What are noble gases? Why are we interested in them? And what do they tell us about how the earth and solar system formed?
Helium, neon, argon, krypton and xenon.
These gases are very rare on Earth and in meteorites. They are unreactive. And each element has many different forms, known as isotopes.
Isotopes are forms of the same element – they have the same number of protons and electrons. But they have different numbers of neutrons which lead to them having different masses. Imagine your friends and family – you are all human beings, but you all have different masses. In the same way, isotopes are the same element but have different masses.
Helium has 2 isotopes: 3He and 4He. Neon has 3: 20Ne, 21Ne and 22Ne. Argon has 3: 36Ar, 38Ar and 40Ar. Krypton has 6 isotopes: 78Kr, 80Kr, 82Kr, 83Kr, 84Kr and 86Kr. Xenon tops the table with a massive 9 isotopes: 124Xe, 126Xe, 128Xe, 129Xe, 130Xe, 131Xe, 132Xe, 134Xe and 136Xe.
The unreactive nature of the noble gases means that they do not readily undergo chemical reactions to form compounds with other elements. Any changes or differences observed in the elemental or isotopic composition are due to physical processes.
Only very tiny amounts of these elements are present in both terrestrial rocks and meteorites. This means small changes in their isotopic and elemental composition can be detected, differences which would not be noticeable against a background of more abundant elements.
Once such example where we can learn about the history of a meteorite is by looking at 129Xe. 129I is a radioactive isotope of iodine which decayed to form 129Xe in the early solar system, with a half-life of just under 16 million years. Once a meteorite has formed, the iodine trapped in that meteorite continues to decay, and the xenon it produces remains trapped inside the rock, leading to an excess of 129Xe relative to the other isotopes in many meteorites. The only other isotope of iodine is stable, 127I. We put meteorites samples into a nuclear reactor to convert the 127I to 128Xe, and then measure the ratio of 128Xe to 129Xe using the RELAX mass spectrometer. This tells us the ratio of 127I to 129I when the meteorite formed, which in turn tells us how old the meteorite is. This process of dating meteorites is known as I-Xe dating, and will be discussed in more detail in the future.
Studying the noble gases helps us to unravel some of the secrets of the solar system, discover more about the 4.5 billion year history of a meteorite, and learn about the journey it made to the Earth.