Climate of Mars

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The climate of Mars has been reasonably well studied. Data has come from above both by flyby and orbital spacecraft as well as some data being gathered by the more powerful recent earth based instruments. On the ground data has been gathered by 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.[1] Several different iterations of MGCM have led to an increase in understanding of Mars as well as the limits of such models. Model limits include observing wheather phenomena that occur at a finer granularity than their cell size, inaccurate or unrealistic assumptions about how Mars works, and a lack of long-term data on how climate works on Mars.

Mars's 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. Mars' atmosphere has a scale height of 11 km (36,000 ft), 60% greater than that on Earth. The climate has been of long standing interest in the question of life on the planet, and briefly received more interest in the news due to NASA measurements indicating that Mars, like the Earth, is undergoing a process of global warming.

Historical

The first historical observation of Mars that was of climatological note is Giancomo Miraldi's 1704 observation of the polar caps and his discovery that the southern cap is not centered on the rotational pole of Mars.[2] This was followed by his 1719 speculation that the "white spots" previously observed are ice caps.

William Herschel was the first to intuit the thinness of 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.[3]

Honore Flaugergues 1809 discovery of "yellow clouds" on the surface of Mars is the first known observation of Martian dust storms.[4] Flaugergas 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 to later be proven dead wrong.

Recent evidence has started to open up information not only about the present climate and atmospheric conditions on Mars but also about its past. Noachian-era Martian atmosphere had long been theorized as a carbon dioxide rich environment. Recent clay studies[5] 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.

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 [6]. 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

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.[7] 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.[8] 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.[9] 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[10] 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.[11] 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.[12]

Effect of dust storms

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[13] 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.[14]

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.[15]

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.[16]

It has been suggested that dust storms on Mars could play a role in storm formation similar to that of water clouds on earth.[citation needed] 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.[17]

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.[18]

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.[19] 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.[20]

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.[21] 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,[22] and contains about 1.6 million cubic km of ice, which if spread evenly on the cap would be 2 km thick.[23] (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.[24] 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.[25][26] 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.[27] 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.[28]

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:

Season Sols
(on Mars)		 Days
(on Earth)
Northern Spring, Southern Autumn: 193.30 92.764
Northern Summer, Southern Winter: 178.64 93.647
Northern Autumn, Southern Spring: 142.70 89.836
Northern Winter, Southern Summer: 153.95 88.997

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 [29].

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. [30]. Current research suggests that Mars is in a warm interglacial period which has lasted more than 100,000 years[31].

Evidence for recent climatic change

Pits in south polar ice cap, MGS 1999, NASA

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 [32].

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) [33]. The pits in the ice continue to grow 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"[34] on Mars.

Attribution

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[35]. 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.[36] This theory is somewhat problematic due to the lack of ice depositation after the 2001 global dust storm[37] 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.

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."[38] Abdusamatov's hypothesis has not been accepted by many other scientists. Amato Evan, an Assistant Researcher at the University of Wisconsin, Madison, stated "the idea just isn't supported by the theory or by the observations." Charles Long, a climate physicist at Pacific Northwest National Laboratories who studies radiative transfer, says "That's nuts...It doesn't make physical sense that that's the case."[39] Other scientists have proposed that the observed variations are caused by irregularities in the orbit of Mars.[40]

In recent decades solar activity has however been relatively stable, (1978-2006 TSI ranges 1365-1368 Wm2)[41] 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 [42]. Unknown factors delaying the reaction to this increase in solar activity in recent decades could be in play.

Others have suggested that comparably high levels of activity have occurred several times in the last few thousand years. [43] 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."[29]

Writing in a Nature news story, Chief News and Features Editor 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"[44][45]

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[46].

