Climate of Mars

The climate of Mars has been a topic of scientific curiosity for centuries, in part because it is the only terrestrial planet whose surface can be easily directly observed in detail from the Earth with help from a telescope.

Although Mars is smaller than the Earth with only one tenth of Earth's mass,^[1] and 50% farther from the Sun than the Earth, its climate has important similarities, such as the presence of polar ice caps, seasonal changes and observable weather patterns. It has attracted sustained study from planetologists and climatologists. While Mars's climate has similarities to Earth's, including periodic ice ages, there are also important differences, such as 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 ever has been present on the planet.

Mars has been studied by Earth-based instruments since 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 landers and rovers have measured atmospheric conditions directly. Advanced Earth-orbital instruments today continue to provide some useful "big picture" observations of relatively large weather phenomena.

The first Martian flyby mission was Mariner 4, which arrived in 1965. That quick two-day pass (July 14–15, 1965) with crude instruments contributed little to the state of knowledge of Martian climate. Later Mariner missions (Mariner 6 and 7) filled in some of the gaps in basic climate information. Data-based climate studies started in earnest with the Viking program landers in 1975 and continue with such probes as the Mars Reconnaissance Orbiter.

This observational work has been complemented by a type of scientific computer simulation called the Mars general circulation model.^[2] Several different iterations of MGCM have led to an increased understanding of Mars as well as the limits of such models.

Giacomo Maraldi determined in 1704 that the southern cap is not centered on the rotational pole of Mars.^[3] During the opposition of 1719, Maraldi observed both polar caps and temporal variability in their extent.

William Herschel was the first to deduce 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 Mars appeared to pass close by two faint stars with no effect on their brightness, Herschel correctly concluded that this meant that there was little atmosphere around Mars to interfere with their light.^[3]

Honore Flaugergues's 1809 discovery of "yellow clouds" on the surface of Mars is the first known observation of Martian dust storms.^[4] Flaugergues also observed in 1813 significant polar-ice waning during Martian springtime. His speculation that this meant that Mars was warmer than Earth proved inaccurate.

There are two dating systems now in use for Martian geological time. One is based on crater density and has three ages: Noachian, Hesperian, and Amazonian. The other is a mineralogical timeline, also having three ages: Phyllocian, Theikian, and Siderikian.

Recent observations and modeling are 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^[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, although the carbonate may later be dissolved by volcanic acidity.^[6]

The discovery of water-formed minerals on Mars including hematite and jarosite, by the Opportunity rover and goethite by the Spirit rover, has led to the conclusion that climatic conditions in the distant past allowed for free-flowing water on Mars. The morphology of some crater impacts on Mars indicate that the ground was wet at the time of impact.^[7] Geomorphic observations of both landscape erosion rates^[8] and Martian valley networks^[9] also strongly imply warmer, wetter conditions on Noachian-era Mars (earlier than about four billion years ago). However, chemical analysis of Martian meteorite samples suggests that the ambient near-surface temperature of Mars has most likely been below 0 °C (32 °F) for the last four billion years.^[10]

Some scientists maintain that the great mass of the Tharsis volcanoes has had a major influence on Mars' climate. Erupting volcanoes give off great amounts of gas, mainly water vapor and CO₂. Enough gas may have been released by volcanoes to have made the earlier Martian atmosphere thicker than Earth's. The volcanoes could also have emitted enough H₂O to cover the whole Martian surface to a depth of 120 m (390 ft). Carbon dioxide is a greenhouse gas that raises a planet's temperature: it traps heat by absorbing infrared radiation. Thus, Tharsis volcanoes, by giving off CO₂, could have made Mars more Earth-like in the past. Mars may have once had a much thicker and warmer atmosphere, and oceans or lakes may have been present.^[11] It has, however, proven extremely difficult to construct convincing global climate models for Mars which produce temperatures above 0 °C (32 °F) at any point in its history,^[12] although this may simply reflect problems in accurately calibrating such models.

Evidence of a geologically recent, extreme ice age on Mars was published in 2016. Just 370,000 years ago, the planet would have appeared more white than red.^[13]

Mars' temperature and circulation vary every Martian year (as expected for any planet with an atmosphere and axial tilt). Mars lacks oceans, a source of much interannual variation on Earth.^{[clarification needed]} Mars Orbiter Camera data beginning in March 1999 and covering 2.5 Martian years^[14] show that Martian weather tends to be more repeatable and hence more predictable than that of Earth. If an event occurs at a particular time of year in one year, the available data (sparse as it is) indicates that it is fairly likely to repeat the next year at nearly the same location, give or take a week.

