Mars
From Wikipedia, the free encyclopedia
This article is about the planet. For other uses, see
Mars (disambiguation).
Mars is the fourth
planet from the
Sun and the second smallest planet in the
Solar System, after
Mercury. Named after the
Roman god of war, it is often referred to as the "Red Planet" because the
iron oxide prevalent on its surface gives it a
reddish appearance.
[15] Mars is a
terrestrial planet with a thin
atmosphere, having surface features reminiscent both of the
impact craters of the
Moon and the volcanoes, valleys, deserts, and
polar ice caps of
Earth. The
rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of
Olympus Mons, the largest
volcano and second-highest known mountain in the Solar System, and of
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.
[16][17] Mars has two
moons,
Phobos and
Deimos, which are small and irregularly shaped. These may be captured
asteroids,
[18][19] similar to
5261 Eureka, a
Mars trojan.
Until the first successful Mars flyby in 1965 by
Mariner 4, many speculated about the presence of liquid water on the planet's surface. This was based on observed periodic variations in light and dark patches, particularly in the polar
latitudes, which appeared to be seas and continents; long, dark
striations were interpreted by some as irrigation channels for liquid water. These straight line features were later explained as
optical illusions, though geological evidence gathered by unmanned missions suggests that Mars once had large-scale water coverage on its surface at some earlier stage of its life.
[20] In 2005, radar data revealed the presence of large quantities of water ice at the poles
[21] and at mid-latitudes.
[22][23] The Mars rover
Spirit sampled chemical compounds containing water molecules in March 2007. The
Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.
[24]
Mars is host to seven functioning
spacecraft: five in orbit—
2001 Mars Odyssey,
Mars Express,
Mars Reconnaissance Orbiter,
MAVEN and
Mars Orbiter Mission—and two on the surface—
Mars Exploration Rover Opportunity and the
Mars Science Laboratory Curiosity. Defunct spacecraft on the surface include MER-A
Spirit and several other inert landers and rovers such as the
Phoenix lander, which completed its mission in 2008. Observations by the
Mars Reconnaissance Orbiter have revealed possible flowing water during the warmest months on Mars.
[25] In 2013, NASA's
Curiosity rover discovered that Mars's soil contains between 1.5% and 3% water by mass (about two pints of water per cubic foot or 33 liters per cubic meter, albeit attached to other compounds and thus not freely accessible).
[26]
Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its
apparent magnitude reaches −2.91,
[6] which is surpassed only by
Jupiter,
Venus, the Moon, and the Sun. Optical ground-based telescopes are typically limited to resolving features about 300 kilometers (190 mi) across when Earth and Mars are closest because of Earth's atmosphere.
[27]
Physical characteristics
Mars is approximately half the diameter of Earth, and its surface area is only slightly less than the total area of Earth's dry land.
[6] Mars is less dense than Earth, having about 15% of Earth's volume and 11% of Earth's
mass. Although Mars is larger and more massive than
Mercury, Mercury has a higher density. This results in the two planets having a nearly identical gravitational pull at the surface—that of Mars is stronger by less than 1%. The red-orange appearance of the Martian surface is caused by
iron(III) oxide, more commonly known as
hematite, or rust.
[28] It can also look like butterscotch,
[29] and other common surface colors include golden, brown, tan, and greenish, depending on the
minerals present.
[29]
Internal structure
Like Earth, Mars has
differentiated into a dense metallic core overlaid by less dense materials.
[30] Current models of its interior imply a core region about 1,794 ± 65 kilometers (1,115 ± 40 mi) in radius, consisting primarily of
iron and nickel with about 16–17%
sulfur.
[31] This
iron(II) sulfide core is thought to be twice as rich in lighter elements than Earth's core.
[32] The core is surrounded by a silicate
mantle that formed many of the tectonic and volcanic features on the planet, but it now appears to be dormant. Besides silicon and oxygen, the most abundant elements in the Martian crust are iron,
magnesium,
aluminum,
calcium, and
potassium. The average thickness of the planet's crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi).
[32] Earth's crust, averaging 40 km (25 mi), is only one third as thick as Mars's crust, relative to the sizes of the two planets. The
InSight lander planned for 2016 will use a
seismometer to better constrain the models of the interior.
[33]
Surface geology
Main article:
Geology of Mars
Mars is a
terrestrial planet that consists of minerals containing
silicon and
oxygen,
metals, and other elements that typically make up
rock. The surface of Mars is primarily composed of
tholeiitic basalt,
[34] 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 show 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 also been found.
[35] Much of the surface is deeply covered by finely grained
iron(III) oxide dust.
[36][37]
Although Mars has no evidence of a current structured global
magnetic field,
[41] observations show that parts of the planet's crust have been magnetized, and that alternating polarity reversals of its dipole field have occurred in the past. This
paleomagnetism of magnetically susceptible minerals has properties that are similar to the
alternating bands found on the ocean floors of Earth. One theory, published in 1999 and re-examined in October 2005 (with the help of the
Mars Global Surveyor), is that these bands demonstrate
plate tectonics on Mars four
billion years ago, before the planetary
dynamo ceased to function and the planet's magnetic field faded away.
[42]
During the
Solar System's formation, Mars was created as the result of a
stochastic process of run-away accretion out of 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 sulphur, are much more common on Mars than Earth; these elements were probably removed from areas closer to the Sun by the young star's energetic
solar wind.
[43]
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,
[44][45][46] 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 km (6,600 by 5,300 mi), or roughly four times larger than the Moon's
South Pole – Aitken basin, the largest impact basin yet discovered.
[16][17] 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.
[47][48]
Artist's impression shows how Mars may have looked about four billion years ago.
[49]
The geological history of Mars can be split into many periods, but the following are the three primary periods:
[50][51]
- Noachian period (named after Noachis Terra): Formation of the oldest extant surfaces of Mars, 4.5 billion years ago 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.
- Hesperian period (named after Hesperia Planum): 3.5 billion years ago to 2.9–3.3 billion years ago. The Hesperian period is marked by the formation of extensive lava plains.
- Amazonian period (named after Amazonis Planitia): 2.9–3.3 billion years ago to present. Amazonian regions have few meteorite impact craters, but are otherwise quite varied. Olympus Mons formed during this period, along with lava flows elsewhere on Mars.
Some geological activity is still taking place on Mars. The
Athabasca Valles is home to sheet-like lava flows up to about 200
Mya. Water flows in the grabens called the
Cerberus Fossae occurred less than 20 Mya, indicating equally recent volcanic intrusions.
[52] On February 19, 2008, images from the
Mars Reconnaissance Orbiter showed evidence of an avalanche from a 700 m high cliff.
[53]
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 gardens on Earth, and they are necessary for growth of plants.
[54] 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.
[55][56][57][58]
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. Sometimes, the streaks start in a tiny area which then spread out for hundreds of metres. They have also 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.
[59] Several explanations have been put forward, some of which involve water or even the growth of organisms.
[60][61]
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