JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, E04004, doi: /2006je002805, 2007

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006je002805, 2007 Mars equatorial mesospheric clouds: Global occurrence and physical properties from Mars Global Surveyor Thermal Emission Spectrometer and Mars Orbiter Camera limb observations R. Todd Clancy, 1 Michael J. Wolff, 1 Barbara A. Whitney, 1 Bruce A. Cantor, 2 and Michael D. Smith 3 Received 2 August 2006; revised 17 October 2006; accepted 15 November 2006; published 19 April [1] We report the occurrence of a new type of cloud in the Mars dayside ( Local Time) atmosphere, apparent as high-altitude (60 to 80 km), vertically discrete aerosol scattering layers. Mars Global Surveyor (MGS) limb observations from the Thermal Emission Spectrometer (TES) visible channel indicate peak frequencies at the beginning and end of the aphelion northern summer season (L S = 30 and 150 ), where they are confined to equatorial (15 S 15 N) latitudes and two longitude ranges (40 E 2 W and 50 W 120 W). Limb images from the MGS Mars Orbiter Camera (MOC) indicate significant horizontal variations in these Mars equatorial mesospheric (MEM) clouds on km scales. On the basis of the distribution of projected limb heights, MEM clouds exhibit peak optical depths over km altitudes, that are substantial (nadir t vis of order 0.01) for the low-pressure region of formation (1 mbar). Averaged TES limb infrared spectral (5 40 mm) and solarband radiance profiles corresponding to MEM occurrence indicate dust aerosols (r eff = mm, t vis 0.4) at 0 25 km altitudes, capped by water ice clouds (r eff = mm, t vis 0.2) at km altitudes. The lack of detectable infrared radiances at MEM cloud heights precludes distinction of water versus CO 2 ice, but indicates 1 mm particle sizes for water or 1.5 mm for CO 2 ice compositions. More recent Mars Express observations point toward Mars mesospheric CO 2 clouds, although current dynamical and radiative models do not indicate sufficiently cold temperatures at MEM cloud locations to produce daytime CO 2 saturation conditions. Citation: Clancy, R. T., M. J. Wolff, B. A. Whitney, B. A. Cantor, and M. D. Smith (2007), Mars equatorial mesospheric clouds: Global occurrence and physical properties from Mars Global Surveyor Thermal Emission Spectrometer and Mars Orbiter Camera limb observations, J. Geophys. Res., 112,, doi: /2006je Introduction [2] One of the most striking aspects of the atmospheric temperature profile retrieved during the Pathfinder descent entry was the detection of very cold temperatures near 80 km altitude, falling below saturation conditions for CO 2 ice formation [Schofield et al., 1997]. Clancy and Sandor [1998] suggested the relatively frequent occurrence of CO 2 ice clouds in the km altitude region, on the basis of observed cold temperatures (Pathfinder [Schofield et al., 1997], and sub-millimeter ground-based retrievals), Pathfinder images of blue (small size) ice clouds in the predawn (LT 0400, L S = 162 ) sky, and Mariner 6 and 7 near-ir (4.3 mm) identification of CO 2 ice in equatorial limb tangent views around L S = 200 in 1969 [Herr and Pimental, 1970]. The latter 4.3 mm spectral feature is now 1 Space Science Institute, Boulder, Colorado, USA. 2 Malin Space Science Systems, San Diego, California, USA. 3 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. Copyright 2007 by the American Geophysical Union /07/2006JE associated with non-lte CO 2 molecular emission [e.g., Lopez-Valverde et al., 2005] and, prior to MGS limb measurements, it remained unclear whether CO 2 or water ice aerosols are ever present at >60 km altitudes. [3] Dust aerosols have been identified at altitudes above 50 km in Mariner 9 [Anderson and Leovy, 1978], Viking [Jaquin et al., 1986], and MGS limb measurements [Smith, 2003; Clancy et al., 2003a; Cantor, 2007], associated with the 1971, 1977, and 2001 planet-encircling dust storms, respectively. Jaquin [1988] describes Viking limb images that exhibit vertically extended brightness maxima between 50 and 70 km altitudes during the southern spring/summer season of elevated dust loading. Similar profiles have been detected in both solarband and IR spectral limb observations by TES during the 2001 dust storm [Clancy et al., 2003a]. The presence of the highest distinctly detached ice cloud identified from Viking limb data occurred at a projected tangent altitude of 55 km, at 16 S, 72 W and L S = 176 [Jaquin et al., 1986], modeled by Montmessin et al. [2002]. The vertical separation of this limb cloud from lower 1of18

2 Figure 1. Spectral radiance weighting of the TES solar band channel, as influenced by the solar spectrum (dotted line) and the TES solar bandpass (dashed line). The net effective spectral weighting of the TES solar band limb observation is shown by the solid line. altitude aerosol scattering, as well as its occurrence in nonelevated dust loading conditions, distinguish this cloud from high-altitude aerosol detections prior to MGS and Mars Express. [4] The seasonal period and location of this detached limb cloud appears consistent with km detached limb clouds that we have observed as prominent in MGS TES solarband limb scans and MGS MOC wide-angle (WA) limb images ( LT). We first reported MGS TES limb observations of these Mars equatorial mesospheric (MEM) clouds in 2003 [Clancy et al., 2003a], and addressed their detailed spatial and temporal distributions in 2004 [Clancy et al., 2004] from analysis of the extended set of TES limb observations. Since these initial reports of MEM clouds, subsequent spacecraft cloud measurements have returned high-altitude cloud detections that appear related to MEM clouds, including Mars Odyssey Themis dayside nadir imaging (T. H. McConnochie et al., THEMIS-VIS observations of clouds in the Martian mesosphere, submitted to Journal of Geophysical Research, 2007) (hereinafter referred to as McConnochie et al., submitted manuscript, 2007); Mars Express SPICAM nightside stellar occultations [Montmessin et al., 2006], and Mars Express dayside OMEGA near-ir spectral measurements (Brigitte Gondet, personal communication, 2006). These observations may eventually define key compositional and local time aspects of MEM clouds, but their direct association with the MEM clouds presented here remains tentative at this point. This paper presents the TES observed distributions of MEM clouds, as well as contemporaneous MOC limb imaging observations, a spherical radiative transfer (RT) analysis of coincident TES limb profiles in solar band and thermal IR spectra, and aerosol property determinations in the context of the RT analysis and thermodynamic/ transport considerations. 