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Climate of planet Mars is an inhospitable place.


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Climate of Mars has a reasonably well studied climate. Studying the climate of Mars started in earnest with the Viking program in 1975 and continuing with such probes as the highly successful Mars Global Surveyor. This observational work has been complemented by a type of scientific computer simulation called the Mars General Circulation Model . Mars' climate has similarities to Earth's, including seasons and periodic ice ages, but also important differences such as the absence of liquid water and the much lower thermal inertia. The climate has been of long standing interest in the question of life on the planet, and has recently become newsworthy due to claims that Mars, like the Earth, is undergoing a process of global warming.

Low atmospheric pressure

Mars ice cap.
Mars climate pits in south polar ice cap, MGS 1999, NASA.

The Martian atmosphere is composed mainly of carbon dioxide and has an average surface pressure of about 6 millibars, much lower than the Earth's 1013 millibars. One effect of this is that Mars' atmosphere can react much more quickly to a given energy input than can our atmosphere . As a consequence Mars is subject to strong thermal tides, similar to the sea tides on Earth, but produced by solar heating rather than a gravitational influence. These tides can be significant, being up to 10% of the total atmospheric pressure. Earth's atmosphere experiences similar diurnal and semidiurnal tides but their effect is less noticeable because of Earth's much greater atmospheric mass.

Although the temperature on Mars can reach above 273K (0ºC), liquid water is unstable as the atmospheric pressure is below water's triple point and water ice simply sublimes into water vapour. An exception to this is in the Hellas Planitia impact crater, the largest such crater on Mars. It is so deep that the atmospheric pressure at the bottom reaches 11.55 millibars, which is above the triple point, so if the temperature exceeded 0ºC liquid water could exist there.

Winds on Mars.

Mars Polar Cyclone.
Hubble, colossal Polar Cyclone on Mars

The surface of Mars has a very low Thermal inertia, which means it heats quickly when the sun shines on it. Typical daily temperature swings, away from the polar regions, are around 100 K. On Earth winds often develop in areas where thermal inertia changes suddenly, such as from sea to land. There are no seas on Mars, but there are areas where the thermal inertia of the soil changes, leading to morning and evening winds akin to the sea breezes on Earth.

At low latitudes the Hadley circulation dominates, and is essentially the same as the process which on Earth generates the trade winds. At higher lattitudes a series of high and low pressure areas, called baroclinic pressure waves, dominate the weather. Mars is dryer and colder than Earth, and in consequence dust raised by these winds tends to remain in the atmosphere longer than on Earth as there is much less precipitation to wash it out. One such cyclonic storm was recently captured by the Hubble space telescope (pictured above)..

Effect of dust storms on the ckimate of Mars.

Mars dust storm.
2001 Hellas Basin dust storm.

When the Mariner 9 probe arrived at Mars in 1979 the world expected to see crisp new pictures of surface detail. Instead they saw a near planet-wide dust storm with only the giant volcano Olympus Mons showing above the haze. The storm lasted for a month, an occurrence scientists have since learned is quite common on Mars. On June 26, 2001, the Hubble Space Telescope spotted a dust storm brewing in Hellas Basin on Mars (pictured right). A day later the storm "exploded" and became a global event. This dust storm raised the temperature of the atmosphere of Mars by 30ºC. The low density of the Martian atmosphere means that winds of 40 to 50 mph are needed to lift dust from the surface, but since Mars is so dry, the dust can stay in the atmosphere far longer than on Earth, where it is soon washed out by rain. The season following that dust storm had daytime temperatures 4ºC below average. This was attributed to the global covering of dust that settled out of the dust storm. This temporarily increased Mars' albedo.

Dust storms are most common during perihelion, when the planet receives 40 percent more sunlight than during aphelion. During aphelion water ice clouds form in the atmosphere, interacting with the dust particles and affecting the temperature of the planet .

It has been suggested that dust storms on Mars could play a role in storm formation similar to that of water clouds on earth. Observation since the 1950s has shown that the chances of a planet-wide dust storm in a particular Martian year are approximately one in three.

Polar caps on Mars.

The polar regions of Mars, in particular the southern pole, are cold enough for carbon dioxide to condense and form polar ice caps together with large amounts of water ice. So much of the atmosphere can condense at the poles in summer and winter that the atmospheric pressure can vary by up to a third of its mean value of 6 millibars. The eccentricity of Mars's orbit affects this cycle, as well as other factors. In the spring and autumn wind caused by this sublimation process is so strong that it can be a cause of the global dust storms mentioned above .

