Shattered hypothesis? (Letters).The brief article on the discovery of sheets of melted sand in Australia ("Desert glass: Is it baked Australia?" SN: 11/24/01, p. 331) mentioned several possible sources of the heat that produced this material, but it failed to mention the most probable source--the impact of a comet on the upper atmosphere. The nature of comets Non-periodic comets are seen only once. They are usually on near-parabolic orbits that will not return to the vicinity of the Sun for thousands of years, if ever. Periodic comets usually have elongated elliptical orbits, and usually return to the vicinity of the Sun after a number is that when they encounter an atmosphere of significant density they deposit their considerable energy in that atmosphere at high altitude Conventionally, an altitude above 10,000 meters (33,000 feet). See also altitude. and leave little or no solid debris. We have evidence of this happening at the Tunguska site in Siberia. The article is tantalizing tan·ta·lize tr.v. tan·ta·lized, tan·ta·liz·ing, tan·ta·liz·es To excite (another) by exposing something desirable while keeping it out of reach. in that it gives no information about the area over which this glass can be found. GEORGE PREHMUS, PRESCOTT, ARIZ ARIZ Arizona (old style) . Although the burst of a comet on the upper atmosphere would have provided enough heat to fuse the glass, the researchers note that such an air burst probably wouldn't have generated the intense pressures needed to shock the quartz grains trapped in the material. Those conditions could only come from some sort of an impact with the ground, an extremely close call from a low-flying meteor meteor, appearance of a small particle flying through space that interacts with the earth's upper atmosphere. While still outside the atmosphere, the particle is known as a meteoroid. Countless meteoroids of varying sizes are moving about the solar system at any time. , or both. --S. PERKINS |
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