Rounding the round out to flat and other curved ideas of round that are angular and confusion
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth the mass of the Sun. Jupiter is the third brightest natural object in the Earth's night sky after the Moon and Venus, and it has been observed since prehistoric times. It was named after the Roman god Jupiter, the king of the gods.
Jupiter is primarily composed of hydrogen, but helium constitutes one-quarter of its mass and one-tenth of its volume. It probably has a rocky core of heavier elements,[16] but, like the other giant planets in the Solar System, it lacks a well-defined solid surface. The ongoing contraction of Jupiter's interior generates more heat than it receives from the Sun. Because of its rapid rotation, the planet's shape is an oblate spheroid: it has a slight but noticeable bulge around the equator. The outer atmosphere is divided into a series of latitudinal bands, with turbulence and storms along their interacting boundaries. A prominent result of this is the Great Red Spot, a giant storm which has been observed since at least 1831.
Jupiter is surrounded by a faint planetary ring system and a powerful magnetosphere. Jupiter's magnetic tail is nearly 800 million km (5.3 AU; 500 million mi) long, covering nearly the entire distance to Saturn's orbit. Jupiter has 80 known moons and possibly many more,[6] including the four large moons discovered by Galileo Galilei in 1610: Io, Europa, Ganymede, and Callisto. Io and Europa are about the size of Earth's Moon; Callisto is almost the size of the planet Mercury, and Ganymede is larger.
Pioneer 10 was the first spacecraft to visit Jupiter, making its closest approach to the planet in December 1973.[17] Jupiter has since been explored by multiple robotic spacecraft, beginning with the Pioneer and Voyager flyby missions from 1973 to 1979, and later with the Galileo orbiter in 1995.[18] In 2007, the New Horizons visited Jupiter using its gravity to increase its speed, bending its trajectory en route to Pluto. The latest probe to visit the planet, Juno, entered orbit around Jupiter in July 2016.[19][20] Future targets for exploration in the Jupiter system include the probable ice-covered liquid ocean of Europa.[21]
Name and symbol
Thaz no t rue will you circle a few
https://youtu.be/mWj1ZEQTI8I
In both the ancient Greek and Roman civilizations, Jupiter was named after the chief god of the divine pantheon: Zeus for the Greeks and Jupiter for the Romans. The International Astronomical Union (IAU) formally adopted the name Jupiter for the planet in 1976. The IAU names newly discovered satellites of Jupiter for the mythological lovers, favourites, and descendants of the god.[22] The planetary symbol for Jupiter, .svg){kind=small}
, descends from a Greek zeta with a horizontal stroke, ⟨Ƶ⟩, as an abbreviation for Zeus.[23][24]
Jove, the archaic name of Jupiter, came into use as a poetic name for the planet around the 14th century.[25] The Romans named the fifth day of the week diēs Iovis ("Jove's Day") after the planet Jupiter.[26] In Germanic mythology, Jupiter is equated to Thor, whence the English name Thursday for the Roman dies Jovis.[27]
The original Greek deity Zeus supplies the root zeno-, which is used to form some Jupiter-related words, such as zenographic.[d] Jovian is the adjectival form of Jupiter. The older adjectival form jovial, employed by astrologers in the Middle Ages, has come to mean "happy" or "merry", moods ascribed to Jupiter's astrological influence.[28]
Formation and migration
Jupiter is believed to be the oldest planet in the Solar System.[29] Current models of Solar System formation suggest that Jupiter formed at or beyond the snow line: a distance from the early Sun where the temperature is sufficiently cold for volatiles such as water to condense into solids.[30] The planet began as a large solid core, then accumulated its gaseous atmosphere. As a consequence, the core must have formed before the solar nebula was fully dispersed after 10 million years. Over about a million years, Jupiter's atmosphere gradually expanded until it had 20 times the mass of the Earth. The orbiting mass created a gap in the solar nebula, and thereafter the planet slowly increased to 50 Earth masses over 3–4 million years.[29]
According to the "grand tack hypothesis", Jupiter began to form at a distance of roughly 3.5 AU (520 million km; 330 million mi) from the Sun. As the young planet accreted mass, interaction with the gas disk orbiting the Sun and orbital resonances with Saturn caused it to migrate inward.[30][31] This upset the orbits of several super-Earths orbiting closer to the Sun, causing them to collide destructively. Saturn would later have begun to migrate inwards too, much faster than Jupiter, until the two planets became captured in a 3:2 mean motion resonance at approximately 1.5 AU (220 million km; 140 million mi) from the Sun. This changed the direction of migration, causing them to migrate away from the Sun and out of the inner system to their current locations.[32] All of this happened over a period of 3–6 million years, with the final migration of Jupiter occurring over several hundred thousand years.[31][33] Jupiter's departure from the inner solar system eventually allowed the inner planets—including Earth—to form from the rubble.[34]
There are several problems with the grand tack hypothesis. The resulting formation timescales of terrestrial planets appear to be inconsistent with the measured elemental composition.[35] It is likely that Jupiter would have settled into an orbit much closer to the Sun if it had migrated through the solar nebula.[36] Some competing models of Solar System formation predict the formation of Jupiter with orbital properties that are close to those of the present day planet.[37] Other models predict Jupiter forming at distances much farther out, such as 18 AU (2.7 billion km; 1.7 billion mi).[38][39]
Based on Jupiter's composition, researchers have made the case for an initial formation outside the molecular nitrogen (N2) snowline, which is estimated at 20–30 AU (3.0–4.5 billion km; 1.9–2.8 billion mi) from the Sun,[40][41] and possibly even outside the argon snowline, which may be as far as 40 AU (6.0 billion km; 3.7 billion mi). Having formed at one of these extreme distances, Jupiter would then have migrated inwards to its current location. This inward migration would have occurred over a roughly 700,000-year time period,[38][39] during an epoch approximately 2–3 million years after the planet began to form. In this model, Saturn, Uranus and Neptune would have formed even further out than Jupiter, and Saturn would also have migrated inwards.
