Splat! Two Moons over Miami?

Did two sub-moons collide to form our Moon?
Did two sub-moons collide to form our Moon? From Jutzi and Asphaug, Nature 476, 4 August 2011

A recent paper suggests that early in the history of the Solar System, two sub-moons collided to create Earth’s present-day Moon.  Several people have asked for my opinion on this new concept, so I will examine how this result was obtained, along with some general remarks on the nature of modern scientific research.

Over 25 years ago, a popular model for the origin of the Moon emerged at a special conference on the Moon held in Kona, Hawaii.  Whenever I mention that we had a conference in Hawaii, snickering about exotic travel boondoggles invariably follows, but you should note that at this particular conference, it was hard to get attendees out of the meeting room – the tension and excitement of a new and revolutionary discovery was that great.  The collective understanding of the then-current models of lunar origin was that they were all inadequate in one way or another.  But at Kona, a “new idea” was advocated – that a giant impact sprayed material into orbit around the Earth and that debris coalesced into the Moon.   This concept was supported by nearly all attendees and affectionately became known as the “Big Whack” model.  It seemed to satisfy most of the important physical and chemical constraints on lunar origin.  Subsequent work elaborated on the details concerning this model, but its salient features were pretty well defined at Kona in 1984.

The Big Whack has subsequently entered the realm of “settled science” in regard to lunar origin, although some dissenters remain.  But a “consensus” of working lunar scientists seemed satisfied that the origin of the Moon had become a “solved problem.”  Much of the detailed information on such a planetary scale collision comes from computer modeling, in which the basic physical parameters such as size of the two bodies, impact speed, angle of encounter, and composition in broad terms are specified as input variables.  The output of the computer model tells us how much material was vaporized, melted and ejected, and how fast the ejecta was squirted out and where it was deposited.  As you might expect, these calculations are extremely involved, requiring advanced supercomputers working day and night for weeks to churn out the results.

Some scientists tend to be skeptical of purely computational results.  In computer modeling, results are only as good as the input values and assumptions, the realism of the model, the inevitable simplification necessary to make the model fit into the computer and how carefully and thoughtfully the results are interpreted.  After the first few Big Whack computer models were run and presented at scientific conferences, various lunar workers would advance questions or problems that weren’t well explained by the existing models.  The models were tweaked to accommodate the difficulties.  In fact, it seemed that the models were amenable to endless tweaking.  If a tweak couldn’t be found, the observation was questioned or deemed irrelevant.  Models should be flexible enough to explain data outliers and the odd inconvenient fact, but they should also make predictions that can be tested by experiment or observation.  A model that is infinitely flexible ultimately is scientifically worthless.

So in regard to the origin of the Moon, we find ourselves with a solved problem for which a strong consensus of the experts exists.  Big Whack skeptics either have poor or irrelevant observations or are right-brained, qualitative geoscientists incapable of understanding complex planetary “physics.”

Which brings us back to Two Moon Junction.  The recent study suggesting that the Moon is the product of the collision of two sub-moons is an outgrowth of the same type of computer modeling done on problems in planetary accretion, including the Big Whack.  What’s unusual in the new scenario is that the two objects are relatively small to begin with (not Earth-sized, but a few hundreds of kilometers across) and collide at relatively low velocities, less than 2 km/sec.  The result of these unusual conditions, it is claimed, is that the impactor “plastered” itself onto the larger object, without forming a crater.  This “spackling” of matter adds an anomalously thick crust to the far side of the Moon and shoves semi-molten, late-stage liquids around to the near side, simultaneously accounting for two major lunar conundrums – the thicker far side crust and the concentration of KREEP (potassium, rare earths, and phosphorus) on the western near side of the Moon.