2. MGS Limb Observations [5] Limb observations of the Mars atmosphere were included as a standard data set within the TES mapping operations [Christensen et al., 1998, 2001]. They were obtained every 10 of latitude throughout each orbit, for Mars local times (LT) of 0200 and These TES limb radiances are returned from radiometer (solar band, mm, Figure 1; and thermal bolometer, mm) and thermal infrared (IR) spectral (6 50 mm range with 5 or 10 cm 1 resolution) 3 2 detector arrays. Each radiometer and thermal IR spectral detector presents a 8.3 mrad square field-of-view (fov) [see Christensen et al., 2001]. The detector array fovs are scan stepped from just below the projected surface limb tangent to several tens of kilometers above a 100 km limb tangent altitude. The projection of these 3 2 arrays of TES detectors at the Mars limb leads to km vertical resolution from multiply overlapping fovs at the Mars limb [e.g., Conrath et al., 1999; Smith, 2003]. This study employs TES solar band radiometer limb scans for definition of the spatial and seasonal frequencies of Mars MEM cloud occurrence. RT modeling for average vertical profiles of dust and ice aerosol optical depths and particle sizes, associated with MEM cloud occurrence, employ averaged (over L S, latitude, and longitude) thermal IR spectral and solar band limb radiances. [6] While MOC wide-angle (WA) images often contain portions of the atmospheric limb, the primary WA data product consists of daily global images constructed from pixel-binned (to 7.5 km/pixel), central portions (nadir to 2of18

3 Figure 2. TES solarband limb scans obtained on ocks 7725 (asterisks) and (diamonds) for the latitude, L S, and longitude coordinates indicated (Mars LT 1400). The inherent vertical resolution of these TES limb observations is 13 km. Limb brightnesses, normalized at a zero (surface) tangent height, are relatively constant up to limb heights of 20 km owing to unity slant aerosol (dust and ice) optical depths. Limb radiances decrease above 20 km tangent altitudes as the limb path optical depths of dust and ice aerosols decrease to near zero above km altitudes. A secondary peak in limb radiances, centered about km tangent altitudes, is associated with MEM clouds. In the case of the L S =21 limb profile, MEM clouds in the fore and/or background of the observed limb tangent altitude project scattered radiance at lower altitudes (50 65 km). 70 emission angles) of the WA blue (wave = mm) and red (wave = mm) WA global (sunlighted latitudes) images [Malin et al., 1992; Cantor et al., 2001]. Limb images of the Mars atmosphere appropriate to this study (full detector readout, corresponding to a pixel resolution of 1.3 km extending to above 100 km altitude) require specifically targeted observations. In response to this study (and Mars Exploration Rover overflight support imaging), MOC WA limb images were scheduled in April/May of 2004 during the Mars season and locations known to present Mars mesospheric clouds (based upon their distributions derived from the TES limb mapping data). These MOC WA limb images were obtained primarily in the blue channel, although two mesospheric clouds were imaged in the WA red channel. Simultaneous red and blue limb imaging at full pixel resolution with the MOC WA cameras, of potential benefit to MEM particle size estimates, was not obtained. The MOC limb images of MEM clouds complements the mapping and spectral definitions provided by TES limb scans through definition of the instantaneous three dimensional (altitude, latitude, and inferred longitude) nature of these high-altitude clouds, which is not defined by the coarse latitudal (10 ) and longitudinal (30 ) grid of the TES limb scan maps. It is also important to note that TES limb scans and MOC limb images do not obtain simultaneous observation of a specific limb region, as TES scans within the orbit plane (dayside, LT 1400) whereas MOC obtains cross track limb views (LT 1315 or 1445, depending on the spacecraft roll direction) TES Solar Band Radiometry Limb Scans [7] The TES solarband limb scans provided the first identification of MEM clouds as a new class of discrete high-altitude clouds [Clancy et al., 2003a]. Figure 2 presents TES solarband limb radiance profiles for orbit numbers (OCK, as counted from MGS orbit insertion [see Christensen et al., 2001] 7725 and 19640, indicated as asterisks and diamonds, respectively. The solar longitudes (L S ) and latitude/longitudes of these solarband limb scans, provided in Figure 2, are representative of the seasonal and spatial occurrences of MEM clouds (LT = 1400). The dates of these observations are 14 July 2000 (7725) and 15 March 2003 (19640). The current analysis regards identification of MEM clouds in TES limb observations from OCKs , corresponding to the beginning of the nominal MGS mapping mission in April 1999 to May of In terms of Mars years (MY) as defined by Clancy et al. [2000], these TES OCK ranges cover MY [8] Key aspects of the MGS observations of MEM clouds, apparent in both TES limb scans and MOC limb images (next section), are demonstrated in the solarband brightness profiles of Figure 2. The presented limb brightness profiles, versus limb tangent height above the reference ellipsoid surface, are normalized to the observed brightness near the surface limb tangent view (height 0 km) of each observation. Below 20 km tangent altitudes the observed limb brightness remains relatively constant owing to the large slant path optical depths of aerosol (ice and dust) scattering. Over the km altitude range, limb brightness decreases as the slant aerosol optical depth decreases 3of18