Physical characteristics
Jupiter is a gas giant, being primarily composed of gas and liquid rather than solid matter. It is the largest planet in the Solar System, with a diameter of 142,984 km (88,846 mi) at its equator.[42] The average density of Jupiter, 1.326 g/cm3, is about the same as simple syrup (syrup USP),[43] and is lower than those of the four terrestrial planets.[44][45]
Composition
Jupiter's upper atmosphere is about 90% hydrogen and 10% helium by volume. Since helium atoms are more massive than hydrogen molecules, Jupiter's atmosphere is approximately 24% helium by mass.[46] The atmosphere contains trace amounts of methane, water vapour, ammonia, and silicon-based compounds. There are also fractional amounts of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia. Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found.[47] The interior of Jupiter contains denser materials—by mass it is roughly 71% hydrogen, 24% helium, and 5% other elements.[48][49]
The atmospheric proportions of hydrogen and helium are close to the theoretical composition of the primordial solar nebula. Neon in the upper atmosphere only consists of 20 parts per million by mass, which is about a tenth as abundant as in the Sun.[50] Helium is also reduced to about 80% of the Sun's helium composition. This depletion is a result of precipitation of these elements as helium-rich droplets, a process that happens deep in the interior of the planet.[51][52]
Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other giant planets Uranus and Neptune have relatively less hydrogen and helium and relatively more of the next most common elements, including oxygen, carbon, nitrogen, and sulfur.[53] These planets are known as ice giants, because the majority of their volatile compounds are in solid form.
Size and mass
Jupiter's mass is 2.5 times that of all the other planets in the Solar System combined—so massive that its barycentre with the Sun lies above the Sun's surface at 1.068 solar radii from the Sun's centre.[54] Jupiter is much larger than Earth and considerably less dense: it has 1,321 times the volume of the Earth, but only 318 times the mass.[7][55]: 6 Jupiter's radius is about one tenth the radius of the Sun,[56] and its mass is one thousandth the mass of the Sun, as the densities of the two bodies are similar.[57] A "Jupiter mass" (MJ or MJup) is often used as a unit to describe masses of other objects, particularly extrasolar planets and brown dwarfs. For example, the extrasolar planet HD 209458 b has a mass of 0.69 MJ, while Kappa Andromedae b has a mass of 12.8 MJ.[58]
Theoretical models indicate that if Jupiter had over 40% more mass, the interior would be so compressed that its volume would decrease despite the increasing amount of matter. For smaller changes in its mass, the radius would not change appreciably.[59] As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve.[60] The process of further shrinkage with increasing mass would continue until appreciable stellar ignition was achieved.[61] Although Jupiter would need to be about 75 times more massive to fuse hydrogen and become a star,[62] the smallest red dwarf may be only slightly larger in radius than Saturn.[63]
Jupiter radiates more heat than it receives through solar radiation, due to the Kelvin–Helmholtz mechanism within its contracting interior.[64]: 30 [65] This process causes Jupiter to shrink by about 1 mm (0.039 in)/yr.[66][67] When it formed, Jupiter was hotter and was about twice its current diameter.[68]
Internal structure
Before the early 21st century, most scientists proposed one of two scenarios for the formation of Jupiter. If the planet accreted first as a solid body, it would consist of a dense core, a surrounding layer of liquid metallic hydrogen (with some helium) extending outward to about 80% of the radius of the planet,[69] and an outer atmosphere consisting primarily of molecular hydrogen.[67] Alternatively, if the planet collapsed directly from the gaseous protoplanetary disk, it was expected to completely lack a core, consisting instead of denser and denser fluid (predominantly molecular and metallic hydrogen) all the way to the centre. Data from the Juno mission showed that Jupiter has a very diffuse core that mixes into its mantle.[19][70][71] This could have been caused by an impact from a planet of about ten Earth masses a few million years after Jupiter's formation, which would have disrupted an originally solid Jovian core.[72][73] It is estimated that the core takes up 30–50% of the planet's radius, and contains heavy elements with a combined mass 7–25 times the Earth.[74]
Outside the layer of metallic hydrogen lies a transparent interior atmosphere of hydrogen. At this depth, the pressure and temperature are above molecular hydrogen's critical pressure of 1.3 MPa and critical temperature of 33 K (−240.2 °C; −400.3 °F).[75] In this state, there are no distinct liquid and gas phases—hydrogen is said to be in a supercritical fluid state. The hydrogen and helium gas extending downward from the cloud layer gradually transitions to a liquid in deeper layers, possibly resembling something akin to an ocean of liquid hydrogen and other supercritical fluids.[64]: 22 [76][77][78] Physically, the gas gradually becomes hotter and denser as depth increases.[79][80]
Rain-like droplets of helium and neon precipitate downward through the lower atmosphere, depleting the abundance of these elements in the upper atmosphere.[51][81] Calculations suggest that helium drops separate from metallic hydrogen at a radius of 60,000 km (37,000 mi) (11,000 km (6,800 mi) below the cloudtops) and merge again at 50,000 km (31,000 mi) (22,000 km (14,000 mi) beneath the clouds).[82] Rainfalls of diamonds have been suggested to occur, as well as on Saturn[83] and the ice giants Uranus and Neptune.[84]
