75th Annual Meteoritical Society Meeting (2012) 5388.pdf

USING WEATHER RADAR DATA FOR RAPID METEORITE RECOVERY: THE “SUTTER’S MILL” METEORITE FALL. Marc D. Fries1, Robert Matson2, Jake Schaefer, and Jeffrey A. Fries3. 1Planetary Science Institute, Tucson, AZ. E-mail: [email protected]. 2SAIC, Seal Beach, CA. 3USAF Weather Agency, 1st Weather Group, Offutt AFB, NE.

Introduction: Weather radar data can accurately and rapidly provide the locations of meteorite falls [1]. Radar data showing “Sutter’s Mill” meteorites falling in Coloma, CA, were released into the public domain before the last meteorites had reached the ground. The same is true for Park Forest, IL, the “Ash Creek” fall in West, TX [2], Mifflin, WI [3], Grimsby, Canada [4], Lorton, VA [5] and other recent meteorite falls. Rapid radar-guided meteorite recovery minimizes terrestrial contamination, maximizes the recovered mass, and facilitates studies which rely on rapid collection such as short-lived isotope analysis. Data and Discussion: The “Sutter’s Mill” meteorite fall occurred at 7:51 a.m. PDT on 22 May 2012 in El Dorado County, California. Falling meteorites were detected using Doppler weather radars of the U.S. National Climatic Data Center (NCDC)’s “NEXRAD” network. Radar signatures of falling meteorites are seen in data from the KBBX (Beale AFB, CA), KRGX (Reno, NV), and KDAX (Sacramento, CA) radars. These radar signatures include features seen in other meteorite falls, to include a “falling” appearance trend, small-scale turbulence, widely variable spectrum width values, general agreement with eyewitness reports both spatially and temporally. The first appearance occurs at 14:51.57 UTC and 16.75 km above ground level (AGL) in KRGX radar data. At least four more radar sweeps record falling meteorites. The last appearance of meteorites in radar data occurs in KDAX data at 14:58.40 UTC and an altitude of 5.94 km AGL. Meteorites are observed to fall in a roughly east-west line passing through, but mostly westward from, the town of Coloma, CA. Seismometer data analysis by Schaefer indicates the fireball terminated within seconds of the first appearance of meteorites on radar, in the 16.75 km AGL altitude KDAX signature. This suggests that large meteorites were generated by this fall, as rapidly-moving (and therefore massive) meteorites penetrated to 16.75 km AGL very rapidly. A dark flight model utilizing the physics of falling objects was used to estimate a strewn field from radar signatures since the exact time, position, and altitude are known for the falling meteorites. Model parameters include CM chondrite density and winds-aloft data collected by a weather balloon launched from Reno, NV at 1200 UTC. Model results reveal that light low-altitude winds generate relatively little lateral movement for all but the 16.75 km AGL radar signature. The modeled flight path of the first- appearance object reveals a predominantly ballistic flight path that would carry a 10-20 kg mass approximately 30 km west of the low-mass meteorite finds reported to date. At the time of this writing, approximately 500g of “Sutter’s Mill” meteorites have been recovered. With location information provided by weather radar data, a small number of meteorites were collected rapidly after the fall and before rain fell in the fall area.

[1] Fries and Fries, MAPS 45,9 (2010) 1476-1487. [2] Fries and Steele 41st LPSC (2010) 1179. [3] Fries and Fries, 73rd MetSoc (2010) 5365. [4] Brown et al MAPS 46, 3 (2011) 364- 378. [5] Corrigan et al 73rd MetSoc (2010) 5353.