Observations of Atmospheric Water

8.1.1 Water Vapour

Mars is covered by a thin atmosphere composed mainly of CO2. The mean atmospheric pressure at the surface is only about 6 hPa (= mbar), roughly 150 times less than on Earth, but varies by 25 % with the seasons because a part of the atmosphere condenses out and is deposited at the poles in their respective winter. Due to the larger distance to the Sun compared to the Earth and the lack of a substantial greenhouse effect, the mean surface temperature is only 210 K, about 80 K lower than on Earth. Since a thin atmosphere can rapidly respond to solar heating both the seasonal and diurnal variation of the air temperature are large. Diurnal temperature variations of more than 100 K are common at low latitudes. Characteristic of the Martian atmosphere is also the large susceptibility of the atmospheric structure to the amount and distribution of suspended dust.

Water vapour is one of the trace species of the atmosphere with a volume fraction of the order of 10-4, and was first detected by spectroscopic measurements from Earth-based telescopes [59]. Atmospheric water vapour was systematically mapped by the Viking Orbiter in the 1970s. The Viking Orbiter was equipped with a spectrometer termed MAWD (Mars Atmospheric Water Detector Experiment)

T. Tokano, Water Cycle in the Atmosphere and Shallow Subsurface. In: Water on Mars and Life, Tetsuya Tokano (ed.), Adv. Astrobiol. Biogeophys., pp. 191-216 (2005)

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devoted to the measurement of the global water vapour amount from the intensity of water absorption lines in the near-infrared spectra [26]. Similar measurements of the vapour abundance have recently been conducted by the Mars Global Surveyor Thermal Emission Spectrometer (TES) as well [57, 58]. The global and seasonal map of the column vapour abundance (vertically integrated amount of vapour at a given place) acquired by MA WD and TES represent the most complete picture of the temporal and spatial variation of atmospheric water, and are the basic data in the assessment of the global water cycle. (The TES results are shown in Fig. 8.1). The column vapour abundance is expressed in precipitable micrometres (pr pm = 10-3 kg m-2), which gives the height of precipitation that would result on the surface if it entirely condenses out. In general MAWD data are biased towards lower values than the TES data because the MAWD spectra were more susceptible to scattering by dust.

Fig. 8.1 Latitudinal and seasonal variation of the dust optical depth, water ice cloud optical depth and atmospheric water vapour column abundance (in pr |m) retrieved by TES reprinted from Smith [58] (reprinted with permission from Elsevier). The data cover more than two Mars years and LS is the areocentric longitude of the Sun (season on Mars) beginning with 0° (northern vernal equinox).

Fig. 8.1 Latitudinal and seasonal variation of the dust optical depth, water ice cloud optical depth and atmospheric water vapour column abundance (in pr |m) retrieved by TES reprinted from Smith [58] (reprinted with permission from Elsevier). The data cover more than two Mars years and LS is the areocentric longitude of the Sun (season on Mars) beginning with 0° (northern vernal equinox).

On an annual average the Martian atmosphere contains a global vapour amount of 2x10" kg-2.3x1012 kg, with a substantial seasonal variation [57]. This is more than 104 times less than the terrestrial atmospheric water content. To a first approximation the atmospheric vapour content on Mars varies with season and latitude and the basic feature does not differ much from year to year. The most apparent feature is the annual maximum of about 100 pr pm observed near the north pole shortly after the summer solstice. A further characteristic feature is the tongue-like feature which extends from the north pole southward with progressing season. For any given latitude in the northern hemisphere the peak water vapour column abundance occurs progressively later in the season for lower northern latitudes. The annual peak also decreases towards the equator. The seasonal variation in the southern hemisphere is not an exact mirror image of the northern counterpart. The annual maximum vapour abundance at the south pole is also found shortly after the southern summer solstice, but is substantially smaller. The southern maximum was 40 pr pm in the TES data [57] and only 10 pr pm in the MAWD data [26]. This is the most significant interannual variability concerning the atmospheric water content. Further interannual variability is discernible in southern summer when planet-encircling dust storms develop. The annual-mean vapour abundance is largest between the equator and 30°N, not near the north pole. This is because the vapour abundance in the polar region drops to almost zero in polar winter, while the equatorial region holds water year-round without any large seasonal variation.

The vertical distribution of water vapour is a critical factor in models of photochemistry and aeronomy (see also Section 8.2.4). Remote observations indicate that the water vapour is uniformly distributed up to 20-25 km, but rapidly diminishes above 25 km [53]. However, the observation from the Martian surface by the IMP (Imager for Mars Pathfinder) suggests that water vapour is more concentrated near the surface [66]. This difference may be a seasonal one. There are also some diurnal variations of the vapour amount, with a maximum in the early afternoon and a minimum in the morning and late afternoon [60, 65]. The diurnal variation is more pronounced in the vicinity of large volcanoes. Besides seasonal and latitudinal variations there is also some longitudinal variation of the annual-mean vapour abundance (Fig. 8.7 and [57, 58]).

The atmosphere of Mars is dry not only in terms of its absolute moisture content, but also with respect to its relative humidity. Under the given temperatures and air pressures the equatorial region could contain 200 times more vapour than is actually observed. It is also quite unlike the Earth in that the atmospheric water content does not steadily decrease towards the poles. However, models indicate that the vapour amount and the vigour of the atmospheric water cycle in general may dramatically increase during repeatedly occurring epochs of high obliquity because the polar cap sublimation enhances [39, 49].

8.1.2 Ice Clouds

Clouds on Mars have been documented for at least 200 years by telescopic observations [23], but they were identified as water ice only 30 years ago [12]. Nowadays, Martian clouds can be detected by visual observation, spectral mapping (MGS TES (shown in Fig. 8.1), Mars Odyssey THEMIS) and laser (MGS MOLA). On Mars both H2O and CO2 clouds are possible, but they can be distinguished from each other if the temperature is known. Morphologically, many clouds resemble thin high-altitude clouds on Earth, which appear faint on weather satellite images. Ice clouds consist of particles with radii of 1-4 pm [12, 46], which are comparable to those of terrestrial ice cloud particles or ice fog, but are substantially smaller than terrestrial snow. Ice particles typically settle with velocities of less than 5 cm s-1 for about one day before they sublime [29]. In many respects the Martian ice particles are considered similar to the so-called diamond dust in the clear Antarctic sky.

A prominent cloud feature is the cloud belt that extends over all longitudes within the 10°S-30°N latitude region in late northern spring and early northern summer (Figs. 8.1 and 8.2) [10, 46, 56, 58, 63, 64, 70]. These clouds are referred to as the equatorial cloud belt or aphelion cloud belt [10] because they form near the equator when Mars approaches its aphelion, the farthest point from the Sun on the orbit of Mars. These bright clouds have optical depths of 0.2-0.6 and are visible, e.g. on images taken by the Hubble Space Telescope. In the initial phase the cloud belt looks fibrous, but later on it contains more convective clouds [70]. A further persistent feature is the grey, opaque polar hood, which is more ubiquitous in the north than in the south. The polar hood sometimes extends south of 60°N due to the action of storm systems in winter [46, 63]. Sometimes there are also large spiral clouds at the edge of the north polar cap. They are virtually the Martian counterparts of the eastward-travelling mid-latitude storm systems on Earth.

Otherwise, relatively more clouds are observed in the vicinity of large mountains such as Olympos Mons, Tharsis volcanoes, Alba Patera or Elysium as well as over the Hellas basin [5, 28, 45, 58, 63]. Orographic clouds are more frequent in (cold) northern summer, and undergo diurnal variations, with thickest clouds observed in the afternoon. Clouds over the summits of volcanoes are the most opaque and highest ones observed on Mars. On the other hand, vertically extensive clouds such as thunderstorms do not develop on Mars because the humidity is too low. A detailed cloud catalogue was created based on images acquired by Mariner 9 and Viking Orbiters [17]. Many of the clouds have analogies on Earth such as wavelike lee waves in the lee of large obstacles, periodical cloud streets or irregular streaky clouds.

The cloud base altitude (condensation level, hygropause) depends on the temperature and water vapour profile, and varies mainly with season rather than with latitude [57]. The cloud base becomes highest (40 km) at perihelion near the northern winter solstice and drops to as low as 10 km at aphelion near the northern summer solstice. The presence of clouds does not imply saturation of the air below the cloud down to the surface because downward transport of water by precipitation considerably lowers the relative humidity of the air [51].

A special type of cloud is the surface fog that develops during the night in some regions and seasons on Mars (Fig. 8.3). The fog was first imaged by the Viking Orbiter [7]. The inflection in the time series of the near-surface air temperature, i.e. an anomalous temperature drop for about an hour in the night, was also interpreted as radiative cooling caused by fog [54].

Fig. 8.2 Mars in early northern summer/southern winter imaged by the Hubble Space Telescope (STScI-PRC1997-15C, courtesy of STScI) on 30 March 1997. A faint aphelion cloud belt is visible near the equator. Also visible are thick clouds over volcanoes near the equator.

Fig. 8.3 False-colour image of morning fog in northern spring near 53°N, 65°W imaged by the Mars Orbiter Camera on 4 June 1998 (MOC2-52, courtesy of NASA/JPL/Malin Space Science Systems). North is up and illumination is from the right.

Fogs on Mars are frequent in topographic depressions, particularly in the large basins Hellas and Argyre [17]. Laser observations reveal that surface fogs of H2O ice are prevalent along the edges of the seasonal polar cap at night [43].

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