Airglow on Mars: Some model expectations for the OH Meinel bands and the O 2 IR atmospheric band

Abstract

This work presents model calculations of the diurnal airglow emissions from the OH Meinel bands and the O 2 IR atmospheric band in the neutral atmosphere of Mars. A time-dependent photochemical model of the lower atmosphere below 80 km has been developed for this purpose. Special emphasis is placed on the nightglow emissions because of their potential to characterize the atomic oxygen profile in the 50–80 km region. Unlike on Earth, the OH Meinel emission rates are very sensitive to the details of the vibrational relaxation pathway. In the sudden death and collisional cascade limits, the maximum OH Meinel column intensities for emissions originating from a fixed upper vibrational level are calculated to be about 300 R, for transitions v ′ = 9 → v ′ ⩽ 8 , and 15,000 R, for transitions v ′ = 1 → v ′ = 0 , respectively. During the daytime the 1.27 μm emission from O 2( a Δ g 1 ), primarily formed from ozone photodissociation, is of the order of MegaRayleighs (MR). Due to the long radiative lifetime of O 2( a Δ g 1 ), a luminescent remnant of the dayglow extends to the dark side for about two hours. At night, excited molecular oxygen is expected to be produced through the three body reaction O + O + CO 2. The column emission of this nighttime component of the airglow is estimated to amount to 25 kR. Both nightglow emissions, from the OH Meinel bands and the O 2 IR atmospheric band, overlap in the 50–80 km region. Photodissociation of CO 2 in the upper atmosphere and the subsequent transport of the atomic oxygen produced to the emitting layer are revealed as key factors in the nightglow emissions from these systems. The Mars 5 upper constraint for the product [H][O 3] is revised on the basis of more recent values for the emission probabilities and collisional deactivation coefficients.