Mars W cloud: Evidence of nighttime ice depositions

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L14204, doi: /2009gl039061, 2009 Mars W cloud: Evidence of nighttime ice depositions Y. Moudden 1 and J. M. Forbes 1 Received 5 May 2009; revised 10 June 2009; accepted 22 June 2009; published 30 July [1] Water clouds on Mars, their current and past connections with other forms of water, along with surface features that suggest the early existence of flowing water, continue to be intriguing aspects of the red planet. Discrete afternoon mountain clouds were one of the earliest atmospheric features to be observed on Mars. Their exclusive formation near the high mountains led to the common belief that air lifting associated with local circulation induced by the mountains underlies their formation. We demonstrate that an additional local source of water vapor is needed for the formation of opaque discrete afternoon clouds on the mountains of Mars, including the so-called W-formation. We suggest that the possible deposition of ice from the thick aphelion cloud belt during the night is capable of providing this additional source. This direct cloud water deposition on mountains on Mars bears many similarities with the well-known interception of water by mountains frequently immersed in fogs on Earth. Citation: Moudden, Y., and J. M. Forbes (2009), Mars W cloud: Evidence of nighttime ice depositions, Geophys. Res. Lett., 36, L14204, doi: /2009gl Introduction [2] Mountain clouds in the Tharsis region on Mars were observed as discrete afternoon brightenings in the Mariner 6 and 7 TV pictures in 1969 [Leovy et al., 1971]. These mountain clouds, labeled the W cloud because of the pattern that they form, were later associated with water vapor condensing over the high volcanoes of the Tharsis region [Peale, 1973]. Mountain clouds on Mars were extensively observed by the Hubble Space Telescope (HST) and the Mars Orbiter Camera (MOC) on-board Mars Global Surveyor (MGS), and are currently monitored by the Mars Color Imager (MCI) onboard the Mars Reconnaissance Orbiter (MRO) spacecraft. The above observations revealed the clouds as discrete daytime patches of high brightness located on or in the vicinity of the mountain peaks. Since they are a quasi-stationary feature of the Tharsis region they were interpreted as a result of air lifting caused by the large Volcanoes [Clancy et al., 1996], but the existence of a local source of water was not ruled out [Peale, 1973]. [3] Mountain clouds are regularly seen over the high mountains on Mars such as the soaring Tharsis volcanoes, the giant Olympus Mons that peaks at 21.1 km high, and Elysium Mons, but also over moderately elevated mountains like Alba Patera (6.8 km) and Apollinaris Patera (5 km). 1 Department of Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, USA. Copyright 2009 by the American Geophysical Union /09/2009GL They acquire their distinctive character from being associated with mountain peaks and regularly appear in daytime in the northern spring and summer seasons when it is rare to observe other clouds at the same lavel of opacity. Figure 1a is a composite image from the Mars Orbiter Camera that shows the afternoon mountain clouds [NASA, 2006]. Due to the Sun-synchronous orbit of the spacecraft the local time at all planet points on Figure 1a is near 14LT. The solar areocentric longitude is Ls = 93 which is near the northern summer solstice. The clouds appear on the northwest flanks of the mountains and extend in the direction of the wind before resublimating in dryer atmosphere. Various GCM simulations [Haberle et al., 1993; Wilson and Hamilton, 1995; Moudden and McConnell, 2005] produce near L s =90inthe northern hemisphere a westward jet that maximizes at km and wherein westward winds gradually increase from the surface. This wind configuration suggests that during the northern summer season westward winds prevail above 10 km in the northern hemisphere which explains the extension of the clouds in the leeward side of the mountains. Olympus and Ascraeus have the largest and most opaque clouds, the other mountains tend to have slightly less opaque clouds. MOC and MARCI images show that there are usually more extended clouds near Olympus and Ascraeus than on Pavonis, Arsia or Alba. During the northern spring and summer seasons both the average atmospheric temperature and water vapor gradually decrease from the northern pole to the southern one according to TES and Viking data [Smith et al., 2001a; Smith, 2002]. Olympus and Ascraeus are in the middle of a latitude corridor that is most favorable to the formation of