On September 29, 2008, the Phoenix lander detected snow falling from clouds 4.5 kilometres (2.8 mi) above its landing site near Heimdal Crater. The precipitation vaporised before reaching the ground, a phenomenon called virga.^[15]

This section needs expansion. You can help by adding to it. (January 2010)

Martian dust storms can kick up fine particles in the atmosphere around which clouds can form. These clouds can form very high up, up to 100 km (62 mi) above the planet.^[16] As well as Martian Dust Storms, clouds can naturally form as a result of dry ice formation or water and ice.^[17] Furthermore, rarer "Mother of Pearl" clouds have formed when all particles of a cloud form at the same time, creating stunning iridescent clouds.^[17] The first images of Mars sent by Mariner 4 showed visible clouds in Mars' upper atmosphere. The clouds are very faint and can only be seen reflecting sunlight against the darkness of the night sky. In that respect, they look similar to mesospheric clouds, also known as noctilucent clouds, on Earth, which occur about 80 km (50 mi) above our planet.

Measurements of Martian temperature predate the Space Age. However, early instrumentation and techniques of radio astronomy produced crude, differing results.^[18][19] Early flyby probes (Mariner 4) and later orbiters used radio occultation to perform aeronomy. With chemical composition already deduced from spectroscopy, temperature and pressure could then be derived. Nevertheless, flyby occultations can only measure properties along two transects, at their trajectories' entries and exits from Mars' disk as seen from Earth. This results in weather "snapshots" at a particular area, at a particular time. Orbiters then increase the number of radio transects. Later missions, starting with the dual Mariner 6 and 7 flybys, plus the Soviet Mars 2 and 3, carried infrared detectors to measure radiant energy. Mariner 9 was the first to place an infrared radiometer and spectrometer in Mars orbit in 1971, along with its other instruments and radio transmitter. Viking 1 and 2 followed, with not merely Infrared Thermal Mappers (IRTM).^[20] The missions could also corroborate these remote sensing datasets with not only their in situ lander metrology booms,^[21] but with higher-altitude temperature and pressure sensors for their descent.^[22]

Differing in situ values have been reported for the average temperature on Mars,^[23] with a common value being −63 °C (210 K; −81 °F).^[24][25] Surface temperatures may reach a high of about 20 °C (293 K; 68 °F) at noon, at the equator, and a low of about −153 °C (120 K; −243 °F) at the poles.^[26] Actual temperature measurements at the Viking landers' site range from −17.2 °C (256.0 K; 1.0 °F) to −107 °C (166 K; −161 °F). The warmest soil temperature estimated by the Viking Orbiter was 27 °C (300 K; 81 °F).^[27] The Spirit rover recorded a maximum daytime air temperature in the shade of 35 °C (308 K; 95 °F), and regularly recorded temperatures well above 0 °C (273 K; 32 °F), except in winter.^[28]

It has been reported that "On the basis of the nighttime air temperature data, every northern spring and early northern summer yet observed were identical to within the level of experimental error (to within ±1 °C)" but that the "daytime data, however, suggests a somewhat different story, with temperatures varying from year-to-year by up to 6 °C in this season.^[29] This day-night discrepancy is unexpected and not understood". In southern spring and summer, variance is dominated by dust storms which increase the value of the night low temperature and decrease the daytime peak temperature.^[30] This results in a small (20 °C) decrease in average surface temperature, and a moderate (30 °C) increase in upper atmosphere temperature.^[31]

