Mars

Mars is a

Although Mars has no evidence of a structured global magnetic field,^[61] observations show that parts of the planet's crust have been magnetized, suggesting that alternating polarity reversals of its dipole field have occurred in the past. This paleomagnetism of magnetically susceptible minerals is similar to the alternating bands found on Earth's ocean floors. One hypothesis, published in 1999 and re-examined in October 2005 (with the help of the Mars Global Surveyor), is that these bands suggest plate tectonic activity on Mars four billion years ago, before the planetary dynamo ceased to function and the planet's magnetic field faded.^[62]

The Phoenix lander returned data showing Martian soil to be slightly alkaline and containing elements such as magnesium, sodium, potassium and chlorine. These nutrients are found in soils on Earth. They are necessary for growth of plants.^[63] Experiments performed by the lander showed that the Martian soil has a basic pH of 7.7, and contains 0.6% of the salt perchlorate,^[64][65] concentrations that are toxic to humans.^[66][67]

Streaks are common across Mars and new ones appear frequently on steep slopes of craters, troughs, and valleys. The streaks are dark at first and get lighter with age. The streaks can start in a tiny area, then spread out for hundreds of metres. They have been seen to follow the edges of boulders and other obstacles in their path. The commonly accepted hypotheses include that they are dark underlying layers of soil revealed after avalanches of bright dust or dust devils.^[68] Several other explanations have been put forward, including those that involve water or even the growth of organisms.^[69][70]

Radiation levels on the surface are on average 0.64 millisieverts of radiation per day, and significantly less than the radiation of 1.84 millisieverts per day or 22 millirads per day during the flight to and from Mars.^[71][72] For comparison the radiation levels in low Earth orbit, where Earth's space stations orbit, are around 0.5 millisieverts of radiation per day.^[73] Hellas Planitia has the lowest surface radiation at about 0.342 millisieverts per day, featuring lava tubes southwest of Hadriacus Mons with potentially levels as low as 0.064 millisieverts per day.^[74]

Geography and features

Although better remembered for mapping the Moon,

Features on Mars are named from a variety of sources. Albedo features are named for classical mythology. Craters larger than roughly 50 km are named for deceased scientists and writers and others who have contributed to the study of Mars. Smaller craters are named for towns and villages of the world with populations of less than 100,000. Large valleys are named for the word "Mars" or "star" in various languages; smaller valleys are named for rivers.^[76]

Large albedo features retain many of the older names but are often updated to reflect new knowledge of the nature of the features. For example, Nix Olympica (the snows of Olympus) has become Olympus Mons (Mount Olympus).^[77] The surface of Mars as seen from Earth is divided into two kinds of areas, with differing albedo. The paler plains covered with dust and sand rich in reddish iron oxides were once thought of as Martian "continents" and given names like Arabia Terra (land of Arabia) or Amazonis Planitia (Amazonian plain). The dark features were thought to be seas, hence their names Mare Erythraeum, Mare Sirenum and Aurorae Sinus. The largest dark feature seen from Earth is Syrtis Major Planum.^[78] The permanent northern polar ice cap is named Planum Boreum. The southern cap is called Planum Australe.^[79]

Mars's equator is defined by its rotation, but the location of its

The vast upland region Tharsis contains several massive volcanoes, which include the shield volcano Olympus Mons. The edifice is over 600 km (370 mi) wide.^[90][91] Because the mountain is so large, with complex structure at its edges, giving a definite height to it is difficult. Its local relief, from the foot of the cliffs which form its northwest margin to its peak, is over 21 km (13 mi),^[91] a little over twice the height of Mauna Kea as measured from its base on the ocean floor. The total elevation change from the plains of Amazonis Planitia, over 1,000 km (620 mi) to the northwest, to the summit approaches 26 km (16 mi),^[92] roughly three times the height of Mount Everest, which in comparison stands at just over 8.8 kilometres (5.5 mi). Consequently, Olympus Mons is either the tallest or second-tallest mountain in the Solar System; the only known mountain which might be taller is the Rheasilvia peak on the asteroid Vesta, at 20–25 km (12–16 mi).^[93]