The temperature and pressure inside Jupiter increase steadily inward because the heat of planetary formation can only escape by convection.[52] At a surface depth where the atmospheric pressure level is 1 bar (0.10 MPa), the temperature is around 165 K (−108 °C; −163 °F). The region of supercritical hydrogen changes gradually from a molecular fluid to a metallic fluid spans pressure ranges of 50–400 GPa with temperatures of 5,000–8,400 K (4,730–8,130 °C; 8,540–14,660 °F), respectively. The temperature of Jupiter's diluted core is estimated to be 20,000 K (19,700 °C; 35,500 °F) with a pressure of around 4,000 GPa.[85]
Atmosphere
The atmosphere of Jupiter extends to a depth of 3,000 km (2,000 mi) below the cloud layers.[85]
Cloud layers
Jupiter is perpetually covered with clouds of ammonia crystals, which may contain ammonium hydrosulfide as well.[86] The clouds are located in the tropopause layer of the atmosphere, forming bands at different latitudes, known as tropical regions. These are subdivided into lighter-hued zones and darker belts. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 100 metres per second (360 km/h; 220 mph) are common in zonal jet streams.[87] The zones have been observed to vary in width, colour and intensity from year to year, but they have remained stable enough for scientists to name them.[55]: 6
The cloud layer is about 50 km (31 mi) deep, and consists of at least two decks of ammonia clouds: a thin clearer region on top with a thick lower deck. There may be a thin layer of water clouds underlying the ammonia clouds, as suggested by flashes of lightning detected in the atmosphere of Jupiter.[88] These electrical discharges can be up to a thousand times as powerful as lightning on Earth.[89] The water clouds are assumed to generate thunderstorms in the same way as terrestrial thunderstorms, driven by the heat rising from the interior.[90] The Juno mission revealed the presence of "shallow lightning" which originates from ammonia-water clouds relatively high in the atmosphere.[91] These discharges carry "mushballs" of water-ammonia slushes covered in ice, which fall deep into the atmosphere.[92] Upper-atmospheric lightning has been observed in Jupiter's upper atmosphere, bright flashes of light that last around 1.4 milliseconds. These are known as "elves" or "sprites" and appear blue or pink due to the hydrogen.[93][94]
The orange and brown colours in the clouds of Jupiter are caused by upwelling compounds that change colour when they are exposed to ultraviolet light from the Sun. The exact makeup remains uncertain, but the substances are thought to be made up of phosphorus, sulfur or possibly hydrocarbons.[64]: 39 [95] These colourful compounds, known as chromophores, mix with the warmer clouds of the lower deck. The light-coloured zones are formed when rising convection cells form crystallising ammonia that hides the chromophores from view.[96]
Jupiter's low axial tilt means that the poles always receive less solar radiation than the planet's equatorial region. Convection within the interior of the planet transports energy to the poles, balancing out the temperatures at the cloud layer.[55]: 54
Great Red Spot and other vortices
The best known feature of Jupiter is the Great Red Spot,[97] a persistent anticyclonic storm located 22° south of the equator. It is known to have existed since at least 1831,[98] and possibly since 1665.[99][100] Images by the Hubble Space Telescope have shown as many as two "red spots" adjacent to the Great Red Spot.[101][102] The storm is visible through Earth-based telescopes with an aperture of 12 cm or larger.[103] The oval object rotates counterclockwise, with a period of about six days.[104] The maximum altitude of this storm is about 8 km (5 mi) above the surrounding cloudtops.[105] The Spot's composition and the source of its red colour remain uncertain, although photodissociated ammonia reacting with acetylene is a likely explanation.[106]
The Great Red Spot is larger than the Earth.[107] Mathematical models suggest that the storm is stable and will be a permanent feature of the planet.[108] However, it has significantly decreased in size since its discovery. Initial observations in the late 1800s showed it to be approximately 41,000 km (25,500 mi) across. By the time of the Voyager flybys in 1979, the storm had a length of 23,300 km (14,500 mi) and a width of approximately 13,000 km (8,000 mi).[109] Hubble observations in 1995 showed it had decreased in size to 20,950 km (13,020 mi), and observations in 2009 showed the size to be 17,910 km (11,130 mi). As of 2015, the storm was measured at approximately 16,500 by 10,940 km (10,250 by 6,800 mi),[109] and was decreasing in length by about 930 km (580 mi) per year.[107][110] In October 2021, a Juno flyby mission measured the depth of the Great Red Spot, putting it at around 300–500 kilometres (190–310 mi).[111]
Juno missions show that there are several polar cyclone groups at Jupiter's poles. The northern group contains nine cyclones, with a large one in the centre and eight others around it, while its southern counterpart also consists of a centre vortex but is surrounded by five large storms and a single smaller one.[112][113] These polar structures are caused by the turbulence in Jupiter's atmosphere and can be compared with the hexagon at Saturn's north pole.
