The origin of the moon and the single impact hypothesis IV

Abstract

Previous papers in this series have described the smoothed particle hydrodynamics method (SPH) which we have employed to explore the conditions in which a major planetary collision may have been responsible for the formation of the Moon. In this paper we have run 41 simulations of the planetary collisions; these constitute an exploration of some of the relevant parameter space. The parameters varied include the mass ratio of the Impactor to the Protoearth (values of 0.14, 0.16, and 0.25), the angular momentum in the collision (in units of the present angular momentum of the Earth-Moon system, from slightly more than 1 to slightly less than 2), and the velocity at infinity (0, 5, and 7 km/sec). For a mass ratio of 0.14, there is only a rather narrow range of conditions in which an iron-free disk can be obtained. For higher mass ratios the range of conditions is much wider. For lower values of the angular momentum, the Impactor is destroyed in the collision and drawn out into a long arc. The inner part of this falls into the Protoearth, usually including all the iron, leaving the outer part of the arc in orbit. For higher values of the angular momentum the Impactor is not completely destroyed on impact, but it draws itself together as it travels beyond the Roche lobe and comes around for a second collision, whereupon the results are essentially as described above, except that the disks can contain more material. Occasionally a body of substantial size (a major fraction of or comparable to the mass of the Moon) is formed and left in a stable orbit beyond the Roche lobe. This presents us with two possible scenarios for the formation of the Moon: either directly in the collision or after dissipation and evolution of the disk. These collisions do very extensive damage to the Protoearth. The iron core of the Impactor plunges through the mantle of the Protoearth and forms a very hot layer (several tens of thousands of degrees) around the surface of the Protoearth core; this will later provide a heat source to drive vigorous mantle convection. The outer part of the mantle of the Protoearth is also strongly heated, with rock decomposition products in gaseous form with an initial photospheric surface at a temperature of 16,000 K or higher and a radius more than 20% greater than that of the Earth. This hot mantle surface should lead to the hydrodynamic escape of the Earth's primordial atmosphere and must be taken explicitly into a account in any calculations of the evolution of the orbiting disk. The results lead naturally to a Giant Blowoff hypothesis. Not only can the primitive atmosphere escape, but so can large amounts of mantle material which flows over the surface of the orbiting disk and blows off in the forward direction. Because of the greater volatility of SiO, the Mg/Si and Fe/Si ratios in the Earth are likely to increase. Large amounts of angular momentum can also be lost. This greatly extends the range of interesting parameters that can be seriously considered in the Giant Impact hypothesis.