OH reactions with interstellar dust, rather than producing excessive levels of H _{2} O. High methanol concentrations could be a signature of reactions between liquid water and metal-laden materials believed to be present in the nucleus.
As 3I/ATLAS orbits the sun and passes perihelion at 1.4 AU and heads into the constellation of Gemini, scientists will be watching for changes similar to what was observed in the Borisov comet and could thus potentially be able to determine differences between volatiles altered through surface processes and pristine ices. Thanks to the Virtual Telescope Project’s live webcam, the comet will be visible online before being lost in interstellar space a star system’s fossil record.
