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Earth's Moon is a Rarity
Smivs writes "Scientists have concluded that moons like the Earth's are actually quite rare. Only 5-10% of planetary systems are likely to contain moons formed by planetary collisions. 'By the time the Earth's moon formed, when the Sun was 30 million years old, the planet formation process in our Solar System should have been approaching its end. In the latest study, Dr Gorlova's team looked at the heat signature of stars using the infrared. This allows astronomers to predict how much of that heat comes from the star itself and how much is re-emitted by dusty material encircling it.'"
Side note: In The End of Eternity, we developed time travel before space travel, and so never colonised the galaxy until we eventually discovered hyperspace in the 130,000th century and found that the galaxy was already full of other species and we had no room to expand. Eventually those from near the human extinction altered history to make sure time travel was not invented and thus ensure the expansion into a galactic empire. Apparently the idiots who wrote the sequel trilogy a few years ago failed to read this book (or Robots and Empire), and retcon'd the robots in as Eternals who killed off all competing intelligences in a bizarre and nonsensical addition.
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That (at least the text on wikipedia) is in the context of our own solar system (and the rare earth hypothesis generally is not a very strongly scientific area, it is a bit too "we haven't yet seen anything like us so we *must* be special")
The evidence brought forth by this science is looking at the current (relative) stars that are forming and finding what percentage are likely to have moon formation occurring at around the time that our moon was formed. The figure is surprisingly low - but like most cutting edge astronomy the errors are large and numerous.
To be honest, I'm not convinced that you can take a SF plot device and run away with it too far in the real world.
1. Radiation. Actually, Earth probably has the least radiation problem in the solar system, because of its strong magnetic field.
Venus, for example, started extremely similar to Earth but was doomed because its dynamo stopped (and was probably weaker to start with). So the solar wind stripped away all hydrogen, leaving it with an atmosphere of CO2.
Mars hardly has a dynamo because its core froze already. Fat lot of good it did for intelligent life there.
Mercury. Ditto. Its magnetic field is at a whole 0.1% of Earths.
So even when you factor in the different mass and conditions, it seems to me like Earth is unique in having too _strong_ shielding, not in needing some plot device to weaken it.
2. (Or 1a.) If allowing more radiation in was better, you don't need a moon for that. Just rotate slower.
(And indeed the way I remember it, the collision theory says that the same collision that created the moon actually accelerated Earth's rotation a lot.)
Or lose your water, which stops plate tectonics, which kills off the dynamo. Easy.
In fact, you need a whole bunch of special conditions to _keep_ your shielding. Losing it seems more like the norm for a rocky planet in the right band to not turn into a snowball. If the moon's positive influence were punching a hole in our shield... heh... then a lot of planets would get there without a moon just as well.
3. Mutations. Longer text, so have patience please.
Well, this is stuff that happens anyway, simply because some UV gets through, there are radioactive elements in the soil, and even because simply errors happen when transcribing DNA. Especially look again at the last parts: even if you kept something under a slab of lead, without UV or cosmic radiation at all, it would still mutate.
Most of the history of life (except for virii, some bacteria and your immune system) was about _preventing_ mutations. Your cells have layers upon layers of defenses against that kind of thing. Starting with the very fact that you're DNA instead of RNA based, and all the repair proteins, and it goes on and on.
Heck, even the fact that you age is a defense against cancer, i.e., against mutation. Your cells start with a max division counter and literally count divisions. So if that mechanism didn't break down too, a tumour would reach a maximum size and stop. Unfortunately that also means that as more and more of your cells reach that limit natuarally, there's more and more damage which can't be repaired, and you discover the fun of old age.
At any rate, any multi-cellular kind of life, actively fights off mutations. Simply because you can't exceed a certain complexity without preventing mutations. You can't have a body consisting of gazillions of cells, if they don't obey the rules. If cells in your palm randomly tried to evolve into a nose, your left foot tried to become a palm, etc, your body would break apart pretty fast.
You also have to understand that this all happens on a "good enough" basis. Your body could evolve even more fool-proof defenses -- and through the billions of years it has, slowly -- but beyond a point they wouldn't be worth the extra complexity and energy requirements. Plus, in the long term, perfect repairs would also mean an inability to evolve. So anything that got too good at it just disappeared later in the next glaciation, when it was unable to evolve.