Impact topography

The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. It is possible that, four billion years ago, the Northern Hemisphere of Mars was struck by an object one-tenth to two-thirds the size of Earth's Moon. If this is the case, the Northern Hemisphere of Mars would be the site of an impact crater 10,600 by 8,500 kilometres (6,600 by 5,300 mi) in size, or roughly the area of Europe, Asia, and Australia combined, surpassing Utopia Planitia and the Moon's South Pole–Aitken basin as the largest impact crater in the Solar System.^[94][95][96]

Mars is scarred by a number of impact craters: a total of 43,000 craters with a diameter of 5 kilometres (3.1 mi) or greater have been found.^[97] The largest exposed crater is Hellas, which is 2,300 kilometres (1,400 mi) wide and 7,000 metres (23,000 ft) deep, and is a light albedo feature clearly visible from Earth.^[98][99] There are other notable impact features, such as Argyre, which is around 1,800 kilometres (1,100 mi) in diameter,^[100] and Isidis, which is around 1,500 kilometres (930 mi) in diameter.^[101] Due to the smaller mass and size of Mars, the probability of an object colliding with the planet is about half that of Earth. Mars is located closer to the asteroid belt, so it has an increased chance of being struck by materials from that source. Mars is more likely to be struck by short-period comets, i.e., those that lie within the orbit of Jupiter.^[102]

Martian craters can have a morphology that suggests the ground became wet after the meteor impacted.^[103]

Tectonic sites

The large canyon,

Images from the Thermal Emission Imaging System (THEMIS) aboard NASA's Mars Odyssey orbiter have revealed seven possible cave entrances on the flanks of the volcano Arsia Mons.^[107] The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters".^[108] Cave entrances measure from 100 to 252 metres (328 to 827 ft) wide and they are estimated to be at least 73 to 96 metres (240 to 315 ft) deep. Because light does not reach the floor of most of the caves, they may extend much deeper than these lower estimates and widen below the surface. "Dena" is the only exception; its floor is visible and was measured to be 130 metres (430 ft) deep. The interiors of these caverns may be protected from micrometeoroids, UV radiation, solar flares and high energy particles that bombard the planet's surface.^[109][110]

Atmosphere

Mars lost its

ppb during the northern winter to about 0.65 ppb during the summer.^[121] Estimates of its lifetime range from 0.6 to 4 years,^[122][123] so its presence indicates that an active source of the gas must be present. Methane could be produced by non-biological process such as serpentinization involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars,^[124] or by Martian life.^[125]

Compared to Earth, its higher concentration of atmospheric CO₂ and lower surface pressure may be why sound is attenuated more on Mars, where natural sources are rare apart from the wind. Using acoustic recordings collected by the Perseverance rover, researchers concluded that the speed of sound there is approximately 240 m/s for frequencies below 240 Hz, and 250 m/s for those above.[127]^[128]

Auroras have been detected on Mars.^{[129][130][131]} Because Mars lacks a global magnetic field, the types and distribution of auroras there differ from those on Earth;^[132] rather than being mostly restricted to polar regions as is the case on Earth, a Martian aurora can encompass the planet.^[133] In September 2017, NASA reported radiation levels on the surface of the planet Mars were temporarily doubled, and were associated with an aurora 25 times brighter than any observed earlier, due to a massive, and unexpected, solar storm in the middle of the month.^[133][134]

Climate

Of all the planets in the Solar System, the seasons of Mars are the most Earth-like, due to the similar tilts of the two planets' rotational axes. The lengths of the Martian seasons are about twice those of Earth's because Mars's greater distance from the Sun leads to the Martian year being about two Earth years long. Martian surface temperatures vary from lows of about −110 °C (−166 °F) to highs of up to 35 °C (95 °F) in equatorial summer.^[15] The wide range in temperatures is due to the thin atmosphere which cannot store much solar heat, the low atmospheric pressure (about 1% that of the atmosphere of Earth), and the low thermal inertia of Martian soil.^[135] The planet is 1.52 times as far from the Sun as Earth, resulting in just 43% of the amount of sunlight.^[136][137]

