Latin1 was the early default character set for encoding documents delivered via HTTP for MIME types beginning with /text . Today, only around only 1.1% of websites on the internet use the encoding, along with some older appplications. However, it is still the most popular single-byte character encoding scheme in use today. A funny thing about Latin1 encoding is that it maps every byte from 0 to 255 to a valid character. This means that literally any sequence of bytes can be interpreted as a valid string. The main drawback is that it only supports characters from Western European languages. The same is not true for UTF8. Unlike Latin1, UTF8 supports a vastly broader range of characters from different languages and scripts. But as a consequence, not every byte sequence is valid. This fact is due to UTF8's added complexity, using multi-byte sequences for characters beyond the general ASCII range. This is also why you can't just throw any sequence of bytes at it and e...
Matter is composed of chemical elements. Every chemical element has its own arrangement of protons, neutrons, and electrons. As a consequence, each element also has its own atomic number, which indicates the number of protons in its nucleus.
Every element also has varying isotopes—differing versions of itself that possess a non-standard number of neutrons in its nuclei. Some of those isotopes are unstable (radioactive), experience decay, and turn into different elements over time.
The process of tracing these radioactive impurities in materials is known as radiometric dating. For example, thanks to meteorite samples, we know that the Earth is around 4.5 billion years old. But how, exactly, do we know this?
There are various types of radiometrics, and the process can involve different elements—from carbon, rubidium, potassium, samarium, uranium, to thorium.
The elements uranium and thorium both decay into lead over billions of years. Thus, it is possible to determine the age of materials like rocks and meteorites by measuring various isotopes of lead and retroactively inferring their age: Pb-206, Pb-207, Pb-208, and Pb-204. All of these, except for Pb-204, are considered radiogenic isotopes.
This trick relies on the fact that uranium and thorium decay in a constant, predictable way over time. For example, uranium-238 has a half-life of about 4.5 billion years, and it decays into lead-206. Uranium-235 has a half-life of approximately 700 million years and decays into lead-207.1
| Parent Isotope | Stable Daughter Product | Currently Accepted Half-Life Values |
|---|---|---|
| Uranium-238 | Lead-206 | 4.5 billion years |
| Uranium-235 | Lead-207 | 704 million years |
| Thorium-232 | Lead-208 | 14.0 billion years |
| Rubidium-87 | Strontium-87 | 48.8 billion years |
| Potassium-40 | Argon-40 | 1.25 billion years |
| Samarium-147 | Neodymium-143 | 106 billion years |
Lead-lead dating does not directly involve uranium. Instead, it involves analyzing the ratios between specific amounts and isotopes of lead, the decay products of uranium and thorium.
Uranium-lead dating, on the other hand, relies on measuring the ratios via the decay routes of uranium and thorium. This method frequently involves sampling the mineral zircon. But it can also involve other minerals, such as monazite.
Why minerals? Why not just measure rocks? When formations like rocks develop, there’s a chance they may contain some preexisting amount of lead. This can derail measurements and produce unwieldy results. Additionally, the Earth is dynamic—magma and rocks are constantly undergoing change and having their geological clocks reset and tampered with.
Zircon, a crystal mineral, unlike rocks, essentially offers a clean starting point for the task of radiometric dating—because any lead found inside zircon almost certainly originated from decayed uranium and wasn’t there beforehand.
Due to zircon’s crystal lattice structure, it’s picky about its elemental friends. The structures like those found in zircon are useful for radiometric dating because they tend to reject lead during their formation while letting uranium in.
Theoretically, lead in an unusual oxidative state, like Pb+4, could potentially make its way into zircon.2 But the most common compounds of lead found are in a +2 oxidative state, not +4—this is due to the inert pair effect.3
Despite the advent of “uranium-lead” dating, “lead-lead” dating is still useful. One of the earliest measurements to determine the age of Earth was a lead-lead measurement. As it turns out, meteorites are quite useful in offering a bigger than earth point-of-view.
Meteorites, which mostly come from the asteroid belt between Mars and Jupiter, are remnants from the formation of the early solar system. As such, they largely remain unchanged. They serve as a useful reference point and cosmic timestamp hinting at when our solar system began.
Lead dating was used to determine the age of the Canyon Diablo meteorite from the Barringer Crater. The result suggested the meteorite was roughly 4.5 billion years old—a value that has been replicated hundreds of times by other tests.
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