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 (1)Sigma(+)- X-1 Sigma(+)) at 1088 angstrom to consist of both "cold" and "hot" components, the "cold" component accounting for similar to 75% of the flux and with a rotational temperature in the range 55 K-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 H-2. This process also leaves the H-2 preferentially populated in excited vibrational levels which are identified by fluorescent H-2 lines in the spectrum excited by solar O VI lambda 1031.9 and solar Ly alpha. The amount of H-2 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-1 Sigma(+)- X-1 Sigma(+)(0,0) band at 1151 angstrom.