Section M 10.3.1.
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Lunar Gravity

Marvin Ostrega

On the moon gravity is comparatively weak. Lunar Surface gravity at the equator is 5.32 ft/sec2 (1.622 m/sec2), compared to 32.174 ft/sec2 (9.806 m/sec2) of Earth. That's 16.5% or nearly exactly 1/6 Earth's gravity.

The lower gravity comes in handy in geological field expeditions. For example, on Apollo 15, Scott and Irwin sampled a large boulder and the ground beside it, and then were able to roll the boulder out of the way and sample the well-preserved material underneath it.

Lunar gravity model from NASA GSFC
Lunar gravity model from NASA GSFC
Space Geodesy Branch
The Lunar gravity field is uneven due to mascons (mass concentrations) near the surface. These are large, positive gravity anomalies which were detected when NASA scientists discovered that doppler radar signals from the Lunar Orbiter satellite varied as the satellite moved over different parts of the moon. The largest gravity field disturbances are in northern hemisphere at Mare Serenitatis and Mare Imbrium (mascons are usually associated with mare terrain). Mascons are yet to be satisfactorily explained.

Of course, low lunar gravity has many medical and engineering implications, as well as recreational uses, discussed in other areas of the Artemis Data Book.

20 Apr 98 Update (Gregory Bennett): The Space Geodesy Branch at the NASA Goddard Space Flight Center are working on a paper presenting the GLGM-2 lunar gravity model based on the results of the Clementine and Lunar Prospecter missions. (The paper might be a bit too technical for many readers.) The image above is from their reseach. Click on the image for a very large (272K, 903x768 pixels) rendering of the lunar gravity field. The map shows how much the moon's surface would vary from a perfect spheroid if the moon had uniform density instead of those mascons.

Since the highest concentrations of gravity occur over the maria, one theory for the origin of these gravitational anomalies is that we're seeing the results of the collisions that formed the maria. The moon's maria are really huge impact craters which have been filled in with lava flows. The gravitational anomalies might be due to high-density asteroids that smacked into the young moon and were buried as the maria formed over them.

Chaikin, Andrew. A Man on the Moon. New York: Viking Penguin, 1994
Heiken et al. Lunar sourcebook: a user's guide to the moon. Cambridge: University of Cambridge Press, 1991


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