Formation of the Post Lunar Earths Atmosphere An Accretion Formula Born with the Moon

The Moon was formed by a giant Mars-sized impact with the Earth, which occurred near the end of the formation time interval of the Earth, A (Earth), as shown by Canup and Asphaug (2001). This impact blew off the pre-lunar atmosphere (cf. Sect. 10, first rubric). It thus left a vacant "niche" for the

Fig. 3.3. Isotopic composition of various types of waters, including the constituent water of the CI and CM hydrous-carbonaceous chondrites (upper histogram), and the constituent water of Antarctic micrometeorites (lower histogram). From left to right, the arrows refer to (i) Antarctic melt ice water showing the lowest terrestrial D/H ratio; (ii) an average of the terrestrial oceans that defines the "SMOW" value; and (iii) an average of the D/H ratios measured in three nonperiodic comets (Courtesy C. Engrand).

Fig. 3.3. Isotopic composition of various types of waters, including the constituent water of the CI and CM hydrous-carbonaceous chondrites (upper histogram), and the constituent water of Antarctic micrometeorites (lower histogram). From left to right, the arrows refer to (i) Antarctic melt ice water showing the lowest terrestrial D/H ratio; (ii) an average of the terrestrial oceans that defines the "SMOW" value; and (iii) an average of the D/H ratios measured in three nonperiodic comets (Courtesy C. Engrand).

accumulation of a new "post-lunar" atmosphere. The invariance of the composition of the micrometeorites with time (Sect. 4.2) allows assuming that the wt% content A(%) of a given volatile, A, in early micrometeorites, back to 4.4 Gyr ago, is just given by the corresponding value measured in Antarctic micrometeorites.

If a method is found to estimate the integrated mass flux of micrometeorites, , deposited on the Earth since the formation of the Moon, the total amount, Ma, of A deposited by micrometeorites in the post-lunar atmosphere is given by a "simple" accretion formula: Ma ~ A(%) x 10~2 x 0o. The next section briefly outlines two fully independent methods that unexpectedly yielded a similar value of 0o.

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