Global Dust Storm in 2001. Hubble Space Telescope image dust storm dual image is NASA, James Bell (Cornell Univ.), Michael Wolff (Space Science Inst.), and the Hubble Heritage Team (STScI/AURA)
Mars has an average atmospheric pressure of about 6 mbar although this varies with elevation and the planet's seasons. Like on Earth, areas of high elevation on Mars have lower atmospheric pressures, for instance, the atmospheric pressure around the summit of the Olympus Mons volcano (21 km) is about 2 mbar. This is an important factor in planning landing missions on Mars. Although the martian atmospheric pressures are hundreds of times lower than those of Earth they are great enough to support dust storms which are a feature of springtime in the southern hemisphere of Mars. Clouds of H2O and CO2 ice also form in the atmosphere - particularly in circumpolar regions where they are known as the Polar Hood - and some of the clearest pictures of this have been taken with the Hubble Space Telescope (Figure below). One of the clearest effects of seasonal variations on Mars is the change in sizes of the polar caps. Over 99% of the volume of the polar caps sublimes from ice to atmospheric CO2 and H2O during the spring and summer (liquid water does not form during this process because it is not stable at the low atmospheric pressures on Mars). This leads to slightly elevated atmospheric pressures especially during southern summer.
Hubble Space Telescope image of polar clouds. The dust-rich clouds can be seen around the northern pole.(STSCI-PRC96-34,James Bell III (Cornell University), Todd Clancy (Space Science Institute), Phil James (University of Toledo), Steve Lee (University of Colorado), Leonard Martin (Lowell Observatory), Michael Wolff (University of Toledo), and NASA).
Mars experiences a great range in surface temperatures. The Viking Infra Red Thermal Mapping data showed temperatures as low as -133oC at the south pole during its winter. The highest temperature recorded was +25oC, in a midlatitude region. There is also a great range in daily temperatures - up to 120oC variation was recorded in one locality.
Thermal inertia is a measure of how rocks retain and conduct heat. For instance, rocks tend to maintain their temperatures and so have high thermal inertias. Dusty areas heat up and cool more readily. Measuring the thermal inertia of the surface helps characterise the type of land surface. Rocky areas have high thermal inertia and this is shown by the fact that the pre-dawn temperatures are highest in these areas because relatively little heat has been lost. Dusty areas have low thermal inertia and so colder pre-dawn temperatures. Maps of thermal inertia have been constructed on the basis of pre-dawn temperatures and used to calculate rock abundances in the planning of missions.
Mars has an eccentric orbit, with a much more elliptical shape than that of the Earth (see Table 1 in Physical and Orbital Parameters and the bibliography for some of the terms used here to describe the parameters of Mars's orbit). At the perihelion of this orbit (the closest point to the Sun during the martian year at Ls = 250o) the southern hemisphere happens to be tilted towards the Sun and so southern summers are hotter than northern summers, which do not coincide with the perihelion of Mars orbit. The large daily variations in temperature between the southern and northern hemispheres during southern summer, together with the slightly increased atmospheric pressures at that time, leads to winds which pick up large amounts of dust. This can result in regional or, more occasionally, global dust storms that last from a few weeks to months. One of the longest documented dust storms took place in 1977 and was observed by the Viking Landers and Orbiters. This storm lasted over 100 martian days. More recently in September 2001, much of Mars was obscured by a large dust storm.
Regions on Mars where dust has settled and accumulated are bright (high albedos) and those where it does not settle are dark, providing distinctive markings for astronomers. There are extensive regions of dust accumulation in the northern hemisphere and great dune fields and layered dust deposits around both poles. These are considered further in the section on martian geology.