10. Meteorite with Fusion Crust
The story of possible fossils from Mars begins with a meteorite, a martian meteorite, found in Antarctica. Meteorites fall to Earth all the time, but very few are found and collected. Even fewer are observed falling and landing on the ground. So how can you tell that a rock is a meteorite if you don't see it fall?
The first thing to look for in a possible meteorite is a fusion crust, a thin layer of black or dark glass that coats the meteorite as if it had been dipped in tar. This meteorite is covered with a black fusion crust, except where it has been broken off to show the meteorite's light gray interior.
The fusion crust forms when the meteorite blazes through our atmosphere on its way from interplanetary space to Earth. We see the light of this passage as a meteor streak in the sky. Most pieces of interplanetary dust or rock make meteors but not meteorites — they burn up completely in the air. As a meteorite blasts through the air, it gets so hot that its outside melts; when the meteorite cools down before landing, the melted rock solidifies to glass, which is the fusion crust. Freshly fallen meteorites have a black, glassy fusion crust, as do most meteorites collected from Antarctica. The Antarctic cold and dryness preserve the glass; elsewhere on Earth, the fusion crust weathers rapidly to become dull and rusty brown, and can look just like many Earth rocks.
Meteorites come in many different varieties. Iron meteorites are probably most familiar; they are typically large and make impressive museum displays. But nearly 90% of all meteorites are stony — made of the same materials and minerals as Earth rocks. But unlike Earth rocks, nearly all meteorites contain some iron metal. Most stony meteorites are classified as chondrites (like this meteorite, ALH 90411). They are made of millimeter- sized balls and clumps of minerals that formed directly from the gas and dust of the early solar nebula. The rest of the stony meteorites are called achondrites (like ALH 84001). They do not contain the millimeter-sized balls and clumps of ancient minerals; rather, they are igneous rocks, formed from molten lava on planets or large asteroids.
ALH 90411, a chondrite meteorite collected in 1990 from the Allan Hills area of Antarctica. The cube at the lower left is 1 centimeter on a side. S91-38433, NASA/JSC.