Mapping an Iron-Meteorite Impact Site with a Magnetometer, and Implications for the Probability of a Catastrophic Impact on Earth

Abstract

The Wabar meteorite impact site in central Saudi Arabia, first visited by Henry St. John (Abdullah) Philby in 1932, is unique in several ways. It is one of only 17 impact sites on the Earth where part of the original object is still present. The Wabar impact event also took place entirely in sand, permitting a much clearer and simpler reconstruction of the physical processes of a hypervelocity impact. A careful magnetic survey conducted there adds crucial information to this reconstruction: almost none of the [Formula: see text] of the original iron-nickel object remains in or beneath the known impact craters. The magnetic data were difficult to acquire because of the hostile environment, and the processing was non-trivial for what in effect was a magnetic survey of a ferrous junkyard. The event sequence, which took place in just a few seconds, generated a unique binary set of impactite that includes a black glass (90% local sand and 10% meteorite) and a shock-generated, coarsely laminar bleached-white sandstone we have informally named “Instarock.” Very little of the original object survived intact, save some small fragments apparently spalled off by the reflected impact shock-wave against the back of the meteoroid. Most of these fragments are in the form of so-called “shale-balls” buried beneath a shallow sand cover. Despite a [Formula: see text] fission-track date, field relationships suggest a much younger age, a conclusion supported by thermoluminescence dates as low as [Formula: see text]. Along with several other small meteoroid impact events that have come to light in the past few decades, this very young age-date suggests a need to reconsider the frequency of these types of “city-buster” impacts.