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Cosmos - "The Clean Room" Clarification Question about Age of the Earth.
June 16, 2014 5:12 PM   Subscribe

My wife and I just watched the seventh episode of Cosmos called "The Clean Room" which tells us (among other things) about how Clair Patterson determined the age of the Earth. The episode explains Patterson used meteorite fragments, such as those from the Canyon Diablo crater, because they were assumed to be leftovers from the creation of the solar system. My question is one of clarification; specifically, how they could be sure that assumption was correct?

I've tried doing some reading on this before coming here and while it's answered my question in part, I'm still not clear on one crucial part of this.

As the Wikipedia entry for the episode says, "... scientists have used the debris from meteor impacts, such as the Meteor Crater in Arizona, knowing that the material from such meteors coming from the asteroid belt would have been made at the same time as the Earth."

That's fine; I completely understand that the asteroid belt is basically the leftovers from the formation of our solar system. But I think I either missed the part of the episode (or they didn't explain it very well) that said how we, and indeed Patterson, knew the Arizona Meteor Crater was a meteorite from the asteroid belt? How can we be sure it didn't come from outside our solar system before eventually landing on Earth?

It doesn't help that most of the stuff I've read on Patterson's work keeps using the words "assume" and "assumption" about the nature of meteorites. To me, who doesn't know any better, that seems like a big assumption to make.

To be clear, I'm not here to dispute Patterson's findings or the age of the Earth. I'm quite certain people far smarter than me are right when they say the Earth is around 4.5 billion years old. I almost have a clear picture of how he worked it out. I just need someone to explain to me (as simply as possible, please) how we know the meteor he used came from the asteroid belt and not from somewhere outside our solar system?
posted by Effigy2000 to Science & Nature (12 answers total) 5 users marked this as a favorite
 
The simple answer is "They can't."

What they do is to try to use multiple means to reach the same answer, and if those all give about the same result then they assume that they're right.

As to the meteor, it's not so much that he needed it to be from the asteroid belt itself as that he needed it to be from the same nebula that created the entire rest of the solar system, and that's a pretty safe bet.

But it's all probabilities. Nothing in this kind of science is ever absolutely certain. And every once in a while there's an amazing discovery which turns everything on its head and forces everyone to reconsider everything they thought they knew. (A recent example of that is the Alvarez hypothesis.)
posted by Chocolate Pickle at 5:20 PM on June 16


Spectrophotometry measurements can be used to determine the mineral composition of asteroids and similar bodies inside and outside the solar system. Perhaps an inference was made by comparing those measurements with meteorite impact samples; if there is a strong pattern match between the from-within-solar-system bodies and the meteorites, then a possible inference is that they are sourced from the same place.
posted by Blazecock Pileon at 5:21 PM on June 16


First of all, something would have had to have been pushing that meteor from outside our solar system. The things randomly flying around in space aren't just floating, they're being pushed and pulled by one or another planets' or star's gravitational field. They can get knocked off path if they run into something, but space is just way mad big, and any boost that an outside-our-solar-system rock would have gotten would probably have petered out way before it got to earth - because it would have to have gotten past the gravitational pull of Pluto, Neptune, Uranus, Saturn, and Jupiter first before it hit the asteroid belt, and so most likely it'd get trapped there, in the gravitational field that keeps most of the asteroids there in the belt in the first place.

But something from outside our solar system that made it there could have been the thing that knocked into another asteroid and sent it towards Earth in the first place.
posted by EmpressCallipygos at 5:23 PM on June 16


EC, you do realize, don't you, that there's a lot more space in the solar system where there aren't any planets than there is where there are planets? A meteor could have come in from galactic north or south and not run into anything interesting. And even if you assume it came in on the ecliptic, it still could easily have avoided anything massive.

Planets, even Jupiter, are itty bitty by comparison to the size of the solar system. And there aren't really all that many of them.
posted by Chocolate Pickle at 5:27 PM on June 16 [1 favorite]


It's my understanding that the asteroid belt is more of an asteroid sphere, though. Isn't it?
posted by EmpressCallipygos at 5:40 PM on June 16


EC - nope, it's a belt. Perhaps you're thinking of the Oort cloud, which is spherical, but contains far more space than matter (and is unlikely to stop anything extrasolar on its way in to the inner Solar System).

Also, gravity doesn't "trap" things in orbits unless their location and velocity are right (otherwise Earth-crossing asteroids and comets would get "trapped" in Earth's orbit, or Jupiter's orbit, or the asteroid belt, or wherever).

As to the original question, anything coming in from outside our Solar System will almost certainly be travelling very fast relative to us, so there's probably also energy related measures that allow known impacts on Earth to be attributed to locally sourced materials.
posted by russm at 5:58 PM on June 16


Also, the asteroid belt is very sparse. Asteroids are thousands of miles apart. The odds of something coming in on the ecliptic hitting an asteroid are vanishingly small.

