Mars
Fourth planet from the Sun / From Wikipedia, the free encyclopedia
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Mars is the fourth planet from the Sun. The surface of Mars is orange-red because it is covered in iron(III) oxide dust, giving it the nickname "the Red Planet".[21][22] Mars is among the brightest objects in Earth's sky and its high-contrast albedo features have made it a common subject for telescope viewing. It is classified as a terrestrial planet and is the second smallest of the Solar System's planets with a diameter of 6,779 km (4,212 mi). In terms of orbital motion, a Martian solar day (sol) is equal to 24.5 hours and a Martian solar year is equal to 1.88 Earth years (687 Earth days). Mars has two natural satellites that are small and irregular in shape: Phobos and Deimos.
Designations | |||||||||||||
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Adjectives | Martian | ||||||||||||
Symbol | |||||||||||||
Orbital characteristics[1] | |||||||||||||
Epoch J2000 | |||||||||||||
Aphelion | 249261000 km (154884000 mi; 1.66621 AU)[2] | ||||||||||||
Perihelion | 206650000 km (128410000 mi; 1.3814 AU)[2] | ||||||||||||
227939366 km (141634956 mi; 1.52368055 AU)[3] | |||||||||||||
Eccentricity | 0.0934[2] | ||||||||||||
686.980 d (1.88085 yr; 668.5991 sols)[2] | |||||||||||||
779.94 d (2.1354 yr)[3] | |||||||||||||
Average orbital speed | 24.07 km/s (86700 km/h; 53800 mph)[2] | ||||||||||||
19.412°[2] | |||||||||||||
Inclination |
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49.57854°[2] | |||||||||||||
2022-Jun-21[5] | |||||||||||||
286.5°[3] | |||||||||||||
Satellites | 2 (Phobos and Deimos) | ||||||||||||
Physical characteristics | |||||||||||||
Mean radius | 3389.5 ± 0.2 km[lower-alpha 2] [6] (2106.1 ± 0.1 mi) | ||||||||||||
Equatorial radius | 3396.2 ± 0.1 km[lower-alpha 2] [6] (2110.3 ± 0.1 mi; 0.533 Earths) | ||||||||||||
Polar radius | 3376.2 ± 0.1 km[lower-alpha 2] [6] (2097.9 ± 0.1 mi; 0.531 Earths) | ||||||||||||
Flattening | 0.00589±0.00015[5][6] | ||||||||||||
1.4437×108 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)[2] | ||||||||||||
25.19° to its orbital plane[2] | |||||||||||||
North pole right ascension | 317.68143°[6] 21h 10m 44s | ||||||||||||
North pole declination | 52.88650°[6] | ||||||||||||
Albedo | |||||||||||||
Temperature | 209 K (−64 °C) (blackbody temperature)[14] | ||||||||||||
| |||||||||||||
Surface absorbed dose rate | 8.8 μGy/h[17] | ||||||||||||
Surface equivalent dose rate | 27 μSv/h[17] | ||||||||||||
−2.94 to +1.86[18] | |||||||||||||
−1.5[19] | |||||||||||||
3.5–25.1″[2] | |||||||||||||
Atmosphere[2][20] | |||||||||||||
Surface pressure | 0.636 (0.4–0.87) kPa 0.00628 atm | ||||||||||||
Composition by volume |
| ||||||||||||
When viewed closely, the relatively flat plains in northern parts of Mars strongly contrast with the cratered terrain in southern highlands – this terrain observation is known as the Martian dichotomy. Mars hosts many enormous extinct volcanos (such as Olympus Mons, 21.9 km or 13.6 mi tall) and one of the largest canyons in the Solar System (Valles Marineris, 4,000 km or 2,500 mi long). Geologically, the planet is fairly active with marsquakes trembling underneath the ground, dust devils sweeping across the landscape, and cirrus clouds. Carbon dioxide is substantially present in Mars's polar ice caps and thin atmosphere. During a year, there are large surface temperature swings on the surface between −78.5 °C (−109.3 °F) to 5.7 °C (42.3 °F)[lower-alpha 3] similar to Earth's seasons, as both planets have significant axial tilt.
Mars was formed approximately 4.5 billion years ago. During the Noachian period (4.5 to 3.5 billion years ago), Mars's surface was marked by meteor impacts, valley formation, erosion, and the possible presence of water oceans. The Hesperian period (3.5 to 3.3–2.9 billion years ago) was dominated by widespread volcanic activity and flooding that carved immense outflow channels. The Amazonian period, which continues to the present, was marked by the wind as a dominant influence on geological processes. Due to Mars's geological history, the possibility of past or present life on Mars remains of great scientific interest.
Since the late 20th century, Mars has been explored by uncrewed spacecraft and rovers, with the first flyby by the Mariner 4 probe in 1965, the first Mars orbiter by the Mars 2 probe in 1971, and the first landing by the Viking 1 probe in 1976. As of 2023, there are at least 11 active probes orbiting Mars or at the Martian surface. Mars is an attractive target for future human exploration missions, though in the 2020s no such mission is planned.
