[meteorite-list] Meteoroid entry angle
From: Rob Matson <mojave_meteorites_at_meteoritecentral.com>
Date: Sat, 18 Sep 2010 17:02:54 -0700 Message-ID: <GOEDJOCBMMEHLEFDHGMMKEJMEFAA.mojave_meteorites_at_cox.net> Hi All, Playing message catch-up after 10 days' vacation/biz trip in Maui. Saw John's message below, and thought I'd make a small correction to a common misconception about meteoroid entry angles. "Gilbert and Barringer both realized that 45 degree impacts are the most probable trajectories for meteoritic material." I find many people make this mistake in solving all kinds of problems that depend on spherical geometry. In *two* dimensions, if the range of possible angles is from 0 to 90 degrees, and the angle is uniformly random, then the mean angle is of course 45 degrees: the midpoint of the uniform distribution. In three dimensions, this is no longer true. If you look at a globe, it is easy to see why. Compare the surface area of the earth from the equator up to 45 degrees north latitude with the surface area from 45 N up to the North Pole. The latter is a much smaller area. In fact, for a sphere, the surface area from the equator up to *30* degrees north latitude is the same as the area from 30N to 90N. Similarly, the average entry angle for a meteoroid coming from a uniformly random direction is 30 degrees from horizontal. That said, the entry angle distribution is not quite uniformly random because earth's gravity bends all trajectories slightly toward the center of the earth. This has the effect of making all entry angles a little bit steeper than they would be if gravity didn't play a part. Bottom line is that the average entry angle for a meteoroid is a little bit more than 30 degrees, but definitely not as much as 45 degrees. To answer Eric's question below about what's considered shallow vs. steep, I consider any entry angle less than 20 deg to be "shallow", anything greater than 40 deg to be "steep", and anything from 20-40 (which is about 30% of all meteoroids) to be average. --Rob -----Original Message----- From: meteorite-list-bounces at meteoritecentral.com [mailto:meteorite-list-bounces at meteoritecentral.com]On Behalf Of Kashuba Sent: Friday, September 10, 2010 8:06 AM To: bernd.pauli at paulinet.de; Meteorite-list at meteoritecentral.com Subject: Re: [meteorite-list] Meteor Crater Shape and Entry Angle Eric, Bernd, Sterling, List, David Kring of LPL put together a great guidebook for the 2007 MetSoc tour of the crater (150 pages). He is Gene Shoemakers successor as advisor to the Barringer family. He and family members lead the tour. Carolyn Shoemaker was there too. Chapter 9. "Trajectory" begins and ends thusly: The trajectory of the impacting asteroid is another issue of considerable debate and still unresolved. Historically, circular plan views of impact craters confounded many investigators who assumed a circular crater requires a vertical impact. They wondered why more craters are not elliptical. Gilbert and Barringer both realized that 45 degree impacts are the most probable trajectories for meteoritic material. Yet Gilbert, like many of his contemporaries, mistakenly thought a 45 degree impact produces an oval crater (Hoyt, 1987). Barringer, on the other hand, realized that a 45 degree impact will produce a round crater (Hoyt, 1987). Despite this insight, Barringer, like Gilbert, initially assumed that the northern Arizona impact had been vertical or nearly vertical and that the asteroid was buried beneath the center of the crater floor. When extensive drilling did not locate a main mass beneath the crater floor and instead only produced traces of the projectile, Barringer began to consider other options. He had already noted several features that seem to have a directional symmetry. - snip - More recently, techniques similar to those of Sutton were applied by Holliday et al. (2005) to the Odessa impact site. They estimated the Odessa craters were produced approximately 63,000 years ago. Although the ages of Barringer and Odessa craters are still not precisely known, these approximate ages suggest Odessa formed earlier, with the caveat that the Barringer crater may be older than 49,000 yrs. (See discussion in Chapter 11). Thus, the two impact events may not be directly related and may not have any bearing on the issue of trajectory. Nonetheless, several other potential indicators of trajectory survive (and even the Odessa connection might be revived). Unfortunately, these indicators cannot be reconciled at the present time and I think it fair to conclude that the trajectory of the impacting asteroid that produced Barringer Crater remains uncertain. Chapter 9: http://www.lpi.usra.edu/publications/books/barringer_crater_guidebook/chapte r_9.pdf Whole "guidebook": http://www.lpi.usra.edu/publications/books/barringer_crater_guidebook/index. shtml Regards, - John Ontario, California -----Original Message----- From: meteorite-list-bounces at meteoritecentral.com [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of bernd.pauli at paulinet.de Sent: Friday, September 10, 2010 3:26 AM To: Meteorite-list at meteoritecentral.com Subject: [meteorite-list] Meteor Crater Shape and Entry Angle Eric wrote: "The crater is not perfectly round as would be expected from an impactor coming in at a sharper angle. In fact the crater is more elliptical in shape." SHOEMAKER E.M. and KIEFFER S.W. (1974, 1979) Guidebook to the Geology of Meteor Crater, Arizona (Publ. No. 17, Center for Meteorite Studies, Arizona State University, Tempe, Arizona): "Regional jointing has controlled the shape of the crater, which is somewhat squarish in outline; the diagonals of the "square" coincide with the trend of the two main sets of joints. The largest tears occur in the "corners" of the crater." Eric also inquired: "What would a "relatively low" impact angle be? 10 degrees, 20 degrees?" I tried to find more precise information on that but was unable to find something that might be of help here. Maybe someone else can shed more light on this! Regards, Bernd Received on Sat 18 Sep 2010 08:02:54 PM PDT |
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