Before and after the Viking missions, newer, more advanced Martian temperatures were determined from Earth via microwave spectroscopy. As the microwave beam, of under 1 arcminute, is larger than the disk of the planet, the results are global averages.^[32] Later, the Mars Global Surveyor's Thermal Emission Spectrometer and to a lesser extent 2001 Mars Odyssey's THEMIS could not merely reproduce infrared measurements but intercompare lander, rover, and Earth microwave data. The Mars Reconnaissance Orbiter's Mars Climate Sounder can similarly derive atmospheric profiles. The datasets "suggest generally colder atmospheric temperatures and lower dust loading in recent decades on Mars than during the Viking Mission,"^[33] although Viking data had previously been revised downward.^[34] The TES data indicates "Much colder (10–20 K) global atmospheric temperatures were observed during the 1997 versus 1977 perihelion periods" and "that the global aphelion atmosphere of Mars is colder, less dusty, and cloudier than indicated by the established Viking climatology," again, taking into account the Wilson and Richardson revisions to Viking data.^[35]

A later comparison, while admitting "it is the microwave record of air temperatures which is the most representative," attempted to merge the discontinuous spacecraft record. No measurable trend in global average temperature between Viking IRTM and MGS TES was visible. "Viking and MGS air temperatures are essentially indistinguishable for this period, suggesting that the Viking and MGS eras are characterized by essentially the same climatic state." It found "a strong dichotomy" between the northern and southern hemispheres, a "very asymmetric paradigm for the Martian annual cycle: a northern spring and summer which is relatively cool, not very dusty, and relatively rich in water vapor and ice clouds; and a southern summer rather similar to that observed by Viking with warmer air temperatures, less water vapor and water ice, and higher levels of atmospheric dust."^[29]

The Mars Reconnaissance Orbiter MCS (Mars Climate Sounder) instrument was, upon arrival, able to operate jointly with MGS for a brief period; the less-capable Mars Odyssey THEMIS and Mars Express SPICAM datasets may also be used to span a single, well-calibrated record. While MCS and TES temperatures are generally consistent,^[36] investigators report possible cooling below the analytical precision. "After accounting for this modeled cooling, MCS MY 28 temperatures are an average of 0.9 (daytime) and 1.7 K (night-time) cooler than TES MY 24 measurements."^[37]

It has been suggested that Mars had a much thicker, warmer atmosphere early in its history.^[38] Much of this early atmosphere would have consisted of carbon dioxide. Such an atmosphere would have raised the temperature, at least in some places, to above the freezing point of water.^[39] With the higher temperature running water could have carved out the many channels and outflow valleys that are common on the planet. It also may have gathered together to form lakes and maybe an ocean.^[40] Some researchers have suggested that the atmosphere of Mars may have been many times as thick as the Earth's; however research published in September 2015 advanced the idea that perhaps the early Martian atmosphere was not as thick as previously thought.^[41]

Currently, the atmosphere is very thin. For many years, it was assumed that as with the Earth, most of the early carbon dioxide was locked up in minerals, called carbonates. However, despite the use of many orbiting instruments that looked for carbonates, very few carbonate deposits have been found.^[41][42] Today, it is thought that much of the carbon dioxide in the Martian air was removed by the solar wind. Researchers have discovered a two-step process that sends the gas into space.^[43] Ultraviolet light from the Sun could strike a carbon dioxide molecule, breaking it into carbon monoxide and oxygen. A second photon of ultraviolet light could subsequently break the carbon monoxide into oxygen and carbon which would get enough energy to escape the planet. In this process the light isotope of carbon (¹²C) would be most likely to leave the atmosphere. Hence, the carbon dioxide left in the atmosphere would be enriched with the heavy isotope (¹³C).^[44] This higher level of the heavy isotope is what was found by the Curiosity rover on Mars.^[45][46] Climate data for the Gale Crater is provided here below, with the seasons normalized to those of Earth.

Martian storms are significantly affected by Mars' large mountain ranges.^[105] Individual mountains like record holding Olympus Mons (26 km (85,000 ft)) can affect local weather but larger weather effects are due to the larger collection of volcanoes in the Tharsis region.

One unique repeated weather phenomenon involving mountains is a spiral dust cloud that forms over Arsia Mons. The spiral dust cloud over Arsia Mons can tower 15 to 30 km (49,000 to 98,000 ft) above the volcano.^[106] Clouds are present around Arsia Mons throughout the Martian year, peaking in late summer.^[107]

Clouds surrounding mountains display a seasonal variability. Clouds at Olympus Mons and Ascreaus Mons appear in northern hemisphere spring and summer, reaching a total maximum area of approximately 900,000 km² and 1,000,000 km² respectively in late spring. Clouds around Alba Patera and Pavonis Mons show an additional, smaller peak in late summer. Very few clouds were observed in winter. Predictions from the Mars General Circulation Model are consistent with these observations.^[107]

Mars has ice caps at its north pole and south pole, which consist mainly of water ice; however, there is frozen carbon dioxide (dry ice) present on their surfaces. Dry ice accumulates in the north polar region (Planum Boreum) in winter only, subliming completely in summer, while the south polar region additionally has a permanent dry ice cover up to eight meters (25 feet) thick.^[108] This difference is due to the higher elevation of the south pole.

