Monte Carlo Simulation of Metastable Oxygen Photochemistry in Cometary Atmospheres

Abstract

Cometary atmospheres are produced by the outgassing of material, mainly H 2 O, CO, and CO 2 from the nucleus of the comet under the energy input from the Sun. Subsequent photochemical processes lead to the production of other species generally absent from the nucleus, such as OH. Although all comets are different, they all have a highly rarefied atmosphere, which is an ideal environment for nonthermal photochemical processes to take place and influence the detailed state of the atmosphere. We develop a Monte Carlo model of the coma photochemistry. We compute the energy distribution functions (EDF) of the metastable O( 1 D) and O( 1 S) species and obtain the red (630 nm) and green (557.7 nm) spectral line shapes of the full coma, consistent with the computed EDFs and the expansion velocity. We show that both species have a severely non-Maxwellian EDF, that results in broad spectral lines and the suprathermal broadening dominates due to the expansion motion. We apply our model to the atmosphere of comet C/1996 B2 (Hyakutake) and 103P/Hartley 2. The computed width of the green line, expressed in terms of speed, is lower than that of the red line. This result is comparable to previous theoretical analyses, but in disagreement with observations. We explain that the spectral line shape does not only depend on the exothermicity of the photochemical production mechanisms, but also on thermalization, due to elastic collisions, reducing the width of the emission line coming from the O( 1 D) level, which has a longer lifetime.

Scientific problems addressed by the Spektr-UV space project (world space Observatory—Ultraviolet)

“…The main component of cometary nuclei-water molecules-is manifest in the coma as atoms of H and O and OH radicals. All three types of particle have very strong transitions in the UV, where these three types of emission can be simultaneously detected with high spatial resolution [242]. Other parent molecules, such as CO and CO 2 , can also be studied through their UV line emission.…”

Transport and Distribution of Hydroxyl Radicals and Oxygen Atoms from H2O Photodissociation in the Inner Coma of Comet 67P/Churyumov–Gerasimenko

“…If we only consider the number distribution of neutral oxygen atoms, a similar curve will result. However, the excited state of O( 1 D) is subject to collisional quench with a quenching cross section of 1.8 9 10 -16 cm 2 (Bisikalo et al 2015; in the innermost region of the coma before the radiative decay time of 130 s. Figure 8 shows the column number densities (N) of OH and OI, respectively. It is noted that N(OH) ) N(O( 1 D)) because the branching ratio of H 2 O ?…”