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

References

  1. ^ NASA. "Mars General Circulation Modeling". NASA. Retrieved 2007-02-22.
  2. ^ Exploring Mars in the 1700s
  3. ^ ibid.
  4. ^ Exploring Mars in the 1800s
  5. ^ [1]
  6. ^ MGCM. "Mars' low surface pressure. ." NASA. Retrieved 2007-02-22.
  7. ^ MGCM. "Mars' desert surface. ." NASA. Retrieved 2007-02-25.
  8. ^ Antares project "Mars Small-Scale Weather" (MSW)
  9. ^ Francois Forget. "Alien Weather at the Poles of Mars" (PDF). Science. Retrieved 2007-02-25.
  10. ^ The Martian tropics
  11. ^ The Martian mountain ranges
  12. ^ http://photojournal.jpl.nasa.gov/catalog/PIA04294
  13. ^ NASA. "Planet Gobbling Dust Storms". NASA. Retrieved 2007-02-22.
  14. ^ Global warming and climate forcing by recent albedo changes on Mars
  15. ^ Mars Exploration Rover Status Report Concern Increasing About Opportunity
  16. ^ "Duststorms on Mars". whfreeman.com. Retrieved 2007-02-22.
  17. ^ Richard Zurek. "Interannual variability of planet-encircling dust storms on Mars". J. Geophys. Res. Retrieved 2007-03-16.
  18. ^ http://www.news.cornell.edu/releases/May99/mars.cyclone.deb.html
  19. ^ Francois Forget. "Alien Weather at the Poles of Mars" (PDF). Science. Retrieved 2007-02-25.
  20. ^ MGCMG. "Mars' dry ice polar caps..." NASA. Retrieved 2007-02-22.
  21. ^ Darling, David. "Mars, polar caps, ENCYCLOPEDIA OF ASTROBIOLOGY, ASTRONOMY, AND SPACEFLIGHT". Retrieved 2007-02-26.
  22. ^ "MIRA's Field Trips to the Stars Internet Education Program". Mira.org. Retrieved 2007-02-26.
  23. ^ Carr, Michael H. (2003). "Oceans on Mars: An assessment of the observational evidence and possible fate". Journal of Geophysical Research. 108 (5042): 24. doi:10.1029/2002JE001963. {{cite journal}}: |access-date= requires |url= (help); |format= requires |url= (help)
  24. ^ Phillips, Tony. "Mars is Melting, Science at NASA". Retrieved 2007-02-26.
  25. ^ Pelletier, J. D. (2004). "How do spiral troughs form on Mars?". Geology. 32: 365–367. Retrieved 2007-02-27.
  26. ^ "MarsToday.Com". Mars Polar Cap Mystery Solved. Retrieved 2007-01-23.
  27. ^ [2]
  28. ^ http://science.nasa.gov/headlines/y2001/ast31jan_1.htm
  29. ^ a b Steinn Sigurdsson. "Global warming on Mars?". RealClimate. Retrieved 2007-02-21.
  30. ^ Jacques Laskar. "Martian 'wobbles' shift climate". bbc.co.uk. Retrieved 2007-02-24.
  31. ^ Francis Reddy. "Titan, Mars methane may be on ice". astronomy.com. Retrieved 2007-03-16.
  32. ^ "MOC Observes Changes in the South Polar Cap". Malin Space Science Systems. Retrieved 2007-02-22.
  33. ^ "Evaporating ice". Astronomy.com. Retrieved 2007-02-22.
  34. ^ Orbiter's Long Life Helps Scientists Track Changes on Mars
  35. ^ "Albedo of the South Pole of Mars. .". Nature. 435: 184–188. 12 May 2005. {{cite journal}}: |access-date= requires |url= (help)
  36. ^ "Year-to-year instability of the Mars Polar Cap". J.Geophys Res. 95: 1359–1365. 1990.
  37. ^ [3]
  38. ^ "Look to Mars for the truth on global warming". National Post. Retrieved 2007-03-02.
  39. ^ http://www.livescience.com/environment/070312_solarsys_warming.html
  40. ^ Kate Ravilious. "Mars Melt Hints at Solar, Not Human, Cause for Warming, Scientist Says". National Geographic Society. Retrieved 2007-03-02.
  41. ^ http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant
  42. ^ S. K. Solanki (2004). "Unusual activity of the Sun during recent decades compared to the previous 11,000 years". Nature. 431: 1084–1087. doi:10.1038/nature02995. Retrieved 2007-02-26.
  43. ^ Muscheler, Raimund; Joos, Fortunat; Müller, Simon A.; Snowball, Ian (2005), "Climate: How unusual is today's solar activity?" (PDF), Nature, 436 (7012): 1084–1087, doi:10.1038/nature04045, retrieved 2007-04-20
  44. ^ http://www.nature.com/news/2007/070402/full/070402-7.html
  45. ^ Fenton, Lori K.; Geissler, Paul E.; Haberle, Robert M. (2007), "Global warming and climate forcing by recent albedo changes on Mars" (PDF), Nature, 446, doi:10.1038/nature05718
  46. ^ [4]

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