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The climate of Mars has been an issue of scientific interest for centuries, due not least to the fact that Mars is the only terrestrial planet whose surface can be directly observed in detail from the Earth. Although Mars is smaller and somewhat further away from the Sun than the Earth, its climate has important similarities, such as the presence of an atmosphere, the polar ice caps, seasonal changes and the observable presence of weather patterns. It has attracted sustained study from planetologists and climatologists.

Martian climatatic conditions have been reasonably well studied. Data has been gathered by Earth-based instruments since as early as the 17th century but it is only since the exploration of Mars began in the mid-1960s that close-range observation has been possible. Flyby and orbital spacecraft have provided data from above, while direct measurements of atmospheric conditions have been provided by a number of landers and rovers.

The first martian flyby mission was Mariner 4 which arrived in 1965. That quick two day pass (July 14-15, 1965) was limited and crude in terms of its contribution to the state of knowledge of martian climate. Later Mariner missions (Mariner 6, Mariner 7, and Mariner 9) filled in some of the gaps in basic climate information. Data based climate studies 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. Several different iterations of MGCM have led to an increased understanding of Mars as well as the limits of such models. Models are limited in their ability to represent atmospheric physics that occurs at a smaller scale than their resolution. They also may be based on inaccurate or unrealistic assumptions about how Mars works and certainly suffer from the quality and limited density in time and space of climate data from Mars.

Although Mars's climate has similarities to Earth's, including seasons and periodic ice ages, there are also important differences such as the absence of liquid water and the much lower thermal inertia. Mars' atmosphere has a scale height of approximately 11 km (36,000 ft), 60% greater than that on Earth. The climate is of considerable relevance to the question of whether life is or was present on the planet, and briefly received more interest in the news due to NASA measurements indicating increased sublimation of the south polar icecap.

Historical

Astronomical observations of Mars of relevance to its climate began as early as 1666 when Cassini hypothesized that he had observed an ice cap near the north pole.

Giancomo Miraldi determined in 1704 that the southern cap is not centered on the rotational pole of Mars. During the opposition of 1719, Miraldi observed both polar caps and temporal variability in their extent.

William Herschel was the first to intuit the low density of the Martian atmosphere in his 1784 paper entitled On the remarkable appearances at the polar regions on the planet Mars, the inclination of its axis, the position of its poles, and its spheroidal figure; with a few hints relating to its real diameter and atmosphere. When two faint stars passed close to Mars with no affect to their brightness, Herschel correctly concluded that this meant that there was little atmosphere around Mars to interfere with their light.

Honore Flaugergues 1809 discovery of "yellow clouds" on the surface of Mars is the first known observation of Martian dust storms. Flaugergues also observed in 1813 significant polar ice melting during Martian springtime. His speculation that this meant that Mars was warmer than earth was neither the first nor the last reasonable speculation later to be proven dead wrong.

Recent observations and modeling is producing information not only about the present climate and atmospheric conditions on Mars but also about its past. The Noachian-era Martian atmosphere had long been theorized to be carbon dioxide rich. Recent spectral observations of deposits of clay minerals on Mars and modeling of clay mineral formation conditions have found that there is little to no carbonate present in clay of that era. Clay formation in a carbon dioxide rich environment is always accompanied by carbonate formation.

Weather

Mars lacks an ocean, a source of much inter-annual variation on earth. Mars Orbital Camera data beignning in March 1999 and covering 2.5 martian years shows that martian weather tends to be more repeatable and hence more predictable than that of earth.

Temperature

The average temperature on Mars is -55 C . Temperatures have been estimated from the Viking Orbiter Infrared Thermal Mapper data; this gives extremes from a warmest of 27 oC to -143 oC at the winter polar caps . Actual temperature measurements from the Viking landers range from -17.2 oC to -107 oC.

Low atmospheric pressure

The Martian atmosphere is composed mainly of carbon dioxide and has a mean 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 (typically ~ 0.5 millibars). 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 (0°C), 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 0°C liquid water could exist there.

Winds

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. The Antares project "Mars Small-Scale Weather" (MSW) has recently identified some minor weaknesses in current GCMs due to the GCMs more primitive soil modeling. Those weaknesses are being corrected and should lead to more accurate assessments going forward but make continued reliance on older predictions of modeled Martian climate somewhat problematic.

At low latitudes the Hadley circulation dominates, and is essentially the same as the process which on Earth generates the trade winds. At higher latitudes 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).

One of the major differences between Mars' and Earth's Hadley circulations is their speed which is measured on an overturning timescale. The overturning timescale on Mars is about 100 Martian days while on Earth, it is over a year.

Mountains

Martian storms are significantly affected by Mars' large mountain ranges. Individual mountains like record holding Olympus Mons (27km) can affect local weather but larger weather effects are due to the larger collection of volcanoes in the Tharsis region.