If Mars had an Earth-like orbit, its seasons would be similar to Earth's because its axial tilt is similar to Earth's. The comparatively large eccentricity of the Martian orbit has a significant effect. Mars is near perihelion when it is summer in the Southern Hemisphere and winter in the north, and near aphelion when it is winter in the Southern Hemisphere and summer in the north. As a result, the seasons in the Southern Hemisphere are more extreme and the seasons in the northern are milder than would otherwise be the case. The summer temperatures in the south can be warmer than the equivalent summer temperatures in the north by up to 30 °C (54 °F).^[138]

Mars has the largest dust storms in the Solar System, reaching speeds of over 160 km/h (100 mph). These can vary from a storm over a small area, to gigantic storms that cover the entire planet. They tend to occur when Mars is closest to the Sun, and have been shown to increase global temperature.^[139]

Hydrology

Water in its liquid form cannot exist on the surface of Mars due to low atmospheric pressure, which is less than 1% that of Earth,^[140] except at the lowest of elevations for short periods.^[47][141] The two polar ice caps appear to be made largely of water.^[142][143] The volume of water ice in the south polar ice cap, if melted, would be enough to cover the entire surface of the planet with a depth of 11 metres (36 ft).^[144] Large quantities of ice are thought to be trapped within the thick cryosphere of Mars. Radar data from Mars Express and the Mars Reconnaissance Orbiter (MRO) show large quantities of ice at both poles,^[145][146] and at middle latitudes.^[147] The Phoenix lander directly sampled water ice in shallow Martian soil on 31 July 2008.^[148]

North polar early summer water ice cap (1999); a seasonal layer of carbon dioxide ice forms in winter and disappears in summer.

South polar midsummer ice cap (2000); the south cap has a permanent carbon dioxide ice cap covered with water ice.^[161]

Mars has two permanent polar ice caps. During a pole's winter, it lies in continuous darkness, chilling the surface and causing the

The caps at both poles consist primarily of water ice. Frozen carbon dioxide accumulates as a comparatively thin layer about one metre thick on the north cap in the northern winter only, whereas the south cap has a permanent dry ice cover about eight metres thick. This permanent dry ice cover at the south pole is peppered by flat floored, shallow, roughly circular pits, which repeat imaging shows are expanding in some places and retreating in others.^[164] The northern polar cap has a diameter of about 1,000 kilometres (620 mi),^[165] and contains about 1.6 million cubic kilometres (5.7×10¹⁶ cu ft) of ice, which, if spread evenly on the cap, would be 2 kilometres (1.2 mi) thick.^[166] (This compares to a volume of 2.85 million cubic kilometres (1.01×10¹⁷ cu ft) for the Greenland ice sheet.) The southern polar cap has a diameter of 350 kilometres (220 mi) and a thickness of 3 kilometres (1.9 mi).^[167] The total volume of ice in the south polar cap plus the adjacent layered deposits has been estimated at 1.6 million cubic km.^[168] Both polar caps show spiral troughs, which a recent analysis of SHARAD ice penetrating radar has shown are a result of katabatic winds that spiral due to the Coriolis effect.^[169][170]

The seasonal frosting of areas near the southern ice cap results in the formation of transparent 1-metre-thick slabs of dry ice above the ground. With the arrival of spring, sunlight warms the subsurface and pressure from subliming CO₂ builds up under a slab, elevating and ultimately rupturing it. This leads to

In 2004, Opportunity detected the mineral

In November 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior (which is 12,100 cubic kilometres^[193]).^[194][195] During observations from 2018 through 2021, the ExoMars Trace Gas Orbiter spotted indications of water, probably subsurface ice, in the Valles Marineris canyon system.^[196]

