Atmospheric collection of extraterrestrial dust at the Earth's surface in the mid‐Pacific

The Kwajalein micrometeorite collection utilized high volume air samplers fitted with polycarbonate membrane filters to capture particles directly from the atmosphere at the Earth's surface. This initial study focused on identifying cosmic spherule‐like particles, conservatively categorizing them into four groups based on bulk compositional data: Group I exhibit a range of compositions designated terrestrial in origin; group II are Fe‐rich and contain only additional O, S, and/or Ni; group III are silicate spherules with Mg‐to‐Si At% ratios less than 0.4; group IV are silicate spherules with Mg‐to‐Si At% ratios greater than 0.4. Spherules in groups I, II, and III have compositions that are also consistent with particles that are produced in great numbers by natural and/or anthropogenic terrestrial activities (e.g., volcanic microspherules, fly ash from coal fired power plants, etc.) and thus are assumed terrestrial in origin. Group IV spherules exhibit compositions closest to those of cosmic spherules identified in other collections and are, therefore, designated cosmic spherule candidates. Detailed analysis of seven group IV spherules found that whilst five exhibited morphology and compositions consistent with S‐type cosmic spherules, two appear unique to this collection and could not be matched to either terrestrial or extraterrestrial spherules studied to date.