Elsevier

Icarus

Volume 222, Issue 1, January 2013, Pages 342-356
Icarus

Climatology and first-order composition estimates of mesospheric clouds from Mars Climate Sounder limb spectra

https://doi.org/10.1016/j.icarus.2012.11.012Get rights and content

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, geographic 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 CO2 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 (Ls < 150°), which is in agreement with previous published observations of mesospheric clouds. During perihelion/dust storm season (Ls > 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 occurrence, which we suggest is consistent with a formation mechanism dictated by interaction of broad temperature 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 H2O or CO2 ice. A significant fraction has nearby temperature retrievals that are within a few degrees of the CO2 frost point, indicating a CO2 composition for those clouds. Perihelion season clouds are spectrally most similar to H2O ice and dust aerosols, consistent with temperature retrievals near to the clouds that are 30–80 K above the CO2 frost point.

Highlights

► We detect mesospheric aerosol layers in limb-spectra from Mars Climate Sounder. ► Aphelion season layers are mostly equatorial, high altitude and cold. ► Layers in perihelion season are mostly mid-latitude, lower altitude and warmer. ► Layers are spectrally similar to H2O or CO2-ice. ► Temperatures proximal to some aphelion layers are close to the CO2 frost point.

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 CO2 ice, but a wide range of physical parameters contribute to their spectral signature, which makes unambiguous identification of the their composition challenging. However, information pertaining to cloud composition is often discussed via retrieval 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 (Ls  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 high dust optical depths above five scale heights (∼50 km) for southern summer.

Detections from orbit of mesospheric clouds at visible wavelengths were made using Mars Orbital Camera (MOC) limb images acquired during Mars Years 24–26. Detections were corroborated by spatially and temporally coincident nadir spectra acquired by TES. Results showed a typical altitude range of 60–80 km but cloud composition could not be determined (Clancy et al., 2004).

However, the lack of detectable infrared radiances at these altitudes led Clancy et al. (2007) to constrain particle sizes to ⩽1 μm for water and ⩽1.5 μm for CO2 ice compositions. CO2 ice clouds have been spectrally identified in data from ESA’s OMEGA imaging spectrometer (Montmessin et al., 2007, Scholten et al., 2010, Määttänen et al., 2010, Vincendon et al., 2011) and NASA’s CRISM imaging spectrometer (Vincendon et al., 2011).

However, some high-altitude clouds that were previously classified as CO2 ice in SPICAM observations (Montmessin et al., 2006) might be instead of water ice composition, as suggested by one OMEGA limb observation of an H2O ice cloud observed at similar longitudes and season (Vincendon et al., 2011). This is consistent with the fact that no instrument sensitive to CO2 ice appears to have observed the peak in occurrence of mesospheric equatorial clouds at 145°  Ls  160° identified by Clancy et al. (2007). Many authors have suggested a water ice composition for some mesospheric clouds below about 80 km (Clancy et al., 2004, Clancy et al., 2007, Fedorova et al., 2009, McConnochie et al., 2010, Vincendon et al., 2011).

So called ‘blue wave’ clouds observed above 70 km altitude were imaged by the Pathfinder camera (Smith et al., 1997) and were suggested to be composed of CO2 ice, rather than water ice aerosols (Clancy and Sandor, 1998), since temperatures derived from a density profile inferred from the spacecraft’s deceleration during descent were below the CO2 condensation temperature between approximately 79–85 km (Schofield et al., 1997). Aerosol layers and proximal super-cold (below CO2 condensation temperature) pockets detected between 80 and 100 km using stellar occultation techniques and data from the SPICAM ultraviolet spectrometer aboard Mars Express, led Montmessin et al. (2006) to hypothesise that they could be CO2 ice clouds.

Early retrievals of martian dayside mesospheric temperatures were performed using data returned by the Mariner 6 and 7 (Herr and Pimentel, 1970) and Viking missions (Seiff and Kirk, 1977) as well as from observations using CO2 laser emission (Deming et al., 1983, Johnson et al., 1976) and stellar occultation techniques (Montmessin et al., 2006). However, the wide range of retrieved mesospheric temperatures has encouraged debate as to whether atmospheric conditions frequently become cool enough to form CO2 ice clouds or whether water ice or dust is a more likely candidate. Using modelled spectra to distinguish between water ice and CO2 ice is often challenging due to the large number of unknowns regarding aerosols such as particle shape, heterogeneous cloud structure, line of sight effects, scattering by low altitude clouds with heterogeneous structure, and scattering of radiance from the unconstrained lower atmosphere.

Potential causes of mesospheric clouds have been reported as convection (Montmessin et al., 2007) thermal tides (Clancy and Sandor, 1998, Clancy et al., 2007, Forbes and Miyahara, 2006, González-Galindo et al., 2011, Lee et al., 2009, Määttänen et al., 2010) and ‘cold pockets’ (Clancy and Sandor, 1998) formed by conditions favouring the propagation of gravity waves into the mesosphere (Spiga et al., 2012).

