In 1784 William Hershel reported to the Royal Society that he had seen the white poles on Mars retreat and expand in yearly cycles, like the seasonal melting and freezing of ice caps at the poles on Earth. He concluded that Mars has seasonal cycles like Earth's. He also noticed changes of bright and dark spots which he recorded as clouds and vapours, because of this he thought that Mars had a substantial atmosphere which we now know is wrong, in part due to the fact that vapours cannot be seen with the naked eye. When Mars was formed it's gravity, of only 3.71m/s2, 40% that of the Earth, was too small for it to hold onto its atmosphere and because of ultraviolet light breaking up their molecules, most of the lighter gases have escaped to space over time. Studying Mars is very significant to scientists as it could hold clues to the evolution of the solar system. It also has one of the key elements for life, water, although currently only found to be in the form of ice, it could prove that life once existed on the planet, and may still be present under the surface.
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Mars just like Earth is affected by the same orbital and rotational experience that results in changes in climate and variation in the climate at different times of the year. A day on Mars is comparable to that of a day on Earth, Cattermole (2001) noted that, like Earth, Mars rotates on its axis from west to east but has a longer solar day of 24 hrs 39mins and 35 seconds compared to 24hrs on Earth. The tilt on Mars' axis is also similar at 25.2 degrees, and 23.45 degrees on Earth. It is the precessional effects of Mars and its orbit around the sun which has the greatest influence on the cycles and seasons that occur there. As a result of Mars being a greater distance from the sun than Earth is, it takes double the amount of time to complete one orbit of the Sun. Martian years last 668.6 solar Martian days (sols) which is equivalent to 687 Earth days and result in seasons that are double the length. Martian months and hours follow the same principle as that on Earth, a month is 1/12th of a year and an hour is 1/24th of a sol (day). In the first month of the Martian year Mars experiences Southern Hemisphere Autumn Equinox, which occurs at a solar longitude of 0° (Ls= 0°). Solar Longitude is the angle that is present between the centre of Mars and the centre of the Sun at a given time of year. In the 4th month, Southern Hemisphere Winter Solstice occurs at Ls= 90°, followed by Southern Hemisphere Spring Equinox in the 7th month (Ls= 180°), and then Southern Hemisphere Summer Solstice in the 10th month (Ls= 270°). The seasons on Mars change from aphelion, the point during its orbit where the planet is the furthest away from the sun, to perihelion, where it is the closest. Aphelion at present takes place during the 3rd month of the Martian year at a solar longitude of 71° (Clancy et al, 1996) and perihelion takes place in the 9th month with a solar longitude of 251° (Clancy et al, 1996). Conversely, in 25000 years (Cattermole, 2001) it will be the Northern Hemisphere in this position. The climate that can be witnessed on Mars at the moment is controlled by seasonal variations of the transportation of dust by the atmosphere, the transfer of water vapour amidst the atmosphere and the surface of Mars, and the growth and recession of the carbon dioxide ice caps at its poles.
The seasons on Mars are of disproportionate length, just like Earth's, but to a much greater extent. In the Southern Hemisphere summer and spring are short and warm and last 154 and 143 sols correspondingly (Sorbjan et al, 2009). On the other hand autumn and winter last much longer and are colder due to Mars orbiting at a much slower speed during aphelion, these seasons therefore last 193 and 179 sols respectively (Sorbjan et al, 2009). As can be seen in figure 1, the beginning of the year, at sol 0, has an average temperature of around, 252k, as it approaches winter solstice the temperature decreases to 225k on sol 275. The start of spring equinox and ensuing Summer Solstice brings an increase in Figure 1: Potential temperature (k) of the atmosphere during the different seasons in the Southern Hemisphere at 20m, 1km and 2km above the surface (Sorbjan et al, 2009).
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temperature up to 252 on sol 424 and further to 258k on sol 484. After this follows a cooling of the planet as Autumn Equinox comes around again, decreasing the temperature back to 246k (Sorbjan et al, 2009). These changes in temperature come about from the change in distance of the planet from the sun.