Sounds pretty good, eh?  Well, there are some issues with it.  The idea that a low velocity impact does not make a crater is counter-indicated by the existence of secondary impact craters on the Moon.  Secondary craters are made when blocks and clouds of debris ejected from an impact crater land on the Moon and dig up new craters, either as isolated single holes or as chains and clusters of multiple craters.  Since these features are formed by material thrown from the Moon’s surface, they cannot have been created at speeds greater than lunar orbital velocity (about 1600 m/sec).  Since the ballistic range for most secondaries is typically less than a few tens of kilometers from the primary, most were formed by impacts at much lower speeds, typically less than 1 km/sec.  Moreover, the addition of the far side crust as a sedimentary layer does not jibe with the observation that the lunar crust is a laterally contiguous global layer, composed everywhere of similar rocks (but varying in proportion).  The authors of the study acknowledge this is an issue, but suggest that the two sub-moons would have already formed their own crusts, probably of the same composition since they come from the same region of the Solar System.  This explanation appears rather ad hoc and elastic to me, an example of the “flexibility” for which computer models are renowned.

The Big Splat has not yet been embraced by most of the lunar science community, but will doubtless be examined and considered by many.  At this stage, it remains a model and not a description of reality, but rather, the description of a possible reality.  The distinction is important.  Neither the “votes” of the lunar science community nor the “elegance” of the model are relevant in terms of its validity.  The authors describe some possible tests of their model in the paper, but these seem to me neither particularly conclusive nor easy to accomplish.

So were there originally two moons over Miami (or rather, where Miami would one day exist)?  Maybe.  But the fact that someone can make a computer model of a complex process is not proof of its reality.  In this and similar cases, the burden is on its proponents to offer experimental tests or observations to prove their case.  In the mean time, nothing is settled and consensus is irrelevant.

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The author with his packed bicycle at San Francisco International Airport at the outset of the journey.

Okay. I’m ready. Beam me home.

If only the end of a bike tour abroad were so easy. Unfortunately, wrapping up a bicycle trip is often the only predictably joyless part—several whirlwind days of logistical duties, not the least of which is the crucial business of finding a cardboard box in which to pack the bike. In the old days (just 10 or 15 years ago, as I understand), airlines provided boxes for travelers with bicycles, much like some train and bus companies do today for a small fee. This meant that a touring cyclist could end his or her trip with a triumphant arrival at the airport, stepping off the bike and rolling it through the sliding entrance doors, fresh out of the dust and grime like a hero on horseback returning from an expedition across the frontier.

But in North America and Europe today, the hassle of locating a box suitable for carrying a bike is the traveler’s burden, usually mandating a visit to the nearest city. Here, one must establish a home base (I was lucky enough to be staying at my friend Irem’s apartment while she traveled in Australia) and then hustle around town in search of bicycle shops. Only those that sell new bikes will likely ever have shipping boxes, and even these shops tend not to keep such material around for long, meaning that spontaneous visits to bike stores rarely turn up a box. In 2006, after a 10-week tour around Greece, I took this approach, deciding to wing it at the last minute. I checked with several shops on my final day in Athens and, tragically, failed to secure a box. My bicycle assumed Greek citizenship and has likely been turned into scrap metal. I take no such chances these days with my beloved Surly Crosscheck, which I have pedaled around the Old World four times and which has rolled some 40,000 miles with me on it, and this year I made arrangements weeks ahead of time with a bicycle shop on Barbaros Boulevard, just three blocks from Irem’s place, to save me a box. Easy.

Yet heavy on my mind was another logistical pain in the neck: How was I going to get to the airport from Irem’s apartment? There is an airport shuttle (company name Havaş) that offers this service to both of Istanbul’s international airports, but its main stop-and-go point at Taksim Square was three hilly miles from the apartment. Should I walk there, I wondered, rolling my bike with one hand and toting the box in the other? Or should I should pack the bike into the box first and then move this tidy, compact piece of luggage through the winding streets of Istanbul? I kid you not: I thought about this for weeks before, at some point down around Izmir, deciding that first boxing, then lugging, my bike would be the best option.