clouds at this time of year. This corridor, roughly between 10 S and 30 N, is where the aphelion cloud belt (ACB) is most seen during this season [Clancy et al., 1996; Smith et al., 2001b]. The lower amounts of water vapor at Pavonis and Arsia latitudes or higher temperatures at Alba latitude are likely to account for the differences in cloud cover seen in the discrete afternoon mountain clouds (AMC). The derived opacities of the AMC at 14LT from TES and MOLA observations [Wilson et al., 2007] show, as explained above, a more extended cloud region near Olympus and Ascraeus with patterns that reflect the predominantly westward winds. Cloud absorption opacities in the thermal infrared (9 mm) derived from TES show a maximum opacity of 0.5 for the clouds associated with Olympus, Ascraeus and Pavonis and nearly 0.3 near Arsia; derived opacities from MOLA are nearly 1 for Olympus and Ascraeus clouds and 0.8 for Pavonis and Arsia. The absorption opacities reported by TES are 35 70% less than the corresponding visible extinction opacities depending on the ice particle sizes [Wolff and Clancy, 2003; Clancy et al., 2003]. The images from HST, MOC and MARCI show that the AMC are capable of completely shadowing the surface which is equivalent to opacities larger than 0.5 (an opacity of 0.5 is equivalent to an atmospheric total extinction of 40% L of5

2 while an opacity of 0.1 is equivalent to an extinction of only 10%.) Figure 1. (a) Composite image of the Tharsis region from the Mars Orbiter Camera (picture PIA08636 [NASA, 2006]). The images were acquired during Ls = 93 in August The annotations indicate the four major volcanoes of the Tharsis region: Olympus, Ascraeus, Pavonis and Arsia. Alba is located further north. All MOC images are taken near 14LT and the largest cloud opacities are associated with the high peaks. Simulated cloud opacity (b) without and (c) with nighttime ice deposition. 2. Nighttime Ice Deposition and Formation of Mountain Clouds [4] The formation of mountain clouds is generally linked to the lifting either by the barrier effect of the mountains, upslope thermal lifting from the daytime near-surface heating or to the lifting produced by orographic waves. It is however believed that more than one effect can simultaneously contribute to the formation of AMC [Michaels et al., 2006]. Using 3D numerical simulations we show that while the combined lifting effects and the amount of water vapor known to exist at this season produce localised discrete mountain clouds, they are incapable of systematically generating the observed opacities of mountain clouds. Additional water is necessary to produce similarly opaque clouds especially near Arsia which is surrounded by lower amounts of water vapor, or Alba which is moderately elevated and warmer than mountains located in the middle of the ACB corridor. [5] Computer simulation of mountain clouds requires moderate to high horizontal resolutions, significantly below 5 since the mountains extend over areas of roughly 10 in diameter. Mesoscale simulations yield results only partly consistent with the observations. While 3D simulations [Wilson et al., 2007; Michaels et al., 2006] of AMC incorporate realistic amounts of atmospheric water vapor, they underestimate the opacity over Arsia and Pavonis relative to Ascraeus or Olympus. MOC images [NASA, 2006] show that mountain clouds associated with Arsia and Pavonis extend over smaller areas but also reveal that they exhibit similar high opacities as Olympus or Ascraeus. This underestimation of opacity outside or at the edges of the ACB corridor is also present in our simulation. Figure 1b is a plot of simulated cloud opacity at 14LT (local time at Pavonis Mons). This simulation is performed by the Global Mars Multiscale Model (GMMM) [Moudden and McConnell, 2005] using a bulk water parameterization [Moudden and McConnell, 2007]. The water in the atmosphere is described by two tracers subject to atmosphere dynamics, the vapor and ice phases of water. In addition to dynamic transport, water vapor and ice undergo a constant exchange between the two phases depending on the saturation state of the atmosphere. The clouds are assumed to be composed of ice spheres with radii of 4 mm [Clancy et al., 2003; Wolff and Clancy, 2003; Pearl et al., 2001], Clancy et al. [2003] and Wolff and Clancy [2003] show that ACB clouds are mostly composed of relatively large ice crystals with a radii of 3 4 mm. These particles are subject to gravitational deposition moderated by air resistance. This is a limiting factor to the accuracy of our simulations: our current knowledge about the ice particles size distributions in martian clouds is limited and without further observations it is hard to improve this aspect of the model. There is a source of water ice at the surface that is increased by ice sedimentation and depleted by sublimation. The model is run at a uniform global resolution of which corresponds to 90 km 90 km, and at Ls = 90. While the cloud opacity in Figure 1b near Olympus is between 0.4 and 0.7 which is comparable to the opaque clouds of MOC 2of5