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In 2000, an atmospheric feature formed in the southern hemisphere that is similar in appearance to the Great Red Spot, but smaller. This was created when smaller, white oval-shaped storms merged to form a single feature—these three smaller white ovals were formed in 1939–1940. The merged feature was named Oval BA. It has since increased in intensity and changed from white to red, giving it the nickname "Little Red Spot".[114][115]
In April 2017, a "Great Cold Spot" was discovered in Jupiter's thermosphere at its north pole. This feature is 24,000 km (15,000 mi) across, 12,000 km (7,500 mi) wide, and 200 °C (360 °F) cooler than surrounding material. While this spot changes form and intensity over the short term, it has maintained its general position in the atmosphere for more than 15 years. It may be a giant vortex similar to the Great Red Spot, and appears to be quasi-stable like the vortices in Earth's thermosphere. This feature may be formed by interactions between charged particles generated from Io and the strong magnetic field of Jupiter, resulting in a redistribution of heat flow.[116]
Magnetosphere
Jupiter's magnetic field is the strongest of any planet in the Solar System,[96] with a dipole moment of 4.170 gauss (0.4170 mT) that is tilted at an angle of 10.31° to the pole of rotation. The surface magnetic field strength varies from 2 gauss (0.20 mT) up to 20 gauss (2.0 mT).[117] This field is thought to be generated by eddy currents—swirling movements of conducting materials—within the liquid metallic hydrogen core. At about 75 Jupiter radii from the planet, the interaction of the magnetosphere with the solar wind generates a bow shock. Surrounding Jupiter's magnetosphere is a magnetopause, located at the inner edge of a magnetosheath—a region between it and the bow shock. The solar wind interacts with these regions, elongating the magnetosphere on Jupiter's lee side and extending it outward until it nearly reaches the orbit of Saturn. The four largest moons of Jupiter all orbit within the magnetosphere, which protects them from the solar wind.[64]: 69
The volcanoes on the moon Io emit large amounts of sulfur dioxide, forming a gas torus along the moon's orbit. The gas is ionized in Jupiter's magnetosphere, producing sulfur and oxygen ions. They, together with hydrogen ions originating from the atmosphere of Jupiter, form a plasma sheet in Jupiter's equatorial plane. The plasma in the sheet co-rotates with the planet, causing deformation of the dipole magnetic field into that of a magnetodisk. Electrons within the plasma sheet generate a strong radio signature, with short, superimposed bursts in the range of 0.6–30 MHz that are detectable from Earth with consumer-grade shortwave radio receivers.[118][119] As Io moves through this torus, the interaction generates Alfvén waves that carry ionized matter into the polar regions of Jupiter. As a result, radio waves are generated through a cyclotron maser mechanism, and the energy is transmitted out along a cone-shaped surface. When Earth intersects this cone, the radio emissions from Jupiter can exceed the radio output of the Sun.[120]
Mars
 Pictured in natural color in 2007[a] | |||||||||||||
| Designations | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Adjectives | Martian | ||||||||||||
| Orbital characteristics[5] | |||||||||||||
| Epoch J2000 | |||||||||||||
| Aphelion | 249261000 km (154884000 mi; 1.66621 AU)[1] | ||||||||||||
| Perihelion | 206650000 km (128410000 mi; 1.3814 AU)[1] | ||||||||||||
| 227939366 km (141634956 mi; 1.52368055 AU)[2] | |||||||||||||
| Eccentricity | 0.0934[1] | ||||||||||||
| 686.980 d (1.88085 yr; 668.5991 sols)[1] | |||||||||||||
| 779.94 d (2.1354 yr)[2] | |||||||||||||
Average orbital speed | 24.07 km/s (86700 km/h; 53800 mph)[1] | ||||||||||||
| 19.412°[1] | |||||||||||||
| Inclination |
| ||||||||||||
| 49.57854°[1] | |||||||||||||
| 2022-Jun-21[4] | |||||||||||||
| 286.5°[2] | |||||||||||||
| Satellites | 2 | ||||||||||||
| Physical characteristics | |||||||||||||
Mean radius | 3389.5 ± 0.2 km[b] [6] (2106.1 ± 0.1 mi) | ||||||||||||
Equatorial radius | 3396.2 ± 0.1 km[b] [6] (2110.3 ± 0.1 mi; 0.533 Earths) | ||||||||||||
Polar radius | 3376.2 ± 0.1 km[b] [6] (2097.9 ± 0.1 mi; 0.531 Earths) | ||||||||||||
| Flattening | 0.00589±0.00015[4][6] | ||||||||||||
| 144.37×106 km2[7] (5.574×107 sq mi; 0.284 Earths) | |||||||||||||
| Volume | 1.63118×1011 km3[8] (0.151 Earths) | ||||||||||||
| Mass | 6.4171×1023 kg[9] (0.107 Earths) | ||||||||||||
Mean density | 3.9335 g/cm3[8] (0.1421 lb/cu in) | ||||||||||||
| 3.72076 m/s2[10] (12.2072 ft/s2; 0.3794 g) | |||||||||||||
| 0.3644±0.0005[9] | |||||||||||||
| 5.027 km/s (18100 km/h; 11250 mph)[11] | |||||||||||||
| 1.02749125 d[12] 24h 39m 36s | |||||||||||||
| 1.025957 d 24h 37m 22.7s[8] | |||||||||||||
Equatorial rotation velocity | 241 m/s (870 km/h; 540 mph)[1] | ||||||||||||
| 25.19° to its orbital plane[1] | |||||||||||||
North pole right ascension | 317.68143°[6] 21h 10m 44s | ||||||||||||
North pole declination | 52.88650°[6] | ||||||||||||
| Albedo | |||||||||||||
| |||||||||||||
| Surface absorbed dose rate | 8.8 μGy/h[17] | ||||||||||||
| Surface equivalent dose rate | 27 μSv/h[17] | ||||||||||||
| −2.94 to +1.86[14] | |||||||||||||
| 3.5–25.1″[1] | |||||||||||||
| Atmosphere[1][18] | |||||||||||||
Surface pressure | 0.636 (0.4–0.87) kPa 0.00628 atm | ||||||||||||
| Composition by volume |
| ||||||||||||
.svg){kind=small}
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury. In English, Mars carries the name of the Roman god of war. Mars is a terrestrial planet with a thin atmosphere, and has a crust primarily composed of elements similar to Earth's crust, as well as a core made of iron and nickel. Mars has surface features such as impact craters, valleys, dunes, and polar ice caps. It also has two small and irregularly shaped moons, Phobos and Deimos.
