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The Seasonal Behavior of Water Ice Clouds in the Tharsis and Valles Marineris Regions of Mars: Mars Orbiter Camera Observations. J

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The Seasonal Behavior of Water Ice Clouds in the Tharsis and Valles Marineris Regions of Mars: Mars Orbiter Camera Observations. J Sixth International Conference on Mars (2003) 3147.pdf THE SEASONAL BEHAVIOR OF WATER ICE CLOUDS IN THE THARSIS AND VALLES MARINERIS REGIONS OF MARS: MARS ORBITER CAMERA OBSERVATIONS. J. L. Benson1, B. P. Bonev1, P. B. James1, K. J. Shan1, B. A. Cantor2, and M. A. Caplinger2, 1Ritter Astrophysical Research Center, Dept. of Physics and Astronomy, Univ. of Toledo, Toledo, OH 43606 ([email protected]; [email protected]; [email protected]), 2Malin Space Science Systems, P. O. Box 910148, San Diego, CA 92191 ([email protected]; [email protected]). Introduction: Clouds in the martian atmosphere The Mars Orbiter Camera (MOC) was used to ob- have been observed and studied for many decades. tain global maps of the Martian surface. The maps Nathaniel E. Green first identified clouds in the mar- used were acquired between March 15, 1999 (LS = tian atmosphere in 1877 when he observed white spots 110º) and July 31, 2001 (LS = 205º), corresponding to near the limb of the planet and concluded that they approximately one and a quarter martian years. In this were morning and evening clouds high in the atmos- work we focused on water ice clouds associated with phere [1]. The Tharsis region is known for the “W” the surface features of Olympus Mons, Ascraeus clouds [2], which are prominent from northern mid- Mons, Pavonis Mons, Arsia Mons, Alba Patera, and spring to midsummer. Smith and Smith [3] studied the the Valles Marineris canyon system. Using these data, diurnal and seasonal behavior of the clouds associated we have made three types of quantitative measure- with Olympus Mons using ground based data collected ments to characterize the cloud activity: 1) cloud area between 1924 and 1971 and found that cloud activity and location, 2) cloud height, and 3) cloud optical was generally confined to late spring and early sum- depth. We have also searched for short period varia- mer seasons in the northern hemisphere. Based on a tions in the cloud areas. comparison of their data with seasonal water abun- MOC Observations: MOC includes two wide- dances, Smith and Smith concluded that Type I dis- angle (WA) cameras, one in the blue (400-450 nm) crete white clouds are composed of water ice. Curran bandpass and the other in the red (575-625 nm) band- et al. [4] gave the first spectroscopic evidence confirm- pass, each with a 140º field of view [7]. This coverage ing the presence of water ice clouds on Mars in the is optimal for separating clouds that are composed of Tharsis region using data collected with the infrared condensates from those that contain dust; while dust interferometer spectrometer on the Mariner 9 space- clouds and ice clouds often appear of similar bright- craft. The Mariner 9 spectral shape was consistent ness in red, condensate clouds are very bright in blue with a mean radius of the water ice particles of ~ 2.0 while dust clouds are dark. The WA camera images µm. are bounded by the horizons, corresponding to nomi- There are several meteorological factors that may nal local times of 1217 to 1543 at the equator. contribute to cloud formation and seasonal variability A variable summing algorithm is used to obtain in the Tharsis region. The seasonal variation in the continuous observations with both WA cameras so that cross-equatorial Hadley cells that characterize the gen- in a 24-hour period a complete global map is obtained eral circulation of the martian atmosphere is an impor- at a resolution of approximately 7.5 km/pixel. To con- tant factor [5]. The local circulation over the volcanoes struct the global map, only roughly the center half of is also an important factor: low thermal inertia causes each orbital swath’s coverage is used; every two hours the volcanoes to act as a heat source in the atmosphere, a new image strip is acquired and added to the previ- and warm upslope winds in the afternoon favor cloud ous strips. The daily global maps (DGMs) that we formation and enhance clouds [6]. Insolation and water have used for this work are composite cylindrical pro- vapor concentrations are also potentially important jections colored with a false green channel. factors. Cloud Area Measurements: Clouds were identi- Mars Global Surveyor (MGS) provides the first fied by visual inspection of the DGMs, and then these complete data set with which to study the properties features were matched with the blue images taken by and seasonal variability of clouds throughout the entire MOC. The clouds appear more prominent in blue im- martian year. MGS instruments do not provide much ages