Mass concentration (astronomy)

Last updated March 01, 2024

Topography (top) and corresponding gravity (bottom) signal of Mare Smythii on the Moon containing a significant mascon. MareSmithiiTG.jpg — Topography (top) and corresponding gravity (bottom) signal of Mare Smythii on the Moon containing a significant mascon.

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, 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.^[1] 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 a negative anomaly, such as the "mascon basins" on the Moon.

Lunar mascons

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.^[2]^[3]

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 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 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 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.

Origin of lunar mascons

Since their identification in 1968 by Doppler tracking of the five Lunar Orbiter spacecraft,^[4] the origin of the mascons beneath the surface of the Moon has been subject to much debate, but they are now regarded as being the result of the impact of asteroids during the Late Heavy Bombardment.^[5]

Effect of lunar mascons on satellite orbits

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, 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 a much lower orbit than initially planned. It was only in 2001 that the mascons were mapped and the frozen orbits were discovered.^[2]

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.^[6] The Lunar mascons were discovered by Paul M. Muller and William L. Sjogren of the NASA Jet Propulsion Laboratory (JPL) in 1968^[7] 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 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 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"^[8]^[9]^[10] that was first applied to Apollo 12 and permitted its landing within 163 m (535 ft) of the target, the previously-landed Surveyor 3 spacecraft.^[11]

Mapping

In May 2013 a NASA study was published with results from the twin GRAIL probes, that mapped mass concentrations on Earth's Moon.^[12]

China's Chang’e 5T1 mission also mapped Moon's mascons. ^[13]

Earth's mascons

Mascons on Earth are often measured by means of satellite gravimetry, such as the GRACE satellites.^[14]^[15] Mascons are often reported in terms of a derived physical quantity called "equivalent water thickness", "equivalent water height", or "water equivalent height", obtained dividing the surface mass density redistribution by the density of water.^[16]^[17]

Related Research Articles

A trans-lunar injection (TLI) is a propulsive maneuver, which is used to send a spacecraft to the Moon. Typical lunar transfer trajectories approximate Hohmann transfers, although low-energy transfers have also been used in some cases, as with the Hiten probe. For short duration missions without significant perturbations from sources outside the Earth-Moon system, a fast Hohmann transfer is typically more practical.

Mare Imbrium is a vast lava plain within the Imbrium Basin on the Moon and is one of the larger craters in the Solar System. The Imbrium Basin formed from the collision of a proto-planet during the Late Heavy Bombardment. Basaltic lava later flooded the giant crater to form the flat volcanic plain seen today. The basin's age has been estimated using uranium–lead dating methods to approximately 3.9 billion years ago, and the diameter of the impactor has been estimated to be 250 ± 25 km. The Moon's maria have fewer features than other areas of the Moon because molten lava pooled in the craters and formed a relatively smooth surface. Mare Imbrium is not as flat as it would have originally been when it first formed as a result of later events that have altered its surface.

The Lunar Orbiter program was a series of five uncrewed lunar orbiter missions launched by the United States from 1966 through 1967. Intended to help select Apollo landing sites by mapping the Moon's surface, they provided the first photographs from lunar orbit and photographed both the Moon and Earth.

Mare Orientale is a lunar mare. It is located on the western border of the near side and far side of the Moon, and is difficult to see from an Earthbound perspective. Images from spacecraft have revealed it to be one of the most striking large scale lunar features, resembling a target ring bullseye.

Mare Humorum is a lunar mare. The impact basin it is located in is 425 kilometers (264 mi) across.

Mare Crisium is a lunar mare located in the Moon's Crisium basin, just northeast of Mare Tranquillitatis. Mare Crisium is a basin of Nectarian age.

Mare Tranquillitatis is a lunar mare that sits within the Tranquillitatis basin on the Moon. It is the first location on another celestial body to be visited by humans.

Mare Fecunditatis is a lunar mare in the eastern half of the visible Moon. The mare has a maximum diameter of 840 km.

Mare Moscoviense is a lunar mare that sits in the Moscoviense basin. It is one of the very few maria on the far side of the Moon. Like Mare Marginis, this mare appears to be fairly thin. However, it is clearly centered within a large impact basin. It is also much lower than either the outer basin floor or the farside highlands.

Mare Nectaris is a small lunar mare or sea located south of Mare Tranquillitatis southwest of Mare Fecunditatis, on the near side of the Moon. Montes Pyrenaeus borders the mare to the east and Sinus Asperitatis fuses to its northwestern edge. It is 84,000 square kilometers in size.

Mare Serenitatis is a lunar mare located to the east of Mare Imbrium on the Moon. Its diameter is 674 km (419 mi).

The Gravity Recovery and Climate Experiment (GRACE) was a joint mission of NASA and the German Aerospace Center (DLR). Twin satellites took detailed measurements of Earth's gravity field anomalies from its launch in March 2002 to the end of its science mission in October 2017. The two satellites were sometimes called Tom and Jerry, a nod to the famous cartoon. The GRACE Follow-On (GRACE-FO) is a continuation of the mission on near-identical hardware, launched in May 2018.

