Climatology and first-order composition estimates of mesospheric clouds from Mars Climate Sounder limb spectra E. Sefton-Nash a,⇑ , N.A. Teanby a , L. Montabone b,c , P.G.J. Irwin b , J. Hurley b , S.B. Calcutt b a School of Earth Sciences, University of Bristol, Queen’s Road, Bristol BS8 1RJ, UK b Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK c Laboratoire de Météorologie Dynamique, Université Pierre et Marie Curie, Paris, France article info Article history: Received 30 April 2012 Revised 7 November 2012 Accepted 7 November 2012 Available online 27 November 2012 Keyword: Mars Mars, climate Mars, atmosphere Infrared observations Atmospheres, dynamics abstract Mesospheric clouds have been previously observed on Mars in a variety of datasets. However, because the clouds are optically thin and most missions have performed surface-focussed nadir sounding, geo- graphic and seasonal coverage is sparse. We present new detections of mesospheric clouds using a limb spectra dataset with global coverage acquired by NASA’s Mars Climate Sounder (MCS) aboard Mars Reconnaissance Orbiter. Mesospheric aerosol layers, which can be CO 2 ice, water ice or dust clouds, cause high radiances in limb spectra, either by thermal emission or scattering of sunlight. We employ an object recognition and classification algorithm to identify and map aerosol layers in limb spectra acquired between December 2006 and April 2011, covering more than two Mars years. We use data from MCS band A4, to show thermal signatures of day and nightside features, and A6, which is sensitive to short wave IR and visible daytime features only. This large dataset provides several thousand detections of mesospheric clouds, more than an order of magnitude more than in previous studies. Our results show that aerosol layers tend to occur in two distinct regimes. They form in equatorial regions (30°S–30°N) during the aphelion season/northern hemisphere summer (L s < 150°), which is in agreement with previous published observations of mesospheric clouds. During perihelion/dust storm season (L s > 150°) a greater number of features are observed and are distributed in two mid-latitude bands, with a southern hemisphere bias. We observe temporal and longitudinal clustering of cloud occur- rence, which we suggest is consistent with a formation mechanism dictated by interaction of broad tem- perature regimes imposed by global circulation and the propagation to the mesosphere of small-scale dynamics such as gravity waves and thermal tides. Using calculated frost point temperatures and a parameterization based on synthetic spectra we find that aphelion clouds are present in generally cooler conditions and are spectrally more consistent with H 2 O or CO 2 ice. A significant fraction has nearby temperature retrievals that are within a few degrees of the CO 2 frost point, indicating a CO 2 composition for those clouds. Perihelion season clouds are spec- trally most similar to H 2 O ice and dust aerosols, consistent with temperature retrievals near to the clouds that are 30–80 K above the CO 2 frost point. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Martian mesospheric clouds have been detected using a variety of infrared, ultraviolet and visible wavelength datasets at various points in the martian day and year, but limited spatial and seasonal coverage of observations has resulted in gaps in our understanding of cloud occurrence and composition over the martian globe and throughout the martian year. Aerosol layers are composed of either dust, water ice or CO 2 ice, but a wide range of physical parameters contribute to their spectral signature, which makes unambiguous identification of the their composition challenging. However, infor- mation pertaining to cloud composition is often discussed via re- trieval and analysis of their associated pressure–temperature conditions and comparison of broad spectral characteristics to those of modelled spectra. Detached layers composed of mesospheric dust aerosols were first observed in Mariner 9 limb measurements (Anderson and Leovy, 1978). Dust clouds up to 50 km have been identified in Viking limb images (Jaquin et al., 1986) and brightness maxima at up to 70 km altitude have been reported during southern summer (L s J 180°)(Jaquin, 1988). Maxima between 50 and 70 km were also observed in visible and infrared limb observations by NASA’s Thermal Emission Spectrometer (TES) during the 2001 dust storm (Cantor, 2007; Clancy et al., 2003), while Smith (2003) calculated 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.11.012 ⇑ Corresponding author. Present address: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA. E-mail address: esn@ucla.edu (E. Sefton-Nash). Icarus 222 (2013) 342–356 Contents lists available at SciVerse ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus