Some interesting Lunar news out this last week, about some folks out in California who came up with a computer model that implied a second Moon at some point in the distant past. It was given surprisingly broad coverage in the media:
Here’s the abstract:
“The most striking geological feature of the Moon is the terrain and elevation dichotomy between the hemispheres: the nearside is low and flat, dominated by volcanic maria, whereas the farside is mountainous and deeply cratered. Associated with this geological dichotomy is a compositional and thermal variation, with the nearside Procellarum KREEP (potassium/rare-earth element/phosphorus) Terrane and environs interpreted as having thin, compositionally evolved crust in comparison with the massive feldspathic highlands. The lunar dichotomy may have been caused by internal effects (for example spatial variations in tidal heating, asymmetric convective processes or asymmetric crystallization of the magma ocean) or external effects (such as the event that formed the South Pole/Aitken basin or asymmetric cratering). Here we consider its origin as a late carapace added by the accretion of a companion moon.
Companion moons are a common outcome of simulations of Moon formation from a protolunar disk resulting from a giant impact, and although most coplanar configurations are unstable, a ~1,200-km-diameter moon located at one of the Trojan points could be dynamically stable for tens of millions of years after the giant impact*. Most of the Moonâ€™s magma ocean would solidify on this timescale, whereas the companion moon would evolve more quickly into a crust and a solid mantle derived from similar disk material, and would presumably have little or no core. Its likely fate would be to collide with the Moon at ~2â€“3 km/s, well below the speed of sound in silicates. According to our simulations, a large moon/Moon size ratio (~0.3) and a subsonic impact velocity lead to an accretionary pile rather than a crater, contributing a hemispheric layer of extent and thickness consistent with the dimensions of the farside highlands and in agreement with the degree-two crustal thickness profile. The collision furthermore displaces the KREEP-rich layer to the opposite hemisphere, explaining the observed concentration.”
Seems fairly straightforward, but there are some elements in the story that just don’t add up for me. For full disclosure I am not a Lunar scientist. We aren’t minting them anymore. There’s no school you can go to right now in the U.S. that offers a degree in Lunar science. Planetary Geology is usually your closest option. However, I have read through a fair chunk of the non-fiction section of the Lunar Library, and you can find reviews of many of them in the Book Reviews menu option over on the left.
So my questions are sincere, and not from ignorance. I do want to point out that there is a sharp elevation dichotomy, as they say. Here’s a video of LRO data:
And here is a scan of a postcard I got from the JAXA folks at an LPSC a couple of years ago. You can note that it is not quite as sensitive elevation-wise as LRO in the Mare Orientale region.
The one on the bottom is a gravity map, which highlights that while the far side has generally higher elevations, that doesn’t translate much to heavier when compared with those mascons on the near side.
One thing to be very careful of here is the Aitken Basin. It’s not just a big blue-purple bruise on the far side, it is an enormous chunk that has been taken out of the rear end of the Moon. The movie above is a bit deceiving, as the data is draped onto a sphere, just like you would see in a classroom. If you want to see what the Moon -really- looks like, feed the elevation data into a 3D printer and take a look at the result. Looking at the traditional near side it looks fine, round even. But then turn it sideways. It’ll blow your mind. Shout out to the JAXA kids that blew my mind at LPSC.
So the basic premise is that the reason there is more green-green, yellow and red on the far side as compared with the near side is because, when the Big Splat of Theia hitting the Earth and sloshing off a whole bunch of material into orbit happened, that material coalesced into not one but two large orbiting bodies. Eventually, the second, smaller moon started drifting into the Moon’s gravity well, smacked it at a non-cosmic velocity, and splashed itself on the Lunar far side. And so the far side is higher than the near side.
For more full disclosure purposes, I put very little faith in computer models. Just because I work in the financial industry doesn’t mean I don’t work with models. Budget projections are but one example. Sensitization of financial results another. Value at Risk (VaR) I laughed at when it was introduced, and had to explain to my bosses why it was nonsense. I’m well aware of the limitations of models, as well as their (limited) value.
The first question I have I asterisked in the abstract: “How does a large object stay stable at an Earth-Moon Lagrange point for tens of millions of years when you’ve got the Sun, Jupiter, Saturn et al tugging at and perturbing it?”
The reference in the abstract is to Cuk, M & Gladman, B.J. The fate of primordial lunar Trojans. Icarus 199.2, 237-244 (2009).
I read through the article, and get their point, although it quickly devolves into mathematical tetrapyloctomy. In essence, when the Moon and moon were much closer, the differences in the gravitational “warps” of space caused by the large bodies (which we know in our simple system as the Lagrangian points after the guy who worked out the mathematics) would have been such that a fair amount of material could have accumulated at the L-4 and L-5 points until the Moon reached a certain distance from the Earth, the resonance of the orbit shifts, and all of a sudden the mathematical models start going apeshit, leading to harmonic instabilities that pitch the moon on its course with destiny and our Moon. They do have a good point – 4.0Bn years ago the Moon would have been a lot closer to the Earth, and orbital resonance is a subtle but important part of orbital mechanics.
Interestingly, the paper cited seems to be arguing that any large moon could have broken apart on its way in, providing the large near-side impacts attributed to the “Great Cataclysm” of impacts that formed the nearside basins nigh on 3.9 billion years or so ago. The thinking there is that since there hasn’t been much evidence of the “Great Cataclysm” much of anywhere else in the rest of the Solar system, at least based on current data, and the impactors that created the great basins must have originated near Earth.
Which brings me to my second question – what is the effect of the transition of the second moon through the Moon’s Roche limit? I’m rather disappointed that none of the science articles seemed to address that question.
The Roche limit is the distance from a large body at which the inverse square law of gravity starts to have a significant effect on another large body approaching the first. The second body is experiencing so much different gravity between the near point and farthest point that it starts getting pulled apart. It’s the same basic thing as you getting pulled into spaghetti as you fall into a black hole, ‘cept on a planetary scale. If the body is as big and moving as slowly as they indicate, I guesstimate that it would have spent about eight minutes or so getting taffy-ed transitioning that Roche limit.
My third question has to do with the model. What was the level of granularity of the second Moon? What were the perturbatory inputs other than the Earth and Sun? There are so many things I would have to know about the model before I could put any small amount of faith in its output. And frankly, I don’t feel like paying the $32 for the paper to find out.
So color me as skeptical of the claim of a second early moon. There’s just too many pieces that don’t feel right for me to lend it much credence. My view may change if I ever do get to read the article. I anticipate a trip to Half-Price Books in a few months should prove fruitful, so I’ll let you know if I change my mind.
Which re-minds me, we do have a second moon, called Cruithne, that is currently on a funky potato chip/horseshoe orbit and so doesn’t visit much (and is why some folks deride it as not even a moon). Here’s a cool picture of our second sister and her wacky orbit: