such as claystones and hardgrounds Are easily dissolved such as salt deposits and limestones Have been mass sorted such as heavy mineral concentrates found in deserts and beach sands. The oldest MMs are totally altered iron spherules found in 140- to 180-million-year-old hardgrounds. Urban micrometeorites In 2016 a new study showed that flat roofs in urban areas are fruitful places to extract micrometeorites. The "urban" cosmic spherules have a shorter terrestrial age and are less altered than the previous findings. Amateur collectors may find micrometeorites in areas where dust from a large area has been concentrated, such as from a roof downspout. Polar depositions Micrometeorites found in polar sediments are much less weathered than those found in other terrestrial environments, as evidenced by little etching of interstitial glass, and the presence of large numbers of glass spherules and unmelted micrometeorites, particle types that are rare or absent in deep-sea samples. The MMs found in polar regions have been collected from Greenland snow, Greenland cryoconite, Antarctic blue ice Antarctic aeolian (wind-driven) debris, ice cores, the bottom of the South Pole water well, Antarctic sediment traps and present day Antarctic snow. Classification and origins of micrometeorites Classification Modern classification of meteorites and micrometeorites is complex; the 2007 review paper of Krot et al. summarizes modern meteorite taxonomy. Linking individual micrometeorites to meteorite classification groups requires a comparison of their elemental, isotopic and textural characteristics. Comet versus asteroid origin of micrometeorites Whereas most meteorites originate from asteroids, the contrasting make-up of micrometeorites suggests that most originate from comets. Fewer than 1% of MMs are achondritic and are similar to HED meteorites, which are thought to be from the asteroid 4 Vesta. Most MMs are compositionally similar to carbonaceous chondrites, whereas approximately 3% of meteorites are of this type. The dominance of carbonaceous chondrite-like MMs and their low abundance in meteorite collections suggests that most MMs derive from sources different from those of most meteorites. Since most meteorites derive from asteroids, an alternative source for MMs might be comets. The idea that MMs might originate from comets originated in 1950. Until recently the greater-than-25-km/s entry velocities of micrometeoroids, measured for particles from comet streams, cast doubts against their survival as MMs. However, recent dynamical simulations suggest that 85% of cosmic dust could be cometary. Furthermore, analyses of particles returned from the comet, Wild 2, by the Stardust spacecraft show that these particles have compositions