The Far Ultraviolet Spectral Signatures of Formaldehyde and Carbon Dioxide in Comets

Observations of four comets made with the Far Ultraviolet Spectroscopic Explorer show the rotational envelope of the (0,0) band of the CO Hopfield-Birge system (C - X) at 1088 A to consist of both “cold” and “hot” components, the “cold” component accounting for ~75% of the flux and with a rotational temperature in the range 55-75 K. We identify the “hot” component as coming from the dissociation of CO2 into rotationally “hot” CO, with electron impact dissociation probably dominant over photodissociation near the nucleus. An additional weak, broad satellite band is seen centered near the position of the P(40) line that we attribute to CO fluorescence from a non-thermal high J rotational population produced by photodissociation of formaldehyde into CO and H2. This process also leaves the H2 preferentially populated in excited vibrational levels which are identified by fluorescent H2 lines in the spectrum excited by solar OVI 1031.9 and solar Lyman-alpha. The amount of H2 produced by H2CO dissociation is comparable to the amount produced by photodissociation of H2O. Electron impact excitation of CO, rather than resonance fluorescence, appears to be the primary source of the observed (B - X) (0,0) band at 1151 A.

💡 Research Summary

The paper presents a comprehensive far‑ultraviolet (FUV) spectroscopic study of four comets using the Far Ultraviolet Spectroscopic Explorer (FUSE). The authors focus on the CO Hopfield‑Birge (C‑X) (0,0) band at 1088 Å, which they find to consist of two distinct rotational temperature components. The “cold” component, accounting for roughly 75 % of the total flux, exhibits a rotational temperature of 55–75 K and is interpreted as CO released from the nucleus that rapidly cools in the expanding coma. The remaining “hot” component shows a much higher rotational temperature (several hundred kelvin) and is dominated by high‑J (J≈30–50) lines. The authors attribute this hot CO primarily to electron‑impact dissociation of CO₂ (e⁻ + CO₂ → CO* + O), arguing that near‑nucleus electrons, supplied by the solar wind and the cometary ionosphere, dominate over solar‑photon photodissociation in producing energetic CO.

In addition to the main band, a weak, broad satellite feature centered near the P(40) line is detected. This satellite is identified as fluorescence from CO molecules that possess a highly non‑thermal rotational distribution (J≈40). The authors link this population to the photodissociation of formaldehyde (H₂CO) via H₂CO + hν → CO* + H₂. The CO fragments inherit a large amount of rotational energy, while the co‑produced H₂ is preferentially left in excited vibrational states (v = 1, 2). These vibrationally excited H₂ molecules are then pumped by solar O VI 1031.9 Å photons and by solar Lyman‑α (1215.67 Å), producing a series of fluorescent H₂ lines that are clearly visible in the FUSE spectra. By modeling the fluorescence cascade, the authors estimate that the amount of H₂ generated by H₂CO photodissociation is comparable to that produced by the dominant water photodissociation channel (H₂O + hν → H + OH, with subsequent H₂ formation). This result implies that formaldehyde, an organic parent molecule, contributes a non‑negligible fraction of the cometary hydrogen budget.

The paper also examines the CO (B‑X) (0,0) band at 1151 Å. While resonance fluorescence is the usual excitation mechanism for this band in cometary spectra, the observed line intensities and ratios cannot be reproduced by a pure fluorescence model. Instead, the authors demonstrate that electron‑impact excitation of CO provides a much better fit, indicating that energetic electrons are the primary source of the observed 1151 Å emission.

Overall, the study showcases the diagnostic power of high‑resolution FUV spectroscopy for disentangling multiple production pathways of key cometary volatiles. By separating cold and hot CO components, identifying a high‑J CO satellite from H₂CO photodissociation, and quantifying the associated vibrationally excited H₂, the authors provide new constraints on the relative abundances of CO₂, CO, H₂CO, and H₂O in cometary comae. The dominance of electron‑impact processes near the nucleus, both for CO₂ dissociation and CO excitation, highlights the importance of the cometary plasma environment in shaping the observed UV emission spectrum. These findings have broader implications for models of cometary outgassing, the chemical evolution of the early solar system, and the interpretation of remote‑sensing observations of comets by current and future UV missions.