Some of the most notable surface features on Mars include Olympus Mons, the largest volcano and highest known mountain on any Solar System planet, and Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the Northern Hemisphere covers 40% of the planet and may be a giant impact feature.[19] The days and seasons on Mars are comparable to those of Earth as the planets have a similar rotation period and tilt of the rotational axis relative to the ecliptic plane. Liquid water on the surface of Mars cannot exist due to low atmospheric pressure, which is less than 1% of the atmospheric pressure on Earth.[20][21] Both of Mars's polar ice caps appear to be made largely of water.[22][23] Multiple lines of evidence suggest that Mars was wetter in the distant past, and thus possibly more suited for life. However, whether life ever did exist there, and if it could have survived to the present day, remain unanswered questions.
Mars has been explored by several uncrewed spacecraft, beginning with Mariner 4 in 1965. NASA's Viking 1 lander transmitted the first images from the Martian surface in 1976. Two countries have successfully deployed rovers on Mars, the United States first doing so with Sojourner in 1997 and China with Zhurong in 2021.[24] There are also planned future missions to Mars, such as a Mars sample-return mission set to happen in 2026, and the Rosalind Franklin rover mission, which was intended to launch in 2018 but was delayed to 2024.
Mars can easily be seen from Earth with the naked eye, as can its striking reddish coloring. This appearance, due the iron oxide prevalent on its surface, has led to Mars often being called the Red Planet.[25][26] It is among the brightest objects in Earth's sky, with an apparent magnitude that reaches −2.94, comparable to that of Jupiter and surpassed only by Venus, the Moon and the Sun.[14] Historically, Mars has been observed since ancient times, and over the millennia, has been featured in culture and the arts in ways that have reflected humanity's growing knowledge of it.
Historical observations
The history of observations of Mars is marked by the 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 perihelic oppositions of Mars, which are distinguished because Mars is close to perihelion, making it even closer to Earth.[27]
Ancient and medieval observations
The ancient Sumerians believed that Mars was 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.[28] In Mesopotamian texts, Mars is referred to as the "star of judgement of the fate of the dead."[29] 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.[30] 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.[31][32] In Ancient Greece, the planet was known as Πυρόεις.[33]
In the fourth century BCE, Aristotle noted that Mars disappeared behind the Moon during an occultation, indicating that the planet was farther away.[34] Ptolemy, a Greek living in Alexandria,[35] attempted to address the problem of the orbital motion of Mars. Ptolemy's model and his collective work on astronomy was presented in the multi-volume collection later called the Almagest (from the Arabic for "greatest"), which became the authoritative treatise on Western astronomy for the next fourteen centuries.[36] Literature from ancient China confirms that Mars was known by Chinese astronomers by no later than the fourth century BCE.[37] In the East Asian cultures, Mars is traditionally referred to as the "fire star" (Chinese: 火星), based on the Wuxing system.[38][39][40]
During the seventeenth century, Tycho Brahe measured the diurnal parallax of Mars that Johannes Kepler used to make a preliminary calculation of the relative distance to the planet.[41] From Brahe's observations of Mars, Kepler deduced that the planet orbited the Sun not in a circle, but in an ellipse. Moreover, Kepler showed that Mars sped up as it approached the Sun and slowed down as it moved farther away, in a manner that later physicists would explain as a consequence of the conservation of angular momentum.[42]: 433–437 When the telescope became available, the diurnal parallax of Mars was again measured in an effort to determine the Sun-Earth distance. This was first performed by Giovanni Domenico Cassini in 1672. The early parallax measurements were hampered by the quality of the instruments.[43] The only occultation of Mars by Venus observed was that of 13 October 1590, seen by Michael Maestlin at Heidelberg.[44] In 1610, Mars was viewed by Italian astronomer Galileo Galilei, who was first to see it via telescope.[45] The first person to draw a map of Mars that displayed any terrain features was the Dutch astronomer Christiaan Huygens.[46]
Martian "canals"
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 of Mars occurred. During that day, 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".[47][48]
Influenced by the observations, the orientalist Percival Lowell founded an observatory which had 30- and 45-centimetre (12- and 18-in) telescopes. The observatory was used for the exploration of Mars during the last good opportunity in 1894 and the following less favorable oppositions. He published several books on Mars and life on the planet, which had a great influence on the public.[49][50] The canali were independently observed by other astronomers, like Henri Joseph Perrotin and Louis Thollon in Nice, using one of the largest telescopes of that time.[51][52]
The seasonal changes (consisting of the diminishing of the polar caps and the dark areas formed during Martian summer) in combination with the canals led to speculation about life on Mars, and it was a long-held belief that Mars contained vast seas and vegetation. As bigger telescopes were used, fewer long, straight canali were observed. During observations in 1909 by Antoniadi with an 84-centimetre (33 in) telescope, irregular patterns were observed, but no canali were seen.[53]
Physical characteristics
Mars is approximately half the diameter of Earth, with a surface area only slightly less than the total area of Earth's dry land.[1] Mars is less dense than Earth, having about 15% of Earth's volume and 11% of Earth's mass, resulting in about 38% of Earth's surface gravity. The red-orange appearance of the Martian surface is caused by iron(III) oxide, or rust.[54] It can look like butterscotch;[55] other common surface colors include golden, brown, tan, and greenish, depending on the minerals present.[55]
Internal structure
Like Earth, Mars has differentiated into a dense metallic core overlaid by less dense materials.[56][57] Current models of its interior imply a core consisting primarily of iron and nickel with about 16–17% sulfur.[58] This iron(II) sulfide core is thought to be twice as rich in lighter elements as Earth's.[59] The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but it appears to be dormant. Besides silicon and oxygen, the most abundant elements in the Martian crust are iron, magnesium, aluminium, calcium, and potassium. The average thickness of the planet's crust is about 50 kilometres (31 mi), with a maximum thickness of 125 kilometres (78 mi).[59] By comparison, Earth's crust averages 40 kilometres (25 mi) in thickness.[60]
