Hello again friends! This time I'm here to share with you a tale of asteroids. Specifically, how the heck do 'they' (whoever those scientists are), know that an asteroid struck the earth around 65 million years ago and contributed to the extinction of the dinosaurs?
Firstly...if you know me well enough, you also know that I have had an obsession with dinosaurs since I was born- no really, ask my mom- my first word was "Stegosaurus" :D
But before your attention span with my tale of dinos goes extinct, I'll cut to the chase. There is an element about halfway down the periodic table (the one with all of the chemicals) called Iridium. It's symbol is IR, and it's atomic number is 77 (simply, it's the 77th element). This otherwise boring metal is very VERY rare on Earth- its estimated that it exists in 1 part per billion in crustal rock (or for every 1 iridium atom, there are about 1 billion other atoms). To put this rarity into context, gold is about 4x as common, platinum about 10x, and mercury and silver are about 80x. Simply- there isn't much iridium on Earth.
But there is one place that iridium is very common- asteroids. When asteroids impact the earth they can leave a layer of burnt (they burn because they release a lot of heat when they impact) crust that is rich in iridium. One such layer exists at the K-T boundry- precisely the period in geological history that the dinosaurs became extinct. It is now known that this was not the only factor in the extinction of the dinosaurs- there were flood basalts, changing global temperatures, anoxic oceans, etc...but the asteroid sure did speed things up quite a bit.
If you're intersted in asteroids, check out the Manson Crater in Manson, Iowa. The crater has since been filled in via erosion over the past ~70 million years.