Meteoroids ˜50-200 μm in size represent the dominant mass fraction of extraterrestrial material accreted by the Earth today. About 20% of them survive as unmelted micrometeorites upon atmospheric entry, and they can be recovered from Antarctica ices and snows. Around 99% of them are related to the rare group of the most volatile-rich chondrites (˜2.5% of the meteorite falls), dominantly composed of a primitive hydrous-carbonaceous material. Micrometeorites contain a mixture of volatiles showing an elemental composition and a water isotopic composition very similar to the corresponding values measured in the contemporary Earth's atmosphere. This surprising micrometeoritic "purity" of the Earth's atmosphere led to an accretion equation to compute the total amount of any meteoroid species accreted by the Earth, during the first ˜100-200 Ma of the post-lunar period of the Early Heavy Bombardment of the solar system, when the mass flux of meteoroids was about one million times higher than today. This equation allows using micrometeorites and other meteoroid "ashes" as tracers of early planetary processes, including: the formation of the atmospheres of the Earth and Mars; the functioning of the Earth's mantle, and; the formation of the prebiotic "soups" and benign climatic conditions required for the birth of life.