Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.562p&link_type=abstract
Meteoritics, vol. 30, no. 5, page 562
Astronomy and Astrophysics
Astrophysics
Chondrites, Craters, Loess
Scientific paper
This crater-like structure was recognized by geomorphologist, Wakefield Dort in 1992 while examining topographic maps [1]. Using the same tradition as for meteorite discoveries, he named it after the nearest town with a post office, Merna, Nebraska, Zip code, 68856. This researcher has made two extensive field trips to the crater. The f1rst was to examine and confirm its nature and the second to field check the results after a computer simulation of the impacting projectile. This area is characterized by low rolling hills which are under cultivation. This area is unglaciated and the closest recent glacial approach was about 240 km. The prevailing winds are from the northwest and there are many parallel eolian features which have an azimuth of approximately 300 degrees. The predominate erosional factors are snowmelt and the spring rains. Most of the 53 cm annual precipitation occurs from March to May. The soil is predominantly Peoria loess with an estimated depth of approximately 260 m.[2]. This is probably underlaid with limestone bedrock. Well records of the area have not been very helpful in resolving this question. The Merna Crater is an approximately 1.6 km diameter, 23 m deep, well preserved depression with a flat bottom. It is located about 18 km west of and 2.4 km south of Merna, Nebraska. This site is on the U.S.G.S. 7.'5 Callaway N.W., Nebraska 1951 topographic map. The crater covers most of section 9 and the eastern portion of section 8. The coordinates of the crater center are approximately longitude 99 degrees 58' 20"W and latitude 41 degrees 27' 30" N. A significant landmark on section 9 is the Cliff Union Church and Cemetery which is on the eastern rim of the crater. Even though the land has been plowed for more than 150 years, the general topographic features have not been seriously disturbed. It is believed that the crater was caused by an air blast similar to Tunguska but of a much larger magnitude. It is therefore believed that there never was a significant raised rim. The nature of contour plowing would reduce any present rim by pulling the raised portion down into the crater. The crater has an average diameter of 5400 feet at the 2950 ft. elevation contour. The crater is elliptical with an eccentricity of approximately 0.71. It is oriented with the major axis at an azimuth of 45 degrees. The minimum age for this crater is approximately 3000 years as determined by one Carbon 14 sample [2]. This was probably a witnessed event as Pawnee Indian legends are rich in phenomena which may relate to this event. In the area surrounding the crater for several km are at least 12 secondary craters. Some of these have a diameter of 550 m. These are distinct from the eolian features by their circular shape and distribution from the primary crater. A 6.8 kg (H4) chondrite was found in the proximity [3]. It has a specific gravity of 3.5. Several magnetic spherules of probable extraterrestrial origin and large quantities of glass flakes have been found in the crater area. Approximately 60 quartz grains were examined by polarizing microscope and about half showed some shocking. There is an active search program for more meteorites in the area. A bore hole to search for breccia and shatter cones is planned. The "IMPACI" software was developed to simulate and model this projectile using the "average" entry velocity of Near Earth Asteroids of 21.8 km per second. The best solutions suggest an approximately 195 m diameter NEA entering the atmosphere and creating an air blast at approximately 8 km altitude. The kinetic energy would have been equivalent to 180 megatons of TNT of which approximately 50 percent would have been absorbed by the atmosphere [4] [5]. References: [1] Dort W. et al. (1992) GSA, 24, 196. [2] Dort W. (1993) personal communication. [3] Graham A. et al. (1985) Catalogue of Meteorites, 4th edition, p. 82, Univ. of Arizona. [4] Chyba C. et al. (1993) Nature, 361, 40-44. [5] Hills J. and Goda M. (1993) Astrophys. J., 105, 1114-1144.
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