3 January, 2003

Meteorite Physics

After a short weather delay, we headed back to That Moraine this morning for a systematic foot search. We all felt that the meteorites were concentrated enough that a systematic search would be a good idea. I don't think any rock was left unturned, nor was any windscoop unexplored. At the end of a very methodical search we uncovered 14 new meteorites and there is still more to search.

We decided to take the afternoon to do a reconnaissance trip about 20 minutes north of camp on a blue ice field only visited once before by Jamie and Danny. Fuel was stored there from the Beardmore Camp, so Jamie and Danny brought some of it back to our MacAlpine Camp. Danny described the ice as "looking like a frozen tsunami." He was exactly right. Where the ice meets the mountain, a large windscoop is formed, causing the blue ice to plunge toward the barren rock. We hiked up a small peak to get an idea as to the extent of the blue ice and how we should go about searching it.

Once again, it was strange to set foot on dry, snow and ice free ground. This rock was formed from iron-rich volcanic rocks and had a slightly rusty appearance. One of the reasons I wanted to come to Antarctica is that it is the closest terrestrial environment to that of Mars. I felt as if I was climbing a mountain on Mars. It actually made me a little homesick in that I miss climbing in the mountains of Colorado. Much like home, the view from the top was incredible. I'm guessing we could see 100 miles!

On the way back down, Jamie found a meteorite on dry soil, and another was found on the blue ice. We spent a lot of time on that ice, but mostly saw small windblown rocks until Dante found the largest meteorite of the day.

It makes me wonder when we find one lone meteorite in a field of blue ice if it was brought there by the flow of the ice like the others, or if it's possible that it came from a recent fall. Meteorites don't weather much in Antarctica, so they retain their black fusion crust long after they fall, as opposed to ones found in wetter regions. It's really remarkable that these rocks from space make it to Earth at all.

It's generally accepted that meteorites come from asteroids. They are produced when asteroids collide, and their debris falls into an orbit that intersects the Earth's orbit. They can come into our atmosphere travelling at speeds anywhere from 17,000 to 40,000 miles per hour. As they enter the upper atmosphere, friction with air parcticles causes the outside of the meteorite to attain temperatures and a luminosity comparable to the surface of the sun! However, rocks are poor conductors of heat, so the inside of the meteorite stays very cold. That's why the meteorites we find have a thin, black fusion crust that looks completely different from the interior of the rock. Some meteorites actually melt back partially, and take on a shape not unlike the bottom of the space capsules used early in the space program. These meteorites are described as "oriented."

In addition to these high temperature contrasts, the meteorite is also slowing down at an extreme rate. This causes it to experience g-forces that are completely outside of the experience of most people. In my classroom, I usually find a student who weighs about a hundred pounds. I explain that just sitting in her desk, she's experiencing 1g of force on her. If she were in a car going around a corner quickly or braking quickly, she could experience 2g's, and therefore, for a moment weighs 200 lbs. On a roller coaster, one can achieve 4g's, so suddenly she weighs 400 lbs. But if she were a meteorite, she could experience as many as 300g's when coming into the atmosphere, such that for the few seconds she's plummeting to Earth, she actually weighs 30,000 lbs!

Meteorites are just rocks, and most of them (especially the stony ones) cannot take this force and break up anywhere between 10 and 20 miles above the surface. This is called the "Point of Retardation" because it is the point where Earth's atmosphere retards the cosmic velocity of the meteorite to the extent that it's rate of descent is controlled only by acceleration due to gravity and air resistance. Fragments of the meteorite rain down over a large area called a strewn field. In most cases, the meteorites soft land without making a crater. People who have recovered meteorites quickly after a witnessed fall often describe them as cold to the touch. After all, they've been in the cold vacuum of space for 4.5 billion years. Although legend and folklore hold that they start fires and are radioactive, the only dangerous ones I know of come from Krypton and are harmful only to Superman.

We often find several meteorites together that look like they could have come from the same fall. The glaciers do a good job of mixing them up, so it's hard to tell just by looking at them. They can be analyzed later by extremely precise instruments that can identify trace elements and compounds that tell us about the chemistry of the early solar system. That's the main reason we are so careful about collecting the meteorite without handling it. We would hate to see a new class of meteorite identified because my candy bar scraped across it. I can see the headlines now, "New meteorite: part rock, part iron, part nougat."

Meteorites from Mars or the Moon are much rarer, but are sent into space by collisions, just like those from the asteroid belt. Mars has 1/3 the gravity of Earth and the moon, 1/8. Therefore, it's easier to displace a rock into space from an impact on those worlds than it would be the Earth. Most asteroids have such low gravity, that Tiger Woods could easily send a golf ball into orbit around one.

With some of the wind we've been seeing here and the amount of ice, Tiger Woods could probably launch a record drive. If the weather cooperates, we'll be back at the mouthy ice tomorrow. We're up to 438 meteorites and are optimistic about reaching 500 before we pull out.

The Beardmore team doing some high altitude reconnaissance. Pictured left to right: Carlton Allen, Andy Caldwell, Scott Messenger, Jamie Pierce, Linda Welzenbach, Daniel Glavin, Dante Lauretta. Nancy Chabot not pictured because she was following her wanderlust and found an irresistible path down the mountain.