Critical Thinking Questions
(Think about these carefully before you consult the answers below)
QUESTIONS
1) Most meteorites are fairly small, with the largest ever discovered only a few meters across. Could meteorites be much larger, say 100 kilometers across? <answer>
2) It is very likely that the Earth was bombarded by large asteroids in the past, as well as many smaller asteroids and meteorites, yet there is a dearth of asteroid craters on our planet. Why? <answer>
3) You may have heard of tiny fossils in some meteorites - - the ones thought to be from Mars. Is it possible that a virus from space could arrive on a meteorite? <answer>
4) Why are meteorites found on Earth composed of rock or metal, while there is a good chance that some of the meteors we see in the sky are actually formed from ice fragments? <answer>
5) When cut open, polished and then etched with an acid, many iron meteorites display crystal-like structures in the metal called Widmanstatten patterns. What might these indicate? <answer>
ANSWERS
1) The answer is no for a couple of reasons. First, objects 100 km across would not be considered meteors, but rather asteroids, regardless of their origin or composition. There really isn't any universally accepted definition of the size boundary between asteroids and meteoroids, although we might consider anything 5 -10 meters across or larger an asteroid, and anything smaller a meteoroid. Secondly, objects as large as 100 km would likely burn up or break up into many smaller bodies when they collided with Earth. The small asteroid that formed the Canyon Diablo or Barringer Meteor Crater in Arizona was likely 50 to 100 meters across, but the explosion caused by its impact left nothing more than small meteorite fragments.
2) More than two-thirds of the Earth's surface is covered by water, which makes smaller permanent craters notoriously difficult to form, and which hides ancient ones that might be there. In addition, the sea floor is constantly evolving, which obliterates existing craters. Ancient craters that formed on the surface of the Earth have been mostly warn down through erosion, leaving only a few more recent ones.
3) There is a theory, not overly well substantiated, that life on Earth was seeded by material from space, probably from comets. However, it is pretty unlikely that any viruses from space -- if any exist -- would come in a meteorite. While it is possible that tiny life forms could penetrate the surface crust and cracks on meteorites, such as may be the case with the Martian samples, bits of iron and rock hardly seem like the best place for it. More importantly, most meteorites spend billions of years out in space before they crash into Earth. It is unlikely that any form of like could survive that long on such small objects, at temperatures usually hundreds of degrees below zero. Finally, the fiery trip through the Earth's atmosphere melts the outer layers of the meteorite, likely killing any life that could have been there. While the Martian meteorites may or may not have tiny fossils embedded in them, these objects certainly could not have survived the trip to Earth, even if they were alive when it started, which is unlikely anyway.
4) There has certainly been the suggestion that some meteors could be the result of chunks of ice from comets. Obviously, any ice meteors would not survive the trip through the atmosphere. They would melt. If any did survive, they would break up and melt on impact. If one managed to fall, intact, in an area where it is always below freezing, it might last, but it would be impossible to identify without chemical analysis. There have been reports of large chunks of ice falling from the sky, for which a cometary origin was suggested, but it probably more likely that they fell off airplanes.
5) In general, the larger the crystals, the slower the cooling. Metals can, indeed form crystals, and that is precisely what the Widmanstatten patters are. But metallic crystals large enough to be visible to the unaided eye take a very long time to form, with very slow cooling. The cooling rate in some cases may have been no more than about one degree per million years. What does this tell you? One idea is that the meteorite material must have solidified within a much larger object -- one that was capable of retaining such heat for such a long time. This gives credence to the belief that meteoroids formed within larger, asteroid-sized or planet-sized, objects, which may have subsequently broken apart.
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