Isotope Park

When it comes to extinctions, animals are extremely over-represented in the public eye. Everybody knows what a dinosaur is. And most people have heard of trilobites. But seed ferns? Not so much. And when it comes to nuclides, the lack of public recognition is even more severe. When’s the last time your non-geological friends told you their 6 year old loves ⁶⁰Fe?

I would like to correct this. Extinct nuclides are vital to our understanding of the solar system, and are every bit as deserving of popularity as a triceratops. I consider this to be an injustice, a disgrace. I will not be satisfied until Hollywood blockbusters have Laura Dern fleeing in terror, as a pack of angry- and radioactive- molecules of ²⁶Al¹²⁹I₃ come chasing in pursuit.

But before this dream can become a reality, I should probably explain what they are.

Extinct nuclides are radioactive isotopes that no longer exist in detectable quantities in the modern solar system, but whose presence in the early solar system can be deduced from their decay products. A table showing several geologically interesting live and extinct isotopes is shown below, with a logarithmic timescale at top.

Extinct nuclides are valuable for establishing timelines for the first 1-50 million years of the solar system’s history. This is the time during which the protoplanetary disk cools enough to start condensing crystalline matter, and this matter then forms condrules, asteroids, planetesimals, and eventually, the terrestrial planets that we know and love today. It is constrains from these isotopic systems that tell us the chondrites and apparent bodies for iron meteorites formed in the first few million years, while the time required for these planetesimals to collide and form the four inner planets was about ten times as long. This is all a result of painstaking isotopic analysis and interpretation using a fairly simple theory. The story goes like this:

Once upon a time, there was a giant molecular cloud. One day, a nearby star exploded in a supernova. This supernova created lots of R-process and P-process nuclides, including all the short lived ones. The shockwave triggered a collapse of the molecular cloud, and initiated the formation of the sun and the rest of the solar system, with all these newly formed nuclides mixed in. The radioactive ones decayed, but the stable ones lived happily ever after.

At least, that’s what we geochemists thought. Unfortunately for us, a gang of tactless astrophysicists crashed our storytime party and spewed numerical garbage all over our tidy white cleanrooms. But that is a discussion for another day.