One unique repeated weather phenomena involving Mountains is a spiral dust cloud that forms over Arsia Mons. The spiral dust cloud over Arsia Mons can tower 15 to 30 kilometers (9 to 19 miles) above the volcano.

Effect of dust storms

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 30°C. 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 4°C below average. This was attributed to the global covering of dust that settled out of the dust storm, temporarily increasing Mars' albedo.

In mid-2007 a series of planet-wide dust storms posed a serious threat to the Spirit and Opportunity Mars Exploration Rovers, greatly reducing the amount of energy provided by the solar panels and necessitating the shut-down of most science experiments while waiting for the storms to clear.

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.

Cyclonic storms

First detected during the Viking orbital mapping program, cyclonic storms similar to hurricanes have been detected by various probes and telescopes. These storms tend to appear during the northern summer and only at high latitudes. Speculation is that this is due to unique climate conditions near the northern pole.

Polar caps

The polar regions of Mars, in particular the southern pole, are cold enough for carbon dioxide to condense and form polar ice caps together over the large accumulations 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. This condensation and evaporation will cause the proportion of the noncondensable gases in the atmosphere to change inversely. 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 km during the northern Mars summer, and contains about 1.6 million cubic km of ice, which if spread evenly on the cap would be 2 km 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 maximum thickness of 3 km. 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.

Methane presence

Methane has been detected in the atmosphere of Mars by ESA's Mars Express probe at a level of 10 ppb. Since breakup of that much methane by ultraviolet light would only take 430 years under current martian conditions, some sort of recent source must be replenishing the gas. Mars' current climate conditions may be destabilizing underground clathrate hydrates but there is at present no consensus on the source of Martian methane.

Solar wind

Mars lost most of its magnetic field about 4 billion years ago. As a result, the solar wind interacts directly with the Martian ionosphere. This keeps the atmosphere thinner than it would otherwise be by stripping away atoms from the outer layer.

Seasons

See also Astronomy on Mars#Seasons

Mars has an axial tilt of 25.2°. This means that there are seasons on Mars, just as on Earth. 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.

The seasons present unequal lengths are as follows:

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 45°. 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 local climatic change

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Mars temperature and circulation vary from year to year (as expected for any planet with an atmosphere). There are presently no time series of Martian data long enough to establish "climate" in a statistically meaningful sense.

There is some evidence for local changes around the south pole over the past few Martian years. 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, retreating about 3 meters in one martian year.

These features are caused by the dry ice layer evaporating exposing the inert water ice layer.

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 continue to grow by about 3 meters per martian 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.

Cause of the observed change

Colaprete et al. conducted calculations with the Mars General Circulation Model which show that the local climate around the Martian south pole may currently be in an unstable period. This computer calculated instability is rooted in the geography of the region, leading the authors 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. The changes in dust loading and in albedo are thought to be synchronous due to feedback.

This theory is somewhat problematic due to the lack of ice deposition after the 2001 global dust storm Another issue is that the more local the phenomena, the less likely the Mars General Circulation Model would measure and predict accurately. Grid sizes are being reduced as more computing power becomes available but until recently, grid sizes of up to 300 kilometers in length were routinely used and 200 kilometer grid sizes were considered detailed. Today grid sizes of 45 kilometers are used by some researchers but this is a very recent development .

Assertion that solar irradiance is causing the change

K.I. Abdusamatov has asserted that the changes are due to increased levels of solar activity, saying 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 published in the peer-reviewed literature and has been dismissed by other scientists, who have stated that "the idea just isn't supported by the theory or by the observations" and that it "doesn't make physical sense." Furthermore, in recent decades solar activity has been relatively stable — 1978-2006 total solar irradiance (TSI) ranged from 1365 to 1368 W/m². Other scientists have proposed that the observed variations are caused by irregularities in the orbit of Mars.

Current Missions

The Mars Reconnaissance Orbiter is currently taking daily weather and climate related observations from orbit. One of its instruments, the Mars climate sounder is specialized for climate observation work.

Future Missions

The Phoenix Mars Lander is due to arrive at Mars on May 25, 2008 and will be engaged in a variety of studies including past and present climate measurements.

The next US led Mars mission will be in 2011 as part of the Mars Scout Program. Both final candidates (MAVEN and Great Escape) will have climate study implications as they are upper atmosphere scientific packages.

See also

• Atmosphere of Mars
• Geology of Mars
• Mars Climate Orbiter
• Planum Australe, the southern polar plain.
• Planum Boreum, then northern polar plain.
• Global warming controversy
• Scientific opinion on climate change

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External links

• NASA Martian Climate page
• Mars Today, current conditions on Mars.
• Nature study explains mystery of Mars icecaps.
• Mars could be undergoing major global warming
• Mars Ski Report: Snow is Hard, Dense and Disappearing
• Mars Global Surveyor MOC2-1151 Release
• Global warming on Mars?
• Images of melting ice cap: Evidence for Recent Climate Change on Mars
• Article from National Geographic on the issue of Martian Global Warming

• Mars
• History of climate