Inconstant Moons: A New Lunar Origin Scenario

A recent snowfall followed by warming temperatures produced a foggy night recently, one in which I was out for my usual walk and noticed a beautiful Moon trying to break through the fog layers. The scene was silvery, almost surreal, the kind of thing my wife would write a poem about. For my part, I was thinking about the effect of the Moon on life, and the theory that a large single moon might have an effect on our planet’s habitability. Perhaps its presence helps to keep Earth’s obliquity within tolerable grounds, allowing for a more stable climate.

But that assumes we’ve had a single moon all along, or at least since the ‘big whack’ the Earth sustained from a Mars-sized protoplanet that may have caused the Moon’s formation. Is it possible the Earth has had more than one moon in its past? It’s an intriguing question, as witness a new paper in Nature Geoscience from researchers at the Technion-Israel Institute of Technology and the Weizmann Institute of Science. The paper suggests the Moon we see today is the last of a series of moons that once orbited the Earth.

“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth,” says co-author Assistant Prof. Perets (Technion). “It’s likely that such moonlets were later ejected, or collided with the Earth or with each other to form bigger moons.”

To explore alternatives to giant impact theories, the researchers have produced simulations of early Earth impacts, varying the values for the impactor’s velocity, mass, angle of impact and the initial rotation of the target. The process that emerges involves multiple impacts that would produce small moons, whose gravitational interactions would eventually cause collisions and mergers, to produce the Moon we see today. Here’s how the paper describes the process:

… we consider a multi-impact hypothesis for the Moon’s formation. In this scenario, the proto-Earth experiences a sequence of collisions by medium- to large-size bodies (0.01–0.1M_⊕). Small satellites form from the impact-generated disks and migrate outward controlled by tidal interactions, faster at first, and slower as the body retreats away from the proto-Earth. The slowing migration causes the satellites to enter their mutual Hill radii and eventually coalesce to form the final Moon. In this fashion, the Moon forms as a consequence of a variety of multiple impacts in contrast to a more precisely tuned single impact.

Here’s a graphic from the paper (listed as Figure 1) that shows the process at work:

Image (click to enlarge): a,b, Moon- to Mars-sized bodies impact the proto-Earth (a) forming a debris disk (b). c, Due to tidal interaction, accreted moonlets migrate outward. d,e, Moonlets reach distant orbits before the next collision (d) and the subsequent debris disk generation (e). As the moonlet–proto-Earth distance grows, the tidal acceleration slows and moonlets enter their mutual Hill radii. f, The moonlet interactions can eventually lead to moonlet loss or merger. The timescale between these stages is estimated from previous works.

The Hill radius mentioned above describes the gravitational sphere of influence of an object; in this case, meshing Hill radii can produce interactions that sometimes lead to mergers. The paper notes that in head-on impacts, the rotation of the planet is important because the disk needs angular momentum resulting from the rotation to stay stable. With increased rates of rotation, the angular momentum of the disks increases. Moons like ours emerge from many of the simulations:

We find that debris disks resulting from medium- to large-size impactors (0.01–0.1M_⊕) have sufficient angular momentum and mass to accrete a sub-lunar-size moonlet. We performed 1,000 Monte Carlo simulations of sequences of N = 10, 20 and 30 impacts each, to estimate the ability of multiple impacts to produce a Moon-like satellite. The impact parameters were drawn from distributions previously found in terrestrial formation dynamical studies. With perfect accretionary mergers, approximately half the simulations result in a moon mass that grows to its present value after ~20 impacts.

If the multi-moon hypothesis proves credible, how would it affect the larger astrobiology question? In Ward and Brownlee’s Rare Earth (Copernicus, 2000), after a discussion of obliquity and the Moon’s effect on the Earth’s early history, the authors say this:

If the Earth’s formation could be replayed 100 times, how many times would it have such a large moon? If the great impactor had resulted in a retrograde orbit, it would have decayed. It has been suggested that this may have happened for Venus and may explain that planet’s slow rotation and lack of any moon. If the great impact had occurred at a later stage in Earth’s formation, the higher mass and gravity of the planet would not have allowed enough mass to be ejected to form a large moon. If the impact had occurred earlier, much of the debris would have been lost to space, and the resulting moon would have been too small to stabilize the obliquity of Earth’s spin axis. If the giant impact had not occurred at all, the Earth might have retained a much higher inventory of water, carbon and nitrogen, perhaps leading to a Runaway Greenhouse atmosphere.

The idea of a series of impacts eventually leading to a larger moon significantly muddies the waters here. It is true that in our Solar System, the inner planets are nearly devoid of moons, but we have no way of extending this situation to exoplanets without collecting the necessary data, which will begin with our first exomoon detections. Certainly if numerous collisions in an early planetary system can produce a large moon, as this paper argues, then we can expect similar collisional scenarios in many systems, making such moons a frequent outcome.

The paper is Rufu, Oharonson & Perets, “A Multiple Impact Hypothesis for Moon Formation,” published online by Nature Geoscience 9 January 2017 (abstract).

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