Aspects of the Atmospheric Water Cycle

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The atmospheric and shallow subsurface water cycle of Mars consists of various components as illustrated in Fig. 8.4. Each of them will be discussed in this section. Atmospheric escape processes discussed in Chap. 2 by Lammer et al. are only relevant on geological time scales, so they are totally negligible in the consideration of the present global water cycle.

Fig. 8.4 Schematic diagram showing the different reservoirs of exchangeable water in the atmosphere and shallow subsurface as well as the fluxes between them. The magnitudes of the fluxes are not shown because of large uncertainties.

8.2.1 Exchange with the Polar Caps

The accumulation of water in polar caps (ice sheets) is a characteristic feature in cold climates, and is also expected to have prevailed to some extent on Earth during the ice ages. In such climates atmospheric water does not readily precipitate out as rain, but it is transported to the poles where it is finally trapped. Mars represents such a state and the polar caps indeed involve the largest water reservoirs on the surface.

On present Mars the sublimation of the H2O polar cap in summer after the retreat of the seasonal CO2 cap overlying the H2O cap provides the largest seasonal source of atmospheric water [3, 49]. The total vapour amount released in one summer may be 1011-1012 kg. At the north pole the sublimation begins near the northern summer solstice and continues until late summer. The vapour emission rate is controlled by the cap surface temperature, the total area of the exposed cap (poleward of about 82°N) and the wind speed near the surface [18]. The water vapour amount over the cap suddenly increases with the onset of the cap sublimation and becomes largest in early summer with 100 pr pm. The maximum is found when the polar cap attains the annual maximum temperature (205 K). This implies that the vapour amount in the polar region in summer would sensitively vary if the polar cap peak temperature changes for some reason, e.g. a change of insolation pattern as a result of secular variation of astronomical parameters or a change of the polar cap albedo as a result of variable dust deposition.

Until recently the southern polar cap was not considered a major seasonal source of atmospheric water, in contrast to the northern cap. However, recent observations with the Mars Odyssey THEMIS detected the exposure of H2O ice in a small part of the southern polar cap in late southern summer [67]. Mars Express OMEGA more directly confirmed the presence of perennial water ice in the southern polar cap [80]. In accordance with this, the water vapour abundance over the south polar cap recently acquired significantly increases in this season, albeit the maximum is only 40 pr pm, about 60 % less than in the north [57]. This smaller maximum may indicate that either the summer peak temperature of the south polar cap is lower or the actual extent of the exposed H2O cap is smaller.

The polar caps are not only an important source of atmospheric water, but also a sink. Atmospheric water in the polar region provided in early summer by sublimation of the polar cap is deposited again to the polar cap in late summer when the temperature drops [3]. In this way much (but not all) of the water from the polar cap is recycled within one season. Presently, (only) the centre of the polar cap is a net sink of atmospheric water [4]. More detailed discussions of the polar caps are given in Chap. 6 by Hvidberg.

8.2.2 Water Vapour Transport

Water vapour in the atmosphere is subject to transport by the wind. The wind system on Mars relevant for the north-south transport consists of several components [e.g. 15, 19, 62, 71] (Fig. 8.5). The first component is the so-called Hadley circulation, which is forced by the global imbalance of solar heating and infrared cooling of the atmosphere. Air is forced to lift in the warm summer hemisphere, flows to the winter hemisphere, descends and flows back to the summer hemisphere near the surface. The circulation pattern reverses twice per Mars year. The Hadley circulation on Mars is about six times stronger than the terrestrial counterpart, and is particularly strong in southern summer when global dust storms develop, with wind speeds of several tens of m s-1. The second component is the so-called CO2 condensation flow, which develops as a result of a periodical cycle of condensation and sublimation of a quarter of the total atmosphere at both poles. Air flows from the spring/summer pole, where the air pressure increases with the CO2 sublimation, to the autumn/winter pole, where the situation is reversed. This flow is strongest (some tens of cm s-1) at the edge of the polar cap and is significant because the Hadley circulation in this region is weak. Otherwise, the condensation flow is insignificant in comparison with the Hadley circulation. The third component is the atmospheric wave activity generated by inherent atmospheric instabilities and external forcing such as topography or strong solar heating (thermal tide). Atmospheric instabilities are caused by strong horizontal temperature variations and wind shear, and are analogous to the weather systems at mid latitudes of the Earth. They manifest themselves in short-term disturbances of wind and temperature.

