ICARUS 42, 211--233 (1980) Effects of Atmospheric Breakup on Crater Field Formation 1 QUINN R. PASSEY AND H. J. MELOSH z Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 Received October 5, 1979; revised February 18, 1980 This paper investigates the physics of meteoroid breakup in the atmosphere and its implications for the observed features of strewn fields. There are several effects which cause dispersion of the meteoroid fragments: gravity, differential lift of the fragments, bow shock interaction just after breakup, centripetal separation by a rotating meteroid, and possibly a dynamical transverse separation resulting from the crushing deceleration in the atmosphere. Of these, we show that gravity alone can produce the common pattern in which the largest crater occurs at the downrange end of the scatter ellipse. The average lift-to-drag ratio of the tumbling fragments must be less than about 10 -3, otherwise small fragments would produce small craters downrange of the main crater, and this is not generally observed. The cross-range dispersion is probably due to the combined effects of bow shock interaction, crushing deceleration, and possibly spinning of the meteoroid. A number of terrestrial strewn fields are discussed in the light of these ideas, which are formulated quantitatively for a range of meteoroid velocities, entry angles, and crushing strengths. It is found that when the crater size exceeds about 1 km, the separation between the fragments upon landing is a fraction of their own diameter, so that the crater formed by such a fragmented meteoroid is almost indistinguishable from that formed by a solid body of the same total mass and velocity. INTRODUCTION After breakup, the fragments fall over an It has been estimated that over area which is roughly elliptical in shape. If 70,000,000 meteoroids enter the Earth's the impacting meteorites have sufficient ki- atmosphere each day. Of these, about 1000 netic energy to produce craters, a crater kg (about 1%) of the meteoric material field is created in this same elliptical form survives the ablative effects of atmospheric and is often referred to as a strewn field or descent and strikes the surface (Baldwin, scatter ellipse. 1963, pp. 6-7). Within this crater field, the individual The meteoroids are subjected to high meteorites, or craters, are distributed in a pressures and stresses while traveling systematic manner; the largest masses or through the atmosphere at velocities of craters are generally located at, or near, the several kilometers per second and often downrange boundary of the crater field break into fragments which may or may not while the smallest masses fall at the up- survive the remaining descent to the sur- range boundary. There is also a cross-range face. The altitude at which breakup occurs distribution of craters which we show can generally varies from 4 to 40 km, and be the result of a transverse velocity sup- appears to be independent of either the plied to the meteoroid fragments at the time mass or class of the meteorite (Krinov, of the interaction of bow shocks coupled 1960, pp. 76-77). with the effects of crushing the meteoroid. Lift can also affect the distribution of cra- Contribution No. 3328 of the Division of Geologi- ters but we find that it is negligible except cal and Planetary Sciences, California Institute of Technology, Pasadena, Calif. 91125. for the case of fragments of masses less 2 Present address: Department of Earth and Space than about 10 z kg. Also, some deviations Sciences, SUNY, Stony Brook, N.Y. 11794. from a regular distribution of craters can be 211 0019-1035/80/050211-23502.00/0 Copyright © 1980 by Academic Press, Inc. All rights of reproduction in any form reserved. 212 PASSEY AND MELOSH explained by multiple breakup or relatively Rather, the information is based entirely steep angles of entry. upon publications. This paper is primarily concerned with The diameters of the craters listed in this crater fields but there are several single paper are generally from the measurements terrestrial craters which may be the result by previous authors. However, a few of the of the almost simultaneous impacts of me- diameters listed herein were estimated from teoroid fragments which, due to the angle published maps of the crater fields. The of entry or altitude of breakup, were not diameters refer to the present rim-to-rim sufficiently separated from one another to crater diameter without respect to a recon- form individual or obviously overlapping structed crater diameter and is probably craters. Some possible examples of these somewhat smaller than the original rim-to- include: Boxhole Crater, Brent Crater, rim diameter