Scientists have theorized 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 on Earth; these elements were probably pushed outward by the young Sun's energetic solar wind.[23]
After the formation of the planets, the inner Solar System may have been subjected to the so-called Late Heavy Bombardment. About 60% of the surface of Mars shows a record of impacts from that era,[24][25][26] whereas much of the remaining surface is probably underlain by immense impact basins caused by those events. However, more recent modelling has disputed the existence of the Late Heavy Bombardment.[27] 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, which would be the largest impact basin yet discovered if confirmed.[28] It has been hypothesized that the basin was formed when 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.[29][30]
A 2023 study shows evidence, based on the orbital inclination of Deimos (a small moon of Mars), that Mars may once had a ring system 3.5 billion years to 4 billion years ago.[31] This ring system may have been formed from a moon, 20 times more massive than Phobos, orbiting Mars billions of years ago; and Phobos would be a remnant of that ring.[32][33]
The geological history of Mars can be split into many periods, but the following are the three primary periods:[34][35]
- 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.[36]
- 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.[36]
- 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.[36]
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.[37] The Mars Reconnaissance Orbiter has captured images of avalanches.[38][39]
Mars is approximately half the diameter of Earth, with a surface area only slightly less than the total area of Earth's dry land.[2] 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. Mars is the only presently known example of a desert planet, a rocky planet with a surface akin to that of Earth's hot deserts. The red-orange appearance of the Martian surface is caused by ferric oxide, or rust.[40] It can look like butterscotch;[41] other common surface colors include golden, brown, tan, and greenish, depending on the minerals present.[41]
- Comparison: Earth and Mars
- Animation (00:40) showing major features of Mars
- Video (01:28) showing how three NASA orbiters mapped the gravity field of Mars
Internal structure
Like Earth, Mars is differentiated into a dense metallic core overlaid by less dense rocky layers.[46][47] The outermost layer is the crust, which is on average about 42–56 kilometres (26–35 mi) thick,[42] with a minimum thickness of 6 kilometres (3.7 mi) in Isidis Planitia, and a maximum thickness of 117 kilometres (73 mi) in the southern Tharsis plateau.[48] For comparison, Earth's crust averages 27.3 ± 4.8 km in thickness.[49] The most abundant elements in the Martian crust are silicon, oxygen, iron, magnesium, aluminium, calcium, and potassium. Mars is confirmed to be seismically active;[50] in 2019 it was reported that InSight had detected and recorded over 450 marsquakes and related events.[51][52]
Beneath the crust is a silicate mantle responsible for many of the tectonic and volcanic features on the planet's surface. The upper Martian mantle is a low-velocity zone, where the velocity of seismic waves is lower than surrounding depth intervals. The mantle appears to be rigid down to the depth of about 500 km, giving mars a very thick lithosphere compared to Earth. Below this the mantle gradually becomes more ductile, and the seismic wave velocity starts to grow again.[43] The Martian mantle does not appear to have a thermally insulating layer analogous to Earth's lower mantle; instead, below 1050 km in depth, it becomes mineralogically similar to Earth's transition zone.[44] At the bottom of the mantle lies a basal liquid silicate layer approximately 150–180 km thick.[53][45]
Mars's iron and nickel core is completely molten, with no solid inner core.[54][55] It is around half of Mars's radius, approximately 1650–1675 km, and is enriched in light elements such as sulfur, oxygen, carbon, and hydrogen.[56][57]
Surface geology
Mars is a terrestrial planet with a surface that 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,[58] 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.[59] Much of the surface is deeply covered by finely grained iron(III) oxide dust.[60]
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 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.[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 theories 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]
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.[75]
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 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.[80][81][82]
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[83] of Mars, analogous to the terrestrial geoid.[84] Zero altitude was defined by the height at which there is 610.5 Pa (6.105 mbar) of atmospheric pressure.[85] 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).[86]
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.[87] In April 2023, The New York Times reported an updated global map of Mars based on images from the Hope spacecraft.[88] A related, but much more detailed, global Mars map was released by NASA on 16 April 2023.[89]
Volcanoes
The vast upland region Tharsis contains several extinct volcanoes, which include the shield volcano Olympus Mons (Mount Olympus). 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, Valles Marineris (Latin for "Mariner Valleys", also known as Agathodaemon in the old canal maps[104]), 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.[105][106]
Holes and caves
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]
Mars lost its magnetosphere 4 billion years ago,[111] possibly because of numerous asteroid strikes,[112] so the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density by stripping away atoms from the outer layer.[113] Both Mars Global Surveyor and Mars Express have detected ionised atmospheric particles trailing off into space behind Mars,[111][114] 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).[115] The highest atmospheric density on Mars is equal to that found 35 kilometres (22 mi)[116] above Earth's surface. The resulting mean surface pressure is only 0.6% of Earth's 101.3 kPa (14.69 psi). The scale height of the atmosphere is about 10.8 kilometres (6.7 mi),[117] which is higher than Earth's 6 kilometres (3.7 mi), because the surface gravity of Mars is only about 38% of Earth's.[118]
The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water.[2][119][113] 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.[120] It may take on a pink hue due to iron oxide particles suspended in it.[21] 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.[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]