The role and intensity of the transport processes for the global water budget as compared to sources and sinks cannot be assessed by the analysis of the observational data alone. Water transport depends on the direction and speed of winds and on the spatial gradient of the water vapour amount. One approach to investigating the atmospheric water transport is to implement water vapour and clouds as prognostic variables in numerical climate models (general circulation models). Typically, model results are verified by comparing the predicted and observed seasonal and latitudinal variation of the vapour abundance. However, some caution is necessary in the interpretation of such numerical models. Different water cycle models seemingly have been able to reproduce the observation by quite different mechanisms. Some models are able to fit the observation only if a strong seasonal exchange of water between the atmosphere and soil is included [24, 25], while other models do not require such exchange at all [49, 51].

The latter model [51] without any water exchange across the surface suggests that the atmospheric transport is one major mechanism in the global water cycle. The condensation flow in northern summer accounts for the southward transport of water vapour released from the northern cap. As the volume of air increases towards the equator, water vapour becomes diluted by longitudinal redistribution. The southward transport is efficient because the latitudinal vapour gradient is particularly large in this season and much of the northern tongue-like shape in Fig. 8.1 is attributed to the equatorward transport. A similar transport pattern is also found in the southern hemisphere. The weather systems in the northern subpolar region cause a strong mixing of water vapour. In late summer this strong mixing brings much water back into the polar region. At lower latitudes the transport is mainly by the Hadley circulation, and is directed from the summer to winter hemisphere. The Hadley circulation is also crucial for the upward transport and subsequent cross-equatorial transport of water at high altitudes.

The net annual interhemispheric transport of water in the atmosphere is likely to be northward. This may partly be due to the astronomically forced asymmetric heating pattern of the hemispheres, but it is also suggested that the global dichotomy of the Martian topography (southern highlands versus northern lowlands) may be responsible for the asymmetric transport pattern [50, 62].

Aphelion

Perihelion n-

South

Winter little vapour transport little vapour transport i cloud belt condensation flow i cloud belt condensation flow

North South

Summer Summer dust storm dust storm

waves

North

Winter

Fig. 8.5 Simplified scheme of the global vapour transport on Mars in two different seasons.

8.2.3 Influence of Clouds and Fogs

Despite their relatively small amount, ice clouds on Mars have a significant effect on the global water cycle. The most important aspect is the aphelion cloud belt near the equator [10]. This cloud belt begins to form when Mars approaches the aphelion, so the equatorial temperatures generally drop. Due to the lower air temperature condensation sets in at lower altitudes than otherwise. In this season the rising branch of the Hadley circulation is located at low northern latitudes, so water vapour in this region is efficiently carried upward and encounters condensation [51]. This gives rise to the observed aphelion clouds, which typically form at altitudes between 5 and 10 km (Figs. 8.2 and 8.5). The cloud belt disappears in late northern summer when the temperature increases. In northern late spring and early summer the cross-equatorial wind is northward near the surface but southward further above. If water vapour were able to enter these high altitudes, it would be transported southward. However, the cloud formation efficiently limits the upward transport of vapour, so water remains relatively near the surface and cannot be transported to the southern hemisphere. A similar reversed feature does not occur near the perihelion because then the air temperature is higher and no cloud belt forms that would otherwise limit the northward transport across the equator in southern summer. Besides this particular aphelion feature, condensation and subsequent sedimentation of ice particles always act to concentrate water vapour near the surface, especially in late northern summer.

Another cloud feature of importance for the water cycle is the thick polar hood that develops along the edge of the polar night. The latitudinal extent and the thickness of the polar hood is controlled by the actual vapour amount, the location of the interface between the polar and subpolar air masses (polar front) and the water transport efficiency across this polar front [51]. The polar hood is likely to act as a seasonal reservoir of atmospheric water in the cold polar winter in which almost no water vapour can exist.

Although clouds on Mars are optically thin, they are able to affect the temperature profile of the thin Martian atmosphere. The first hint on the radiative effect of clouds came from the vertical temperature profile measured at the Mars Pathfinder site [34]. During the night the temperature profile showed a marked inversion up to 20 km altitude, which could not be explained by atmospheric waves or night-time cooling under clear sky. Infrared cooling by the cloud during the night creates a large temperature inversion of 5-10 K near the cloud top, initiating further cloud formation and cooling [11]. Moreover, clouds reduce the amount of sunlight arriving at the surface during the day, lowering the surface temperature. On the other hand, heat exchange associated with condensation/dissipation of clouds is negligible on Mars because of the tiny cloud mass. Otherwise, clouds are excellent markers of the atmospheric motion, and often reveal the circulation pattern or turbulence in the atmosphere, although the clouds are of minor meteorological importance compared to dust.

The formation mechanism of fog is qualitatively different from the clouds observed near the summit of Martian volcanoes, although both show a marked diurnal behaviour. The diurnal temperature near the surface is quite large, so the relative humidity can vary between less than 1 % to 100 % within a Martian day [55, 76]. The fog begins to form in the evening or at night and disappears soon after sunrise. It may extend from the surface up to a height of several tens of metres. In contrast, orographic clouds usually develop in the afternoon when the upslope wind transports water vapour to higher, colder regions where water vapour encounters saturation. The optical depth of the surface fog is anti-correlated to the surface temperature [74], indicating that the temperature variation is the main driver for the fog formation and dissipation.

The surface fog is a temporary night-time reservoir of atmospheric water that cannot be accommodated in the vapour phase. It is light enough to remain suspended the whole night without precipitation to the surface, except very near the surface.

8.2.4 Photochemistry Related to Water

Water vapour is subject to photolysis which produces odd hydrogen (e.g. OH, H, HO2), a catalytic driver of carbon and oxygen chemistry on Mars [42]. The decomposition of water molecules in the atmosphere is a prerequisite of all the atmospheric escape processes discussed in Chap. 2 by Lammer et al., although on the time scales discussed in this chapter the atmospheric loss of water to space can safely be neglected.

In most parts of the Martian atmosphere the water vapour abundance is not sensitive to photochemistry [42]. On the other hand, the budget of several other species, particularly in the upper atmosphere, is controlled by the varying abundance and vertical distribution of water.

One important species whose abundance is very sensitive to water is ozone (O3). Photochemical models predict that atomic H reacts with O2, thereby reducing the amount of O2 available to reform O3 from collisions between O and O2 [e.g. 42]. Odd hydrogen chemistry accounts for the destruction of O3 and as a whole O3 and water vapour abundance are expected to be anti-correlated. For instance, the reduction in H-atom densities above the hygropause as a result of water condensation can permit higher O3 densities [9]. However, since the altitude of the hygropause varies with season (Section 8.1.2) the ozone number density at 10-40 km altitude is predicted to vary by an order of magnitude in the course of a Martian year. Simultaneous telescopic observations of the global ozone and water distribution are quite useful in this context and they are beginning to confirm some of these predictions [44].

Another major product of water vapour photolysis is H2O2 (hydrogen peroxide). H2O2 is produced by reaction of two HO2 radicals and is rapidly destroyed by photodissociation, and its abundance strongly correlates with the water vapour abundance [13]. Observations seem to indicate that H2O2 is substantially lower at aphelion than at perihelion, reflecting the well-known fact that there is less water vapour in the upper atmosphere at aphelion compared to perihelion as a result of the aphelion cloud belt. This species was suggested as a possible candidate oxidant in the Martian soil because H2O2 can condense to the surface [36] or diffusion of gaseous H2O2 may be possible [8]. However, see Chap. 2 by Lammer et al. for an alternative, more likely oxidant (superoxides) in the soil.

8.2.5 Surface Frost

Viking 2 Lander observed an accumulation of a white thin layer on the surface in winter, which was interpreted as surface frost composed of H2O [21, 61]. The images (Fig. 8.6) themselves do not reveal whether this frost was composed of H2O or CO2. It is also unknown whether this ice fell as snow or formed in situ as surface frost. Since the surface temperature at this site and season was not low enough to form and retain CO2 ice, the frost was assumed to be H2O ice.

Basically, surface frost forms when the near-surface air gets saturated with respect to H2O ice. However, this is not a self-evident process, as fog formation is an alternative process in such a situation. The formation (condensation) and retreat (sublimation) of the frost critically depend on the surface thermal balance and transport efficiency of vapour near the surface. Condensation begins when water vapour comes into contact with the cold surface. This may be favourable, e.g. in the shadow of large rocks [61]. The long survival of the frost is possible because water vapour near the surface is not readily transported away unlike the fog, so it can be re-deposited on the surface during the winter many times.

The surface frost formation/sublimation is considered a regular process in several atmospheric water cycle models and to account for a part of the seasonal water cycle [24, 49]. The thickness of the surface frost depends on season, and increases with latitude in both hemispheres. Maximum surface frost coverage of about 150 pr pm in polar winter is predicted. On the other hand, observations seem to indicate that the frost may be much thinner [46]. The frost deposition at the poles is ultimately responsible for the net accumulation of the polar cap, and simulating this process is an important but difficult task.