Mars's current climate is caught in a cycle of sub-zero temperatures and most activity on the planet is constrained to the Northern and Southern Polar Regions. Mars' climate used to be a lot warmer; this is thought to be as a result of an increase in reduced greenhouse gases such as methane and ammonia. The subsequent formation of CO2 and H2O would have kept the planet warm if the atmosphere had been filled with CO2 clouds, which scatter infrared radiation instead of absorbing it and re-radiating it. These clouds would also tend to form in the upper troposphere and as a result would have a strong warming effect on the planet. Mars has a very thin atmosphere, which consists primarily of Carbon Dioxide, with nitrogen and argon next in abundance, made up of 95.32%, 2.7% and 1.6% respectively (Cattermole, P. 2001). The extreme thinness of the Martian air (less than 100th the size of Earth's) means that it has low heat capacity; it therefore heats and cools up much more quickly than Earth's atmosphere. Instruments onboard Viking Landers launched in 1975 measured atmospheric structure. Viking Landers 1 and 2 measured surface pressures and temperatures at their point of impact, pressures detected were 7.62 and 7.81 mbar and temperatures were 238 K and 236 K in that order (Seiff and Kirk, 1977). 'Mean temperature was found to decrease with a lapse rate of around 1.6 K/km (significantly sub adiabatic) from above the boundary layer to around 40 km' (Seiff and Kirk, 1977). The pressure on Mars translates to about 0.7% of the atmospheric pressure at the Earth's surface and can vary with season by around 20-30% (Cattermole, 2001).
Viking Landers in 1975 and 1978 detected a temperature for that of the Martian atmosphere from the surface up to 60k that was 15-20 ° k warmer than it usually is during late spring and measurements in 1972 were 10-15 ° k warmer than they should have been. Clancy writes that these irregularities are due to a difference in dust activity which absorbs heat from the sun causing the surface temperatures to be much lower than predicted (Clancy et al, 1995). The part of the atmosphere where water vapour is present changes between seasons; this is due to the presence of dust storms between months 9 and 12, in perihelion. The consequent absence of dust found in aphelion results in a much colder atmospheric temperature because the dust reflects solar radiation. It is this change in temperature that results in a reduction of the altitude that water vapour can be present, from around 5 to 10km compared with 25km (Clancy et al, 1996). As a result of a decrease in the saturation level of the Martian atmosphere, clouds can globally be found as a belt at these lower altitudes during aphelion. The formation of such clouds is thought to be associated with causing irregularity in the transfer of atmospheric water vapour between the two poles. On the other hand, the warmer temperatures of the atmosphere during perihelion on Mars results in a saturation level that is found at much higher altitudes, this resultantly does not interfere with the transfer of water vapour from the South pole to the North pole. The water continues to move from the south to the north in perihelion, which increases the amount of water vapour present in the Northern Hemisphere (Clancy et al, 1996). It is thought that when aphelion and perihelion switch seasons after around 51000years that the ice will mainly accumulate at the other Pole. Viking Lander observations showed that water vapour present in the atmosphere closest to the surface of Mars decreased during the night; this was consistent with the movement of water vapour from the atmosphere to the surface as a result of it possessing 'adsorptive and diffusive properties' (Jakosky et al, 1997).
Mars also experiences seasonal variations of global dust storms; these storms occur in perihelion season. Dust, although not completely understood, plays an important role as it reflects solar radiation and absorbs and emits longwave radiation from Mars' surface. Dust also significantly enhances cloud nucleation as ice forms around dust particles. When dust is present in the atmosphere it is thought to affect the water vapour as it causes water ice clouds to condense. As previously mentioned the Northern Hemisphere has a greater amount of atmospheric mass, this creates cloud particles that are generally 25% bigger than those in the south (Colaprete, 2007). Cloud particle size varies with season; in the north the largest particles can be found during the expansion and retreat of the ice caps. The retreat is simultaneous with the formation of a band of CO2 clouds as the CO2 is exchanged for the surface to the atmosphere; however in the south the atmosphere near the pole is relatively clear of such clouds (Colaprete, 2007). Every winter, the condensation of CO2 on the southern polar cap results in a larger decrease in atmospheric mass than that of this north, this is because this season is much colder and longer than its northern equivalent. This change results in atmospheric pressure altering by up to 25% (Tillman et al. Figure 2: The zonal average mass and height of CO2 clouds by latitude over the Martian year (Colaprete, 2007).
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The climate currently found on Mars is mainly as a result of the seasonal changes in the suspension of dust in the atmosphere, the cycle of CO2 between the polar caps and the atmosphere as well as the movement of water vapour through the system. The interactions of these with each other at different times of year affect the solar radiation that reaches the planet, where it is absorbed by the atmosphere or transmitted by it and results in a temperature that varies drastically. Overall, the climate on Mars is a lot more simple than the climate on Earth and there is little variation in weather. However, it is still not explicitly understood the processes involved in each of the cyclic variations; water vapour is considered the least understood of them all but is thought to have some non-linear relationship with the dust in the atmosphere as a result of nucleation of crystals.