Formation of the lunar crust: An electrical source of heating

Electrical heating of the outer layers of the Moon just after formation is shown to lead to melting in a time of order 10 5–10 7 yr. The heating mechanism is based upon eddy current induction from disordered magnetic fields swept outwards by an intense (T Tauri-like) plasma flow from the Sun. Threshold temperature for the development of the intense electrical heating lies between 300–400°C depending upon the bulk electrical conductivity function used. The electrical mechanism is an alternate to intense short period accretion as a source of heat for the evolution of lunar maria and highlands, provided that long-lived radioactives are not swept to the surface from too large a melt volume during the initial thermal episode. Thus formation of the lunar highlands does not intrinsically require rapid accretion, nor on this basis is the time of formation of the planets generally restricted to a very short time. The threshold temperature for eddy current heating is attained by either a solar nebula at 300–400°C during formation of the Moon or a very low-energy long-period accumulation of the Moon. Variations among laws for mass flux during accretion are unimportant in attaining the threshold temperature for electrical heating. Eddy currents correspond to the transverse electric mode; they are complementary to the transverse magnetic mode discussed earlier for asteroidal parent body melting. Joint excitation of these two modes would modify and intensify lunar heating.