MCS Climatology of Detached, Localized Haze Layers

Introduction: Multiple instruments have detected haze layers in the martian atmosphere using a variety of techniques, wavelengths and observing geometries (see, for example, Montmessin et al. 2006,2007; Clancy al. 2007, Inada et al. 2007, Scholten et al. 2010, Maattanen et al. 2010, McConnochie et al. 2010). Based on these detections, the hazes are detached from the bulk of the (dust) aerosols and cover a limited horizontal extent. In addition to a multitude of morphologies usually of limited latitudinal extent, the clouds sometimes exhibit complex internal structures. Some of the measurement techniques can directly or indirectly provide altitude information (Montmessin et al. 2006, 2007; Scholten et al. 2010, McConnochie et al. 2010). Some of these hazes have been observed to be at very high altitudes (90-100 km; Montmessin et al. 2006), however many are in the lower or middle atmosphere (20 km to 80 km; Montmessin et al. 2007, Clancy et al. 2007, Scholten et al. 2010, McConnochie et al. 2010). Many of the high hazes have been identified as CO2 ice (Montmessin et al. 2007; Maattanen et al. 2010), however not all techniques allow for the species to be identified. Due to the observing technique (reflected visible or near-IR sunlight), most of the observations are only available during the daytime (Montmessin et al. 2007; Clancy et al. 2007; Inada et al. 2007; Scholten et al. 2010; Maattanen et al. 2010, McConnochie et al. 2010). There are a few observations from SPICAM stellar occultations of nighttime clouds (Montmessin et al. 2006). Furthermore, many of the instruments are unable to perform daily global monitoring. MCS Observations: The Mars Climate Sounder (MCS) [McCleese et al, 2007] on the Mars Reconnaissance Orbiter (MRO) [Zurek and Smrekar, 2007] is a 9 channel filter radiometer (ranging from 0.3 to 43 μm) designed for continuous limb observations. The instrument has a 21 detector array oriented perpendicular to the limb of Mars for each channel. The arrays generally cover the limb from the surface to ~90 km. The primary use of the observations is for the retrieval of temperature, dust and water ice profiles [Kleinbohl, et al., 2009], but radiances are measured and calibrated for all infrared detectors. MRO is in a near polar, sun-synchronous orbit with a local mean solar time of 3 AM/3 PM at the equator [Zurek and Smrekar, 2007]. The standard for MCS is to observe the forward limb (in the direction of the velocity vector). 16 second limb stare periods are acquired every ~30 seconds throughout the orbit. There are routine short interruptions (~45 seconds) for blackbody calibration views and occasional longer interruptions due to MRO spacecraft activities. While looking at the limb, MCS is quite sensitive to aerosols. The limb view increases the opacity (for a well mixed aerosol), by a factor of ~50. The mid-IR window channels have good SNR performance and they are viewing the aerosols against cold space. The mid-IR also allows MCS to observe the atmosphere during both the daytime and during the nighttime. “Arches”: When looking at the limb, detached localized haze layers have a very distinct signature in the MCS radiances. The layer creates a local radiance maximum in the limb radiance profiles. More importantly, from the perspective of MCS, the layer rises above the limb as the S/C approaches, reached a maximum altitude and then sets below the S/C. Figure 1 shows a schematic of the different views. This viewing geometry allows MCS to determine the altitude of the layer directly (to the 5 km vertical limb resolution). The actual altitude is the highest apparent height of the layer.