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Gravity anomalies on the near-earth (left) and distant side of the moon: red = positive, blue = negative.

Mascons are large areas of increased rock density under the surface of the moon . The most massive of them are in the area of ​​the large circular Maria (lunar seas) in the northern hemisphere. The name is from the English word for mass concentrations ( mas s con derived centration).

The density within a mascon is 3.3 g / cm³, for the surrounding rock it is 3.0 g / cm³. The mass of all mascons makes up about 0.01 to 0.03 percent of the total mass of the moon. Areas that are less dense than their surroundings are called negative mascons.

The biggest mascons

Comparison of the topography of the surface (above) and the gravitational field of the Mascons from Mare Serenitatis ( Lunar Prospector , NASA ).
Topography (above) and gravitational field of Mare Smythii.

The five most important mascons are located under the large, round lunar seas on the side facing the earth (the red areas in the picture above). There are also smaller mascons on the back of the moon , negative (blue areas) under some very large impact craters .

The places ranked according to the extent of the mascons:

  1. Mare Imbrium , largest lunar sea, northwest of the center of the moon
  2. Mare Serenitatis northeast of the middle of the moon (see picture on the right)
  3. Mare Crisium in the east
  4. Mare Nectaris southeast of the center of the moon
  5. Mare Humorum in the southwest

    Smaller and more irregular:
  6. Mare Humboldtianum on the northeastern edge of the moon
  7. Mare Orientale on the western edge, mostly on the back of the moon
  8. Mare Smythii on the eastern edge (see picture on the right)
  9. Ring around the giant Korolev crater on the back of the moon
  10. Region around Gagarin on the back of the moon
  11. Mendel- Ryberg region on the back of the moon

Negative mascons (mass deficits) characterize many whale levels and large craters:

Discovery story

Already at the first moon satellite Luna-10 were perturbations discovered in the form of regional curvatures, on a non-uniform mass distribution pointing of the moon. Only with a spherically symmetrical mass distribution does a satellite orbit the celestial body on an elliptical orbit, so the irregular orbits of the probes could only be explained with gravity anomalies. In Lunar Orbiter 1 an excess of the forecast was shortly thereafter Doppler effect in the telemetry transmission detected. With Luna-11 and Lunar Orbiter 3 , a regular pattern in the orbital disturbances could be recognized for the first time. The Lunar Prospector Mission later mapped these areas and made a detailed map of the Mascons.

Theories of origin

Since mascons lie below impact craters and Maria, their formation goes back to the time of the last Great Bombardment , about 4 to 3.8 billion years ago. In this era, giant meteorites fell on the moon and broke through its rock crust. The rising magma poured over the surface of the moon and formed the Maria, below the mascons emerged. Some scientists assume that the Mascons are remnants of the iron cores of these meteorites, others assume that lava bubbles rose as a result of the impacts.

Measurements from the Apollo missions, however, suggest that the round basins were only partially filled with magma at first. These basalt masses (rock density 3.3 g / cm³) sank in the surrounding crustal rock (2.9 to 3.4 g / cm³) until an isostatic equilibrium was reached. The layer package then solidified, so that later basalt pourings, which filled the Maria up to the present height, no longer reached isostasis and now represent mass surpluses. In the center of the round Maria the basalt layer is about 25 km thick, it decreases towards the edges.

The irregularly shaped lunar seas, on the other hand, have no mascons and are less deep. They were probably created at the same time as the uppermost lava effusions of the round Maria, which overflowed there.

Influence of mascons on orbits of satellites

Lunar mascons influence the gravitational field around them in such a way that low satellite orbits are unstable without orbit corrections. This can also cause the satellite to crash.

In the Apollo program , Mascons caused navigation errors of up to 2 km; Deviations of max. 200 m. For this reason the software was Apollo Guidance Computer of the lunar module of Apollo 12 revised and all subsequent missions, to compensate for the deviations by mascons. Otherwise it would probably have been impossible for Apollo 12 to land directly next to the landing site of Surveyor 3 , at a distance of only 360 m.

Mascons on the earth

No mass concentrations have been detected below the large impact craters on earth, such as the Nördlinger Ries . It is assumed that the core, which was heated up by the flight in the atmosphere, evaporated on impact or that the debris was distributed over a large area on impact. The earth's crust also extends much deeper than the earlier lunar crust and was not penetrated as was the case when the mare basin was formed. In Antarctica in Wilkes Land a large was in 2006 gravity anomaly discovered which could be a mascon.


Web links

Commons : Mascons  - collection of images, videos and audio files