P31D-01
Three Martian Years of MGS-TES Aerosol Limb Sounding: Vertical Distributions, Diurnal Variations, and the Polar Night.
We apply a forward radiative transfer model to Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) limb-pointed radiance spectra, retrieving, for each limb-scan sequence, dust and water ice aerosol mixing ratios at six levels in the martian atmosphere between 10 and 60 km altitude. MGS-TES typically performed a limb-scan sequence every 10 degrees of latitude along its polar, sun-synchronous, 2pm orbit, and MGS orbit tracks were separated by approximately 30 degrees in longitude. Thus, we derive a three-dimensional aerosol data set, with a resolution of roughly 30 degrees in longitude, 10 degrees in latitude, and 10 km in altitude, for every day and every night of the three martian years of MGS mapping operations. In comparison to the familiar nadir-sounding TES aerosol retrievals [e.g. Smith, 2004, Icarus, 167, 148.], our limb- sounding retrievals have very sparse sampling in latitude (1 per 10 degrees, versus roughly 70 per 10 degrees), and thus, in addition to poorer resolution, maps based on the limb-sounding retrievals have less statistical precision due to a much smaller number of samples available for any given region and time period. However, the limb-sounding retrievals have three key advantages: they provide vertical resolution; they provide more accurate opacity estimates, because the vertical profile is derived rather than assumed and because scattering is included in the model; and, they are effective when the surface is cold, such as at night, and over the winter pole. To illustrate the value of the new information provided by the MGS-TES limb-sounding data set, we focus on two prominent features: the equatorial cloud belt and the polar hood. The equatorial cloud belt, as seen in the limb- sounding data set, persists throughout the martian year, but the altitude of maximum mixing ratio increases from 30 to 45 km from aphelion to perihelion solstice, and the maximum mixing ratio decreases by a factor of three. The diurnal pattern of the equatorial cloud belt is also seen to vary: near perihelion the equatorial cloud belt intensifies and shifts slightly southward at night; while in the equinox seasons it exhibits a more complicated diurnal behavior in which the daytime equatorial ice optical depth maximum bifurcates and shifts to northern and southern mid-latitudes at night. The northern winter polar hood is revealed to be most optically thick in an annulus surrounding the pole, with a maximum optical thickness near 70 degrees north latitude. It is apparently isolated from the dusty conditions occurring at the same time over the rest of the planet: the limb-sounding data set observes sharp drop in dust mixing ratio at 50 degrees north latitude. The optical depth of the northern winter polar hood is substantial, peaking at more than 1.5 at 825 cm-1 in mid-fall. In contrast, the southern winter polar hood is barely recognizable, with an optical depth of less than 0.1.
P31D-02
Retrieval of Mars atmospheric temperature and dust profiles from Mars Climate Sounder measurements
Mars Climate Sounder (MCS) is a mid- and far-infrared thermal emission radiometer on board the Mars Reconnaissance Orbiter (MRO). Since September 2006, MCS has been taking measurements of the Martian surface and atmosphere in limb and nadir geometry. With its 8 infrared and one visible channel, MCS measures vertical profiles of atmospheric temperature, water vapor, dust and condensates from 0 to 80 km altitude with a vertical resolution of ~5 km. Here we present first results of simultaneous temperature and dust profile retrievals. We will describe the algorithm which is based on a modified Chahine method, and discuss changes of temperature and dust with latitude, altitude and season, focusing on the first few months of the MRO mission (Ls ~ 110-170). Preliminary results show the dust confined to the lower part of the atmosphere at the beginning of the mission, and reveal potential tidal signatures in the temperature profiles.
