Discovery Of Mascons

Press, Volume CIX, Issue 31986, 13 May 1969, Page 6

Discovery Of Mascons

Mascons have been found on the Moon. Twelve have now been positively identified. Now that their locations have been discovered they should not threaten the safety of future lunar expeditions.

From their hidden positions 40 miles beneath the lunar basins the mascons extend invisible gravitational tentacles which can waft and wend the path of a spacecraft sweeping down to land on the inoon. Otherwise they are rather harmless. Mascons are, in fact, nothing other than large mass concentrations (hence the name mascon) underlying the depressed lunar basins known for centuries as maria or seas. Were it not for the peculiar perturbations they Induced in the motion of lunar-orbiting spacecraft they might have lain undetected indefinitely. The mascons were discovered as a result of a painstaking analysis of tracking data from the Lunar Orbiter series of spacecraft For some yeara astronomers had suspected that the moon possesses an uneven gravitational field as a result of interior irregularities. If is believed that the moon was never in a molten state as was the Earth several thousand million years ago. In 1956, a Nobel Prize winner, Harold Urey, predicted the existence of mascons beneath the ringed lunar maria. It remained for Paul Muller and William Sjogren of the Jet Propulsion Laboratory in California to find them. The six new mascons announced last month at a meeting in Washington, together with the first six reported last year, are enough to account for most of the maria on the near side of the moon Including the partially visible Mare Orientate on the western limb and Mare Smythii on the eastern limb. It is impossible to say how many maseons are buried beneath the far-side of the moon because orbiting spacecraft cannot be tracked while they are hidden from the view of earth-based tracking antennae. Fresh Insight Mascons provide a fresh insight on questions concerning the Moon’s origin and evolution. Most investigators support the fairly obvious hypothesis that each mascon is a former asteroid which crashed into the moon; the impact forming the associated mare. The mascon underneath the Mare Imbrium is estimated to be as massive as a nickel-iron object about 70 miles in diameter. It is thought to be the most recent of the 12 because the Mare Imbrium is a very prominent, well-defined feature. Some of the maria with associated mascons are partially obliterated, indicating a very ancient collision. Other scientists, including the co-discoverers of the mascons, are unhappy with impact theories. They have revived the old original concept of oceans on the moon and consider the mascons to be heavy deposited material in the beds of primordial lunar seas. On looking at some of the detailed pictures of the moon’s surface taken from tbe Lunar Orbiters and Apollo 8 it is hard not to believe that water must have played a major role in moulding the lunar features. These include meandering ice or water-cut rilles, levees, deltas, beach terraces and submerged “ghost” craters. There is also the theory that the ringed maria were formed by flows of lava welling up from within the moon when the crust was ripped asunder by catacylsmic impacts from asteroidal bodies. Whatever the origin of the mascons their existence indicates that the moon has a more rigid internal structure than the earth, otherwise the mascons would have sunk deeper or melted and become mixed with the surrounding material. Here on earth the continents float on a viscid liquid mantle, because the continents are lighter than the mantle material, whereas the mascons are much more dense than tbe average lunar constituents. It is this disparity that made the mascons detectable. More May Be Found For those interested, the 12 known mascons are situated under the ringed Maria Crisium, Humorum, Humboldt, Imbrium, Nectaris, Orientate, Serenitatis and Smythii; the Sinus Aestatum; the western edge of the Oceanus Procellarum near the crater Grimaldi; and two unnamed mare areas at 27 de-

grees East, 5 South and 70 East, 15 South. No doubt a few more will be revealed when precise tracking data are available from future Apollo flights. During the 10 lunar orbits of Apollo 8 noticeable speed increases were observed as the spacecraft flew over several of the mapped mascons. The accelerations involved were measurable in minute fractions of the lunar gravity field: the largest was only a little over two millimetres per second squared. Such exact results are a testimony to the fantastic accuracy of modern tracking systems which can readily measure changes in frequency of one part in 20,000 million.