Much of the atmosphere can condense at the winter pole so 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.^[54] The eccentricity of Mars' orbit affects this cycle, as well as other factors. In the spring and autumn wind due to the carbon dioxide sublimation process is so strong that it can be a cause of the global dust storms mentioned above.^[109]

The northern polar cap has a diameter of approximately 1,000 km during the northern Mars summer,^[110] and contains about 1.6 million cubic kilometres of ice, which if spread evenly on the cap would be 2 km thick.^[111] (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.^[112] Both polar caps show spiral troughs, which were initially thought to form as a result of differential solar heating, coupled with the sublimation of ice and condensation of water vapor.^[113][114] Recent analysis of ice penetrating radar data from SHARAD has demonstrated that the spiral troughs are formed from a unique situation in which high density katabatic winds descend from the polar high to transport ice and create large wavelength bedforms.^[115][116] The spiral shape comes from Coriolis effect forcing of the winds, much like winds on earth spiral to form a hurricane. The troughs did not form with either ice cap; instead they began to form between 2.4 million and 500,000 years ago, after three-fourths of the ice cap was in place. This suggests that a climatic shift allowed for their onset. Both polar caps shrink and regrow following the temperature fluctuation of the Martian seasons; there are also longer-term trends that are better understood in the modern era.

During the southern hemisphere spring, solar heating of dry ice deposits at the south pole leads in places to accumulation of pressurized CO₂ gas below the surface of the semitransparent ice, warmed by absorption of radiation by the darker substrate. After attaining the necessary pressure, the gas bursts through the ice in geyser-like plumes. While the eruptions have not been directly observed, they leave evidence in the form of "dark dune spots" and lighter fans atop the ice, representing sand and dust carried aloft by the eruptions, and a spider-like pattern of grooves created below the ice by the outrushing gas.^[117][118] (see Geysers on Mars.) Eruptions of nitrogen gas observed by Voyager 2 on Triton are thought to occur by a similar mechanism.

Both polar caps are currently accumulating, confirming predicted Milankovich cycling on timescales of ~400,000 and ~4,000,000 years. Soundings by the Mars Reconnaissance Orbiter SHARAD indicate total cap growth of ~0.24 km³/year. Of this, 92%, or ~0.86 mm/year, is going to the north,^[119] as Mars' offset Hadley circulation acts as a nonlinear pump of volatiles northward.

Mars lost most of its magnetic field about four billion years ago. As a result, solar wind and cosmic radiation interacts directly with the Martian ionosphere. This keeps the atmosphere thinner than it would otherwise be by solar wind action constantly stripping away atoms from the outer atmospheric layer.^[120] Most of the historical atmospheric loss on Mars can be traced back to this solar wind effect. Current theory posits a weakening solar wind and thus today's atmosphere stripping effects are much less than those in the past when the solar wind was stronger.^{[citation needed]}

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 orbits 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 relatively warm.^{[citation needed]}

It is now thought that ice accumulated when Mars' orbital tilt was very different from what it is now. (The axis the planet spins on has considerable "wobble", meaning its angle changes over time.)^{[121][122][123]} A few million years ago, the tilt of the axis of Mars was 45 degrees instead of its present 25 degrees. Its tilt, also called obliquity, varies greatly because its two tiny moons cannot stabilize it like Earth's moon.

Many features on Mars, especially in the Ismenius Lacus quadrangle, are thought to contain large amounts of ice. The most popular model for the origin of the ice is climatic change from large changes in the tilt of the planet's rotational axis. At times the tilt has even been greater than 80 degrees.^[124][125] Large changes in the tilt explains many ice-rich features on Mars.