P31D-03
Investigating the Wavelength Dependence of the Single-Scattering Albedo of Martian Dust Aerosols with CRISM and MARCI Observations of the Very Dusty Atmosphere in 2007
The very dusty Martian atmosphere during the 2007 perihelion season offers an excellent opportunity to revisit the microphysical nature of dust aerosols. Through the use of MARCI wide-angle images and CRISM "emission phase function" sequences, one can robustly derive the mean single scattering albedo from the ultraviolet through the near-infrared (~4 micrometers). The detailed behavior of this particular dust property over the relatively wide wavelength coverage afforded by MRO has application to the detailed energy balance of the Martian atmosphere (i.e., solar energy input), as well as to the composition of the dust itself (e.g., iron oxidation state) and subsequent geological implications. Our general analysis approach combines CRISM and MARCI observations during the decay phase of the dust activity (to attempt to minimize column variability) with a DISORT-based radiative transfer retrieval scheme (Wolff et al., 2006; Wolff et al., 2007). We derive the optical depth and the single scattering albedo while fixing the other key parameters in the following ways: 1) The surface properties are specified using a generic "Hapke function" approach that includes the roughness parameter, thetabar. Constraints on the individual "Hapke" parameters are taken from both "ground-truth" analyses of MER observations (Seelos, 2005; Johnson et al., 2006a,b) and analyses of pre-perihelion observations of the same or similar regions. 2) The dust single scattering phase functions is computed using the Tomasko et al. (1999) phase function at 965 nm and scaling it to the desired wavelength using a ratio of non-spherical particle phase functions (e.g., oblate disks) calcuated with the T-Matrix method (Mishchenko and collaborators, http://www.giss.nasa.gov/~crmim/). Our methodology is iterative in the sense that once we derive a single scattering albedo spectrum, we adjust the refractive indices to reproduce the albedo values and then recompute the T-matrix phase functions. In our presentation, we will provide the details of the derived mean single scattering albedo and the associated estimates of precision and accuracy. We will also discuss some potential implications for dust composition and atmospheric energy balance.
P31D-04
Great Dust Storms on Mars: What Have We Learned?
Great dust storms on Mars are dramatic episodes of dust-raising and widespread transport that may have climatic effects on intra-seasonal, inter-annual, and ultimately much longer (e.g., obliquity) timescales. This paper will address to what extent the unprecedented observational coverage of Mars in recent times from orbiting spacecraft has changed our understanding of regional dust storms and hazes. During this period two nearly global dust storm events have occurred, in 2001 and in 2007; these are considered within the historical context of such storms with emphasis on the frequency of occurrence and on the mechanisms involved in their onset and decay.
P31D-05
Some Coolness on Martian Global Warming and Reflections on the Role of Surface Dust
Recent comparisons of global snap-shots of Mars' surface taken by the Viking and Mars Global Surveyor (MGS) cameras have been used to suggest that Mars has darkened, and hence has warmed, between the 1970's and 1990's. While this conclusion is not supported by more quantitative analysis of albedo data, the idea of Martian darkening and warming has found its way into the terrestrial climate change debate. Through blogs and other opinion pieces it has been used, both amusingly and disturbingly, to argue that Mars' apparent natural warming should alleviate our concerns about anthropomorphic climate change on Earth. Relating planetary research results to terrestrial analogs is instructive and promotes public understanding, but this example provides a cautionary tale of misinterpretation in this age of politicized science. The dust cycle is the dominant short-term component of the Martian climate. The atmosphere is strongly forced via dust's modification of atmospheric radiative heating rates, while dust loading displays dramatic interannual variability, from background opacity to aperiodic global dust storms. Until recently, the atmospheric component of the dust cycle was better documented than the surface component (which on Mars can be gauged via albedo). But now thanks to the combination of regional imaging, spot thermal infrared spectra, and spot short-wavelength photometry sampled at synoptic time and length scales by MGS, a rich new view of the relationship between specific meteorological phenomena and the patterns of surface dust is emerging. Seasonal cap winds, local, regional, and global dust storms, and monsoonal circulations all redistribute surface dust on large spatial scales, while dust devils are surprisingly shown to be insignificant. Rapid and widespread albedo modification is accomplished by storms that darken relatively bright regions through dust removal, and deposit dust upon largely dust free areas, brightening them. (It is not possible with existing data to infer dust deposition or erosion in perennially dusty areas.) However, most of the dust deposited on darker regions is removed within one Martian year. This rapid cleaning suggests that darker areas retain their dust-free albedo over decadal time scales because any dust deposited there can be eroded at commonly experienced wind speeds. Bright regions recover more slowly, sometimes requiring several martian years. The depletion of these dust sources in some years may play an important role in the interannual variability in dust storm occurrence and intensity by introducing a multiyear "memory" into the system. The observation of the 2001 global storm and its wake allows predictions to be made for the recovery following the 2007 global storm: the southern hemisphere should retain a transient brightening until after the seasonal cap has advanced and retreated. The MGS data show that albedo is a dynamic and evolving meteorologically and climatologically active variable, not a static boundary condition. Overall, the major story that albedo has to tell is one of major dust storms and recovery from them – not of secular changes – and that the changes are mostly cyclic such that surfaces tend to return to their pre-storm albedos. We speculate that this system of fine balances is dynamically controlled, such that interannual occurrence of dust storms and the partial dust coating of the surface should be robust against the expected large changes of orbital parameters throughout Martian geological history.