4 Figure 3. Accumulated TES solarband limb scans that contain MEM clouds within the latitude, L S, and longitude ranges indicated. Forty individual MEM cloud occurrences are observed in TES limb scans over the L S, spatial ranges indicated for MY For each TES limb profile, the observed radiance has been normalized to the maximum observed radiance at tangent altitude below 15 km. toward zero. Above 50 km tangent altitudes, two characteristic cases of MEM cloud profile measurement are represented by the selected TES limb scans, indicating the effect of the spatial extent of the MEM cloud along the observational limb path (in orbit track, and so roughly along lines of longitude for TES limb scans). In the L S = 152 profile of Figure 2 (diamonds), a secondary peak in limb brightness at 70 km tangent height is separated from radiance associated with scattering from lower altitude aerosols by zero level limb brightnesses at km tangent altitudes. Such a zero-level minimum in limb brightness requires that the upper altitude (70 km) aerosol layer is confined horizontally ±20 km along the limb path line-of-sight at the limb tangent position of the 70 km MEM cloud height. Otherwise, the projection of a horizontally extended MEM cloud in the fore and background of the limb tangent position projects the cloud scattering at lower observed tangent altitudes. This is the case presented in the second, L S =21 (asterisks), TES limb profile of Figure 2. [9] Figure 3 presents the accumulated set of TES limb scans that contain MEM cloud layers within the L S, latitude, longitude bin indicated in the figure. As described in following sections, this bin represents one of several distinct spatial and temporal concentrations of MEM cloud occurrence, all of which are centered at near equatorial latitudes. The frequency of MEM cloud appearance in TES limb scans in such cases can exceed 50% within such narrow spatial and seasonal ranges, although it is important to note that such limb views sample >100 km path lengths. Forty individual MEM clouds are contained in the TES limb radiances of Figure 3, each of which are normalized according to maximum observed radiance levels below 15 km altitude. MEM clouds appear at tangent altitudes as high as 85 km. Their true distribution at altitudes below 70 km is obscured by the projection of their inhomogeneous distribution along the limb fov in the fore and background of the observed tangent altitude. The coarse vertical resolution of the TES limb scans (13 km) also under-resolves the vertical structure of the MEM cloud. Conversely, the coarse TES vertical resolution prevents identification of discrete (as opposed to continuous) MEM clouds along the viewed limb path, which are suggested in the L S =21 profile (asterisks) of Figure 2 but not clearly separated. MOC limb imaging allows such vertical discrimination with its 1.3 km per pixel vertical sampling, as well as distinguishing the cloud two dimensional structure in its cross track (roughly versus latitude) image definition. This resolved cross track MEM cloud structure also serves to corroborate the horizontal scale variations of MEM cloud structure that are inferred, but not unambiguously measured, along the in-track TES limb views MOC Limb Imaging [10] Two MOC limb profile views of atmospheric aerosol scattering associated with MEM cloud layers are presented in Figure 4. These limb radiance profiles are extracted from WA blue images observed at full resolution during MGS spacecraft off-nadir rolls, which place the atmospheric limb within well-characterized regions of the WA fov (in terms of flat field corrections and geometric registration) and provide optimum vertical resolution (1.3 km/pixel). The locations and seasons of these limb profiles are indicated on Figure 4, the corresponding Mars local time (LT) is To improve the signal-to noise ratios of these limb profiles, we ve averaged 20 pixels in track (parallel to the limb). Consequently, these MOC limb profiles represent 30 km scale horizontal averages in the latitudinal direction. Nevertheless they provide significantly improved vertical resolution over the TES limb scans for identification of MEM 4of18

5 Figure 4. MOC limb radiance profiles obtained from WA blue limb images on 29 April 2004 (r : UT = 17:14:39, asterisks) and 1 May 2004 (r : UT = 22:13:55, diamonds) for the latitude, L S, and longitude coordinates indicated (Mar LT 1315). The inherent vertical resolution of these MOC limb observations, which were targeted with spacecraft limb rolls, is 1.3 km. However,the presented limb profiles are constructed from along-track (in latitude) 20-pixel averages to increase signalto-noise ratios. Limb brightnesses, normalized at a zero (surface) tangent height, are relatively constant up to limb heights of 20 km owing to unity slant aerosol (dust and ice) optical depths. Limb radiances decrease above 20 km tangent altitudes as the limb path optical depths of dust and ice aerosols decrease to near zero above km altitudes. The highest-altitude peaks in limb radiances, centered about km tangent altitudes, are associated with MEM clouds. In the case of the L S =26 limb profile, MEM clouds in the fore and/or background of the observed limb tangent altitude project a second peak in scattered radiance centered at apparent km limb altitudes. cloud vertical and projected (along the limb fov) horizontal path distributions. The two MOC limb profiles of Figure 4 present comparable geometries of MEM cloud identification to the TES limb profiles of Figure 2. The 65 W (dotted line) MOC profile of Figure 4 indicates a single, horizontally discrete (20 km along the limb fov) MEM cloud centered at an altitude of 73 km. This vertical location presumes that the cloud is centered, horizontally, at this tangent altitude of the MOC limb fov, which is suggested by the peak brightness and sharp topside boundary in the MOC image. The horizontal scale of this MEM cloud along the limb fov is inferred from the lack of limb projected radiance below 65 km altitudes. The latitudinal extent of this cloud (perpendicular to the limb fov), as displayed in the MOC WA image (not shown), is 2 S to 0.5 S. The 11 W (solid line) MOC profile of Figure 4 presents a double peaked MEM cloud structure projected at km altitudes. The brightness and sharp topside of 70 km projected altitude feature suggests a discrete MEM cloud at this tangent altitude, whereas the weaker feature is suggestive of comparable altitude (i.e., 70 km) clouds in