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24-637: Mearcair Planitia is one of four named plains that surround the Caloris basin (with Odin Planitia , Stilbon Planitia , and Tir Planitia ). All of these plains are classified as smooth , as opposed to intracrater plains which have rougher topography. They also contain areas where kilometer-scale knobs protrude above the plains, and these areas are called the Odin Formation . The Odin Formation

48-430: A mascon , is centered on Caloris Planitia. Most large impact basins on the moon, such as Mare Imbrium and Mare Crisium , are also the site of mascons. The giant impact believed to have formed Caloris may have had global consequences for the planet. At the exact antipode of the basin is a large area of hilly, grooved terrain, with few small impact craters that are known as chaotic terrain (also "weird terrain"). It

72-521: A much lower orbit than initially planned. It was only in 2001 that the mascons were mapped and the frozen orbits were discovered. The Luna 10 orbiter was the first artificial object to orbit the Moon, and it returned tracking data indicating that the lunar gravitational field caused larger than expected perturbations, presumably due to "roughness" of the lunar gravitational field. The Lunar mascons were discovered by Paul M. Muller and William L. Sjogren of

96-747: A negative anomaly, such as the "mascon basins" on the Moon . The Moon is the most gravitationally "lumpy" major body known in the Solar System. Its largest mascons can cause a plumb bob to hang about a third of a degree off vertical, pointing toward the mascon, and increase the force of gravity by one-half percent. Typical examples of mascon basins on the Moon are the Imbrium , Serenitatis , Crisium and Orientale impact basins, all of which exhibit significant topographic depressions and positive gravitational anomalies. Examples of mascon basins on Mars are

120-556: A ring of mountains approximately 2 km (1.2 mi) tall. Caloris was discovered on images taken by the Mariner 10 probe in 1974. Its name was suggested by Brian O'Leary , astronaut and member of the Mariner 10 imagery team. It was situated on the terminator —the line dividing the daytime and nighttime hemispheres—at the time the probe passed by, and so half of the crater could not be imaged. Later, on January 15, 2008, one of

144-444: A significant source of sodium and potassium, indicating that the fractures created by the impact facilitate the release of gases from within the planet. The unusual terrain is also a source of these gases. Mass concentration (astronomy) In astronomy , astrophysics and geophysics , a mass concentration (or mascon ) is a region of a planet's or moon's crust that contains a large positive gravity anomaly . In general,

168-529: Is a plain within a large impact basin on Mercury , informally named Caloris , about 1,550 km (960 mi) in diameter . It is one of the largest impact basins in the Solar System . "Calor" is Latin for " heat " and the basin is so-named because the Sun is almost directly overhead every second time Mercury passes perihelion . The crater, discovered in 1974, is surrounded by the Caloris Montes ,

192-525: Is a series of geologic formations thought to have been produced by the basin's ejecta, collectively called the Caloris Group . Mercury has a very tenuous and transient atmosphere, containing small amounts of hydrogen and helium captured from the solar wind , as well as heavier elements such as sodium and potassium . These are thought to originate within the planet, being "out-gassed" from beneath its crust. The Caloris basin has been found to be

216-501: Is filled by lava plains, similar to the maria of the Moon . These plains are superposed by explosive vents associated with pyroclastic material. Outside the walls, material ejected in the impact which created the basin extends for 1,000 km (620 mi), and concentric rings surround the crater. In the center of the basin is a region containing numerous radial troughs that appear to be extensional faults , with an unrelated 40 km (25 mi) crater, Apollodorus , located near

240-551: Is interpreted as a mixture of impact melt and blocky basin ejecta, formed by the Caloris impact event. The large crater Kerouac lies near the center of Mearcair Planitia. This article about the planet Mercury is a stub . You can help Misplaced Pages by expanding it . This article about an extraterrestrial geological feature is a stub . You can help Misplaced Pages by expanding it . Caloris Planitia Caloris Planitia / k ə ˈ l ɔːr ɪ s p l ə ˈ n ɪ ʃ ( i ) ə /

264-460: Is that these anomalies are due to dense mare basaltic lavas , which might reach up to 6 kilometers in thickness for the Moon. While these lavas certainly contribute to the observed gravitational anomalies, uplift of the crust-mantle interface is also required to account for their magnitude. Indeed, some mascon basins on the Moon do not appear to be associated with any signs of volcanic activity. Theoretical considerations in either case indicate that all

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288-409: Is thought by some to have been created as seismic waves from the impact converged on the opposite side of the planet. Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin's antipode. This hypothetical impact is also believed to have triggered volcanic activity on Mercury, resulting in the formation of smooth plains. Surrounding Caloris

