In this case, the team believes the primary impactor would have been at least a mile (1.6 km) wide. For reference, the iron-nickel meteorite that slammed into Earth to create the 0.75-mile-wide (1.2 km) Meteor Crater in Arizona was only about 160 feet (50 m) across.
A closer look at the craters
The 31 secondary craters found so far range from 32 to 229 feet (10 to 70 m) wide and fan out over an area forming a triangle bounded by the cities of Laramie, Casper, and Douglas, Wyoming. The secondaries are located some 93 to 124 miles (150 to 200 km) beyond the suspected main crater. And according to the team, a single meteoroid air burst could not have created such an expansive set of craters.
The secondary craters all were formed by ejecta (from the primary impact) ranging from about 13 to 26 feet (4 to 8 m) wide. These ejected fragments struck Earth with velocities ranging from roughly 1,500 mph (2,400 km/h) to more than 2,200 mph (3,500 km/h).
The smaller strikes line up in a typical secondary chain, and some of the craters are elliptical, indicating a low-angle impact. The classic “herringbone” pattern typical of secondary impacts also may be present.
The craters the team has studied so far show shock features associated with impacts, but another 60 putative depressions still await further scrutiny. The native chert (a fine-grained sedimentary rock) found in some craters also has inclusions of accretionary lapilli, which are tiny spherical objects made of concentric layers of ash that form around condensing liquids or other particles. They form in the giant plumes above volcanic eruptions or impacts in the seconds and minutes immediately following the violent events. Occurring in a sandstone formation with varying degrees of preservation, some of the secondaries even show ejecta blankets, which consist of impact-strewn materials close to a crater rim.
Confirming Wyoming Crater Field’s origin story
Despite mounting evidence, before the team is able to confidently state Wyoming Crater Field is truly a secondary impact site, more work must be done.
First and foremost, the investigators want to find the massive primary impact crater, which has been hidden by sediment deposited over the past couple hundred million years. Furthermore, the team also plans to search for more associated secondary craters that could further constrain the parameters used in their impact models.
Meanwhile, one team member, Sundell, is particularly intrigued by the possibility that the Wyoming Crater Field might be the result of “a meteorite storm that struck the entire Earth over a substantial period of time,” he said. Although such a storm would have surely led to numerous sets of strikes around the world, Sundell suggests that “we just found an area that preserved these fast-moving small impactors very well.”
However, despite some of the data implying the impacts were spread out over a non-negligible period of time, as well as evidence of the Moon falling victim to an increased cosmic barrage some 290 million years ago, Kenkmann and Cook don’t agree with Sundell’s meteorite storm hypothesis.