the fore or background of the limb path, projected to lower (55 60 km) apparent limb tangent altitudes (similar to the L S =21 TES profile of Figure 2). [11] Figures 5 and 6 present MOC limb images of MEM cloud layers, in which the latitudinal structure of these clouds becomes apparent. The double peaked cloud structure presented in cross section by the solid line of Figure 4 extends over a significant latitudinal range in Figure 5 (equator at top to 6 S at bottom), within which the projected altitude of the upper layer rises from 66 to 73 km and the (projected) lower layer fades considerably at the northern extent of the image. Only a portion of the latitudinal coverage obtained in the MOC WA image of Figure 6 is shown, in order to enhance the presented cloud vertical and horizontal structure. The isolated cloud presented in Figure 6 is accompanied by separate, single cloud layers projected at similar projected altitudes (68 70 km) near 3.5 S and 2.5 S, and double layered clouds at km (projected) altitudes over 2 S to 0.5 S latitudes. Eight additional MOC WA observations, not represented in Figures 5 and 6, were targeted for imaging of MEM clouds. The majority of these images were obtained in May 2005 (L S =27 35 ), with the final observation occurring on 7 July (L S =60 ). In 7 out of 12 of these WA images (all blue filter), prominent MEM clouds are apparent. A lower altitude (50 60 km projected) MEM cloud is faintly present in one of two MOC WA red images, corresponding to the final (L S =60 ) observation. MEM clouds appear less prominent or not detected in the three images obtained for the LT = 1445 limb, including both of the red and one blue MOC limb images. This result may suggest a local time dependence in MEM cloud formation (i.e., favoring LT = versus 1500), but the LT sampling remains too limited for strong conclusions. 5of18

6 Figure 5. MOC WA blue image of the Mars atmospheric limb, observed on 29 April 2004 (r : UT = 17:14:39). In the portion of the MOC image presented, MEM clouds are apparent over 3.5 S to0.5 S latitudes, at a longitude of 11 W, L S = 26.4, and Mars LT These clouds extend from 6 S to the equator in the full MOC image (not shown). A vertical (projected) cross section of these MEM cloud layers is presented by the Figure 4 solid line profile for the southern (bottom) portion of the presented image. The bright, upper cloud layer appears at a (true?) altitude of km, whereas the fainter second layer may reflect MEM clouds in the fore or background of the limb tangent point which project to lower (55 65 km) altitudes. Below 45 km altitudes, lower atmospheric dust and ice aerosols contribute to the uniform background of the bright limb TES Thermal Infrared Spectral Limb Scans [12] TES thermal IR spectra, obtained coincidentally with TES solarband limb scans, provide the potential to detect MEM clouds if their particle sizes are sufficiently large (e.g., r eff 1 mm) and their composition is water ice. In principle, CO 2 ice aerosol signatures might also be detected within TES IR spectra for such larger ice particles presented at high enough altitudes where transmission within the CO 2 15 mm gas band is possible. We include the TES IR spectra in our analysis to explore such particle size and compositional limits on MEM clouds, as well as to define the dust and water ice aerosol abundance and particle sizes that characterize the lower atmosphere (below 50 km) when MEM clouds are present. The application of TES IR limb spectra for atmospheric temperature [Conrath et al., 1999] and aerosol [Smith, 2003] profile retrievals requires adoption of TES nadir column retrieval constraints, as a consequence of limb transmission limitations below 10 km (for aerosols) to 30 km (for temperature) tangent altitudes. It is also necessary to sum over large spatial and L S ranges in TES limb spectra observations to obtain appropriate signalto-noise ratios at higher-tangent altitudes, a limitation that is especially relevant to the current study. [13] In Figure 7 we present TES limb spectra, versus limb tangent altitudes over km, in which ten individual limb observations have been averaged. The selected limb profiles are sampled from low latitude, northern spring TES observations which exhibit MEM clouds in coincident solarband limb radiance profiles. Specific dust and ice spectral features are indicated, as well as the 15 mm CO 2 molecular band which appears in absorption at tangent altitudes below 25 km owing to colder atmospheric regions observed above the tangent altitude in the foreground of the limb path fov (i.e., limb path t CO2 1). For moderate aerosol optical depths, dust and ice spectral limb features appear in emission. Furthermore, scattering of strong thermal continuum emission from the warm dayside surface dominates the aerosol limb radiance, especially above km altitudes where decreasing atmospheric temperatures reduce direct thermal emission by aerosols. As demonstrated by the 45 km limb tangent spectrum of Figure 7, in which only the 15 mmco 2 molecular band rises above noise levels, summation of as many TES IR limb spectra as possible is necessary to extend aerosol measurements into the 60 km altitude region of MEM cloud levels. 3. Global and Temporal Distributions [14] Accumulated TES solarband limb observations over the MY24 26 period form the primary basis for the identifications of temporal and spatial distributions of MEM cloud occurrence presented below. The general spatial and seasonal characters of MEM clouds can be seen in Figures 8a 8c, which presents latitude-longitude contour maps of TES solarband 70 km tangent limb brightnesses, averaged over three separate L S bins encompassing Mars aphelion. Similar maps for the L S range (not shown) exhibit significant dust aerosol loading that extends above 70 km altitudes during periods within the 2001 planet-encircling dust storm [Clancy et al., 2003a]. [15] The contoured brightness units within these maps, consistent among the three panels, are scaled absolute units 6of18

