Tuesday, March 8, 2011

To Wish Upon a Shooting Star...

First off, apologies for not posting anything for a while. I’d make excuses, something about a PhD, but no-one would listen anyways.

Meteorites, aren’t they cool! I reckon, maybe it’s because I’ve decided to dedicate at least three years of my life to studying the things, but they are cool anyways. So, what are they?

Have you ever stared up at the night sky for a while on a clear night when the moon isn’t out? If you have, then chances are you’ve seen a shooting star, a little flash of light which briefly cuts across your view of the stars. Well, this little flash of light is actually a small piece of rock which is flying into our atmosphere. The Earth collects about 40000 tons of the stuff every year, so every night you should be able to see a couple of meteors if you hang out for an hour or two. Most of the stuff is really small, from about a few thousandths of a millimetre to maybe the size of your fist or a beach ball.
A meteor shower, the Leonids in 2001

The light itself is basically a trace of the rocks journey through our atmosphere. When they come towards Earth, these little rocks are travelling really fast, somewhere in the order of 10 to 70 km per second. By comparison, a bullet from an average gun travels at about 300 to 900 metres a second. Because they are going so quickly, the atmosphere of Earth acts like sandpaper on the rocks, heating them up and stripping material off them. So, the light we see as a shooting star is basically a rock which is getting cooked at a really high temperature.
A meteorite fireball. Pretty, huh? This one could have reached the ground

Most of these rocks don’t make it to Earth; they completely burn up in the atmosphere. Only something which is a bit bigger has a chance of reaching the surface. Things about the size of a small car will often reach the surface as a rock about the size of your fist. But, this is all dependant on when the meteorite falls. Because of the way the Earth is rotating, those meteorites which fall during the evening are travelling at much lower speeds than those which fall in the morning. Some, those which reach the ground with about the size of a truck, can leave a crater. The meteorite which created Meteor Crater in Arizona, some 1200m in diameter, was probably only about 50m in size. By comparison, the asteroid which caused the Chicxulub crater, about 200km across and the event which wiped out the dinosaurs, was probably only about 8km in diameter.
So, what are they?
Meteor Crater in Arizona

Well, basically they are pieces of asteroids or planets which have drifted into an Earth-crossing orbit. Some of them, the chondrites, represent materials which never formed into proper planets. They contain things called chondrules, small bits of molten rock which crystallised under zero-gravity. They also contain free metal, graphite and sometimes diamonds, all of which are incredibly rare in rocks on Earth. These chondrites can tell us about a lot about how planets, moons and asteroids were able to form during the first million years of the solar system. Others, are completely made of iron and nickel metal, and represent the cores of protoplanets which have been completely destroyed.
A chondrule, a melt drop which crystallised under zero-gravity

An iron meteorite, with metal crystals showing. This came from the core of an asteroid!

Perhaps the most interesting of the lot are those ones called the SNC Meteorites. This stands for Shergottite-Nakhlite-Cassignites and these are thought to represent bits of Mars which have been thrown free of the planet careening towards Earth. So, Mars really has attacked Earth, but has only really thrown rocks. But these bits of another planet give us some really interesting insights into what Mars is like. The fact that they are quite a bit like some rocks found on Earth is intriguing for understanding how planets form and evolve. We also have meteorites from the Moon, the aptly names Lunar Meteorites, and since we’ve sent people there, we also have rocks collected on the Moon. These rocks show some key differences to rocks on Earth, but are generally pretty similar to some volcanic rocks.

To get them from their asteroids or planets to Earth, you have to first break off a meteorite. The way to do this is to throw something really big at it and hope some smaller pieces fly off in the right direction. All meteorites are basically the remnants of collisions between asteroids, or between asteroids and planets. This is the only process which can throw rocks into an orbit which will cross the Earth’s, and allow people like me to walk around a desert and find a meteorite to study. They represent one of the most violent processes in the solar system, one which we haven’t seen in action until recently.
A collision between asteroids. This sort of thing makes meteorites.
Image from the Hubble Space Telescope, courtesy NASA

Meteorites are great treasure chests for scientific knowledge. If we can understand how these rocks came into being we will understand how comets, asteroids, moons and planets formed, but in our own solar system, and in others. Today, we know of over 500 planets orbiting other stars, and this number is set to skyrocket due to the Kepler Space Telescope, which in its two years in space has found 15 known planets and over 1200 possible planets. Understanding how these planets may have formed can help us understand the possible potential for life in these systems. We may not be able to go there and sample meteorites from these far off places, but we can get an understanding from back home.
Yes, meteorites can tell us about how this could actually be likely. More on that in the future...

So, now when you see a bright streak of light cross the sky on a clear night, you’ll know that might just have been a piece of another planet.

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