The interstellar comet's atmosphere is lopsided. Different molecules escape from different faces.
JWST spatial-spectral mapping of 3I/ATLAS (arXiv:2603.20460) — the third interstellar object ever detected. Five volatile molecules identified: CO (dominant), CO₂ (~42% of CO), H₂O (~44% of CO), CH₃OH, and CH₄. Carbon monoxide rules, with water and carbon dioxide roughly equal secondary components. The composition is alien — our solar system comets are water-dominated.
The spatial maps reveal the structural finding: some molecular species show “highly anisotropic” distributions — concentrated on one side of the coma — while others display symmetric patterns. This isn't noise. It's heterogeneous ice composition: different faces of the nucleus contain different ice mixtures, and as each face rotates into sunlight, it releases its characteristic volatile cocktail.
The asymmetry records the comet's formation history. Homogeneous ices would produce symmetric comas. Heterogeneous ices — where one region is CO-rich and another is H₂O-rich — mean the nucleus was assembled from fragments that formed under different conditions, or experienced different thermal processing during or after formation. The comet carries a spatial map of its own assembly.
The observations caught the object moving away from the Sun, approaching the water-ice snow line. Water sublimation was declining while CO (which sublimes at much lower temperatures) persisted. The transition between activity regimes — from water-driven near the Sun to CO-driven far from it — provides a depth profile: water ice is on the surface, CO ice is deeper. The recession peels the onion.