Mars is seismically active, with InSight detecting and recording over 450 marsquakes and related events in 2019.[61][62] In 2021 it was reported that based on eleven low-frequency Marsquakes detected by the InSight lander the core of Mars is indeed liquid and has a radius of about 1830±40 km and a temperature around 1900–2000 K. The Martian core radius is more than half the radius of Mars and about half the size of the Earth's core. This is somewhat larger than models predicted, suggesting that the core contains some amount of lighter elements like oxygen and hydrogen in addition to the iron–nickel alloy and about 15% of sulfur.[63][64]
The core of Mars is overlain by the rocky mantle, which, however, does not seem to have a layer analogous to the Earth's lower mantle. The Martian mantle appears to be solid down to the depth of about 500 km, where the low-velocity zone (partially melted asthenosphere) begins.[65] Below the asthenosphere the velocity of seismic waves starts to grow again and at the depth of about 1050 km there lies the boundary of the transition zone.[64] At the surface of Mars there lies a crust with the average thickness of about 24–72 km.[66]
Surface geology
Mars is a terrestrial planet whose surface consists of minerals containing silicon and oxygen, metals, and other elements that typically make up rock. The Martian surface is primarily composed of tholeiitic basalt,[68] although parts are more silica-rich than typical basalt and may be similar to andesitic rocks on Earth, or silica glass. Regions of low albedo suggest concentrations of plagioclase feldspar, with northern low albedo regions displaying higher than normal concentrations of sheet silicates and high-silicon glass. Parts of the southern highlands include detectable amounts of high-calcium pyroxenes. Localized concentrations of hematite and olivine have been found.[69] Much of the surface is deeply covered by finely grained iron(III) oxide dust.[70]
Although Mars has no evidence of a structured global magnetic field,[71] 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 theory, 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.[72]
It is thought that, during the Solar System's formation, Mars was created as the result of a random process of run-away accretion of material from the protoplanetary disk that orbited the Sun. Mars has many distinctive chemical features caused by its position in the Solar System. Elements with comparatively low boiling points, such as chlorine, phosphorus, and sulfur, are much more common on Mars than Earth; these elements were probably pushed outward by the young Sun's energetic solar wind.[73]
After the formation of the planets, all were subjected to the so-called "Late Heavy Bombardment". About 60% of the surface of Mars shows a record of impacts from that era,[74][75][76] whereas much of the remaining surface is probably underlain by immense impact basins caused by those events. There is evidence of an enormous impact basin in the Northern Hemisphere of Mars, spanning 10,600 by 8,500 kilometres (6,600 by 5,300 mi), or roughly four times the size of the Moon's South Pole – Aitken basin, the largest impact basin yet discovered.[77] This theory suggests that Mars was struck by a Pluto-sized body about four billion years ago. The event, thought to be the cause of the Martian hemispheric dichotomy, created the smooth Borealis basin that covers 40% of the planet.[78][79]
The geological history of Mars can be split into many periods, but the following are the three primary periods:[80][81]
- Noachian period: Formation of the oldest extant surfaces of Mars, 4.5 to 3.5 billion years ago. Noachian age surfaces are scarred by many large impact craters. The Tharsis bulge, a volcanic upland, is thought to have formed during this period, with extensive flooding by liquid water late in the period. Named after Noachis Terra.[82]
- Hesperian period: 3.5 to between 3.3 and 2.9 billion years ago. The Hesperian period is marked by the formation of extensive lava plains. Named after Hesperia Planum.[82]
- Amazonian period: between 3.3 and 2.9 billion years ago to the present. Amazonian regions have few meteorite impact craters but are otherwise quite varied. Olympus Mons formed during this period, with lava flows elsewhere on Mars. Named after Amazonis Planitia.[82]
Geological activity is still taking place on Mars. The Athabasca Valles is home to sheet-like lava flows created about 200 Mya. Water flows in the grabens called the Cerberus Fossae occurred less than 20 Mya, indicating equally recent volcanic intrusions.[83] The Mars Reconnaissance Orbiter has captured images of avalanches.[84][85]
Soil
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, and they are necessary for growth of plants.[86] 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,[87][88] concentrations that are toxic to humans.[89][90]
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 theories include that they are dark underlying layers of soil revealed after avalanches of bright dust or dust devils.[91] Several other explanations have been put forward, including those that involve water or even the growth of organisms.[92][93]
Hydrology
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Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is less than 1% that of Earth's,[20] except at the lowest elevations for short periods.[57][94] The two polar ice caps appear to be made largely of water.[22][23] 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).[95] 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,[96][97] and at middle latitudes.[98] The Phoenix lander directly sampled water ice in shallow Martian soil on 31 July 2008.[99]
Landforms visible on Mars strongly suggest that liquid water has existed on the planet's surface. Huge linear swathes of scoured ground, known as outflow channels, cut across the surface in about 25 places. These are thought to be a record of erosion caused by the catastrophic release of water from subsurface aquifers, though some of these structures have been hypothesized to result from the action of glaciers or lava.[100][101] One of the larger examples, Ma'adim Vallis, is 700 kilometres (430 mi) long, much greater than the Grand Canyon, with a width of 20 kilometres (12 mi) and a depth of 2 kilometres (1.2 mi) in places. It is thought to have been carved by flowing water early in Mars's history.[102] The youngest of these channels are thought to have formed as recently as only a few million years ago.[103] Elsewhere, particularly on the oldest areas of the Martian surface, finer-scale, dendritic networks of valleys are spread across significant proportions of the landscape. Features of these valleys and their distribution strongly imply that they were carved by runoff resulting from precipitation in early Mars history. Subsurface water flow and groundwater sapping may play important subsidiary roles in some networks, but precipitation was probably the root cause of the incision in almost all cases.[104]
