The O 2 nightglow in the martian atmosphere by SPICAM onboard of Mars-Express A.A. Fedorova a,b,⇑ , F. Lefèvre c , S. Guslyakova a,b , O. Korablev a,b , J.-L. Bertaux c , F. Montmessin c , A. Reberac c , B. Gondet d a Space Research Institute RAS, Profsoyuznaya 84/32, Moscow 117997, Russia b Moscow Institute of Physics and Technology (MIPT), 9 Institutsky Per., 141700 Dolgoprudny, Moscow Region, Russia c LATMOS – UVSQ/UPMC/CNRS, 11 bd d’Alembert, 78280 Guyancourt, France d IAS, Centre universitaire d’Orsay, Bât 120-121, 91405 Orsay Cedex, France article info Article history: Received 26 August 2011 Revised 27 March 2012 Accepted 27 March 2012 Available online 03 April 2012 Keywords: Mars Atmosphere Infrared observations Aeronomy abstract We present observations of the O 2 (a 1 D g ) nightglow at 1.27 lm on Mars using the SPICAM IR spectrom- eter onboard of the Mars Express orbiter. In contrast to the O 2 (a 1 D g ) dayglow that results from the ozone photodissociation, the O 2 (a 1 D g ) nightglow is a product of the recombination of O atoms formed by CO 2 photolysis on the dayside at altitudes higher than 80 km and transported downward above the winter pole by the Hadley circulation. The first detections of the O 2 (a 1 D g ) nightglow in 2010 indicate that it is about two order of magnitude less intense than the dayglow (Bertaux, J.-L., Gondet, B., Bibring, J.-P., Montmessin, F., Lefèvre, F. [2010]. Bull. Am. Astron. Soc. 42, 1040; Clancy et al. [2010]. Bull. Am. Astron. Soc. 42, 1041). SPICAM IR sounds the martian atmosphere in the near-IR range (1–1.7 lm) with the spec- tral resolution of 3.5 cm 1 in nadir, limb and solar occultation modes. In 2010 the vertical profiles of the O 2 (a 1 D g ) nightside emission have been obtained near the South Pole at latitudes of 82–83°S for two sequences of observations: L s = 111–120° and L s = 152–165°. The altitude of the emission maximum var- ied from 45 km on L s = 111–120° to 38–49 km on L s = 152–165°. Averaged vertically integrated intensity of the emission at these latitudes has shown an increase from 0.22 to 0.35 MR. Those values of total ver- tical emission rate are consistent with the OMEGA observations on Mars-Express in 2010. The estimated density of oxygen atoms at altitudes from 50 to 65 km varies from 1.5  10 11 to 2.5  10 11 cm 3 . Compar- ison with the LMD general circulation model with photochemistry (Lefèvre, F., Lebonnois, S., Montmessin, F., Forget, F. [2004]. J. Geophys. Res. 109, E07004; Lefèvre et al. [2008]. Nature 454, 971–975) shows that the model reproduces fairly well the O 2 (a 1 D g ) emission layer observed by SPICAM when the large field of view (>20 km on the limb) of the instrument is taken into account. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction The dayglow of molecular oxygen O 2 (a 1 D g ) in 1.27 lm band on Mars is known for a long time. Formed as a result of ozone photol- ysis it serves as a tracer of the ozone abundance in the martian atmosphere, allowing to observe its spatial and seasonal variations. The emission was predicted after the discovery of ozone in the atmosphere of Mars in UV spectra recorded from Mariner 7 (Barth and Hord, 1971). For the first time the emission was detected by Noxon et al. (1976) from the ground using Doppler shift of martian lines in the spectrum recorded with resolving power of 400,000. These observations were continued by Traub et al. (1979). Since then the study of the oxygen dayglow has been supported by ground-based observations (Krasnopolsky and Bjoraker, 2000; Krasnopolsky, 2003, 2009; Novak et al., 2002), and from the orbit, using SPICAM and OMEGA spectrometers on Mars Express and CRISM on Mars Reconnaissance Orbiter (Altieri et al., 2009; Fedorova et al., 2006a; Clancy et al., 2011). The maximal values observed in the early spring in both hemispheres near the polar regions reach 30 MR. On the nightside the nature of the emission is different. Singlet oxygen is a product of atomic oxygen recombination that in turn is a product of photolysis of atmospheric gases on the dayside. This process is well-known both on Earth, and on Venus. In the upper atmosphere of Venus the oxygen emission on the nightside allows to study a circulation of the atmosphere at altitudes of 90–110 km (in the upper mesosphere and the lower thermosphere) (Bougher and Borucki, 1994). In mesosphere the superrotation is dominat- ing, while in thermosphere a motion from a subsolar to antisolar point is prevailing (so-called SS-AS circulation) with some variable contribution of the superrotation zone. The SS-AS circulation brings the oxygen atoms, formed as a result of CO 2 photodissocia- tion in the dayside, to the nightside. At the antisolar point, the air descends, and the excited singlet delta state of oxygen molecules 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.03.031 ⇑ Corresponding author at: Space Research Institute RAS, Profsoyuznaya 84/32, Moscow 117997, Russia. E-mail address: fedorova@iki.rssi.ru (A.A. Fedorova). Icarus 219 (2012) 596–608 Contents lists available at SciVerse ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus