Mars atmosphere

In many ways the climate of Mars resembles that of the Earth, particularly in its daily cycle and yearly sequence of seasons. These affinities result from the many coincidences in the celestial motions of the two planets: the Martian day, or sol, is 24h40m long; Mars completes an orbit around the Sun in approximately 2 earth years; its axis of rotation is tilted with respect to the orbital plane only slightly more than the Earth’s (25.2º and 23º respectively). However, the eccentricity of the Martian orbit is much higher and at a mean distance from the Sun of 1.5 AU, over one complete orbit Mars receives only about half as much sunlight as the Earth. As a consequence, and also because there are no oceans on Mars, the Martian surface is colder and experiences greater seasonal temperature changes and more pronounced diurnal variations, with differences between the night minimum and afternoon maximum of 70 degrees or more.

Mars Atmosphere

When we consider the variability of the planetary atmospheres in the solar system, the similarities between the atmospheres of Mars and Earth appear more impressive than their differences. However, both in thickness and composition the Martian atmosphere bears no resemblance to its terrestrial counterpart. It is 95 % carbon dioxide with Nitrogen and Argon accounting for almost all of the remaining 5 %. Altogether, Oxygen and water vapor, which constitute a significant part of the air we breathe, make up less than 0.2 % of Martian air.

The atmosphere of Mars is also extremely tenuous, with an average surface pressure of only 6.1 mbar (the mean surface pressure on Earth is 1013 mbar). Accidentally, this value is remarkably close to the triple point of water. Since a liquid can only exist in a stable state above the triple point pressure the presence of liquid water on the surface of Mars is uncertain. However, Mars topography is very pronounced and uneven, with lowlands spreading over most of the Northern hemisphere and highlands mainly South of the equator. Thus, the pressure at the surface varies considerably from the top of the highest mountains and extinct volcanoes, where it drops to around 4 mbar, to low lying areas – such as canyons or deep impact craters – where it reaches 10 mbar. This is certainly more than enough for liquid water to be stable provided, of course, that the temperature gets temporarily above 0ºC, which indeed occurs in regions not too far from the equator during summer afternoons. Until today, however, and in spite of all the efforts put to the task, proof of liquid water in present-day Mars remains elusive, although there is circumstantial evidence which points to the presence of liquid water confined to a shallow layer below the surface.

The scarcity of water vapor in the Marian atmosphere and the absence of bodies of liquid water, large or small, means that present-day Mars does not have an hydrological cycle anywhere close to that of the Earth (where water vapor accounts for an impressive 1 – 4 % at the surface). Nevertheless, the Martian atmosphere reaches such low temperatures, that water vapor temporarily exceeds the maximum amount the atmosphere can hold – the atmosphere becomes saturated. High in the martian sky very thin white clouds made of crystals of water ice may form, like cirrus on Earth. On the surface, frost deposits during the night and usually sublimates the next morning. However frosty patches may persist for a significant fraction of the day, depending on the season, particularly when formed on poorly illuminated areas such as the inner walls of craters which, due to their slope and orientation, spend the most part of winter days in the shade. Fogs also form during the night but usually have completely dissipated by mid-morning. Besides all that happens above the surface, water vapor diffuses through the topmost layers of the soil. There it becomes trapped during the coldest hours before being released back to the atmosphere in daytime, as sunlight strikes the surface and triggers a heat wave that slowly penetrates the ground down to a depth of ~ 30 cm. Thus, Mars does have a sober, but crucial, diurnal hydrological cycle. There is a seasonal hydrological cycle as well, characterized by a peak of water vapor abundance at the North pole in the Northern hemisphere mid-summer, during which the ground ice reaches a minimum, and a weaker maximum at the South pole at northern winter solstice. In between those extremes water is transported equatorward by the atmosphere. Again, the total amount of water vapor involved is very small; the Northern polar maximum is only ~ 100 precipitable microns (pr-µm) but in relative terms it is quite significant since it corresponds to more than twice the average. The mean water vapor abundance is estimated to be less than 50 pr-µm, most probably lying in the 10 – 40 pr-µm range. To have a sense of these values, consider that if all the water present in the atmosphere of Mars was to condense at once it would produce at most a 50-µm thick film, half the thickness of a human hair! (On Earth, total water vapor column abundance is measured in precipitable millimeters!)

In response to the large temperature excursion in the diurnal cycle, the surface pressure and wind direction vary in a correlated way with time of day. Although the wind direction displays a pronounced variability, resulting from small-scale gusts, the diurnal behavior of winds on Mars is characterized by a contrast between almost steady winds during the night and a progressive 360˚ rotation in daytime. This pattern of surface winds is a result of the large scale atmospheric tides – global-scale waves driven by the day-night contrast in solar heating – which dominate the Martian weather. On a global scale, easterly winds prevail in the tropics and in the summer hemisphere at the solstices while westerlies are predominant in the winter hemisphere, at middle and high latitudes, during the equinoxes. The Viking and Pathfinder observations showed that the mean wind speed on Mars is fairly weak: 1 – 4 m/s (about 4 – 15 km/h). However, under some extraordinary conditions – such as in the event of global or local dust storms – winds are expected to blow at speeds over 30 m/s or even more (> 110 km/h ).

One feature that clearly differentiates the climates of Earth and Mars is the existence of a global and permanent dust cycle on Mars. There, the absence of vegetation and weathering by liquid water means that what we usually call soil, namely a layer of compact, cemented mixture of mineral and organic constituents, is simply inexistent. Instead, the surface of Mars appears everywhere covered with a layer of fine red dust and rocky debris – a global desert. Only the most arid of Earth’s deserts come close to the bareness of the Martian surface. The sustained action of the surface winds and convection results in large amounts of the smallest dust particles, ranging from a few microns up to millimeter-sized, being lifted to the atmosphere thus causing an increase in atmospheric turbidity . On Mars, obscuration due to aerosol particles can be detected almost every day. Even in on occasions when major dust storms have been absent for weeks there is always a background level of dust in the the atmosphere which for an observer on the surface appears as an almost permanent haze. The fact that incoming and reflected solar radiation is absorbed by the suspended dust provides a direct mechanism for heating the atmosphere.