EC, I'm very sorry to be blunt, but your answer is total nonsense.
posted by Chocolate Pickle at 6:26 PM on June 16


I've managed to track down some more articles which say that meteorites from outside our own solar system (called extrasolar metorites) are very rare... so much so that they think the chances of an extrasolar metorite hitting Earth is something that would only occur once every 10 million years.

Most meteorites, like the ones in the asteroid belt, are "isolated and closed systems" which means (I think) that meteorites pretty much stay in their own solar system. I note, however, that the word "assumption" is used a lot in that essay.

It also turns out that Patterson also didn't rely on just the Arizona meteor. It was his primary source (not sure why) but he also measured several other samples across the world (which I think Cosmos mentioned but probably didn't make explicit enough for dumbshits like me). Doing this he could, as Blazecock Pileon mentioned, get a strong pattern which backed up his measurements.

So basically I'm guessing that since extrasolar impacts are rare, and since most meteorites stay in their own system, that's how he made the (relatively safe?) assumption that it was from our own asteroid belt.

Do I have that right or does anyone want to add/correct me on this?
posted by Effigy2000 at 7:38 PM on June 16 [1 favorite]


The only correction is your implicit assumption that the asteroid belt is the only source of random rocks in the solar system. Actually, the Oort cloud and the Kuiper belt are much larger and contain more mass.

EC: Please read about momentum. There isn't any propelling going on.
posted by Chocolate Pickle at 8:28 PM on June 16


So basically I'm guessing that since extrasolar impacts are rare, and since most meteorites stay in their own system, that's how he made the (relatively safe?) assumption that it was from our own asteroid belt.

Pretty much. There have been studies about extrasolar meteorites and the conclusion is that they are there, but also extremely rare. If you know the objects speed and its orbital path, you can sort of conclude where it came from, making some assumptions about perturbations in the interstellar medium.

This paper gives the number of particles coming from outside the solar system and concludes "meteoroids arriving at the Earth of 6 · 10^-6 meteoroids/km2/h. For our 2r results, the lower limit was 0.003%, with a flux of 2 · 10^-5 meteoroids/km2/h. The total number of events was too low to be statistically meaningful in determining any temporal or directional variations." This is estimated to be about 0.0008% of all the meteor events they studied.

As it happens, extrasolar objects are mainly detected through their very high velocities, high enough that it is unlikely the result of a slingshot effect from a solar system object. They may have been shot out from somewhere else at a reasonable velocity but with another reference frame.
posted by vacapinta at 1:36 AM on June 17 [1 favorite]


since most meteorites stay in their own system

This is part way there (and it's true; asteroids are gravitationally bound to the sun and only ejected via interactions with planets/other objects, which is infrequent but non-zero), but I think the bigger issue is that the space between stars is really, really empty compared to the planetary systems around stars.

To make a wacky illustration, think about throwing (very hard, ignore gravity) a rock towards the sky. What are the odds of it being aimed so precisely that it ends up hitting a planet around another star? You can flip this around and ask, since throwing a rock (very hard) is so very unlikely to hit a planet, then even with all of the nearby stars flinging rocks randomly it's still very unlikely that they are going to hit us in a substantial number.

When you compare that with, say, the number of asteroids we see in the solar system (which are easily scattered about within the solar system) and the number of meteors we see in the sky, it doesn't add up that they are all extra-solar, so it makes sense that they are mostly of solar origin.

Caveat: this is a hokey explanation that may be ahistorical; I don't know the history of planetary science to know what their thoughts would have been at the time. I think you're right that part of the reason why this assumption was trusted was because the observed data fit into the picture well. If meteorite ages had turned out to be all over the place, then they might have gone back and changed their assumptions.
posted by kiltedtaco at 7:47 AM on June 17


Imagine that I have a giant bag of letters received by a person. They're from all sorts of dates in that person's life.

Some very few stand out, because they appear to be (probably, we bet) the oldest in the pack.

Suppose that almost all in that group were mailed in 1893; no other letter we happen to look at is nearly that old.

It's reasonable to assume that these are the oldest letters in the bag, and by inference, possibly the first ones the person ever received.... since it looks like they kept EVERY damn piece.

That's not proof, of course, that they were the first, but it's an interesting point that most of them are from the same point in the past.

Now, as others have noted, that we have "the oldest" letters(meteorites), we can analyze them for content to see if they were written to a very young person. If they begin "Happy 1st Birthday!", we're probably right about these being from the person's first year.

Or, for the meteorites, spectroscopically, look at their radioactive footprint, etc... to see if they match up with our concept of the early Universe makeup.

Happy 13,798,000,000+/-37,000,000 Birthday, Universe! Now let me get away from here before you blow out the candles on that cake...
posted by IAmBroom at 7:58 AM on June 17


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