Orbital motion

Mars's average distance from the Sun is roughly 230 million km (143 million mi), and its orbital period is 687 (Earth) days. The solar day (or sol) on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds.^[197] A Martian year is equal to 1.8809 Earth years, or 1 year, 320 days, and 18.2 hours.^[2] The gravitational potential difference and thus the delta-v needed to transfer between Mars and Earth is the second lowest for Earth.^[198][199]

The axial tilt of Mars is 25.19° relative to its

Mars has a relatively pronounced orbital eccentricity of about 0.09; of the seven other planets in the Solar System, only Mercury has a larger orbital eccentricity. It is known that in the past, Mars has had a much more circular orbit. At one point, 1.35 million Earth years ago, Mars had an eccentricity of roughly 0.002, much less than that of Earth today.^[200] Mars's cycle of eccentricity is 96,000 Earth years compared to Earth's cycle of 100,000 years.^[201]

Mars has its closest approach to Earth (

perihelion in 2003, 2018 and 2035, with the 2020 and 2033 events being particularly close to perihelic opposition.^{[203][204][205]}

As Mars approaches opposition, it begins a period of retrograde motion, which means it will appear to move backwards in a looping curve with respect to the background stars. This retrograde motion lasts for about 72 days, and Mars reaches its peak apparent brightness in the middle of this interval.^[209]

Moons

Enhanced-color HiRISE image of Phobos, showing a series of mostly parallel grooves and crater chains, with Stickney crater at right

Enhanced-color HiRISE image of Deimos (not to scale), showing its smooth blanket of regolith

Mars has two relatively small (compared to Earth's) natural moons, Phobos (about 22 kilometres (14 mi) in diameter) and Deimos (about 12 kilometres (7.5 mi) in diameter), which orbit close to the planet. The origin of both moons is unclear, although a popular theory states that they were asteroids captured into Martian orbit.^[210]

Both satellites were discovered in 1877 by

From the surface of Mars, the motions of Phobos and Deimos appear different from that of the Earth's satellite, the Moon. Phobos rises in the west, sets in the east, and rises again in just 11 hours. Deimos, being only just outside synchronous orbit – where the orbital period would match the planet's period of rotation – rises as expected in the east, but slowly. Because the orbit of Phobos is below a synchronous altitude, tidal forces from Mars are gradually lowering its orbit. In about 50 million years, it could either crash into Mars's surface or break up into a ring structure around the planet.^[212]

The origin of the two satellites is not well understood. Their low albedo and carbonaceous chondrite composition have been regarded as similar to asteroids, supporting a capture theory. The unstable orbit of Phobos would seem to point toward a relatively recent capture. But both have circular orbits near the equator, which is unusual for captured objects, and the required capture dynamics are complex. Accretion early in the history of Mars is plausible, but would not account for a composition resembling asteroids rather than Mars itself, if that is confirmed.^[213] Mars may have yet-undiscovered moons, smaller than 50 to 100 metres (160 to 330 ft) in diameter, and a dust ring is predicted to exist between Phobos and Deimos.^[214]

A third possibility for their origin as satellites of Mars is the involvement of a third body or a type of impact disruption. More-recent lines of evidence for Phobos having a highly porous interior,

The history of observations of Mars is marked by oppositions of Mars when the planet is closest to Earth and hence is most easily visible, which occur every couple of years. Even more notable are the

The ancient Sumerians named Mars Nergal, the god of war and plague. During Sumerian times, Nergal was a minor deity of little significance, but, during later times, his main cult center was the city of Nineveh.^[218] In Mesopotamian texts, Mars is referred to as the "star of judgement of the fate of the dead".^[219] The existence of Mars as a wandering object in the night sky was also recorded by the ancient Egyptian astronomers and, by 1534 BCE, they were familiar with the retrograde motion of the planet.^[220] By the period of the Neo-Babylonian Empire, the Babylonian astronomers were making regular records of the positions of the planets and systematic observations of their behavior. For Mars, they knew that the planet made 37 synodic periods, or 42 circuits of the zodiac, every 79 years. They invented arithmetic methods for making minor corrections to the predicted positions of the planets.^[221][222] In Ancient Greece, the planet was known as Πυρόεις.^[223] Commonly, the Greek name for the planet now referred to as Mars, was Ares. It was the Romans who named the planet Mars, for their god of war, often represented by the sword and shield of the planet's namesake.^[224]

