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 observed in detail from Earth using a telescope.
Although Mars is smaller than Earth, with only one-tenth of Earth's mass, and is 50% farther from the Sun than Earth, its climate has important similarities with Earth's, 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' 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 , which is 60% greater than that of 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 with 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 (MGCM). Several different iterations of MGCM have led to an increased understanding of Mars as well as the limits of such models.
Historical climate observations Giacomo Maraldi determined in 1704 that the southern cap is not centered on the rotational pole of Mars. During the opposition of 1719, Maraldi observed both polar caps and the 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 there was little atmosphere around Mars to interfere with their light.
Honore Flaugergues's 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 waning during Martian springtime. His speculation that this meant that Mars was warmer than Earth proved inaccurate.
Martian paleoclimatology There are two dating systems currently 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, which also has 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 have found that there is little to no carbonate present in the 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.
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 indicates that the ground was wet at the time of impact. Geomorphic observations of both landscape erosion rates and Martian valley networks 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 for the last four billion years.
Some scientists maintain that the great mass of the Tharsis volcanoes has had a major influence on Mars's climate. Erupting the 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 . Carbon dioxide is a greenhouse gas that raises a planet's temperature: it traps heat by absorbing infrared radiation. Thus, Tharsis the 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.Hartmann, W. 2003. A Traveler's Guide to Mars. Workman Publishing. NY NY. It has, however, proven extremely difficult to construct convincing global climate models for Mars that produce temperatures above at any point in its history, 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.
Weather
Mars's temperature and circulation vary over the course of 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. Mars Orbiter Camera data beginning in March 1999 and covering 2.5 Martian years 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 above its landing site near Heimdal Crater. The precipitation vaporised before reaching the ground, a phenomenon called virga.
Clouds
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 above the planet. As well as these dust storms, clouds can form as a result of dry ice formation or water and ice, or other chemicals. Furthermore, rarer "Mother of Pearl" clouds have formed when all particles of a cloud form at the same time, creating iridescent clouds. 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 above our planet.
Temperature
Measurements of Martian temperature predate the Space Age. However, early instrumentation and techniques of radio astronomy produced crude, differing results. 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's 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). The missions could also corroborate these remote sensing datasets with not only their in situ lander metrology booms, but with higher-altitude temperature and pressure sensors for their descent.
Differing in situ values have been reported for the average temperature on Mars, with a common value being . Surface temperatures may reach a high of about at noon, at the equator, and a low of about at the poles. Actual temperature measurements at the Viking landers' site range from to . The warmest soil temperature estimated by the Viking Orbiter was .James E. Tillman Mars – Temperature Overview The Spirit rover recorded a maximum daytime air temperature in the shade of , and regularly recorded temperatures well above , except in winter.
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. 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.William Sheehan, The Planet Mars: A History of Observation and Discovery, Chapter 13 (available on the web ) This results in a small (20 °C) decrease in average surface temperature, and a moderate (30 °C) increase in upper atmosphere temperature.
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. 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," although Viking data had previously been revised downward. 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.