* I don't think the direction of spin of planetoids affects their charge or surface potential. The charge of the interplanetary plasma seems to determine that.

* Here's what Juergens said about the size of the Aristarchus discharge [and later he said the Tycho discharge was likely about 40 times larger].

... we may usefully ask how much electric charge might have been exchanged in the postulated Aristarchus event. Would this charge, for example, be a reasonably small fraction of the total charge carried by each of the two planetary bodies involved?

Suppose we approach this problem by taking the measure of an ordinary lightning bolt, which hopefully is the nearest thing to an interplanetary discharge likely to be observable in our time. The energy of a fairly average lightning discharge, according to Viemeister (59), is about 250 kilowatt-hours-roughly 9 x 10exp8 joules.

On Earth, most of this energy is dissipated in the atmosphere. But what might happen if such a bolt were to strike an airless body like the Moon?

From Baldwin's analysis of lunar and terrestrial explosion craters (60), it would appear that such a bolt ought to produce a lunar crater about 85 meters in diameter (see Figure 1). Aristarchus, as indicated in the figure, was probably formed by an explosion releasing some 2 x 10exp21 joules of energy. So we are talking about an interplanetary discharge a few million million [a few trillion] times as energetic as ordinary lightning.

Cloud-to-ground electric potentials in thunderstorms reach values near 10exp9 volts (61). Presumably the potential drop across an interplanetary spark gap would be considerably greater than this, but by how much we can only guess for now. Let us assume that it would be at least a thousand times greater -- say, 10exp12 volts. On this basis, since the energy of a discharge is the simple product of the potential drop between electrodes and the total charge transferred, we can estimate that a spark transferring 10exp9 coulombs of charge would suffice to produce an Aristarchus on the Moon and wreak corresponding havoc, though of a different kind, on Mars (62).

Some recent estimates of total electric charges carried by solar-system bodies include Bailey's 10exp18 coulombs for the Sun (63) and Michelson's 10exp13 coulombs for the Earth (64). Michelson's figure is derived from Bailey's on the assumption that the specific charges -- total charges divided by total masses -- of all bodies in the solar system might be alike. The same assumption would imply total charges of about 10exp12 and 10exp11 coulombs for Mars and the Moon, respectively. However, as pointed out elsewhere (65), the ubiquitous interplanetary plasma can be expected to equalize surface potentials rather than specific charges; except during near collision episodes, and perhaps even then to large degree, the potentials of all the planets (or at least the inner planets of the system) should be pretty much alike and equal to that of the Sun.

Nor need one put too much stress on Bailey's estimate of the Sun's net charge. Most of his arguments assume that electric fields propagate across interplanetary space, and this seems ruled out by the plasma. Nevertheless, for present purposes we might take Bailey's figure as a minimum value for solar charge and deduce from it a minimum value for the Sun's surface "potential" -- 10exp9 volts. (In passing, it is well to note that this "potential" is relative to some "zero" of potential that probably does not apply anywhere in the solar system, and may not apply anywhere within the limits of the local galaxy, either. Bailey contended that the Sun maintains a potential of this magnitude relative to its immediate surroundings ("empty space"), but his analysis of the solar-charge problem was made before Mariner 2 demonstrated the all pervasive nature of the interplanetary plasma.)

On this basis, then, since the plasma effectively "grounds" the planets to the Sun, each of them ought to be charged so as to have this same 10exp9 -volt surface potential. The charge on each of them, expressed as a fraction of the Sun's charge, should be proportional to the planet's radius, expressed as a fraction of the Sun's radius. Earth, Mars, and the Moon should then carry respective "normal" charges of approximately 10exp5, 5 x 10exp4, and 2.5 x 10exp4 coulombs.

Given such charges -- and it bears reemphasizing that these figures may be substantially on the low side -- we can see that the postulated Aristarchus discharge, transferring 10exp9 coulombs between Mars and the Moon, would alter the "normal" charge of Mars by only about two parts in a million, and that of the Moon by some four parts in a million. Quite a few such bolts might pass between the two bodies during a single encounter without significantly affecting the electrical balance between either of them and the interplanetary plasma.