Behind the Beat
June 2, 2002 (c) Greg Utrecht
Part one - The Roche limit, accretion and Binary stars

It was Charlie who first talked to me about Pangea.  Those kind of ideas had always seemed strange to me, that you could tell much about how things were 5 billion years ago: this theory was the one that said there was originally only one continent.  Hard for me to imagine how anyone could have enough evidence to be sure, sort of like the people who smashed atoms and parts of atoms and then after looking at the pieces said "this is how they must have used to work, before we smashed them". 

But Charlie believed in Pangea, only one continent, most of it in the northern hemisphere, and a lot of it around longitude 0.  Then one day I was reading about the Roche limit, an idea from an early astrophysicist, he must have been German, but if anyone knows I'd like to hear where he was from.  Roche computed that there was a limit of how close two large astronomical bodies could get to each other, and that this limit was a function of mass, and density of the two objects.

The specifics of the Roche argument was that if the moon and the earth got close enough the higher density and gravity attraction of the earth would pull the moon apart.  This was especially bad if the near part of the moon was within the destruction zone, and the farther part of the moon was far enough away that it was unaffected, not attracted fatally toward the earth.

The Roche limit, how close the two objects could get, was used in the Roche theory, that the moon had actually once not been in orbit of the earth, but had passed close enough to be captured.  The Roche theory states that the moon had passed above the earth, been pulled around into earth's gravity, and then was pulled partially apart by the Roche Limit before finally settling down into an orbit around the earth, but not over the north pole like at first. The theory says this happened two and a half billiion years ago, about half of the lifetime of the earth.  

Some speculate in writing that this pulling apart of the moon due to earth gravity and being too close is what caused the lunar oceans - these are what is left after the lunar mass stabilized - the parts of the moon that were once pulled out and some landed on earth and some on the moon.  The craters of the moon were created by parts falling back down, the Pangea was created from the parts that fell to earth.  The lunar seas are the scars left behind on the moon.  I suppose that just after this happened that the moon was not nearly a sphere like it is today.

Some have speculated that this Roche disaster is the reason that the earth's crust and the lunar geology are so similar.  According to this theory the earth's crust - the Pangea - is actually lunar in origin.  So the Apollo astronauts in addition to finding that the moon was geologically active also found that it was rich in metals like nickel, titanium, bauxite aluminum and soil that was rich in nitrogen hydrogen and oxygen compounds, just like earth.

One little known observation of the moon teams was that when Apollo 14 visited one of the 5 year old Pioneer spacecraft, they observed that the soil and dust that had fallen on Pioneer was brown, not white.  Other Apollo astronauts had noted how extremely bright the moon was, and that it affected their vision for days after leaving there.  

But lunar soil under normal lighting conditions looked brown to Alan Bean, and I have noticed that if you place a mirror or if light reflects off a mirrored window onto dirt (such as behind Kofax near the smoking section) then the earth soil looks white and bright and not brown like we expect it to.  The moral here is that if do go to the moon, take some ray-bans.

To summarize: the Roche limit theory says that after 2.5 Billion years the moon got near the earth - somehow- and was captured in an orbit the included having the moon partially destruct, letting the whole of Pangea fall onto a watery earth just south of England, leaving the lunar seas and a cloud of asteroid sized objects that eventually cratered the rest of the moon or fell onto earth.  Without this complicated history places like Poughkeepsie would not exist.

Accretion 

The Roche theory raises some questions which reputable people do not write down - for example the idea that earth soil is from the moon is acceptable, but the idea the Pangea was the moon is going too far.  So no one has written down the Pangea connection before, not even Gerald O'Neill who has written very carefully and comprehensively about all these kinds of ideas

But the Roche Pangea idea begs the question of how we got oceans and atmosphere, how we keep it, and how come the moon doesn't have it.  My understanding of this question is simply that the moon was not dense enough to have an atmosphere, its surface is cold because it does not have the blanket of atmosphere to refract light and hold in the heat.  Without the atmosphere no oceans.

