A Multiple Impact Hypothesis for Moon Formation: Target Spin and Disk Properties

Abstract:

We investigate aspects of the multiple impact hypothesis for Moon’s formation, whereby the proto-Earth suffers successive collisions, each forming a debris disk that accretes to form a moonlet. The moonlets tidally advance outward, and potentially coalesce to form the Moon. In addressing the fundamental problem of the Moon's formation, we consider smaller impactors than previously studied, and investigate the effect of new geometries using a Smoothed Particles Hydrodynamics (SPH) code.

For impacts within the equatorial plane, we find multiple impactors are effective in draining angular momentum from the target’s initial spin due to the often-neglected angular momentum carried by escaping mass. Our simulations reveal new consequences of non-equatorial inclination of the impactor, also previously neglected. We note relationships with the resulting disks of corresponding equatorial cases, but find that the target’s axis of rotation can now be tilted by a significant amount (10’s of degrees) with sub-Mars size impactors. Importantly for distinguishing among competing Moon formation hypotheses, our results imply that (i) the rotational acceleration of the proto-Earth by successive impacts may be limited by angular momentum drain if the impacting population contains multiple members of medium size, and (ii) impacts onto such a non-rapidly rotation proto-Earth (well below break-up speed) can produce disks compatible with sub-Moon fragments in mass, momentum, and composition.