7 Figure 6. MOC WA blue image of the Mars atmospheric limb, observed on 5 May 2004 (r : UT = 00:48:32). In this extracted portion of the full image, a discrete MEM cloud extends from 5.2 S to4.8 S (25 km), at a longitude of 75 W, L S = 28.9, and LT of The projected limb tangent altitude of this cloud is 68 km. (600 TES bolometer units of W cm 2 str 1 ) with stepping factors-of-two between each contour level. It is important to emphasize that the presented background level of reflects a scattered light offset in TES solarband brightness profiles that varies with latitude and L S (as apparent in Figure 8). Multiple (10 20) limb profile averages, under conditions when MEM clouds are not identified and dust lifting to high altitudes is minimal (e.g., most of the regions in Figure 8), show this background to be altitude-independent within 10 20% noise variability over km altitudes. All of the grey-black contour levels of Figure 8 indicate nondetection of aerosol scattering at the 70 km tangent altitude, consistent with the limited profile averaging represented in Figure 8 and independent profile-by-profile searches. [16] This background offset level is times weaker than the brightness level of a typical MEM cloud observed at the 70 km limb tangent altitude. Consequently, to the degree this background may be removed in multi-limb scan averages, MEM clouds present brightnesses more than 2 ordersof-magnitude above the inherent detection levels of TES solarband limb data. The bright contour peaks at low latitudes and within longitude ranges of W and 30 W 60 E for Figures 8a (L S =0 50 ) and 8c (L S = ) result from the occurrence of multiple (2 20) MEM clouds incorporated in modest-to-substantial fractions (1 20%) of TES limb observations observed for these locations and L S ranges. As the Figures 8a 8c sequence demonstrates, MEM clouds bound but do not occur within the aphelion climate season (L S =71 at aphelion), which is Figure 7. TES IR limb spectra can provide dust and water ice profiles with coarse vertical resolution (13 km). Ten individual limb observations, corresponding to the season and location of MEM clouds, are averaged for limb tangent altitudes of 15, 25, 35, and 45 km. The spectral resolution is 10 cm 1 and the radiance units are W cm 2 str 1 /cm 1. Absorption/emission spectral features are noted for dust and water ice aerosol and CO 2 gas atmospheric constituents. In order to retrieve aerosol properties above 35 km tangent altitudes, substantially larger spatial and temporal averages of TES IR limb spectra are required. 7of18

8 Figure 8. Contour plots of the average TES solarband limb brightness (scaled TES units, 600 W cm 2 str 1 ) at a limb tangent altitude of 70 km (measurements between 68 and 72 km) binned over L S ranges of A) 0 50, B)60 110, and C) These averages were constructed from (MY24 26) TES limb data. MEM cloud scattering is observed in the L S periods (a) preceding and (c) following the (b) aphelion season at equatorial latitudes in longitudinal regions of 320 W 20 E and W. All of these dayside TES limb measurements correspond to a Mars LT of characterized by the formation of the aphelion cloud belt at lower altitudes [Clancy et al., 1996]. They are also distinctly equatorial as compared to the northern subtropical location of the aphelion cloud belt and exhibit greater longitudinal confinement relative to the aphelion cloud belt [e.g., Clancy et al., 1996, Figure 18]. More detailed descriptions of the temporal and spatial distributions of MEM clouds as characterized by the TES solarband limb scan data are provided in the following sections Distribution Versus Latitude-Longitude [17] The observed spatial distribution of MEM clouds in TES solarband limb observations is presented in Figure 9, in which the dual L S character is distinguished by two symbols (triangles and crosses). Three distinct regions centered around the Mars equator exhibit MEM clouds, although two of these regions ( W and W) display MEM clouds less prominently in the L S = , northern fall season. Within the 30 W 10 E longitudinal region, MEM clouds are frequent over both L S periods. Apart from such L S details in longitudinal cloud frequencies, the general longitudinal character of MEM clouds remains quite similar before and after the intervening aphelion period (L S = ). The latitudinal distribution is distinctly equatorial. The majority of MEM cloud detections are obtained between 10 S and 10 N latitudes, and essentially all of them between 15 S and 15 N. There appear to be more MEM clouds in the N versus S latitudinal range, but such a bias is, at best, marginally defined in these observations Distribution Versus Mars Season (Ls) [18] The seasonal variation of MEM clouds is emphasized in Figure 10, including distinction of the three longitudinal regions by symbol. The altitude of peak brightness for all 30 S 30 N TES limb scans in which radiance in any altitude above 55 km exceeds (TES units, Wcm 2 str 1 ), is plotted versus Mars solar longitude. Each point in Figure 10 effectively denotes the projected altitude and season of a distinct cloud layer, with the exception of points near the 55 km lower boundary. [19] The concentration of points below 57 km at the beginning and end of the presented L S range (0 180 ) reflects the vertical extent of dust aerosols lifted in association with the perihelion dusty season. Extension of dust aerosols above 50 km is especially prevalent in late northern 8of18

9 Figure 9. Location of MEM cloud layers identified in TES solarband limb scans, versus longitude (W) and latitude on Mars, as present during L S periods of 0 50 (triangles) and (crosses). Each location represents a detached cloud layer with a projected limb tangent altitude at or above 60 km. MEM clouds are equatorial and restricted to longitude regions corresponding to the Tharsis and Meridioni regions on Mars. winter [Smith, 2003], associated with the interannually repeating cross-equatorial regional dust storms observed along the Acidalia storm track over L S = [Cantor et al., 2001]. A decay in the top altitude level of dust loading is evident at km over L S = 0 20 in Figure 10. Peak limb brightnesses at km around L S = coincide with the decay of the aphelion climate, perhaps associated with distinct ice cloud layers that Figure 10. Projected limb tangent altitudes of bright aerosol scattering layers observed over L S =0 180 within TES solarband limb scans. The Mars (W) longitudes of the individual measurements are discriminated by symbols, as indicated. MEM clouds appear as high (60 80 km projected) altitude detached, bright scattering layers with distinctive seasonal peaks in occurrence around L S =20 35 and of18

