Scientists had to look deep inside, in the salts contained within the rock. I popped the halite in the machine and got this amazing peak showing an abundance of xenon-129. I hadn't really expected that, so it was quite stunning. Xenon is a noble gas, which was key to dating the rock, Gilmour said. Noble gases do not readily combine with other chemicals.
Noble gases are very volatile.
The xenon-129 was a dead giveaway. That means half of it decays in 15 million years, half of that in another 15 million, and so on. After 100 million years, essentially none is left. It was present when the solar system began to form 4.5 billion years ago. It was gone 100 million years after.