I always say that I won't complain about my age once I get older. Basically because I'm annoyed by people that, at their late or even mid-twenties, keep saying that they're already old... If it makes you feel better, you haven't even lived a billionth of the time that the Earth already exists. I'm sure that fact is a great help. Sure.
Click here to buy your own 4.4 billion-year-old crystal!
The Earth is believed to be about 4.5 billion years old, or more accurately, at least 4.5 billion years old. So that crystal is quite close, huh? How do scientists know about the minimum age of our planet, though? It's as simple as that: they discovered rocks that could be proven to be at least that old. And once again: how do they tease out that kind of information?
The standard process geologists apply is analising the amount of specific uranium isotopes. You probably know uranium because of their use in nuclear reactors in power plants. An isotope is nothing but one possible "type" of a specific element, in that case uranium. An element is defined by the number of protons its atoms contain, but the number of neutrons is variable. Depending on the number of neutrons we talk about different isotopes, for example U-235 or U-238, "U" being the symbol of uranium. Some isotopes are unstable and decay. That is what we know by radioactivity. And now there comes the clue: every isotope has a characteristic life span, scientists refer to this with the concept of "half life". The function that describes the amount of decayed isotopes over time isn't linear but has a curvy form. The half life of a certain isotope is defined as the time it takes for exactly 50% of the quantity of that isotope in question to decay. In other words, let's say the half life of U-235 was a year. If you have 10 grams of U-235, in one year, there would be only 5 grams left, which means a loss of 5 grams. Once another year will have passed, the leftovers would weigh 2.5 grams, a loss of 2.5 grams. Three years from now you'd have 1.25 grams and so on. As you can see, the amount of U-235 you lose isn't a constant number. It's always the half of your previous weight of isotopes, thus not linear. By doing several calculations, we can determine the age of a rock by analysing the amount of an isotope it still contains, implying its half life.
The technique that allowed them to find out the age of this particular crystal, an atom-probe tomograhpy, was a little more complex since it also considers the possible migration of the studied element, in that case lead instead of uranium. Nonetheless, the basic idea is the same. Curious enough is the fact that the biggest dimension between the crystal pieces they discovered was only 400 micrometers in length, which equals 0.4 millimeters. That is just a tad larger than a house dust mite or more a less 4 human hairs.
Finally, I think it's quite interesting to point out that most rocks that were studied in processes like the one I explained earlier aren't even from Earth, which is why the title reads "oldest piece of Earth" (the crystal the article talks about was actually a part of the Earth). According to this website, the "best" prove we have of the minimum age of the Earth was deduced by measuring the lead ratio in iron meteorites. Also, moon rocks have been a great help, too. By the way, did you know that the moon was in fact a part of the Earth? One day, more or less at the very "earliest time" we know for sure the Earth already existed (the 4.5 billion years I mentioned at the beginning of this post), a meteorite with a similar size like Mars crashed into the Earth. The impact projected huge pieces of the Earth a long distance away from the planet's center. Gravitional force was responsible for those pieces to concentrate, until finally shaping the moon as we know it today. I wonder if all moons of every planet in our solar system have been "born" like this. I should do some research.
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| Talking about "the elder ones" |
Don't expect me to write a funny sentence to introduce you the new words. I said I'm busy doing research!
tad
tease out
mite
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