Hi all, in my intro I said I was a cosmochemist. What does this mean? For practical purposes, it means I use meteorites and other extraterrestrial samples to understand the origin and evolution of the chemical elements in space. So I think I’ll start with a primer on meteorites 
Meteorites are rocks that fall from outer space. Really large ones kill dinosaurs, somewhat large ones make big holes in the ground, but most are small (from sub-millimeter to meter size) and provide extremely valuable scientific information
The vast majority of meteorites come from asteroids (a tiny fraction are known to come from the Moon and Mars), small rock bodies orbiting the Sun between the orbits of Mars and Jupiter . How do we know this?
We know origin of some meteorites because of orbit reconstruction. When meteorite comes thru atmosphere, it creates a dramatic fireball that can be seen for 100s km or more. If this is observed from mult. directions, trajectory can be determined and extrapolated back into space.
This has been done for many meteorite falls now (there are automated networks all over the world, e.g., fireballs.ndc.nasa.gov) and they consistently show them originating in the asteroid belt.
Meteorites fall all over the Earth, sometimes very dramatically like Peekskill meteorite from anim. 2 tweets ago or the Chelyabinsk fall witnessed by thousands of Russia dash cams in 2013, but …
youtube.com/watch?v=CfS3q3…
youtube.com/watch?v=CfS3q3…
… most are now collected in deserts, since there are few terrestrial rocks around and dry conditions allow meteorites to last a long time. These include both hot deserts like the Sahara, and Antarctica
Antarctica is a particularly good place to find meteorites since glacial processes concentrate them on ice fields. The US and other countries regularly send teams there to search. Here I am in early 2001 in a place called Meteorite Hills, collecting a meteorite #ANSMET
Tens of thousands of meteorites, falls and finds, have now been collected and vast majority are held by museums/governments and available for scientists to study. They generally fall in three types: stony, iron, and stony-irons
Most meteorites are stony, primarily rock (though many have a fair amount of iron-nickel metal in them & divided into “chondrites” and “achondrites.” Achondrites are volcanic rocks whereas chondrites are "primitive" and from undifferentiated asteroids. Much more on them later
Iron meteorites are big chunks of iron-nickel metal. Because they are strong, many are bigger than typical stony meteorites. They were once the metal cores of asteroids that melted and differentiated and got smashed apart & thus provide clues about the deep interiors of planets.
Stony-irons are mixtures of metal & silicate rock. Most beautiful are pallasites, with green olivine (peridot) crystals in metal matrix. Their origin is highly debated. Maybe they come from core-mantle interface of a differentiated asteroid, maybe they form by asteroid impacts
More later on science of meteorites, specifically why I study them …