P31D-06
OMEGA Mars Express: Water Vapour Daily Variability on the South Pole
We report on an analysis of the Water vapour on the South Pole for the period LS=250°-270°. This period and region is characterized by a temperature and pressure that vary around the saturation point of the water. We observe an enhancement of the water vapour during the day, going from 5 - 10 ppt-µm in the morning side to 15 - 18 ppt-µm in the evening. This may suggest the following scenario: in the morning the water ice is condensed on/under the surface; as soon as it is illuminated by the Sun, the temperature difference makes it sublimate; the production continues up to the evening with a "quasi" constant slope (0.5 ppt-µm/hour); the evening observations are characterized by a homogeneity of the water vapour which could be explained by a mixing of the atmosphere which redistributes the sublimated morning water vapour in a few hours. The lack of data in the late evening does not allow to argue on a possible condensation during the night and even if no water ice is detected by OMEGA on the ground, it is the most plausible explanation to the low value of water vapour content in the early morning.
P31D-07
CRISM Observations of Water Vapor and Other Atmospheric Gases
Near-infrared spectra returned by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on-board the Mars Reconnaissance Orbier (MRO) contain the clear spectral signature of several atmospheric gases including carbon dioxide (CO2), water vapor (H2O), and carbon monoxide (CO). Here we describe preliminary work on the seasonal and spatial mapping of atmospheric gases using CRISM, and the vertical profiling of water vapor and carbon monoxide made possible by one CRISM observation of the limb. The column abundance of atmospheric gases is retrieved by radiative transfer modeling of the CRISM spectra. We compute synthetic spectra using the correlated-k approximation for gas absorption and a discrete-ordinates approach to model aerosol scattering and the solar beam. Both CRISM hyperspectral (emission phase function) and multispectral observations can be used to map the column abundance of atmospheric gases as a function of season (Ls), latitude, and longitude. Results for water vapor show that the peak in water vapor abundance over the northern summer pole and its subsequent decrease and movement to the south are similar to what was observed by TES in previous martian years. The spatial variation of water vapor follows the expected positive correlations with surface pressure and albedo, and negative correlation with thermal inertia. The main observed variation of carbon monoxide is a direct correlation with surface pressure (or CO2 abundance), with a mixing ratio of about 700 ppm. A single observation by CRISM of the limb (Ls=149°, near the north pole) shows a close correspondence between the vertical dependence of CO and CO2 indicating that at this location CO is well-mixed with the background CO2 gas. On the other hand, the vertical distribution of water vapor appears to be noticeably more confined toward the surface than CO2 given the more rapid decrease with height of the water vapor signal.
P31D-08
MARCI Observations of Mesospheric Cloud Trails and Their Association with Extreme Vertical Ascent within Localized (10's of km), Short Duration (1-2 days) Dust Lifting Events
The Mars Color Imager (MARCI) on board the Mars Reconnaissance Orbiter (MRO) employs wide-angle imaging (180 deg) in five visible color bands (420-750 nm) and two ultraviolet (uv) bands (260-320 nm). Due to the very dark uv, violet albedo of Mars surface and suspended dust, clouds appear with high contrast in MARCI uv, violet daily global image maps. During MARCI imaging prior to the July 2007 planet encircling dust veil (Ls=230-255 versus 265-), a set of striking cloud streaks or trails was located at mesospheric altitudes (50-80 km) based on distinct ground shadows cast from their sharp eastern termini (origins). These bright cloud trails are very linear, with >600 km longitudinal lengths accompanied by <60 km latitudinal widths and small but distinct NE-SW trends. Additional key aspects of these cloud trails are their association with specific regions on Mars (most notably at 20S, 35-43W) and their short timescales of formation and decay (hours). These spatial, temporal aspects of the observed mesospheric cloud trails suggest abrupt, localized surface forcing in which dust particles are lifted into the mesosphere and nucleate water ice particles which are rapidly transported westward as cloud trails in the strong easterly zonal winds of the Mars upper atmospheric circulation in this season. We present MARCI image observations of the cloud trails and their surface forcing dust lifting events, in the context of implied vertical dust propagation and mesospheric zonal wind velocities.