10 continue to cap dust loading at increasing altitudes. The absence of measurable aerosol scattering above 55 km altitudes over most of the L S = period reflects the role of the low altitude (10 40 km) aphelion cloud belt in capping dust (and water vapor) vertical extension during the aphelion climate season [Clancy et al., 1996; Montmessin et al., 2002]. [20] MEM clouds appear as a distinct population of detached scattering layers exhibiting an extensive range of projected limb tangent altitudes (55 to 83 km). Apart from these clouds, no other aerosol scattering is observed above 60 km tangent altitudes over the L S = period between 40 S and 60 N latitudes (e.g., Figure 8) in the TES (LT = 1400) limb scan data. As indicated earlier, MEM clouds are restricted to relatively narrow L S ranges preceding and following the aphelion climate season. During the northern spring period when MEM clouds are most prominent, they are identified as early as L S =5, and are observed as late as L S = However, MEM cloud formation peaks during a fairly narrow range of L S = Within this L S range, MEM clouds are apparent in nearly half of TES limb scans observed in the 10 S 10 N latitude range. The northern fall period of MEM cloud formation is distinctly less active than the northern spring period, but exhibits a similarly narrow L S peak about preceded by isolated MEM clouds as early as L S = 105. As indicated above, the northern fall period of MEM cloud formation is more active over the 0 20 W longitudinal region Distribution Versus Altitude [21] The substantial range in peak limb brightness heights indicated in Figure 10 incorporates real altitude variations in MEM cloud occurrence with negative altitude biases that arise in limb geometry from the projection of discrete (along the fov) MEM clouds in the fore and background of the viewed limb tangent altitude. The more distant in fore or background such horizontally discrete clouds lie from the limb tangent position (which is also a function of limb tangent altitude), the greater the downward bias introduced in their assigned altitude. For the geometry of MGS limb views (400 km orbit altitude about Mars), a discrete cloud present 20 km in the fore or background of the limb tangent altitude will appear 10 km below its true altitude. If the actual altitude of MEM cloud formation is not substantially variable from 70 to 75 km, then the km altitude range of limb brightness peaks (Figure 10) implies discrete clouds of km horizontal scales, viewed over a range of horizontal offsets ±40 km about the limb tangent position along the limb fov. Such a case is at least consistent with the MOC limb image presented in Figure 6, which reveals a 20 km horizontal scale for an individual MEM cloud along the in track direction of TES limb views. This doesn t rule out larger (or smaller) spatial scales for MEM clouds, which certainly exist (e.g., Figure 5). However, it does indicate that 20 km horizontal scales are present to explain the complex and variable limb brightness profiles of MEM cloud observations. [22] In addition, as shown in the following RT modeling section, the averaged limb brightness profile obtained by summing all observed TES limb scans with MEM clouds is well fit with the assumption of an aerosol scattering layer confined to km altitudes. In such an averaged MEM cloud brightness profile, a symmetric thin shell geometry is likely to apply, and significant aerosol opacity below 70 km altitudes is not required to model the observed vertical distribution of this brightness profile. This does not rule out individual cases of MEM clouds below 70 km altitudes but suggests they are infrequent, perhaps similar in frequency to the number of MEM clouds that appear above 75 km tangent altitudes in Figure Distribution Versus Local Time [23] The TES and MOC observations in this study do not allow definition of local time variation in MEM cloud occurrence or brightness, given their limitation to Local Time of measurement. However, MEM clouds almost certainly exhibit substantial local time variations owing to their likely condensate composition (water or CO 2 ice) and the strength of solar tide temperature and vertical wind variations in the Mars mesosphere [e.g., Wilson, 2002]. For comparison, the vertical distribution and column optical depth of the aphelion cloud belt varies substantially between 0200 and 1400 Local Time [Smith, 2003; Hinson and Wilson, 2004] [also Colburn et al., 1989] at altitudes below 30 km, where thermal tide amplitudes are weaker relative to their amplitudes above 60 km. In terms of potential AM mesospheric clouds, Clancy and Sandor [1998] argued that the Pathfinder image of a predawn bluish cloud indicated direct solar illumination of mesospheric CO 2 ice clouds rather than indirect illumination of lower altitude (30 40 km) water ice clouds as a consequence of multiply scattered solar flux around the Mars terminator [Smith et al., 1997]. However, direct illumination of mesospheric clouds at the LT of this Pathfinder observation (LT 0400) requires cloud altitudes above 100 km [Smith et al., 1997]. [24] The Pathfinder AM blue cloud falls occurs reasonably close to the spatial (19 N, 33 W) and L S (162 ) domain of the PM, lower altitude MEM clouds. In the context of this Pathfinder 0400 LT cloud image and MGS observations of MEM clouds, we also note the nighttime ultraviolet (UV, nm) measurements of high altitude (>90 km) aerosol layers recently obtained from the SPICAM stellar extinction experiment on Mars Express [Montmessin et al., 2006]. Four distinct cases of strong aerosol layer extinctions of stellar UV flux were observed with topside altitudes of km, at L S = , and local times of Two SPICAM cloud detections at 15 S fall within the MEM longitude range of occurrence, whereas two at S do not. Both the Pathfinder and SPICAM observations are very limited in terms of establishing global and seasonal distributions for nighttime mesospheric clouds at km. Consequently, it remains unclear whether these AM upper mesospheric cloud layers are related to the PM middle mesospheric MEM clouds via the spatial and LT dependences of Mars atmospheric thermal tides Distribution Versus Brightness [25] MEM clouds exhibit visible limb brightnesses that can approach those of the Mars disk and bright lower atmospheric limb, indicating significant limb path optical depths (e.g., Figures 2 4). Figure 11 presents the relative frequency of individual MEM peak limb brightnesses 10 of 18

