A History of Satellite Capture Studies As Experienced by the Author: A Chronology of Events that Eventually led to a Somewhat Comprehensive Gravitational Satellite Capture Model

I consider the Prograde Gravitational Capture Model (PGCM) to be a default model in that it need not be accepted until all other candidate models are tested and retired. The Giant Impact Model (GIM) was also presented as a default model because the proposers thought that fission, co-formation, and capture models could be retired from consideration. But the GIM does not relate very well to the facts to be explained by a successful model and I think that this model should soon be retired. The PGCM is not just another capture model, it is a complex model in both geological timing and in Solar System space. The model I am presenting attempts to explain several problems that were associated with earlier versions of capture such as (1) Where did the body of the Moon come from?; (2) What is the energy sink for capture?; and (3) How does the capture model relate to the rock and mineral records of Earth and Moon? Furthermore, my version of the capture model relates to Harold Urey’s speculation that the Moon may be a “Rosetta Stone” for interpretation of the history of the Solar System. This capture model was developed over a period of five decades. The initial idea that the Moon has left an imprint on the geological record of Earth was gained from Preston Cloud’s summary articles on the Primitive Earth in the 1968–1972 era. After studying lunar photographs and lunar globes I noticed an obvious surface pattern: i.e., many of the “circular” maria were located very near to a great-circle on the lunar globe. I interpreted this pattern to be compatible with partial tidal disruption of the Moon during a close, but non-collisional, encounter with Earth. A set of numerical simulations (in 1972) of particles lifted from the lunar surface during a close gravitational encounter illustrated under what conditions this could happen. An outstanding question from the simulations was whether this close encounter was simply a non-capture (fly-by) encounter or was it a close encounter associated with a capture sequence of events? By 1977 I had worked out a tentative time-scale for a capture sequence of events that related to the rock records of Earth and Moon. I tentatively set the capture event at 3.95 Ga. But my colleagues and I could not demonstrate that capture is physically possible. We did know that the energy for capture could be stored by tidal deformation processes in the body of the Moon but we could not justify a low Q value of 1, 2, or 3 for the lunar body until 1986. At that time Ross and Schubert (UCLA) published a set of calculations in the Proceedings Volume for the 16th Lunar and Planetary Science Conference justifying a very low Q value for a lunar-like body. Then in 1987 we had our first numerical simulations of capture. At this point we knew that capture of a lunar mass body could be accomplished from a heliocentric orbit that is very similar to that of the Earth’s orbit (within 3% eccentricity). The question now was: “Where did the body of the Moon come from?” After another 10-year waiting period an answer came through in the form of a short article in the journal NATURE on the origin of Vulcanoid planetoids. Devolatilized bodies like Luna and sibling planetoids could form in the zone between Mercury and the Sun as Al Cameron presaged in two articles in 1972 (before he embarked on the GIM excursion). If Luna formed as a Vulcanoid planetoid then it could be transferred to Earth orbit for capture if enough “Lunas” (candidate planetoids) were formed in the source region. My guess is that we need to start with 6 to 8 lunar-like bodies for one to survive the hazardous trip to Earth orbit. The final problem to be worked on was the relation to the rock record of Earth. John Valley (Univ. of Wisconsin) published his ideas on the Cool Early Earth in the journal GEOLOGY in 2002. In my view there could be a Cool Early Earth before the capture episode and that Cool Early Earth era would have an abrupt end at the time of Lunar Capture at 3.95 Ga. The older lunar mare basalts and breccias yielded dates of crystallization at about that time and the oldest and strongest remanent magnetic signature in lunar basalts is of that some general age. So, for me, the concept of a COOL EARLY EARTH was a good one and the story of a PROGRADE GRAVITATIONAL CAPTURE EPISODE was taking shape after a period of several decades. I also see now why no one had been able to piece this together at an earlier time. I still consider it to be a DEFAULT MODEL!