Fig. 8.6 Surface frost observed by Viking 2 Lander in Utopia Planitia (47.7°N, 48°W) in May 1979 (image reference: PIA00533, NASA Planetary Photojournal). At other landing sites (Viking 1 Lander, Mars Pathfinder) located at lower, warmer latitudes no surface frost was observed.

The frost deposition in autumn and the sublimation in spring may constitute a small temporary sink and source of atmospheric water at mid and high latitudes. As the leading edge of the seasonal ice cap is sublimed, water that is released into the atmosphere has some chance of being mixed poleward over the nearby seasonal ice cap [49]. This process is continually repeated, allowing surface frost to creep poleward. This mechanism has been suggested to explain the moderate vapour abundance maximum near the south pole in summer, i.e. a southern H2O cap would not be required.

8.2.6 Liquid Water?

Present Mars is obviously devoid of an ocean and not a single standing or flowing body of liquid water has ever been observed on the surface of Mars with certainty. In 1980 it was speculated that there was a seasonal appearance of liquid water in the near-surface soil in the Solis Lacus region (14-22°S, 84-100°W) because of an unusually high radar reflectivity in southern summer [79]. This feature was interpreted as evidence of brines formed by the melting of ice at depths of a few tens of centimetres. However, this interpretation has been strongly questioned because such brines in the shallow subsurface would be severely out of equilibrium and would barely survive even a single freeze-thaw cycle [78]. Instead, the anomalous radar reflectivity was probably caused by some combination of unique scattering properties of some yet unidentified process other than ice melting [78]. Moreover, recent fast neutron data of Mars Odyssey indicate that this region belongs to the driest places on Mars with water contents of only 2 wt % or so (Fig. 5.8 botttom in Chap. 5). There is also no coincidence at all with the locales of gullies observed by [35].

Recent detection of small surface runoff features (gullies) within the walls of some impact craters at mid latitudes [35] gave rise to reconsideration of the transient occurrence of liquid water. The phase diagram of H2O dictates that liquid water is thermodynamically stable if the temperature and air pressure are above the triple point (6.11 hPa, 273.16 K), but at the same time below the boiling point, which increases with air pressure. Since the maximum surface pressure on Mars does not exceed 12 hPa, the maximum boiling point on Mars is only 283 K, in stark contrast to 373 K on Earth at sea level. At most places the boiling point is even lower. In a nutshell, a surface pressure higher than 6.11 hPa and a surface temperature slightly above 273 K would be required for pure liquid water to be stable. Because of the low temperature and low air pressure on Mars the applicable range is very limited.

Due to the large diurnal temperature variation this cannot be achieved for an extended period. Nevertheless, certain regions of Mars fulfil this criterion for a total of a few days within a Martian year [20, 33]. This applies to the Hellas and Argyre basin at southern mid latitudes and a large portion between the equator and 40°N. In the presence of salts in the soil the applicable area further extends because of the freezing point depression, depending on the concentration and composition of the salts.

It is important to point out that on real Mars liquid water could temporarily exist even it is not stable according to the phase diagram. For energetic reasons both evaporation and freezing of liquid water would be sluggish on Mars [22, 31]. Water is then said to be metastable. Once ice melts, liquid water may persist for a longer period than would be expected. Whether melting can actually occur depends on the surface heat balance, which significantly varies from place to place due to differences in meteorology, topography or geology, but it may occur on slopes nearly anywhere on Mars [22] (see also Chap. 9 by McKay et al.).

The global map of liquid water stability on the surface of Mars is not invariant, but changes due to astronomically caused precession ([69], see also Section 8.4). If the perihelion occurs in another season than is the case presently, the insolation pattern and hence the surface temperature change. This causes a substantial change in the liquid water stability. For instance, when the perihelion occurs in northern summer, the liquid water stability disappears in the southern hemisphere except for in the equatorial canyon systems, while in large parts of the northern lowlands the stability increases. This result illustrates that the thermal condition favouring the existence of Mars may substantially change over time scales of 50000 years, even have done so in the recent past. The highest liquid water stability throughout the precession cycle is achieved in the Hellas basin, the southern rim of Amazonis Planitia, Chryse Planitia and southeast of Elysium.

With the exception of the Hellas basin, the stability is always higher in the northern hemisphere.

The presence of liquid water, however, not only requires the right temperature and pressure range at the surface, but also the availability of abundant water at these places at the right time. It turns out that higher liquid water stability is always accompanied with low soil water content because of the high temperature. Therefore, the open question is whether there is a mechanism to bring water from somewhere to these places (see Chap. 9 by McKay et al. and Chap. 10 by Cabrol and Grin).