due to erosion of the rim. Dalgaranga Crater, Holleford Crater, Me- The distances recorded in the tables were teor (Barringer) Crater, New Quebec derived from maps or from written descrip- (Chubb) Crater, and Wolf Creek Crater tions as supplied by previous authors, and (Baldwin, 1963; Barringer, 1967; Heide, represent the distance from the center of 1963; Krinov, 1960, 1966; Millman, 1971). the crater in question to the center of the We first discuss the principal features of largest crater of the field. The largest crater several well-known terrestrial strewn is used as the origin of coordinates because fields, illustrated by maps of the fields, and it usually marks the distant end of the establish a measure of the crater dispersion scatter ellipse. Where the largest crater in these fields. We then review the physics does not lie near one end of the field (as of atmospheric entry and summarize our with the Campo del Cielo crater field) no computational scheme. We study the ef- distances are listed. fects of gravity, lift, bow shock interaction, spinning meteoroids, and crushing deceler- Campo del Cielo ation within the context of this model. The theory is then applied to the observed cra- The Campo del Cielo crater field is lo- ter fields and it is concluded that gravity, cated within the Chaco and Santiago del bow shock interactions, and possibly spin- Estero Provinces, Argentina (27038 'S, ning play the major roles in strewn field 61°42'W), and is composed of at least 20 formation with the effects of crushing de- craters (Romafia and Cassidy, 1973). The celeration and lift playing minor roles. The- distribution of craters is along a line trend- oretical plots are constructed showing the ing SW-NE, with the largest crater in the fate of meteoroids of given masses, veloci- middle, rather than at one end, of the crater ties, yield strengths, and angles of entry field (Cassidy et al., 1965) (refer to Table I). into earth's atmosphere. It is found that It has been suggested that the impact strewn fields are important only for craters angle for the meteorite involved in the less than about 1 km in diameter; in the formation of crater 9 was about 22° and that case of larger craters, the fragments fall so the final impact velocity was not greater close together (for entry angles greater than than 5.8 km sec -1 (Cassidy and Renard, approximately 10°) that the crater is almost 1970; Cassidy, 1971; Renard and Cassidy, indistinguishable from that made by a single 1971). solid meteoroid. Meteorites found in the immediate area are composed of iron (hexahedrite class) TERRESTRIAL CRATER FIELDS and samples have been recovered ranging Explanation in mass from 50 g to 4210 kg (Milton, 1963). No field study for any crater field dis- Studies have also revealed a meteorite with cussed in this paper was undertaken. an estimated mass of 22,000 kg in crater 10 CRATER FIELD FORMATION 213 TABLE I overlapping craters (Milton, 1968) (refer to Fig. 1 and Table II). CAMPO DEL CIELO a This crater field is one of the best pre- Crater number Crater diameter n Distance from served examples of a scatter ellipse. The (m) largest crater c direction of the impact is inferred to have (m) been from the SW to the NE, as evidenced by the location of the largest craters with l 85 NA a 2 71 NA respect to the smaller craters. 3 103 NA Meteoritic iron (octahedrite class) has 4 89 NA been found at the site (Hodge, 1965; 5 45e NA Krinov, 1966) and the largest meteorite 6a 35 NA recovered has a mass of about 150 kg 6b 20 NA 7 85 NA (Baldwin, 1963). 8 37 NA 9 40 NA Herault 10 221 NA The Herault craters are located in south- 11-20 Not given NA ern France (43°30'N, 3°15'E) near the " Data from Cassidy and Renard (1970), Cassidy towns of Faug~res and Cabrerolles (Beals, (1971), Romaiaa and Cassidy (1973), Cassidy et al. 1964; G~ze and Cailleux, 1950; Janssen, (1975). 1951; Hofl]eit, 1952). Average rim-to-rim diameter. In studying the crater profiles, Beals e Center-to-center. a Not applicable (reference point is uncertain since states that there is a possibility that the the largest crater is not near the downrange end of the craters are not of meteoritic origin because crater field). none of them exhibited a raised rim. If, e Average floor diameter. r Average reconstructed diameter. TABLE II HENBURY a (Cassidy, 1970, 1978 personal communica- tion). Crater numbern Crater diameter c Distance from (m) largest cratera Clearwater Lakes (m) In northern Quebec (56°N, 74 k°W) are 1 147 -- two nearly circular lakes with diameters of 2 119 55 32 and 26 km. These are known as Clearwa- 3 79 122 4 54 143 ter Lakes and the center-to-center separa- 5 47 393 tion is approximately 31 km.