MOONS MAGNETISM - FROM IOS AND GANYMEDES PRESENT MAGNETIC SIGNATURES TO THE ANCIENT LUNAR DYNAMO

Intrinsic magnetic fields, corresponding to virtual axial dipole momen ts of the order of 10(20) Am-2, have recently been evidenced for the J ovian satellites lo and Ganymede. By reviewing the rock magnetic and p alaeomagnetic: properties together with the history of lo, the hypothe sis of a moment due to induced or remanent magnetization of crustal ro cks acquired within the ambient Jovian field is clearly eliminated. Th is demonstration is all the more valid for Ganymede, which experiences a much lower Jovian field. The demonstration of a present dynamo acti on for these Jovian moons, possibly sustained by Jupiter through tidal heating and background magnetic field, may be an actualistic model fo r the early lunar history. The hypothesis of a lunar dynamo, active at 3-4 Ga, seems to be strongly supported by this analogy.