312-477: The Argyre , Isidis , and Utopia basins. Theoretical considerations imply that a topographic low in isostatic equilibrium would exhibit a slight negative gravitational anomaly. Thus, the positive gravitational anomalies associated with these impact basins indicate that some form of positive density anomaly must exist within the crust or upper mantle that is currently supported by the lithosphere . One possibility

336-436: The Moon has only four " frozen orbit " inclination zones where a lunar satellite can stay in a low orbit indefinitely. Lunar subsatellites were released on two of the last three Apollo crewed lunar landing missions in 1971 and 1972; the subsatellite PFS-2 released from Apollo 16 was expected to stay in orbit for one and a half years, but lasted only 35 days before crashing into the lunar surface since it had to be deployed in

360-469: The Moon's geological development and explain the current lunar internal structures. At that time, one of NASA's highest priority " tiger team " projects was to explain why the Lunar Orbiter spacecraft being used to test the accuracy of Project Apollo navigation were experiencing errors in predicted position of ten times the mission specification (2 kilometers instead of 200 meters). This meant that

384-484: The NASA Jet Propulsion Laboratory (JPL) in 1968 from a new analytic method applied to the highly precise navigation data from the uncrewed pre-Apollo Lunar Orbiter spacecraft. This discovery observed the consistent 1:1 correlation between very large positive gravity anomalies and depressed circular basins on the Moon. This fact places key limits on models attempting to follow the history of

408-436: The center of the pattern. The exact cause of this pattern of troughs is not currently known. The feature is named Pantheon Fossae . The impacting body is estimated to have been at least 100 km (62 miles) in diameter. Bodies in the inner Solar System experienced a heavy bombardment of large rocky bodies in the first billion years or so of the Solar System. The impact that created Caloris must have occurred after most of

432-463: The first photos of the planet taken by the MESSENGER probe revealed the crater in its entirety. The basin was initially estimated to be about 810 mi (1,300 km) in diameter, though this was increased to 960 mi (1,540 km) based on subsequent images taken by MESSENGER . It is ringed by mountains up to 2 km (1.2 mi) high. Inside the crater walls, the floor of the crater

456-634: The heavy bombardment had finished, because fewer impact craters are seen on its floor than exist on comparably-sized regions outside the crater. Similar impact basins on the Moon such as the Mare Imbrium and Mare Orientale are believed to have formed at about the same time, possibly indicating that there was a 'spike' of large impacts towards the end of the heavy bombardment phase of the early Solar System. Based on MESSENGER ' s photographs, Caloris' age has been determined to be between 3.8 and 3.9 billion years. A gravitational high, also known as

480-448: The lunar mascons are super-isostatic (that is, supported above their isostatic positions). The huge expanse of mare basaltic volcanism associated with Oceanus Procellarum does not possess a positive gravitational anomaly. Since their identification in 1968 by Doppler tracking of the five Lunar Orbiter spacecraft, the origin of the mascons beneath the surface of the Moon has been subject to much debate, but they are now regarded as being

504-605: The predicted landing areas were 100 times as large as those being carefully defined for reasons of safety. Lunar orbital effects principally resulting from the strong gravitational perturbations of the mascons were ultimately revealed as the cause. William Wollenhaupt and Emil Schiesser of the NASA Manned Spacecraft Center in Houston then worked out the "fix" that was first applied to Apollo 12 and permitted its landing within 163 m (535 ft) of

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528-500: The result of the impact of asteroids during the Late Heavy Bombardment . Lunar mascons alter the local gravity above and around them sufficiently that low and uncorrected lunar orbits of satellites around the Moon are unstable on a timescale of months or years. The small perturbations in the orbits accumulate and eventually distort the orbit enough for the satellite to impact the surface. Because of its mascons,

552-503: The target, the previously landed Surveyor 3 spacecraft. In May 2013 a NASA study was published with results from the twin GRAIL probes, that mapped mass concentrations on Earth's Moon. China's Chang’e 5T1 mission also mapped Moon's mascons. Mascons on Earth are often measured by means of satellite gravimetry , such as the GRACE satellites . Mascons are often reported in terms of

576-423: The word "mascon" can be used as a noun to refer to an excess distribution of mass on or beneath the surface of an astronomical body (compared to some suitable average), such as is found around Hawaii on Earth. However, this term is most often used to describe a geologic structure that has a positive gravitational anomaly associated with a feature (e.g. depressed basin) that might otherwise have been expected to have

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