Cross-section of an urban micrometeorite

(type: porphyritic spherule) found on a Minneapolis rooftop. Micrometeorites are sub-millimeter extraterrestrial dust particles that have survived atmospheric entry. Micrometeorites found on urban rooftops are often less altered and better suited for study of recent extraterrestrial dust flux and the composition of asteroids and comets in general. Backscattered electron scanning image acquired at 10 kV in a electron probe microanalyzer.