11 Figure 11. Relative frequency of MEM cloud peak brightness, as displayed in the TES solarband limb scans and in TES units. As indicated, MEM clouds can be prevalent within specific regions and time bins. A solarband peak limb brightness of corresponds to a nadir optical depth of 0.01, dependent on modeled MEM cloud particle properties. greater than in TES solarband units. This corresponds to a normalized limb brightness level of 0.08 in Figures 2 and 3, below which characterization of MEM cloud brightnesses requires subtraction of the variable scattered light background in TES solarband limb profiles (see above). A limb brightness of W cm 2 str 1 translates roughly to a MEM nadir optical depth of 0.01, assuming particle scattering properties described in RT analysis that follows this section. This further corresponds to a reflectance of 0.12 in terms of I/F units. The vertical scale in Figure 11 reflects relative frequencies, as the absolute frequency of MEM cloud occurrence depends strongly on the spatial and L S bins considered. As indicated on Figure 11, MEM clouds can be prevalent over narrow L S and spatial ranges within the TES limb observations, although this result is further qualified by the long horizontal path of the limb fov. Figure 11 indicates that the observed limb brightness distribution of MEM clouds is fairly broad and somewhat linear, perhaps indicative of variable spatial MEM cloud filling along the limb path fov. The large horizontal sampling scale precludes quantitative separation of spatial and optical depth distributions. Figure 12 presents the observed (projected) height of individual MEM clouds versus their peak brightnesses, distinguished according to the two broad L S ranges of occurrence. The two L S populations are not particularly distinguished in peak brightness or altitude, although northern fall MEM clouds may present lower projected altitudes within the limited sampling available (see also Figure 10). 4. Profiles of Dust and Cloud Aerosols Associated With MEM Clouds [26] The observed MEM cloud brightnesses in TES solarband limb scans and MOC limb images indicate substantial scattering opacities, but do not support quantitative determinations of these opacities or MEM particle sizes and composition. The observed brightnesses depend on the product of the particle single scattering phase functions and cross sections, both of which vary with particle size and/or shape in a nontrivial fashion. TES thermal IR spectra present specific dust and water ice spectral signatures, and a scattering dependence on aerosol particle sizes that is complimentary to visible-wave measurements. Smith [2003] has analyzed TES IR spectral limb data to obtain thermal IR dust and water ice optical depths and vertical profile constraints (Conrath mixing parameter for dust, lower altitude boundary of ice clouds). Here we provide the first combined analysis of TES solarband and IR spectral limb data. The objectives of this RT analysis are coarse (10 15 km resolution) vertical profiles of dust and ice opacities (visible and IR) and particle sizes for altitudes below 50 km; and constraints on the particle sizes (upper limits) and opacities (lower limits) of MEM clouds above 60 km altitudes where IR signal is not detected. The analysis of TES nadir and emission-phase-function (EPF) sequences in solarband [Clancy et al., 2003b] and spectral IR [Wolff and Clancy, 2003] indicates the utility of combined TES solarband and IR spectral observations to retrieving column dust and ice aerosol properties, including their optical depths, particle sizes, and IR spectral indices Radiative Transfer Analysis [27] Combined visible/thermal IR analysis of TES limb scan data requires a full spherical, multiple scattering RT code that supports both scattering and thermal emission source functions. For this purpose, we employ the Mars Monte Carlo code developed by Whitney et al. [1999], with atmospheric inputs and vertical resolution appropriate to TES limb profile thermal IR and solarband simulations. 11 of 18

12 Figure 12. Projected limb altitude of individual MEM clouds observed in TES solarband limb scans is plotted versus the peak cloud brightness (in TES units), distinguished by separate symbols for the two L S periods of MEM cloud occurrence. This model has been applied in simulations of Hubble Space Telescope (HST) and Pathfinder visible imaging data sets [Whitney et al., 1999], TES solarband observations in support of the MER Pancam aerosol analysis [Wolff et al., 2006], and thermal IR modeling for dust profiles from TES limb spectra during the 2001 dust storm [Clancy et al., 2003a]. Although the model is fully 3D capable, our simulations are 1D (vertical coordinate) in terms of aerosol distributions. Specified inputs include an average TES limbretrieved temperature profile [Conrath et al., 1999], surface temperature and albedo from the TES nadir observations, and dust and ice IR optical properties versus wavelength (visible through 50 mm). [28] In the case of the IR spectral simulations, these optical properties must be specified with self-consistent wavelength dependences for dust and ice single scattering albedos and scattering phase function. We generate these employing Mie scattering calculations for variable ice and dust mean particle sizes (cross section weighted radius, r eff ), fixed size distribution functions (modified gamma [Deirmidjian, 1964]) and effective size distribution variances (0.4 for dust, 0.1 for ice), and specified dust and ice indices versus wavelength. We do not directly employ the Mie single scattering phase functions in the Monte Carlo IR simulations, owing to cpu limitations and the inappropriateness of the detailed Mie phase structure for the nonspherical Mars dust and ice aerosols. Rather, we apply the single scattering asymmetry parameter fit to the Mie phase function (versus wavelength) with a simple Henyey- Greenstein phase function. For the TES solarband simulations, where Mie phase functions are even less applicable, we employ existing measurements of visible single scattering asymmetry parameters for Mars dust and ice aerosols (as described below). However, we do use Mie calculations to specify the ratio of visible-to-ir extinction optical depths versus aerosol particle size. Model biases contributed by such approximate treatments of the aerosol are alleviated by the hemispheric illumination provided from the lower surface IR illumination and the constancy of the observational scattering angle with the vertical coordinate for both IR and solarband limb observations. [29] For water and CO 2 ice spectral indices, we employ the laboratory measurements of Hansen [1997] and Warren [1984], respectively. For dust spectral indices, we employ a modified (as described below) spectral set originally derived from the TES analysis of Wolff and Clancy [2003] and more recently updated through analysis of MER (Mars Exploration Rover) minites aerosol observations by Wolff et al. [2006]. Our analysis is strictly focused on aerosol simulations such that we model wavelength regions which avoid significant molecular absorption/emission and adopt temperature profiles retrieved from TES nadir [Smith et al., 2002] and limb [Conrath et al., 1999] CO 2 absorption band analyses. Model inputs for atmospheric and surface temperatures, as well as surface albedo/emissivity, are average values corresponding to the season (northern spring/fall) and region (equator, 40 E to 120 W) of MEM cloud occurrence. We note that the precise values for these inputs are not critical to this modeling effort, as the derived vertical distributions of dust/ice opacity and particle size are not particularly sensitive to these inputs. Above km altitudes, observed daytime (1400 LT) limb IR radiances associated with aerosols are dominated by scattering of continuum emission from the warm surface of Mars. Aerosol thermal emission is subdued owing to the significantly colder atmospheric temperatures, such that the vertical dependence of the observed aerosol radiance is not affected strongly by the vertical profile of atmospheric temperature. Furthermore, the limited transmission of the TES limb 12 of 18

13 Figure 13. Averaged (see text) TES IR limb spectra for tangent altitudes of 15 km (solid line), 25 km (dotted line), 35 km (dashed line), 45 km (solid line), and 75 km (dash-dotted line) are fit with RT model simulations (colored, solid circles) employing a spherical, multiscattering Monte Carlo code. In this dayside limb geometry above km tangent altitudes, IR scattering by dust and ice aerosols dominates the observed signal. Several key spectral features are designated. Dust and water ice opacity profiles and particle sizes (Figure 16) are derived from combined RT modeling of TES IR spectral and solarband (Figure 14) limb scattering profiles. MEM cloud IR scattering is not detected at km tangent altitudes where they exhibit strong visible scattering, indicating small (1 1.5 mm) particle radii (Figure 15). profiles below km altitudes requires that we constrain the aerosol profile retrievals to obtain specified visible and IR column aerosol optical depths. These assumed column optical depths are based on TES nadir IR measurements [e.g., Smith, 2004] and MER Pancam solar extinction imaging [Lemmon et al., 2004]. This assumption, our approximate models for aerosol phase functions, and the significant spatial/temporal averaging of limb data modeled, all serve to reduce the relevance of precise absolute radiance model-data comparisons Composition and Size Constraints [30] In Figure 13 we present averaged TES limb scan spectra (various lines) at observed tangent altitudes of 15, 25, 35, 45, and 75 km; as compared to RT model calculations (colored solid circles). Approximately 150 individual TES limb scans are incorporated in these averaged limb spectra, selected on the condition that MEM clouds are identified in the coincident solarband limb scans. Consequently, this average emphasizes the L S ranges of (75 spectra) and (25), latitudes between 10 S 10 N (110), and longitude ranges of 20 E 20 W (90) and W (35). However, the essential goal is to provide the maximum potential IR spectrum of MEM clouds with as high signal-to-noise ratios as possible. The characteristic dust and ice spectral features are labeled on Figure 13 as on Figure 7. With the increased spectral averaging, prominent water ice scattering features near 12 and 40 mm wavelength regions, and a distinct lack of dust scattering at 9 and 20 mm wavelength regions, are now evident in the observed 45 km tangent altitude spectrum. The observed spectrum at 75 km tangent altitude exhibits only noise, which places upper limits on MEM cloud particle sizes of water ice (or, to a lesser degree, CO 2 ice) composition, in the context of the observed TES solarband MEM cloud radiances, as indicated below. [31] The model IR limb radiances of Figure 13 (solid circles) correspond to best fit opacity profiles and particle sizes for dust and ice aerosols; constrained to obtain a dust 9.3 mm (1075 cm 1 ) column optical depth of This dust column IR optical depth constraint is adopted from the TES nadir absorption measurements [Smith, 2004], scaled by 1.3 to yield extinction optical depths [Wolff and Clancy, 2003]. This constraint is also roughly consistent with the average of the mini-tes dust extinction optical depths derived from the two MERs over L S =20 30 [Smith et al., 2004]. Our best-fit water ice 12.1 mm (825 cm 1 ) column optical depth is Somewhat distinct from the dust case, this ice column optical depth is reasonably constrained by the limb spectra, owing to the decreasing ice opacities at limb tangent altitudes below 15 km (with correspondingly higher limb transmissions relative to the dust spectral features). The derived ice optical (IR) column of 0.08 is 20% smaller than obtained by scaling the average TES column absorption optical depth of ice to an equivalent extinction optical depth (also factor of 1.3). By comparison, mini- TES spectra over both MERs do not detect measurable ice optical depths, partly associated with their reduced sensi- 13 of 18