Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007dps....39.3208c&link_type=abstract
American Astronomical Society, DPS meeting #39, #32.08; Bulletin of the American Astronomical Society, Vol. 39, p.475
Astronomy and Astrophysics
Astronomy
Scientific paper
Prior models of lunar-forming giant impacts assume that the impactor and the target protoearth were not rotating prior to the Moon-forming event. However, planet formation models suggest that such objects would have had substantial rotation rates during the late stages of terrestrial planet accretion. Here I explore the effects of pre-impact rotation on impact outcomes through SPH simulations that consider a range of impactor masses, impact angles and impact speeds. Pre-impact rotation, particularly in the target protoearth, can substantially alter collisional outcomes and leads to a more diverse set of final systems than seen previously. However, the subset of these impacts that are also lunar-forming candidates--i.e. that produce a sufficiently massive and iron-depleted protolunar disk--have properties that are remarkably similar to those determined for collisions of non-rotating objects (Canup & Asphaug 2001, Canup 2004). With or without pre-impact rotation, a lunar-forming impact requires an impact angle near 45 degrees, together with a low impact velocity that is not more than 10% larger than the Earth's escape velocity, and produces a protolunar disk containing up to about two lunar masses that is composed predominantly of material originating from the impactor. The most significant differences in the successful cases involving pre-impact spin occur for impacts into a retrograde rotating protoearth, which allow for larger impactors (containing up to 20% of Earth's mass) and provide an improved match with the current Earth-Moon system angular momentum compared to prior results. The most difficult state to reconcile with the Moon's origin from a giant impact is that of a low obliquity, rapid prograde rotation in the protoearth prior to the lunar forming event, because this requires a smaller impactor and/or less oblique impact that typically produces a disk too small to yield the Moon.
Support from NASA's Origins of Solar Systems program is acknowledged.
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