Mars possesses ice caps at both poles, which mainly consist of water ice; however, there is dry ice present on their surfaces. Frozen carbon dioxide (dry ice) accumulates in the northern polar region in winter only, melting completely in summer, while the south polar region additionally has a permanent dry ice cover up to eight metres (25 feet) thick. This difference is due to the higher elevation of the south pole.

The northern polar cap has a diameter of approximately 1,000 kilometres during the northern Mars summer, and contains about 1.6 million cubic kilometres of ice, which if spread evenly on the cap would be 2 kilometres thick. (This compares to a volume of 2.85 million cubic kilometres for the Greenland ice sheet.) The southern polar cap has a diameter of 350 km and a thickness of 3 km, and contains enough water ice to cover the planet with liquid water 36 ft deep. Both polar caps show spiral troughs, which are believed to form as a result of differential solar heating, coupled with the sublimation of ice and condensation of water vapor. Both polar caps shrink and regrow following the temperature fluctuation of the Martian seasons.

Seasons on Mars and the effect on its climate.

The eccentricity of Mars' orbit is 0.1, much greater than the Earth's present orbital eccentricity of about 0.02. The large eccentricity causes the insolation on Mars to vary as the planet passes round the Sun (the Martian year lasts 687 days, roughly 2 Earth years). As on Earth, Mars' obliquity dominates the seasons but, because of the large eccentricity, winters in the southern hemisphere are long and cold while those in the North are short and warm. precession in the alignment of the obliquity and eccentricity lead to global warming and cooling ('great' summers and winters) with a period of 170,000 years .

Like Earth, the obliquity of Mars undergoes periodic changes which can lead to long-lasting changes in climate. Once again, the effect is more pronounced on Mars because it lacks the stabilising influence of a large moon. As a result the obliquity can alter by as much as 45º. Jacques Laskar, of France's National Centre for Scientific Research, argues that the effects of these periodic climate changes can be seen in the layered nature of the ice cap on the planets north pole. . Current research suggests that Mars is in a warm interglacial period which has lasted more than 100,000 years.

Evidence for recent climatic change

In 1999 the Mars Global Surveyor photographed pits in the layer of frozen carbon dioxide at the Martian south pole. Because of their striking shape and orientation these pits have become known as swiss cheese features. In 2001 the craft photographed the same pits again and found that they had grown slightly larger .

More recent observations indicate that Mars' south pole is continuing to melt. "It's evaporating right now at a prodigious rate," says Michael Malin, principal investigator for the Mars Orbiter Camera (MOC) . The pits in the ice are growing by about 3 meters per year. Malin states that conditions on Mars are not currently conductive to the formation of new ice. NASA has suggested that this indicates a "climate change in progress" on Mars.

Attribution.

Calculations with the Mars General Circulation Model show that the local climate around the Martian south pole is currently in an unstable period. This instability is rooted in the geography of the region, leading Colaprete et al. to speculate that the melting of the polar ice is a local phenomenon rather than a global one. The researchers showed that even with a constant solar luminosity the poles were capable of jumping between states of depositing or losing ice - the trigger for a change of states could be either due to increased dust loading in the atmosphere or an albedo change due to a deposition of water ice on the polar cap.

K.I. Abdusamatov of the Pulkovo Observatory has attributed the changes to increased levels of solar activity, asserting that "parallel global warmings -- observed simultaneously on Mars and on Earth -- can only be a straightline consequence of the effect of the one same factor: a long-time change in solar irradiance." Abdusamatov's hypothesis has not been accepted by other scientists. Amato Evan, a climate scientist at the University of Wisconsin, Madison, stated that "the idea just isn't supported by the theory or by the observations." Other scientists have proposed that the observed variations are caused by irregularities in the orbit of Mars.

In recent decades solar activity has however been relatively stable, though researchers at the Max Planck Institute have inferred that solar activity over the past 60 to 70 years may have been at its highest level in 8,000 years . Others have suggested that comparably high levels of activity have occurred several times in the last few thousand years. Alternatively, it has been argued that "observed regional changes in south polar ice cover are almost certainly due to a regional climate transition, not a global phenomenon, and are demonstrably unrelated to external forcing."

Writing in Nature, Oliver Morton said "The warming of other solar bodies has been seized upon by climate sceptics; but oh how wrong they are. .. On Mars, the warming seems to be down to dust blowing around and uncovering big patches of black basaltic rock that heat up in the day"




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