Conditions associated with cloud formation have been compared to output from the Laboratoire de Météorologie Dynamique (du CNRS) global circulation model (LMD-GCM) (Forget et al., 1999, Forget et al., 2008) by González-Galindo et al. (2011) and to output from the LMD Mesoscale GCM (Spiga and Forget, 2009) by Spiga et al. (2012). In general mesospheric temperature retrievals are occasionally below the condensation point of CO2, but models do not usually predict sub-CO2 frost point temperatures at mesospheric altitudes (work by Colaprete et al. (2008) is an exception to this trend). This disagreement may be explained by interaction of cold atmospheric regions created by global circulation and the propagation to the mesosphere of small-scale dynamics such as gravity waves and thermal tides, which adiabatically reduce temperatures in rarefactions (González-Galindo et al., 2011, Määttänen et al., 2010, Spiga et al., 2012).

A full understanding of temporal and geographic distribution of clouds remains unconstrained due to limitations in data coverage, since clouds are observed coincidentally in nadir or rare limb observations. However, based on previously available detections, two distinct mesospheric cloud populations are observed in latitude-Ls space: equatorial clouds observed during northern summer, and mid-latitude clouds that tend to occur around the winter solstices in both hemispheres. There are a small number of observations of both types of clouds during local autumn (Clancy et al., 2004, Clancy et al., 2007, Formisano et al., 2006, Inada et al., 2007, Määttänen et al., 2010, McConnochie et al., 2005, McConnochie et al., 2010, Montmessin et al., 2006, Montmessin et al., 2007, Scholten et al., 2010).

We present new detections of mesospheric clouds using data from NASA’s Mars Climate Sounder (MCS) aboard Mars Reconnaissance Orbiter (MRO). MRO’s Sun-synchronous polar orbit does not allow MCS to resolve daily cloud formation trends, but does provide near continuous atmospheric coverage. In addition, MCS’ limb-sounding observation strategy enables greater sensitivity to optically thin clouds.

In this paper, we describe an algorithm to detect mesospheric aerosol layers based on the fact that they form high radiance arch-shaped loops in gridded limb spectra (due to observation geometry). The algorithm’s detection and classification criteria are used to discern loops from background noise and non-loop features. We discuss the spatiotemporal distribution of positive detections made using more than two Mars years of gridded limb-spectra in two wavebands (MCS channels A4 and A6). Finally, we consider possible aerosol composition by analysis of the nearest available temperature–pressure retrievals for each loop and comparison of loop spectra with a wide range of synthetic spectra calculated for dust, water ice and CO2 ice aerosol layers.

Section snippets

Observations

Mars Climate Sounder is a filter radiometer that measures thermal radiance in 9 spectral bands, with 21 pixels in each band (McCleese et al., 2007). Spectra are acquired in nadir sounding, polar buckshot scanning and along track limb-staring observation modes. Limb sounding, with pixel arrays oriented perpendicular to Mars’ limb, is the primary observation mode and has returned a large dataset of atmospheric observations between 0 and 80 km altitude, acquired nearly continuously since Mars

Cloud detection

A characteristic of the limb observation geometry is such that measured radiance contains contributions from the instrument’s entire line of sight, which is tangential to the surface and passes through a range of altitudes in the atmosphere. As a result, in successive spectra obtained as the spacecraft orbits, discrete radiance features such as aerosol layers/clouds show different apparent altitudes in the instruments’ focal plane. Their altitude projected onto the instruments will at first lie

Preprocessing

We started with calibrated and geometrically registered level 1B data from the standard pipeline (Henderson et al., 2007, McCleese et al., 2007). First, averaging was applied to spectra to increase the signal to noise ratio. Limb spectra were taken in groups of eight, however the first three spectra after a black body calibration could be affected by high temperature transients (Kleinbohl et al., 2009a). We therefore averaged the last five points in each sequence of eight and rejected the first

Results

Fig. 6 shows the location of detected loop features in bands A4 and A6. We attribute these features to discrete high altitude (>50 km) aerosol layers or clouds. To remove observation bias we count the number of detections in bins of 5° latitude and 5° solar longitude, then divide by the number of observation blocks acquired in each bin (Fig. 7).

Atmospheric temperature

Fig. 10 shows the MCS-derived temperature at 60 km and 80 km above the aeroid for Mars years 29 and 30 (as available in PDS level 2 DDRs derived by Kleinbohl et al. (2009b)) averaged for all observations within latitude-Ls bins of 5° i.e. over all longitudes and all local times. Some inter-annual variation is evident between the two Mars years, but seasonal trends are similar. We note similarities between the temperature distribution over Ls and latitude for detected clouds (Fig. 6) and the

Conclusions

Using more than two Mars years of limb spectra, we detect aerosol layers in the martian mesosphere and determine their spatiotemporal distribution. We suggest composition of these layers based on nearby temperature retrievals and the diagnostic spectral ratio B1/B2. The distribution of detected features compares well to previous detections of clouds in Mars’ mesosphere (Clancy et al., 2007, Määttänen et al., 2010, Montmessin et al., 2006, Montmessin et al., 2007, McConnochie et al., 2010,

Acknowledgments

This work was funded by the UK Science and Technologies Facilities Council (STFC) and the Leverhulme Trust. Thanks are due to David Kass for insightful feedback on an early draft of the manuscript, and to two anonymous reviewers whose comments helped to improve the paper.

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