due to the relatively high single scattering albedo diurnal coverage in the equatorial regions, however, (~0.9) of these particles relative to the low surface because the orbit is fixed at ~ 2pm at the nadir. Only albedo at these wavelengths. To measure the areas of the early afternoon part of the cycle, in which previous the clouds an IDL program was used, and it took a observations have suggested growth in the large cloud subset of each DGM that corresponded to the particu- features, can be studied, but this does make it possible lar surface features studied and measured the number to determine the seasonal behavior, augmented by lim- of pixels in the blue image which had a threshold digi- ited local time coverage. tal number value greater than 120. This value was cho- Sixth International Conference on Mars (2003) 3147.pdf sen through visual inspection of cloud boundaries in ity is between LS = 227º-235º, which corresponds ex- the blue images. Because the resolution varies with actly to the largest regional dust storm seen by [8] and latitude for a cylindrical projection, each line is multi- [9]. TES observations showed that the dust storm ex- plied by a correction factor. These lines were then panded into the region around Arsia Mons at LS = 228° added and multiplied by the resolution of the projec- and was accompanied by a roughly 15 K increase in tion at the equator to give the true cloud area. temperature. By LS = 237º the dust storm had mi- Cloud Area Results: A plot of cloud area vs. LS grated much further south of Arsia, and the atmos- for Olympus Mons is presented in Figure 1. The cloud pheric temperature in the region had decreased. area maximum near LS = 100º is likely due to cooler Richardson et al. [10] demonstrated that cloud ice con- atmospheric temperatures and maximum upward ad- tent decreases strongly as a function of increasing dust vecting velocities in the solstice Hadley circulation. optical depth. The temperature increase lowers the The cloud area decrease at LS = 120º is probably relative humidity, suppressing cloud activity [11]. caused by the rise in air temperature due to the season (moving away from aphelion) and increased atmos- pheric dust [8]. Figure 2: Arsia Mons clouds area versus Ls. Note that the abrupt disappearance of clouds near Ls = 185º is due to the global dust storm of 2001, not to a lack of data. Figure 1: Olympus Mons cloud area versus Ls. Cloud activity ends earlier at all of our sites in the Observations of Ascraeus Mons show the same second martian year than in the first. This is most general trend for cloud area as Olympus Mons, with a clearly seen in Fig. 2. In the first martian year, cloud peak near L = 100º. At L = 140º, there is a minimum S S activity ceased near LS = 210º at Alba Patera, Olympus in cloud area that is especially prominent in the first Mons, and Ascraeus Mons, and near LS = 220º at Pa- martian year of observations. vonis Mons. However, in the second martian year, Alba Patera cloud activity displays a double peak clouds disappeared in all regions between LS = 183º feature that was not observed in any of the other re- and LS = 190º (June 24 and July 4, 2001). This earlier gions. The peaks occur at LS = 60º and 140º, with a cessation of cloud activity almost certainly results minimum near LS = 85º. The peaks in the seasonal from the global martian dust storm of 2001. As seen activity of Alba Patera occur at the times that the cloud from the TES data, atmospheric temperatures near the areas associated with Olympus and Ascraeus Mons Tharsis region began to rise around June 25 and the volcanoes are rapidly increasing or decreasing; this dust storm had expanded into the region by July 2 anticorrelation is one of the most interesting and unex- [12]. As noted above, the increased dust and atmos- pected results of this study. pheric temperature promote the dissipation of water ice Clouds over Pavonis Mons have maximum area clouds. near L = 100º. There is a local minimum in cloud S Valles Marineris: Clouds associated with the area at L = 140º observed during both martian years. S western Valles Marineris canyon system and other Then cloud area increases for a second peak between small surface features were also observed during the L = 140º-220º, which corresponds to the time of S time period of this study. The clouds associated with maximum cloud area at Arsia Mons. Valles Marineris generally were located in the latitude Cloud area vs. L for Arsia Mons is shown in Fig- S range -4° to +4° N and longitude range 70° to 80° W. ure 2. Observations of Arsia Mons reveal more or less Clouds were also seen around 12° S latitude, 100° W continuous cloud activity. The only period of inactiv- Sixth International Conference on Mars (2003) 3147.pdf longitude, in the region of Syria Planum, near 27° N local time at the topographic feature.
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