3 Figure 2. East-west cross sections of simulated water ice mixing ratio for (a) Alba, (b) Ascraeus, (c) Pavonis, (d) Arsia and (e) Olympus, at 07 LT at Pavonis longitude. images, it is nearly 0.1 near Ascraeus, and less than 0.05 near other mountains. Opacities lower than 0.1 are mostly transparent and do not agree with the rather opaque afternoon clouds. This shortcoming is present in other simulations [Wilson et al., 2007; Michaels et al., 2006]. The dynamic lifting by the mountains seems insufficient to produce by itself the observed opacities with the given water vapor amounts. It is especially difficult to explain the existence of substantial clouds near Arsia Mons, where atmospheric water columns are less than 10 mm of precipitable water vapor (according to TES and Viking water columns [Smith, 2002]), by any dynamical lifting (see Appendix A). Cloud climatology studies using MOC images [Benson et al., 2003, 2006] show that Arsia has comparable cloud cover as other Tharsis mountains and confirm the existence of persistent daytime opaque clouds. The existence of a local source of water vapor therefore appears necessary to produce the observed AMCs. Observations of underground water-equivalent-hydrogen by the Gamma-Ray Spectrometer on Mars Odyssey [Boynton et al., 2002] do not show any elevated amounts of ground water ice near the mountain peaks which leads us to suggest the deposition of nighttime ice on the high mountains as a probable source. This deposition finds its source in the clouds associated with the ACB known to exist at this time of year with particularly high levels of ice [Clancy et al., 1996; Smith et al., 2001b]. The addition of a local source of ice in the model (Figure 1c) enhances the opacity values over all four mountains and produces comparable afternoon clouds that extend in the wind direction as seen in MOC images. [6] Figure 2 shows east-west cross-section plots of water ice mixing ratio at 07LT present over Alba, Ascraeus, Pavonis, Arsia and Olympus. The clouds are part of the Aphelion Cloud Belt that forms during the northern spring and summer. It is usually located in the tropical region between 10 S and 30 N. TES observations show that the longitudinal average opacity resulting from the ACB is generally between 0.1 and 0.15 [Smith et al., 2001b]. Ground-based millimeter wave temperature and water vapor profiling observations established that the ACB has a low altitude lower boundary, typically around 10 km. Hubble Space telescope images supported this locally for the Tharsis region [Clancy et al., 1996]. Figure 2 shows that the only surfaces on Mars directly exposed to the thick persistent nighttime clouds of the ACB are mountain surfaces. This direct contact with suspended water ice clouds, coupled with fast horizontal winds at these altitudes (typically m/s based on our model simulations) is likely to produce an accumulation of water ice on the cold mountain surfaces at night. These surfaces exhibit typical nighttime temperature near 160 K based upon our model predictions and TES-retrieved nighttime temperatures [Wilson et al., 2007]. The physics of the surface interaction with the ice-laden air is currently unknown, but the mass flux of ice is likely to depend on the ice mixing ratio in the model s first layer, the speed of the horizontal wind and degree of exposition to wind: windward-facing surfaces are likely to intercept more ice than leeward ones. The nighttime ice interception by high mountains is currently parameterized simply by allowing mountain surfaces exposed to nighttime clouds to hold enough ice to keep the first model layer saturated in daytime. [7] This method gives an upper limit estimate of daytime vapor transfer from surface to air. There are numerical schemes that estimate the hydrologic input from fog in mountainous terrain on Earth but their application on Mars is not relevant because vegetation plays an important role in collecting fog water, and air pressure and water content are orders of magnitude larger than on Mars. [8] Figure 3 shows east-west cross sections of water ice mixing ratios and vertical velocities at 16LT along Alba, Ascraeus, Pavonis, Arsia