In the fourth century BCE,

By the 19th century, the resolution of telescopes reached a level sufficient for surface features to be identified. On 5 September 1877, a perihelic opposition to Mars occurred. The Italian astronomer Giovanni Schiaparelli used a 22-centimetre (8.7 in) telescope in Milan to help produce the first detailed map of Mars. These maps notably contained features he called canali, which were later shown to be an optical illusion. These canali were supposedly long, straight lines on the surface of Mars, to which he gave names of famous rivers on Earth. His term, which means "channels" or "grooves", was popularly mistranslated in English as "canals".^[238][239]

Influenced by the observations, the orientalist

As of 2023[update], Mars is host to ten functioning

• NASA's EscaPADE spacecraft, planned to launch in late 2024.^[254]
• The Rosalind Franklin rover mission, designed to search for evidence of past life, which was intended to be launched in 2018 but has been repeatedly delayed, with a launch date pushed to 2028 at the earliest.^{[255][256][257]}
• A current concept for a joint NASA-ESA mission to return samples from Mars would launch in 2026.^[258][259]

As of February 2024^[update], debris from these types of missions has reached over seven tons. Most of it consists of crashed and inactive spacecraft as well as discarded components.^[260][261]

Beagle 2

Curiosity ※

Deep Space 2

Rosalind Franklin ⁂

InSight

Mars 2

Mars 3

Mars 6

Mars Polar Lander ↓

Opportunity

Perseverance ※

Phoenix

Schiaparelli EDM

Sojourner

Spirit

Zhurong

Viking 1

Viking 2

Habitability and the search for life

During the late 19th century, it was widely accepted in the astronomical community that Mars had life-supporting qualities, including the presence of oxygen and water.

The environmental conditions on Mars are a challenge to sustaining organic life: the planet has little heat transfer across its surface, it has poor insulation against bombardment by the solar wind due to the absence of a magnetosphere and has insufficient atmospheric pressure to retain water in a liquid form (water instead sublimes to a gaseous state). Mars is nearly, or perhaps totally, geologically dead; the end of volcanic activity has apparently stopped the recycling of chemicals and minerals between the surface and interior of the planet.^[266]

Evidence suggests that the planet was once significantly more habitable than it is today, but whether living

Small quantities of methane and formaldehyde detected by Mars orbiters are both claimed to be possible evidence for life, as these chemical compounds would quickly break down in the Martian atmosphere.^[269][270] Alternatively, these compounds may instead be replenished by volcanic or other geological means, such as serpentinite.^[124] Impact glass, formed by the impact of meteors, which on Earth can preserve signs of life, has also been found on the surface of the impact craters on Mars.^[271][272] Likewise, the glass in impact craters on Mars could have preserved signs of life, if life existed at the site.^{[273][274][275]}

Human mission proposals

Several plans for a

Mars is named after the Roman god of war. This association between Mars and war dates back at least to Babylonian astronomy, in which the planet was named for the god Nergal, deity of war and destruction.^[284][285] It persisted into modern times, as exemplified by Gustav Holst's orchestral suite The Planets, whose famous first movement labels Mars "the bringer of war".^[286] The planet's symbol, a circle with a spear pointing out to the upper right, is also used as a symbol for the male gender.^[287] The symbol dates from at least the 11th century, though a possible predecessor has been found in the Greek Oxyrhynchus Papyri.^[288]

The idea that Mars was populated by intelligent

• Astronomy on Mars
• Outline of Mars – Overview of and topical guide to Mars
• List of missions to Mars
• Magnetic field of Mars – Past magnetic field of the planet Mars
• Mineralogy of Mars – Overview of the mineralogy of Mars

Notes

References