Transient Events on the Moon

In addition to eclipses, the Moon has occasionally produced other phenomena such as glowing or sudden flashes of light. But on these phenomena, there is less agreement as to the cause. The extensive cratering of the Moon indicates that the surface is constantly being bombarded by meteoroids, although at much lower frequency than that prevailing during the first billion years of the Moon's existence. The impact of the larger bodies must be accompanied by a flash of light as the material is vaporized during the explosion on impact. The source of energy for the explosion and creation of the crater is the kinetic energy of the body that encounters the Moon at or beyond the Moon's escape velocity (2375m/s). The energy of impact would be at least 3 x 106 joules per kilogram of mass of the impacting body, but may be much larger.25 The energies compare to the chemical energy in TNT: 4.2 x 106 J/kg. Such a collision has calculable effects.

For an object with a density of 1g/cm3, 10m in radius, impacting the Moon with a velocity of 9 Km/s, the maximum energy to be dissipated would be ~1.7 x 1014 joules. If 10% of this energy were converted to visible light, and the radiated energy was released over a 1-second interval, the average power radiated would be ~1.7 x 1013 watts during that time (note, however, that a "luminous efficiency" of 10% may be too high; Bello Rubio, Ortiz, and Sada (2000) derive 0.2%, from observed impacts of Leonid meteoroids on the Moon in November, 1999, but with an uncertainty of a factor of 10). Viewed from Earth, the explosion would produce a radiant flux of ~1 x 10-5W/m2. The integrated light of the full moon over the V bandpass is about 5 x 10-3 Wm-2m-1 (C.W. Allen 1973, pp. 142ff); so there would be a negligible increase in the brightness of the Moon during the event. However, the surface brightness of the Moon decreases away from full Moon phase in dramatic fashion. As a function of phase angle on the Moon, a, the angle measured at the Moon between the Earth and the Sun, the V magnitude increases by ~0.25 magnitude26 (becoming ~23% dimmer) for every 10° interval from full moon phase, where a = 0°. The event would be still more apparent if it was to occur on the night side of the Moon, i.e., on the part unillu-minated by the Sun. In fact, seen against the dark face of the Moon, the sample event would be about as bright as a 5th magnitude star, visible, but not spectacular. The energy dis

25 The kinetic energy is V2 x mv2. The Moon's mass is M = 7.3483 x 1022kg, so that, using only the velocity of escape from the Moon's surface, v. = [2GM/R]1'2, with G = 6.672 x 10-11, and the Moon's radius, R = 1.738 x 106m, the energy of impact is 2.82 x 106J/kg;adding the escape velocity of the Earth at the distance of the Moon, 1440m/s, the energy becomes 7.28 x 106 J/kg; and finally, adding the speed relative to the Earth-Moon system of an object on a typical Earth-crossing asteroid orbit, ~5km/s, the speed of impact becomes ~9000m/s, and the impact energy becomes ~4 x 107 J/kg. Thus, orbit details of the impacting body determine to a large extent the resulting energy of explosion. See Melosh (1989) for more detailed treatment.

sipated in the collision is proportional to the mass; in general, the mass is unknown, but M = (4pp/3)R3, where M is the mass, p the density, and R the radius; so given the density and radius, the mass and thus the energy can be computed. If the object was denser, say, ~3gm/cm-3, typical of a rocky rather than an icy body, the collision energy would be proportionately greater. An object ten times the diameter of our test object would produce ~1000 times more energy; 100 times larger (~2km across) would produce a million times more energy. The larger the object, however, the rarer it is. An object 20 km across is likely to impact only once in tens of millions of years. Strong and widespread evidence suggests that such events have punctuated the geological record, the most recent such major event occurring at the end of the Cretaceous period of the Mesozoic era, coinciding with the end of the age of dinosaurs.27

Observations of sudden brightenings on the Moon were recorded in the medieval annals of European monasteries (Newton 1972). They probably represent impacts of modest or intermediate size: a bright light within the lunar disk at "new moon" (Rampona, 1058); a light on the Moon's disk during a total eclipse (Cavenses, Aug. 6, 1096); and "the upper horn of the new moon seemed to split in two and a flame shot from it" (Gervaise, June 18, 1178). The phase "new moon" in these observations is probably a 1-day-old waxing crescent in modern astronomical terms. Such observations are consistent with the impact of an object ~200m across that would appear to be a thousand times brighter, with a visual magnitude of at least -2.5, and would take on the appearance of a very bright star for a brief period of time. Recent investigations have denied an association with the crater Bruno as has sometimes been claimed, and it has been argued that atmospheric phenomena rather than a col-lisional event is what is being described. Whether or not the latter is the case, a verification of meteoritic transient events, albeit telescopic, was provided for the first time in 1999, by observers organized by David Dunham to look for Leonid meteor shower (see §5.6, especially, Table 5.8) impacts on the Moon.

A different sort of bombardment event could, in principle, be observed also. A luminescence mechanism has been demonstrated in the laboratory on meteoritic material, which was bombarded by high-velocity charged particles like those of the solar wind. The bombarding particles through ionizing interactions cause the material to emit a continuous light upon recombination. Observations of reddish glows on the Moon are reported in Kopal (1966, 388-402, 436-439), especially on the full Moon, when the Moon sweeps through the magnetic "tail"28 of the Earth. Lunar luminescence has not been linked in any certain way with the recorded "transient lunar phenomena," however. Careful examination of lunar material gathered on the Apollo 11 expedition failed to show strong luminescence

27 The Chicxulub crater on the western edge of the Yucatan peninsula is widely held to be the impactor site for the K-T event.

28 The tail is down-wind of the solar wind, which compresses the Earth's magnetosphere on the sunward side and causes its extension on the leeward side.

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