And in rare cases, even conversely: if it's of advantage to mutate faster (if still in a controlled manner), mechanisms evolve to create just that. E.g., there are cells in your immune system which actively mutate certain genes randomly, to try to produce a protein that exactly matches a target protein. (E.g., a piece of a new virus's capsid.) There's literally an enzyme in there whose sole role is to junk a random codon (think: byte) of DNA, so the repairs would kick in and some of them would get i
A polar bear is a cartesian bear after a coordinate transform.
As you probably know, Asimov wrote not only fiction, but non-fiction for the masses, and was rightly well-known for the way in which he made science not just understandable but interesting. He explained in a number of works, including The Tragedy Of The Moon, explaining how unique the moon is.
As noted in the parent post, Asimov will often incorporate real science into his fiction.
So, what's this about how the Earth's moon is unique? Is this something new?
404555974007725459910684486621289147856453481154 in hex is "You sank my Battleship?"
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We could have had a That's no moon for a Moon.
One line blog. I hear that they're called Twitters now.
So potentially 2/9 so far...
The moon isn't just a rarity in terms of formation. It's also a rarity in terms of fortune, I think. How awesome is it that there's a big ball of rock only 200,000 miles away where we can practice our space technology on till kingdom come? How awesome is it that it has enough gravity (and water!) to make a moon base possible? I think in the next 5000 years, we'll look up at the moon and see next year's resort spa trip. Though it's a huge, lucky win, we also kind of got screwed by being so far from the next nearest star.
If I remember correctly, they claimed that without a large moon, Earth's rotational angle would wobble wildly at times and a single pole would point toward the sun all year round for millions of years, like Uranus. (Recent research suggests that Mars has done this in the past.) This allegedly would slow the formation of life.
Uranus doesn't point one pole at the sun year round. It points one pole at the same area in its "sky" year round.
Imagine if our north star wasn't Polaris, but instead, say, Aldebaran (Which is in Taurus). When the sun is in Taurus, the North pole would point at the sun. 6 months later, the South pole would point at the sun. In "spring" and "autumn" the sun would be over the equator.
So north and south of the equator, you'd have 6 months of darkness (read: COLD) and 6 months of light (read: HOT). On the equator, the sun would, over the course of a year, go from the southern horizon to the northern horizon, and back. When it was significantly above the horizon, it would rise and set in much the way it does now.
No idea if that'd be habitable or not, but it would assuredly not be "fairly stable"
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The amazing thing, to me, is that the Moon's diameter as viewed from the Earth is almost exactly the same as that of the Sun. I've heard that, of the moons in the Solar System, only a handful subtend the same arc as the Sun when viewed from their primary's surface (though of course "surface" is a tricky concept when we're talking about the gas giants), and of those, I don't think many of them are spherical. The kind of diamond rings we get during eclipses are probably quite rare.
And we're back to the moon allowing complex life.
Plot device vindicated. In all of one short line of text.
You're right of course but everything under the crust and outside of the core is molten. Our planet is very geologically active and that's helped by the moon acting on the liquid mantle. Perhaps if the moon wasn't there the mantle would cool and settle down more quickly. But what do I know, I am not a geologist...
You do realise that this is middle-school physics, don't you?
I certainly wasn't taught about this directly in school at all, 15 years ago or so, and judging by your spelling, I'm from the same place as you (UK). The inverse square law people should know (though it is not, even in places like /., commonly applied knowledge), but extrapolating it to planets etc. is not immediately intuitive. Don't assume people are trolls simple because their field of expertise is not your's (I include amateur expertise in this).
Ehm, no, I don't live in the UK... Continental Europe, though. That said, this was really standard stuff in my high school and I can guarantee you that the calculation I did here was on the curriculum. Actually, this together with Keplers laws was often illustrated with planets.
Sure, that's over 15 years ago. I guess, the curricula have been dumbed down by now.
This *is* basic physics and goes hand in hand with F=m*a. In this special case F=m*g, and you guessed that I actually calculated g for both Earth and The Moon! Since m2 was 1kg F=g in this case. Frankly, I can't imagine doing F=m*g without explaining where the g in that formula comes from. Hence you automatically come to F=(G*m1*m2)/r^2.
I'm not an amateur physicist, nor an amateur astronomer. I'm a mere dumb computer scientist, remembering what he had in school (and double-checking with wikipedia because frankly, I didn't know the value of G by heart anymore)
Oh, and finally, I just *asked* if he was a troll. I assumed he his innocence or he wouldn't have gotten the detailed reply that I gave.
Besides, aren't we supposed to be Nerds???
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