Radar Subsurface Imaging Leads to Discovery of Rare Meteorite
November 22, 2006
The recent discovery of a 70-kilogram (154-pound) pallasite in a wheat field just east of Greensburg, Kansas, in the Brenham strewn field, was the result of a collaborative effort between the Houston Museum of Natural Science (HMNS), the Lunar and Planetary Institute (LPI), the Rice Space Institute, and the Brenham Meteorite Company. The goal of the team was to develop new methods for finding and archiving meteorites. The team was led by Dr. Carolyn Sumners, senior director of the Astronomy Department at HMNS.
Pallasites are iron-rich meteorites and are very rare, comprising less than 1% of the world’s found meteorite collection. What made this discovery particularly important was the method used to find and image the buried meteorite. LPI scientist Dr. Essam Heggy, who was responsible for the radar survey, developed an innovative algorithm that combined the use of multiple-frequency ground-penetrating radar (GPR) with three-dimensional polarimetric imaging to more narrowly focus the search, which allowed more accurate and faster reconnaissance of the object and allowed the team to obtain a record of the meteorite in the subsurface before it was extracted from the soil. This approach will allow scientists to maximize the data that was collected during meteorite recovery, which will be of critical importance when trying to model the fall that generated the strewn field.
The Brenham strewn field provided an optimal test site, as a number of relatively large pallasites have been found at this location (ranging in size from a few centimeters to a few tenths of centimeters).
The largest known pallasite on Earth, weighing 635 kilograms (1400 pounds), was discovered there in 2005 by meteorite hunter Steve Arnold. Previous meteorite searches at this location relied on the use of metal detectors. However, the history of agricultural use of this land meant that the site contained a significant number of man-made metal artifacts that could not readily be distinguished from potential meteorite locations. A new method was needed that could distinguish meteoritic targets from dozens of other undesirable targets over the 30-acre site. Radar three-dimensional polarimetric imaging was suggested as a means of determining a potential meteorite’s precise location and provide geometrical characterization of the buried meteorite before digging began.
Using laboratory electromagnetic characterization on soil samples previously collected from the Kansas soil in the strewn field, along with meteoritic samples from the LPI collection, Heggy identified the optimal penetration depth and dielectric contrast between the pallasites and the surrounding soil, and then set the optimal frequency and surveying parameter to search for the potential presence of buried meteorites at the study site.
The survey used a cross-polarized and multiple frequency sounding GPR surveying technique to reduce ambiguities regarding the physical and geometrical properties of the identified object in the subsurface. Due to the oriented shape of the meteorite, cross-polarization at multiple frequencies was determined to be a very efficient tool to use in its identification. Once the potential meteorite was located, three-dimensional mapping at three different frequencies (270, 500, and 900 MHz) and two polarizations of a 6 meter × 6 meter area (20 feet × 20 feet) were performed in order to better establish the orientation of the meteorite in the subsurface. Armed with this knowledge, paleontologists from HMNS were able to fine-tune the extraction process to the precise orientation and location of the meteorite, thereby allowing them to preserve critical information about the impact and date the event of the meteorite strike. Soil samples and organic materials immediately surrounding the meteorite were preserved for dating purposes.
Comparison of the meteorite as imaged by the 900-MHz cross-polarized ground-penetrating radar with the actual meteorite retrieved by the paleontology team of the Houston Museum of Natural Science. The dimensions of the meteorite as inferred from the radar signal match that revealed by the dig.
Preliminary information obtained at the dig has already dispelled the prevailing wisdom that the Brenham meteorite fall may have occurred 20,000 years ago. The meteorite’s location in a Pleistocene epoch soil layer sets the date of the event as closer to 10,000 years ago.
The high-resolution 900-MHz three-dimensional GPR image revealed using Heggy’s technique indicated that the dimension of the meteorite was 0.5 × 0.3 × 0.3 meters (18 × 12 × 12 inches), a dimension that closely matched the direct measurements obtained after the meteorite was retrieved from the soil.
This same radar-polarization and multiple-frequency technique is being proposed for inclusion on the ExoMars Rover mission currently in planning stages by the European Space Agency (ESA). The technique would perform high-resolution shallow subsurface mapping on Mars, which would not only support the planned drilling experiment, but would also reveal crucial information about the sedimentation process in small impact craters.
The radar equipment for this study was supported by Exploration Instruments (EXI) and Geophysical Survey Systems Inc. (GSSI). The dielectric measurements and field work were partially supported through the Houston Museum of Natural Science, NASA Grants PGG04-0000-0059 and EPO-05-561, and the Rice University Space Institute.
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Last updated May 21, 2007