and Olympus. The left panels are from an experiment that includes only gravitational ice sedimentation while the right panels experiment includes nighttime ice deposition. At 16LT and without nighttime ice deposition the amounts of ice in the vicinity of mountains is nearly five times lower than at 07LT (Figure 2) because of higher temperatures, and ice mixing ratios near 10 5 at altitudes above the mountains tops do not produce enough water ice column opacity. From the vertical velocity plots it is clear that upslope anabatic winds (Alba), upward movement in the mountain gravity waves (Ascraeus) or a combination of both (Pavonis) are providing the necessary uplift for the formation of discrete afternoon clouds. The amounts of water vapor present in the lower atmosphere are however not enough to produce the observed opacity. The additional water vapor intercepted by surfaces exposed to nighttime clouds (panels f, g, h, i and j) is sufficient to increase the water content within the clouds formed by the same lifting mechanisms. 3. Discussion [9] Large amounts of water are known to exist on Mars especially in the topmost layers of the soil. With the 3of5

4 Figure 3. East-west cross sections of simulated water ice mixing ratio and vertical velocity (m/s) for (a and f) Alba, (b and g) Ascraeus, (c and h) Pavonis, (d and i) Arsia and (e and j) Olympus at 16LT at Pavonis longitude. (right) Simulation that includes nighttime ice deposition in the water parameterization in addition to gravitational sedimentation, (left) simulation where nighttime deposition is neglected. The yellow contour indicates a vertical velocity of 0 m/s and separates upward and downward moving air masses. Solid and dashed contours indicate positive (upward) and negative (downward) velocities respectively, contour interval is 0.5 m/s for Ascraeus and Olympus and 0.25 m/s for other mountains. 4of5

5 mapping of water vapor by Viking and MGS orbiters and the estimates of buried water ice from Odyssey, we possess a reasonable knowledge of the amounts of water on Mars. Modelling efforts and cloud opacities from TES have improved our understanding of the global water cycle. TES observations confirmed what was clearly established by Clancy et al. [1996], i.e., the existence of a tropical cloud belt near the aphelion in the northern spring and summer seasons located slightly north of the equator. Mountain clouds observed by various imaging instruments in the immediate vicinity of major volcanoes are part of the seasonal water cycle. They also appear during the same time of the year as the aphelion cloud belt and were interpreted as a result of the dynamical lifting produced by the mountains. Using model simulations we show that while the combined lifting effect and the amount of water vapor known to exist at this season produce localised discrete mountain clouds, they are incapable of systematically generating the observed opacities of mountain clouds. Additional water is necessary to produce similarly opaque clouds especially near Arsia which is surrounded by lower amounts of water vapor, or Alba which is moderately elevated and warmer than mountains located in the middle of the ACB corridor. According to our simulations nighttime deposition of ice appears to be a valid source of additional water. Appendix A [10] A simple analytical calculation indicates that the water vapor available in the atmosphere is unlikely to produce the opacities of mountain clouds in daytime. Water column observations from Viking, TES and recently CRISM instrument onboard MRO indicate that during the northern spring and summer there is less than C =10mm of precipitable water vapor at Arsia s latitudes. This amount increases gradually with latitude to 15 mm in the Tharsis region according to CRISM data and 25 mm according to TES. Assuming an average surface pressure of p s =6mb and a pressure at the top of Arsia of p s = 1.5 mb then the water mixing ratio r w (assuming a constant mixing ratio and a hydrostatic exponential decrease of density) would be: r w = C g/p s = where C is the water vapor column in kg m 2 and g is gravity assumed to be 3.7 m s 2. The column of water vapor above Arsia would be C = r w p s /g = kg m 2. If all this water vapor were to be converted to ice particles with a radii of 4 mm then the optical depth of the resulting cloud would roughly be t = 0.5. 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Res., 108(E9), 5097, doi: /2003je J. M. Forbes and Y. Moudden, Department of Aerospace Engineering Sciences, University of Colorado, UCB 429, Boulder, CO 80309, USA. (youssef.moudden@colorado.edu) 5of5