Hertzsprung is an enormous lunar impact crater, or impact basin, that is located on the far side of the Moon, beyond the western limb. In dimension, this formation is larger than several of the lunar mare areas on the near side. It lies in the northwestern fringe of the blast radius of the Mare Orientale impact basin. Nearby craters of note include Michelson across the northeast rim, Vavilov across the western rim, and Lucretius to the southeast.

In astronomy and spaceflight, a lunar orbit is an orbit of an object around Earth's Moon. In general these orbits are not circular. When farthest from the Moon a spacecraft is said to be at apolune, apocynthion, or aposelene. When closest to the Moon it is said to be at perilune, pericynthion, or periselene. These derive from names or epithets of the moon goddess.

The acceleration due to gravity on the surface of the Moon is approximately 1.625 m/s², about 16.6% that on Earth's surface or 0.166 $ɡ$ . Over the entire surface, the variation in gravitational acceleration is about 0.0253 m/s². Because weight is directly dependent upon gravitational acceleration, things on the Moon will weigh only 16.6% of what they weigh on the Earth.

<span class="mw-page-title-main">GRAIL</span> 2011–12 NASA mission to study the Moons geology

The Gravity Recovery and Interior Laboratory (GRAIL) was an American lunar science mission in NASA's Discovery Program which used high-quality gravitational field mapping of the Moon to determine its interior structure. The two small spacecraft GRAIL A (Ebb) and GRAIL B (Flow) were launched on 10 September 2011 aboard a single launch vehicle: the most-powerful configuration of a Delta II, the 7920H-10. GRAIL A separated from the rocket about nine minutes after launch, GRAIL B followed about eight minutes later. They arrived at their orbits around the Moon 25 hours apart. The first probe entered orbit on 31 December 2011 and the second followed on 1 January 2012. The two spacecraft impacted the Lunar surface on December 17, 2012.

The Freundlich-Sharonov Basin is a Pre-Nectarian impact basin on the far side of the Moon. It is named after the younger craters Freundlich near the northwest margin and Sharonov near the southwest margin. It lies east of Mare Moscoviense basin and northwest of Korolev basin.

The Schiller-Zucchius Basin is a Pre-Nectarian impact basin on the near side of the Moon. It is named after the elongated crater Schiller at the northeast margin and fresh crater Zucchius near the southwest margin. This basin has received the unofficial designation 'Schiller Annular Plain' among lunar observers.

The Coulomb-Sarton Basin is a Pre-Nectarian impact basin on the far side of the Moon. It is named after the crater Coulomb northeast of the center of the basin and the smaller crater Sarton just south of the center. The basin is not obvious on the lunar surface. There are only small fragments of inner rings and a rim, and the most indicative topographic feature is a smooth, low plain at the center.

The gravity of Mars is a natural phenomenon, due to the law of gravity, or gravitation, by which all things with mass around the planet Mars are brought towards it. It is weaker than Earth's gravity due to the planet's smaller mass. The average gravitational acceleration on Mars is 3.72076 ms⁻² and it varies.

References

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↑ Jennifer Ross-Nazzal (7 December 2006). "NASA JOHNSON SPACE CENTER ORAL HISTORY PROJECT Oral History 2 Transcript" (PDF). NASA Johnson Space Center. Retrieved 12 November 2015. Somewhere about this time Wilbur R. Wollenhaupt, who went by Bill, joined our group. He had extensive background in ground-based navigation at JPL. He was pretty familiar with the JPL Deep Space Network (DSN) Trackers after which the Apollo trackers were patterned.
↑ Malcolm Johnston; Howard Tindall (31 May 1996). "Tindallgrams" (PDF). Collect Space.com. Retrieved 12 November 2015. If this determination, using the LM data, disagrees substantially with the other data sources, we must consider the possibility that it's due to gravity anomalies. The sort of differences we are willing to tolerate is 0.3° in longitude, which is more or less equivalent to 0.3° pitch misalignment in the platform. True alignment errors in excess of that could present ascent guidance problems. Since 0.3° is equivalent of about five miles, you'd expect the crew's estimate of position could probably be useful in determining the true situation. All they'd have to do is tell us they are short or over-shot the target point a great deal.
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↑ Chow, Denise (30 May 2013). "Mystery of Moon's Lumpy Gravity Explained". Space.com . SPACE.com. Retrieved 31 May 2013.
↑ Yan, Jianguo; Liu, Shanhong; Xiao, Chi; Ye, Mao; Cao, Jianfeng; Harada, Yuji; Li, Fei; Li, Xie; Barriot, Jean-Pierre (2020). "A degree-100 lunar gravity model from the Chang'e 5T1 mission". Astronomy & Astrophysics. EDP Sciences. 636: A45. Bibcode:2020A&A...636A..45Y. doi: 10.1051/0004-6361/201936802 . ISSN 0004-6361.
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