In the context of the global atmospheric water cycle it is only important whether occasional liquid water on the surface would affect the water cycle. The volume of liquid water that would have been required to carve the 120 observed gullies is conservatively estimated to 2500 m3 (2.5x106 kg) per gully [35]. A more detailed analysis yielded an estimated volume of 60000 m3 (6x107 kg) per gully [72]. While this may appear to be a big local source of water, even a simultaneous and rapid evaporation of all the liquid water is still tiny compared to the present total vapour abundance (1012 kg). This comparison shows that the analysis of the seemingly tiny cycle of water vapour is more important in the consideration of the long-term global water cycle.

8.2.7 Exchange with the Soil

The atmosphere is bounded on its bottom by the planetary surface, which provides the interface between a gaseous and solid/liquid medium. In principle, water can be exchanged across the surface, changing the water content both in the atmosphere and subsurface. On Earth this exchange mostly takes place in the form of rainfall, infiltration, evaporation from the ocean or land surface. Snow does not enter the subsurface unless it melts. By this mechanism the atmospheric water on Earth is recycled within about 9 days [47].

The lack of liquid water in the atmosphere of Mars greatly reduces the efficiency of surface water exchange. Nevertheless, some exchange is supposed to take place because otherwise the present global distribution of subsurface water would be difficult to explain [68].

There is so far no in situ measurement of the water exchange at the Martian surface that could confirm such a mechanism. The observed temporal variation of the atmospheric water vapour content can certainly set an upper limit on the exchange rate, but we have to ascertain that the vapour content simultaneously changes due to many other effects, so the relative significance of the surface exchange is not readily estimated.

One basic reason to suggest the exchange of water at the surface is the dependence of the water holding capacity of the soil on basic climatological parameters such as temperature or vapour amount [14]. Mineral grains in the soil have the ability to attach water vapour by van der Waals forces between the negatively charged mineral grains and dipolar water molecules. The attached water molecules form a thin film of adsorbed water around the grains [41].

Adsorbed water does not form a crystal lattice, so it is unfrozen. As a general rule the amount of adsorbed water increases with the atmospheric vapour amount adjacent to the mineral grains and decreases with temperature (see Fig. 4.5 in Chap. 4), and depends on the mineralogy. The mineralogical composition of the Martian soil and its global distribution is not well known (see Chap. 4 by Bishop), but it is likely that hydrated minerals include montmorillonite (smectite) or zeolites. The adsorptive capacity of Na-montmorillonite and palagonite under Mars-like conditions has been experimentally determined [75]. The adsorptive capacity of montmorillonite (a clay mineral) or zeolites is much larger than that of palagonite (coating observed on alterated basaltic glass) because of the much larger specific surface area, i.e. there is more space for water molecules. Zeolites can retain more water than smectites at temperatures higher than 200 K for a given vapour pressure, but also at very low atmospheric vapour pressures typical for Mars [6]. Water can be reversibly exchanged between the cavities and channels within the zeolite structure and the atmosphere. Zeolites are suggested as one likely component of the dust particles as well as of the dust mantle on the surface.

The exchange of water between the soil and atmosphere requires not only adsorption and desorption, but also transport of water molecules. As liquid water usually does not exist on the surface of Mars and no infiltration of liquid water takes place, the transport of water in the shallow subsurface takes the form of diffusion of water vapour through narrow soil pores. The vapour diffusion can be upward or downward, depending on the actual vertical gradient of vapour density. The amount of water transported in this way is incredibly tiny in comparison with water percolating in Earth's soil. The vapour diffusion may be limited if the soil is cemented, e.g. by salts or ice, the latter probably being the case in the polar region. Migration of the adsorbed water film itself is negligible under Martian conditions [37].

One could assume that in the warm daytime or in warm seasons the adsorptive capacity of the soil decreases, so the water molecules released as vapour would diffuse to the atmosphere and enter the soil again as the air gets colder in the afternoon or in autumn. In arid regions of the Earth the diurnal variation of the water vapour adsorption by the soil indeed affects the soil water content on diurnal time scales, particularly in clayey soils [30]. In sandy soils (dune), however, the diurnal cycle is due to the evaporation and condensation of liquid water and the diffusive exchange of water vapour between the atmosphere and soil rather than adsorption and desorption of water [73].