Earth has an atmosphere because we are dense enough, even without the Roche disaster, to hold onto hydrogen and nitrogen and oxygen molecules when we hit them while orbiting around the sun.

The process is called accretion and if you care to look up the numbers and do the math you will find that as the earth moves in its orbit, and as the sun moves toward the constellation Cygnus, we bump into a lot of molecules every second.  The rough number is that there are 100 molecules per cubic centimeter inside the solar system, and roughly a tenth as many if you are between the stars.

This isn't many when compared to Avogadro's number - that there are 10 with 23 zeroes of molecules in a small space - but still the earth is running into a lot of air molecules all the time.  Eventually this accretion ended up with earth having an atmosphere, and no matter how many air molecules we run into we are losing about the same amount.  Smaller planets and moons don't accrete, bigger ones do.

Our atmosphere is really quite complicated with varying temperature and pressure properties according to height, as well as various radiation belts such as the van Allen, and magnetic belts and electrical fields and Alfven numbers.  It's all related to accretion and the bigger astronomical bodies all have it.

It is what protects us from meteorites, many of which are probably left over from the Roche disaster, small asteroids orbiting in something similar to an earth orbit, although many of them are from some similar disaster that must have resulted in the asteroid belt out past Mars on the way to Jupiter.  

Asteroids nearly in an earth orbit were predicted by an American astrophysicist in the 1960's and experimental observations proved that such things probably exist.  Since the Roche theory predicts these will be similar to lunar material, they will be rich in common metals.  Since they are already nearly in earth orbit and are not attached to some heavy object like the moon or the earth, the material nearly in earth orbits will be the cheapest material for use when constructing large objects in low earth orbit.  

Some funny numbers get passed around, like it is 4000 times more expensive to lift something 100 miles from the earth as it is to left it from the moon and put it near the earth.  Lunar gravity is just lower; the energy cost is 4000 times less.  

But the asteroids following us on the other side of the sun will be cheaper still; at least a few hundred times cheaper than mining metal on the moon.  People like Gerald O'Neill who studied all the original work on this topic has written that we can make mile across habitats in space, make enough of them to support 6 times the land area that's available on earth, and still not crowd the skies, or cause too many shady days for anyone.  O'Neill implies that it would be reasonable to support 40 billion people in habitats close to earth.  Area 51, outer space, will require a new system for social security numbers.

Binary Stars.

Like the theory about why some planets have atmospheres, I have a theory about why some stars have Binary suns.  I'm not an expert in solar physics any more than I am an expert in atmospheres or radiation belts.  But when those guys lit off the test nuclear device at Alamogordo, some of them were aware that we really didn't know that much about how fusion stars like our sun get started. 

The idea that worried Rabi was that for some planets with atmospheres it might be possible to ignite a solar fusion reaction - like lighting a star - and no one knew if the atomic device at Alamogordo was going to turn the earth into a miniature sun.  Smarter people would have tested the thing underground.

Fusion powers our sun, and when we get into space it will be nice to have a fusion reactor only 93 million miles away giving light and heat and power to the mile across habitats that we are going to build.  

The idea behind binary stars is that somebody might have thought that if one fusion engine was good, then a second one might be useful too.  In our system the best candidate for a second fusion engine would be Jupiter.  Perhaps we could shoot a few thermonuclear missiles into the red spot, igniting the whole atmosphere into a man made sun and heating up the colder parts of the solar system.  This would be nice because the moons of Saturn, especially the earth sized moon called Titan, might become warm enough to live on.  Two fusion engines are better than one, an idea that Arthur C Clarke wrote about in his novel 2060, calling the Jupiter-sun by the name of Lucifer.

Lots of binary stars exist, and from an engineering standpoint a binary sun can double the habitable potential of a star system, so that the argument could be made to the Ceti people that binary stars might be more likely to have intelligent life, not less.  Binary systems could be man made.

But if Binary systems were man made, wouldn't it be possible that the Roche disaster of 2.5 billion years ago was also man made?


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Next time on Behind the Beat:  

Seems like an eternity:  klystrons, phasitrons and blasters - getting from here to there