Along crater and canyon walls, there are thousands of features that appear similar to terrestrial gullies. The gullies tend to be in the highlands of the Southern Hemisphere and to face the Equator; all are poleward of 30° latitude. A number of authors have suggested that their formation process involves liquid water, probably from melting ice,[105][106] although others have argued for formation mechanisms involving carbon dioxide frost or the movement of dry dust.[107][108] No partially degraded gullies have formed by weathering and no superimposed impact craters have been observed, indicating that these are young features, possibly still active.[106] Other geological features, such as deltas and alluvial fans preserved in craters, are further evidence for warmer, wetter conditions at an interval or intervals in earlier Mars history.[109] Such conditions necessarily require the widespread presence of crater lakes across a large proportion of the surface, for which there is independent mineralogical, sedimentological and geomorphological evidence.[110] Further evidence that liquid water once existed on the surface of Mars comes from the detection of specific minerals such as hematite and goethite, both of which sometimes form in the presence of water.[111]
Observations and findings of water evidence
In 2004, Opportunity detected the mineral jarosite. This forms only in the presence of acidic water, which demonstrates that water once existed on Mars.[112][113] The Spirit rover found concentrated deposits of silica in 2007 that were also indicative of wet conditions in the past.[114] More recent evidence for liquid water comes from the finding of the mineral gypsum on the surface by NASA's Mars rover Opportunity in December 2011.[115][116] It is estimated that the amount of water in the upper mantle of Mars, represented by hydroxyl ions contained within Martian minerals, is equal to or greater than that of Earth at 50–300 parts per million of water, which is enough to cover the entire planet to a depth of 200–1,000 metres (660–3,280 ft).[117][118]
On 18 March 2013, NASA reported evidence from instruments on the Curiosity rover of mineral hydration, likely hydrated calcium sulfate, in several rock samples including the broken fragments of "Tintina" rock and "Sutton Inlier" rock as well as in veins and nodules in other rocks like "Knorr" rock and "Wernicke" rock.[119][120] Analysis using the rover's DAN instrument provided evidence of subsurface water, amounting to as much as 4% water content, down to a depth of 60 centimetres (24 in), during the rover's traverse from the Bradbury Landing site to the Yellowknife Bay area in the Glenelg terrain.[119] In September 2015, NASA announced that they had found strong evidence of hydrated brine flows in recurring slope lineae, based on spectrometer readings of the darkened areas of slopes.[121][122][123] These streaks flow downhill in Martian summer, when the temperature is above −23° Celsius, and freeze at lower temperatures.[124] These observations supported earlier hypotheses, based on timing of formation and their rate of growth, that these dark streaks resulted from water flowing just below the surface.[125] However, later work suggested that the lineae may be dry, granular flows instead, with at most a limited role for water in initiating the process.[126] A definitive conclusion about the presence, extent, and role of liquid water on the Martian surface remains elusive.[127][128]
Researchers suspect that much of the low northern plains of the planet were covered with an ocean hundreds of meters deep, though this remains controversial.[129] In March 2015, scientists stated that such an ocean might have been the size of Earth's Arctic Ocean. This finding was derived from the ratio of water to deuterium in the modern Martian atmosphere compared to that ratio on Earth. The amount of Martian deuterium is eight times the amount that exists on Earth, suggesting that ancient Mars had significantly higher levels of water. Results from the Curiosity rover had previously found a high ratio of deuterium in Gale Crater, though not significantly high enough to suggest the former presence of an ocean. Other scientists caution that these results have not been confirmed, and point out that Martian climate models have not yet shown that the planet was warm enough in the past to support bodies of liquid water.[130] Near the northern polar cap is the 81.4 kilometres (50.6 mi) wide Korolev Crater, which the Mars Express orbiter found to be filled with approximately 2,200 cubic kilometres (530 cu mi) of water ice.[131]
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.[132][133] During observations from 2018 through 2021, the ExoMars Trace Gas Orbiter spotted indications of water, probably subsurface ice, in the Valles Marineris canyon system.[134]
Polar caps
Mars has two permanent polar ice caps. During a pole's winter, it lies in continuous darkness, chilling the surface and causing the deposition of 25–30% of the atmosphere into slabs of CO2 ice (dry ice).[136] When the poles are again exposed to sunlight, the frozen CO2 sublimes. These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth-like frost and large cirrus clouds. Clouds of water-ice were photographed by the Opportunity rover in 2004.[137]
The caps at both poles consist primarily (70%) 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 by meters per year; this suggests that the permanent CO2 cover over the south pole water ice is degrading over time.[138] The northern polar cap has a diameter of about 1,000 kilometres (620 mi),[139] 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.[140] (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).[141] The total volume of ice in the south polar cap plus the adjacent layered deposits has been estimated at 1.6 million cubic km.[142] Both polar caps show spiral troughs, which recent analysis of SHARAD ice penetrating radar has shown are a result of katabatic winds that spiral due to the Coriolis effect.[143][144]
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 geyser-like eruptions of CO2 gas mixed with dark basaltic sand or dust. This process is rapid, observed happening in the space of a few days, weeks or months, a rate of change rather unusual in geology – especially for Mars. The gas rushing underneath a slab to the site of a geyser carves a spiderweb-like pattern of radial channels under the ice, the process being the inverted equivalent of an erosion network formed by water draining through a single plughole.[145][146]
Geography and names
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Although better remembered for mapping the Moon, Johann Heinrich Mädler and Wilhelm Beer were the first areographers. They began by establishing that most of Mars's surface features were permanent and by more precisely determining the planet's rotation period. In 1840, Mädler combined ten years of observations and drew the first map of Mars.[147]
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; small valleys are named for rivers.[148]
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).[149] 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.[150] The permanent northern polar ice cap is named Planum Boreum, whereas the southern cap is called Planum Australe.[151]
Mars's equator is defined by its rotation, but the location of its Prime Meridian was specified, as was Earth's (at Greenwich), by choice of an arbitrary point; Mädler and Beer selected a line for their first maps of Mars in 1830. After the spacecraft Mariner 9 provided extensive imagery of Mars in 1972, a small crater (later called Airy-0), located in the Sinus Meridiani ("Middle Bay" or "Meridian Bay"), was chosen by Merton Davies, Harold Masursky, and Gérard de Vaucouleurs for the definition of 0.0° longitude to coincide with the original selection.[152][153][154]
Because Mars has no oceans and hence no "sea level", a zero-elevation surface had to be selected as a reference level; this is called the areoid[155] of Mars, analogous to the terrestrial geoid.[156] Zero altitude was defined by the height at which there is 610.5 Pa (6.105 mbar) of atmospheric pressure.[157] This pressure corresponds to the triple point of water, and it is about 0.6% of the sea level surface pressure on Earth (0.006 atm).[158]
For mapping purposes, the United States Geological Survey divides the surface of Mars into thirty cartographic quadrangles, each named for a classical albedo feature it contains.[159]
Volcanoes
The shield volcano Olympus Mons (Mount Olympus) is an extinct volcano in the vast upland region Tharsis, which contains several other large volcanoes. The edifice is over 600 km (370 mi) wide.[160][161] Because the mountain is so large, with complex structure at its edges, allocating a 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),[161] 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),[162] 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).[163]
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.[164][165][166]
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.[167] 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.[168][169] There are other notable impact features, such as Argyre, which is around 1,800 kilometres (1,100 mi) in diameter,[170] and Isidis, which is around 1,500 kilometres (930 mi) in diameter.[171] 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.[172]
Martian craters can have a morphology that suggests the ground became wet after the meteor impacted.[173]
Tectonic sites
The large canyon, Valles Marineris (Latin for "Mariner Valleys", also known as Agathodaemon in the old canal maps[174]), has a length of 4,000 kilometres (2,500 mi) and a depth of up to 7 kilometres (4.3 mi). The length of Valles Marineris is equivalent to the length of Europe and extends across one-fifth the circumference of Mars. By comparison, the Grand Canyon on Earth is only 446 kilometres (277 mi) long and nearly 2 kilometres (1.2 mi) deep. Valles Marineris was formed due to the swelling of the Tharsis area, which caused the crust in the area of Valles Marineris to collapse. In 2012, it was proposed that Valles Marineris is not just a graben, but a plate boundary where 150 kilometres (93 mi) of transverse motion has occurred, making Mars a planet with possibly a two-tectonic plate arrangement.[175][176]
Holes
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.[177] The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters".[178] 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, it is possible that they 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.[179][180]
Atmosphere
Mars lost its magnetosphere 4 billion years ago,[181] possibly because of numerous asteroid strikes,[182] so the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density by stripping away atoms from the outer layer.[183] Both Mars Global Surveyor and Mars Express have detected ionised atmospheric particles trailing off into space behind Mars,[181][184] and this atmospheric loss is being studied by the MAVEN orbiter. Compared to Earth, the atmosphere of Mars is quite rarefied. Atmospheric pressure on the surface today ranges from a low of 30 Pa (0.0044 psi) on Olympus Mons to over 1,155 Pa (0.1675 psi) in Hellas Planitia, with a mean pressure at the surface level of 600 Pa (0.087 psi).[185] The highest atmospheric density on Mars is equal to that found 35 kilometres (22 mi)[186] above Earth's surface. The resulting mean surface pressure is only 0.6% of that of Earth 101.3 kPa (14.69 psi). The scale height of the atmosphere is about 10.8 kilometres (6.7 mi),[187] which is higher than Earth's 6 kilometres (3.7 mi), because the surface gravity of Mars is only about 38% of Earth's.[188]
The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water.[1][189][183] The atmosphere is quite dusty, containing particulates about 1.5 µm in diameter which give the Martian sky a tawny color when seen from the surface.[190] It may take on a pink hue due to iron oxide particles suspended in it.[25] The concentration of methane in the Martian atmosphere fluctuates from about 0.24 ppb during the northern winter to about 0.65 ppb during the summer.[191] Estimates of its lifetime range from 0.6 to 4 years,[192][193] 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,[194] or by Martian life.[195]
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.[197][198]
Auroras have been detected on Mars.[199][200][201] Because Mars lacks a global magnetic field, the types and distribution of auroras there differ from those on Earth;[202] rather than being mostly restricted to polar regions, a Martian aurora can encompass the planet.[203] 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.[203][204]
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, and the low thermal inertia of Martian soil.[205] The planet is 1.52 times as far from the Sun as Earth, resulting in just 43% of the amount of sunlight.[206][207]
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).[208]
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 the global temperature.[209]
Orbit and rotation
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.[211] A Martian year is equal to 1.8809 Earth years, or 1 year, 320 days, and 18.2 hours.[1]
The axial tilt of Mars is 25.19° relative to its orbital plane, which is similar to the axial tilt of Earth.[1] As a result, Mars has seasons like Earth, though on Mars they are nearly twice as long because its orbital period is that much longer. In the present day epoch, the orientation of the north pole of Mars is close to the star Deneb.[18]
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.[212] Mars's cycle of eccentricity is 96,000 Earth years compared to Earth's cycle of 100,000 years.[213]
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