Planetary nebulae are hostile environments. The central white dwarf floods the surrounding gas with ultraviolet radiation intense enough to ionize atoms, dissociate molecules, and destroy the delicate chemical bonds that hold ices together. Models of planetary nebula chemistry generally assume that ices — water, carbon dioxide, carbon monoxide frozen onto dust grains — cannot survive in this radiation field. The ices formed in the cool envelope of the asymptotic giant branch star, but once the envelope was ejected and the hot core exposed, the UV should have sublimated and photolyzed everything frozen.
Bhatt and collaborators (arXiv 2602.22366, February 2026) detect CO2 ice in NGC 6302 using JWST's mid-infrared instrument. The absorption signature is crystalline — a double-peaked profile characteristic of ordered CO2 ice, not amorphous. The ice is cold, in a region where the dust temperature is 20 to 50 Kelvin. Gas-phase CO2 coexists along the same line of sight, with a gas-to-ice ratio substantially higher than in young stellar objects where CO2 ice is commonly found.
The ice survives because of geometry. NGC 6302 has a dense equatorial torus — a thick waist of dust concentrated in the orbital plane of the original binary system. The torus is optically thick to ultraviolet radiation. Material embedded deep within it is shielded from the central star's radiation field, maintaining temperatures low enough for ices to persist. The bipolar lobes above and below the torus are irradiated and hot. The torus interior is cold and dark. The ice lives in the shadow.
This matters for astrochemistry because it means ice-surface reactions — the grain-surface chemistry that dominates in cold molecular clouds — must also operate in planetary nebulae, at least in their shielded regions. Chemical models of these environments have focused on gas-phase reactions driven by the UV field. The detection of crystalline CO2 ice indicates that a parallel cold chemistry operates in the protected interior, potentially producing complex molecules through surface catalysis on dust grains. The chemistry of a planetary nebula is not uniform — it is stratified by geometry into irradiated gas-phase zones and shielded solid-phase zones, each with its own reaction network.
The crystallinity of the ice provides additional information. Amorphous ice — the form that condenses directly from the gas phase at low temperatures — converts to crystalline ice upon heating above roughly 70 Kelvin. The crystalline signature suggests the ice experienced a thermal pulse at some point, likely during the late AGB phase when the star's luminosity spiked, then recooled. The ice remembers being heated. Its current structure records a thermal event that preceded the planetary nebula phase by thousands of years.
The fragile thing survived because something dense stood between it and the thing that would have destroyed it. The shelter was not engineered. It was a consequence of the system's angular momentum distribution — the same rotation that shaped the bipolar nebula also concentrated dust in the equator, and that concentration happened to be thick enough to block UV. Protection as geometry, not intention.