roughly 50 times higher than that estimated for meteorites, which represent approximately 50 t/yr, and the huge number of particles entering the atmosphere each year (~1017 > 10 µm) suggests that large MM collections contain particles from all dust-producing objects in the Solar System including asteroids, comets, and fragments from the Moon and Mars. Large MM collections provide information on the size, composition, atmospheric heating effects and types of materials accreting on Earth while detailed studies of individual MMs give insights into their origin, the nature of the carbon, amino acids and pre-solar grains they contain. Chemical analysis of the microscopic chromite crystals, or chrome-spinels, retrieved from micrometeorites in acid baths has shown that primitive achondrites, which represent less than half a percent of the MM reaching Earth today, were common among MMs accreting more than 466 million years ago. Collection sites Micrometeorites have been collected from deep-sea sediments, sedimentary rocks and polar sediments. They were previously collected primarily from polar snow and ice because of their low concentrations on the Earth's surface, but in 2016 a method to extract micrometeorites in urban environments was discovered. Ocean sediments Melted micrometeorites (cosmic spherules) were first collected from deep-sea sediments during the 1873 to 1876 expedition of HMS Challenger. In 1891, Murray and Renard found "two groups [of micrometeorites]: first, black magnetic spherules, with or without a metallic nucleus; second, brown-coloured spherules resembling chondr(ul)es, with a crystalline structure". In 1883, they suggested that these spherules were extraterrestrial because they were found far from terrestrial particle sources, they did not resemble magnetic spheres produced in furnaces of the time, and their nickel-iron (Fe-Ni) metal cores did not resemble metallic iron found in volcanic rocks. The spherules were most abundant in slowly accumulating sediments, particularly red clays deposited below the carbonate compensation depth, a finding that supported a meteoritic origin. In addition to those spheres with Fe-Ni metal cores, some spherules larger than 300 µm contain a core of elements from the platinum group. Since the first collection of HMS Challenger, cosmic spherules have been recovered from ocean sediments using cores, box cores, clamshell grabbers, and magnetic sleds. Among these a magnetic sled, called the "Cosmic Muck Rake", retrieved thousands of cosmic spherules from the top 10 cm of red clays on the Pacific Ocean floor. Terrestrial sediments Terrestrial sediments also contain micrometeorites. These have been found in samples that: Have low sedimentation rates