The Close Approach – A Lunar Model
What if the moon weren’t a three days travel from the earth? What if it didn’t always show one face towards us? Have you ever thought about how a change in the orientation or dynamics of our moon would impact our everyday lives?
The past evolution of the moon’s orbit has interested scientists since at least George Darwin’s study of rotating viscous spheres in the late 19th century. It is typically thought that the moon has been steadily retreating from the earth for the past four plus billion years and that it will continue to retreat. Strangely though, today when the moon’s current rate of retreat is calculated backwards it is found that the moon approaches the earth much more recently than four billion years ago – rather only 1.8 billion years ago! It is the investigation of this discrepancy which has ultimately led to the hypothesis of a close lunar approach in the history of the earth-moon system.
Let’s look at things from the perspective of the CALM. Consider that at one time, in the not so distant past, the moon approached the earth, coming much closer than it is now. This happened during a great calm in our earth’s history. For hundreds of millions of years the earth saw temperate weather, mild seasons, and a relatively consistent environment. But this long period of calm was jarringly disrupted when the moon literally got “too” close. The close proximity of the two bodies set off a series of cataclysmic heating events that resulted in what is known as the Great Dying (the greatest extinction of all time).
But like two magnets of the same charge being pressed together, the earth repulsed the moon, bouncing it back out again. The moon was gravitationally ejected to the position in which we find it now.
The legacy of the close approach and the time of the moon’s “second” retreat is clearly reflected in the story of life on Earth and specifically the rise and fall of the dinosaur. The close approach and resultant Permian extinction left an evolutionary vacuum for the dinosaurs to fill. However, the story does not end there.
The steady retreating of the moon and the changing climatic conditions were a major contributor, but not the only contributor, to the ultimate demise of the dinosaurs. With the dinosaurs then out of the picture, our opportunist mammalian ancestors stepped into this new seasonal void to repopulate the world as we have come to know it.
What might a lunar approach have looked like?
The Close Approach Lunar Model (CALM) is based on the hypothesis of an intermediate or secondary “close approach” of the moon to the earth. It proposes that the moon retreated from the earth initially, as in classic origin models, but that a significant intermediary “regression event” altered its course, causing the moon to fall back in towards the earth. In other words, the moon was slowed in its original orbit by tidal exchange with the earth, and subsequently began to spiral back in towards the earth.
But today we clearly see that the moon is retreating from the earth. Rather than “crashing” into the earth during this regression event, the moon experienced just a short high energy close approach to the earth. Dynamically speaking the moon bounced off the earth, something like trying to force two like charged magnets together. This close temporary and high energy orbit of the moon about the earth then evolved out to where we see the moon today.
The current lunar orbit, the model proposes, is the result of the close approach dynamics of this eviction event, not the dynamics of the “origin” of the earth-moon system as classic origin models assume. Though the dynamics of this event have the signature of a classic capture event, it is better described as a secondary capture and heating event in the evolution of the earth-moon system.