Mass concentration

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Topography (top) and corresponding gravity (bottom) signal of lunar Mare Smythii containing a significant mascon.

A mass concentration or mascon is a region of a planet or moon's crust that contains a large positive gravitational anomaly. In general, the word "mascon" can be used as a noun to describe an excess distribution of mass on or beneath the surface of a planet (with respect to some suitable average), such as Hawaii. However, this term is most often used as an adjective to describe a geologic structure that has a positive gravitational anomaly, such as the "mascon basins" on the Moon.

Type examples of mascon basins on the Moon are the Imbrium, Serenitatis, Crisium and Orientale impact basins, all of which possess prominent topographic lows and positive gravitational anomalies. Examples of mascon basins on Mars include the Argyre, Isidus, 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. However, while these lavas certaintly contribute to the observed gravitational anomaly, uplift of the crust-mantle interface is also required to account for its magnitude. Indeed, some mascon basins on the Moon do not appear to be associated with any signs of volcanic activity, suggesting that the mantle uplift might even be super-isostatic (that is, uplifted above its isostatic position). It should be noted that the huge expanse of mare basaltic volcanism associated with Oceanus Procellarum does not possess a positive gravitational anomaly.

The lunar mascons alter the local gravity in certain regions sufficiently that low and uncorrected satellite orbits around the Moon are unstable on a timescale of months or years. This acts to distort successive orbits, causing the satellite to ultimately impact the surface. The lunar mascons were discovered by Paul M Muller and William Sjogren of the NASA Jet Propulsion Laboratory (JPL) in 1968^[1] from analysis of the highly precise navigation data from the unmanned pre-Apollo Lunar Orbiter spacecraft. 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 resulting from strong gravitational perturbations 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 30 meters of the target, the previously-landed Surveyor 3 spacecraft.

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