However, in the case of Mars there is considerable uncertainty as to whether a substantial exchange of water across the surface takes place on seasonal or even diurnal time scales. Some models support a strong seasonal exchange [24, 25], while others discard it [3, 49]. Similarly, some studies suggest a substantial diurnal exchange [6, 27, 65], while others do not consider this to be significant [55, 68, 75, 76, 77].

The first uncertainty concerns the time required for the adsorption and desorption of water molecules. For kinetic reasons, the adsorption of H2O by smectites is a relatively slow process, which typically takes several days until equilibrium is achieved [77]. On the other hand, zeolites hydrate and dehydrate quickly (on time scales of an hour) [6]. Therefore, it is possible but not certain that the water contents of minerals in the Martian soil change on diurnal time scales. Another concern is that water desorbed from minerals has to be transported within the soil and into the atmosphere and vice versa. The efficiency of this process depends on properties of the soil, the near-surface air (boundary layer) and the actual amount of water vapour near the surface. The vapour amount near the surface can simultaneously vary by surface frost deposition/sublimation, surface fog formation/dissipation, horizontal and vertical transport and vapour flux into and out of the soil, so the temporal variation of the vapour amount near the surface alone cannot be automatically attributed to the water exchange with the soil.

Enhanced diurnal exchange of water with the surface by adsorption/desorption in the Tharsis region was suggested on the basis of previous measurements as reviewed by [65], but it was not clear why this should be unique to Tharsis [77]. Recent GCM simulation of the atmospheric water cycle [51] reproduced substantial thickening of clouds in the Tharsis region, as observed, without any water exchange with the surface. The Tharsis clouds were ascribed to the regional daytime upward motion and radiative cooling in the vicinity of Tharsis, giving rise to an intensive cloud activity and water accumulation there. The enhanced water vapour abundance at this place is likely to be caused by the sublimation of these clouds.

The positive and negative arguments for the significance of seasonal or diurnal water exchange between the soil and atmosphere are mostly based on the ability or inability of the numerical models under consideration to reproduce the observation. It is fair to state that no model has been able to consider all the relevant details for the surface-atmosphere water exchange and, more importantly, the knowledge about the thermodynamic behaviour of hydration and dehydration of minerals and of the global distribution of minerals is tentative.

8.2.8 Impact of Dust on the Water Cycle

Dust is a ubiquitous, non-volatile component of the Martian atmosphere with a huge impact on the meteorology. In principle the dust particles are weathering products of the soil, and are readily suspended if the wind speed at the surface exceeds some threshold. The dust content in the atmosphere is quite variable, and mainly depends on season, but varies dramatically from year to year.

The atmospheric water cycle is affected by dust in several important ways. Dust particles are likely to act as condensation nuclei for H2O ice. Without such nuclei either the vapour amount would have to become much higher or the temperature must drop much more strongly to initiate ice condensation and cloud formation at all. If there had been no ice condensation due to the lack of dust particles, the global distribution of water vapour would have been more uniform than it is today, as can be imagined from the previous discussion in this chapter. However, ions and micrometeorites may be alternative condensation nuclei, although their abundance in the Martian atmosphere is less uncertain than that of dust [38].

If the surface can adsorb water molecules, dust particles should be able to adsorb water as well. Evidence for such a mechanism came from the simultaneous observation of the diurnal cycle of dust and water vapour in southern summer [16]. While the water vapour content increases in the morning and decreases in the afternoon, the dust opacity decreases in the morning and increases in the afternoon, i.e. the diurnal cycle of dust and vapour is anti-correlated. This behaviour is best understood by adsorption/desorption of water molecules by the dust particles added to the atmosphere, causing a decrease/increase of the water vapour content. The increase of the dust opacity in the afternoon is probably induced by the convective instability near the surface, which gives rise to suspending of a large number of dust particles from the surface. Water adsorption by suspended dust ought to be easier than by soil because water vapour need not be transported into or out of the regolith. Hence, this may be regarded as a possible mechanism to account for the observed diurnal cycle, in addition to the surface fog formation. The attachment of water molecules by the dust particles may also be important for the global water cycle, considering the intensity of planet-encircling dust storms, but has not yet been quantified.

Furthermore, the occurrence of large dust storms affects the water transport substantially. Since dust raises the temperature, ice clouds dissipate and the dust particles no longer transport water with themselves (see also Fig. 8.1). At the same time, dust strengthens the Hadley circulation. The northward transport of water vapour is greatly increased in years with strong dust storms [52, 68]. The amount of water transported each year across the equator may vary from year to year as with the intensity of the dust storms themselves.

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