Studies have shown that when the tilt of Mars reaches 45 degrees from its current 25 degrees, ice is no longer stable at the poles.^[126] Furthermore, at this high tilt, stores of solid carbon dioxide (dry ice) sublimate, thereby increasing the atmospheric pressure. This increased pressure allows more dust to be held in the atmosphere. Moisture in the atmosphere will fall as snow or as ice frozen onto dust grains. Calculations suggest this material will concentrate in the mid-latitudes.^[127][128] General circulation models of the Martian atmosphere predict accumulations of ice-rich dust in the same areas where ice-rich features are found.^[125] When the tilt begins to return to lower values, the ice sublimates (turns directly to a gas) and leaves behind a lag of dust.^[129][130] The lag deposit caps the underlying material so with each cycle of high tilt levels, some ice-rich mantle remains behind.^[131] Note, that the smooth surface mantle layer probably represents only relative recent material. Below are images of layers in this smooth mantle that drops from the sky at times.

• 
  Smooth mantle covers parts of a crater in the Phaethontis quadrangle. Layering suggests the mantle was deposited multiple times.
• 
  Enlargement of previous image of mantle layers. Four to five layers are visible. Picture taken under HiWish program.

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.^[132]

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 stabilizing 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 at the Martian north pole.^[133] Current research suggests that Mars is in a warm interglacial period which has lasted more than 100,000 years.^[134]

Because the Mars Global Surveyor was able to observe Mars for 4 Martian years, it was found that Martian weather was similar from year to year. Any differences were directly related to changes in the solar energy that reached Mars. Scientists were even able to accurately predict dust storms that would occur during the landing of Beagle 2. Regional dust storms were discovered to be closely related to where dust was available.^[135]

There have been regional changes around the south pole (Planum Australe) 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 larger, retreating about 3 meters in one Martian year.^[136] These features are caused by the sublimation of the dry ice layer, thereby exposing the inert water ice layer. More recent observations indicate that the ice at Mars' south pole is continuing to sublimate.^[137] The pits in the ice continue to grow by about 3 meters per Martian year. Malin states that conditions on Mars are not currently conducive to the formation of new ice. A NASA press release indicates that "climate change [is] in progress"^[138] on Mars. In a summary of observations with the Mars Orbiter Camera, researchers speculated that some dry ice may have been deposited between the Mariner 9 and the Mars Global Surveyor mission. Based on the current rate of loss, the deposits of today may be gone in a hundred years.^[135]

Elsewhere on the planet, low latitude areas have more water ice than they should have given current climatic conditions.^{[139][140][141]} Mars Odyssey "is giving us indications of recent global climate change in Mars", said Jeffrey Plaut, project scientist for the mission at NASA's Jet Propulsion Laboratory, in non-peer reviewed published work in 2003.^{[citation needed]}

Though at its current state, Mars is unhabitable to humans, many people have suggested terraforming Mars to change the climate to make it more habitable to humans. Notably, Elon Musk has suggested detonating nuclear weapons on the ice caps of Mars to release water vapor and carbon dioxide, which would warm the planet significantly enough to possibly make it habitable for humans.^[145]

Terrestrial Climate zones first have been defined by Wladimir Köppen based on the distribution of vegetation groups. Climate classification is furthermore based on temperature, rainfall, and subdivided based upon differences in the seasonal distribution of temperature and precipitation; and a separate group exists for extrazonal climates like in high altitudes. Mars has neither vegetation nor rainfall, so any climate classification could be only based upon temperature; a further refinement of the system may be based on dust distribution, water vapor content, occurrence of snow. Solar Climate Zones can also be easily defined for Mars.^[146]

The 2001 Mars Odyssey is currently orbiting Mars and taking global atmospheric temperature measurements with the TES instrument. 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. The MSL was launched in November 2011 and landed on Mars on August 6, 2012.^[147] Orbiters MAVEN, Mangalyaan, and TGO are currently orbiting Mars and studying its atmosphere.

Curiosity rover – Temperature, Pressure, Humidity at Gale Crater on Mars (August 2012 – February 2013)

Temperature

Pressure

Humidity

• Current Weather Report on Mars by the Curiosity rover
• Current Weather Report on Mars by the InSight lander

• Geology of Mars
• Mars analog habitat
• Mars Climate Orbiter
• Mars MetNet, a proposed meteorological network on Mars
• Water on Mars
• Terraforming of Mars
• Mars carbonate catastrophe