The micrometeoritic purity of the Earth's atmosphere

The atmosphere includes the air, the ocean and CO₂ trapped in carbonates. Its composition was defined by the Ne/N₂, H₂O/N₂ and CO₂/N₂ ratios inferred from the total quantities of these four volatiles in the atmosphere. This composition, which refers to a total mass of volatiles of about 2 × 10²⁴ g, was found to be unexpectedly similar to that of a tiny "puff" of micrometeoric atmosphere of ∼ 1 mg, which would be released upon the atmospheric entry of the small aliquot of about 200 Antarctic micrometeorites (AMMs) with sizes ∼ 100 μ m, which were individually analyzed as to get the wt% content, A(%), of each one of their volatile species, A. This was the first hint about the role of juvenile micrometeorites in the formation of the atmosphere. The following "accretion" formula yielded the total amount, M(A), of a species, A, which is released during the aerodynamical braking of micrometeorites in the thermosphere: M(A) ∼ A(%)x Φ (Earth), where Φ (Earth) is the integrated micrometeorites mass flux accreted by the whole Earth, since the formation of the Moon by a giant impactor, which did blow off the pre-lunar atmosphere. The value of Φ (Earth) was first estimated from the total amounts of either neon or nitrogen in the Earth's atmosphere and the corresponding values of A(%). To get a completely independent estimate, we next assumed that the impactors responsible for the formation of lunar craters originate from a family of bodies, which were also the parent bodies of juvenile micrometeorites. In this case, the variations of the lunar cratering rates K(t) with time, which was conjectured by W.Hartmann prior to 3.9 Gyr, scales to that of the amplification of the micrometeorite flux with time, relatively to the present day value (∼ 40,000 tons per year for the whole Earth). The integration of this curve yields a value Φ (Earth) ∼ 5.6 × 10²⁴ g, which well corresponds to the average value of the neon and nitrogen estimates of about 4 × 10²⁴ and 7 × 10²⁴ g, respectively. This formula predicts M(A) values that are similar to those measured in Earth's atmosphere within a factor 2 (except water that might be depleted by a factor 2-3). This is quite unexpected. Indeed, the values of A(%) greatly differ (up by a factor of ∼ 10 millions) and these volatiles have very different origins. For example solar neon was implanted in AMMs during their flight times to the Earth, while water originates from hydrous silicates. Therefore the Earth's atmosphere was probably formed by the post-lunar accretion of a giant storm of juvenile micrometeorites probably originating from comets, which was mostly effective during the first ∼ 100 Myr of the post-lunar period, around 4.4 Gyr ago.