Big mistake. Mountain lions have been known to haul dead elk or cows weighing several times their own weight up mountainsides (see paragraph seven here). And leopards frequently muscle dead prey twice their own weight up trees to stash them in the branches. Comparatively, humans are pitiful athletes. On the appointed morning of departure, I coffeed up and faced the feat ahead. The box, stuffed with clothing and my sleeping bag to pad the disassembled bike, also contained a pannier, my tools and various items like wrenches and knives prohibited in carry-on baggage. It weighed at least 50 pounds. Additionally, I had the other saddlebag and my front handlebar basket to carry, each stuffed. It was a cumbersome load.

I shoved it all out into the dark hallway and behind me the door to Irem’s apartment closed for the last time. She lives below ground level, and it took me a frightful lot of effort to climb one flight of stairs and leave the building. Just 50 feet out the door I was sweating. Along the sidewalk, I moved in spurts, lifting the long, awkward box and shuffling about 50 yards at a time before stopping to pant and shake out my cramping arms.

“Man up, Ally!” I griped at myself. “A leopard would lift this thing in its mouth, climb an apartment building, and leap all the way to the airport over the rooftops!”

It was humiliating. After moving silently and rather effortlessly some 2500 miles—like the world was a ballet and I was the gleaming star—I was suddenly clumsy, graceless and immobilized. With each push I went less far than the last, and after half a kilometer of all the heroic effort I could muster, I was out of gas.

The lesson learned? Putting one’s very means of transportation into a box while miles of travel still remain is about as clever as stepping into a canvas shopping bag and attempting to carry oneself to the market.

I looked at the sky to gain my bearings, but the sun was blocked by clouds. Indeed, it began to rain. I shimmied under an awning and surveyed my surroundings. I chanced to be standing at the entrance to a pastry shop. I asked the owner inside if he could help me find a cab. He was cold at first—he a man of business and I just another grubby soul in a city of 12 million.

So I charmed him. I explained I was from America, which always got the small-town folks giddy, and that I’d pedaled a bicycle around Turkey for two months, gone as far as Aydin, and that I needed to go to the airport.

“Aydin!” he exclaimed. “My mother lives there!”

“Many figs in Aydin!” I said. I was reeling him in.

“Beautiful figs!” he answered—and so his village nature emerged. He called for tea from the shop next door, asked a hundred questions about me and my journey, and finally flipped open his cell phone. He made arrangements with a friend to drive me in his station wagon to the bus stop.

The man arrived and we quickly bargained. “Fifteen lira,” I said. “Twenty,” he answered with a grin.

“Let’s go!”

We loaded the box in the hatchback and away we went. Nearing Taksim Square, the man, named Miko, asked that I pay him before we arrived. I slipped him the cash while he explained that paying a person for a ride other than a licensed cabbie was illegal. “We must look like friends,” he said.

“I understand, Miko. We are friends!”

Still, he was nervous as we rolled up behind the buses—and there was a policeman on the sidewalk. As we stepped out of the car, Miko said theatrically, “Alex, my friend, call again next time you are in Turkey! Now, let us get your luggage!”

And I said, “Miko! It is great to have a friend in Turkey! Friends! Friends! Friends!”

It was a ridiculous sham—but Miko was clearly intimidated by the police presence. We actually hugged each other to further the deception before he saluted and drove away. The bus was rumbling to leave. I bought a quick cup of carrot juice from a kiosk and had a last look around at the hotels, the cabs, the businessmen, the police officers and the traffic. “So long, Istanbul, city of dogs, cats and chaos. You’re not so scary after all.” I stepped aboard, and mile by mile, I was beaming myself home.

Note: I recently heard tell of Asian airline companies that require no box at all and, what’s more, gently handle their customers’ bicycles. I need to research this more, but this is phenomenal news to me, as I have plans to travel in the area in the near future to hunt durians (which, on a related note, are often forbidden by airlines in luggage because of their smell). I also have heard that some airlines, though they require boxes, may sell the boxes at the airport. This, too, is a huge convenience. But don’t take my word for it, and trust no airline. Call in advance. Meanwhile, I’ll learn more. Back soon.

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