A micrometeorite is the smallest type of object that reaches the surface of the Earth (or some other planet or moon) from space. Two varieties occur: cosmic spherules and interplanetary dust particles. The former are round and about 1 mm or less in diameter. Interplanetary dust particles are irregular in shape and less than 0.05 millimeters in size. Micrometeorites are mineralogically different from larger meteorites and probably originate in comets. Their small size allows them to radiate heat away so that they are not melted during their passage through the atmosphere. Most are recovered from the clay beneath the oceans far from land and ice from Antarctica and Greenland. Others have been collected in the stratosphere.