ATMOSPHERE EVOLUTION 199 giant impact hypothesis envisages total Earth melting, blow-out of the tattered remnants of primary atmosphere captured during accretion, and devolatilization of the material that coalesced to form the Moon (see Solar System: Moon) Abatement and Cooling Figure Noble gas abundances on Earth in comparison with cosmic abundances (adapted with permission from D Hunten et al in Meteorites and the Early Solar System, eds John F Kerridge and Mildred Shapley Matthews ß 1988 The Arizona Board of Regents) of 10–15% of its present value Whatever this primary atmosphere was, intense solar wind from the young Sun and later energetic impacts, culminating in the Moon-forming event, would have led to significant gas loss Evidence for this can be seen in the concentrations of the noble gases (helium, neon, argon, krypton, xenon) Abundances of these chemically unreactive gases in Earth’s atmosphere are far less than their solar or cosmic equivalents (Figure 3) This could be due to either solar wind or impact-induced erosion of planetary atmospheres (or both) Formation of the Moon: Loss of the Primary Atmosphere Evidence from a combination of lunar samples, meteoritics, interplanetary probes, isotopic studies, and physical models shows that the accretionary phase of the planets was intense and short-lived, lasting for less than 100 Ma after the initial collapse of the solar nebula Under conditions of intense early bombardment, the conversion of the kinetic energy of millions of impacting planetesimals to thermal energy would have heated the proto-Earth to melting, further facilitating differentiation into core, mantle, crust, and atmosphere (see Earth: Mantle; Crust) Sometime between 4.5 Ga and 4.45 Ga an object the mass of Mars entered an Earth-crossing orbit The consequences of this event were profound The As planetary accretion slowed, heat rapidly radiated from the surface into space, and the Earth’s surface rapidly cooled, forming a solid chill crust Outgassing due to the internal differentiation of the planet into core and mantle resulted in the formation of a dense atmosphere, probably composed of carbon dioxide, steam, and other gases Condensation of oceans onto the primordial surface could then have occurred as a consequence of either intrinsic water from planetary outgassing or later contributions from comets and meteorites Comets and meteorites are primitive undifferentiated objects rich in volatiles and reduced organic compounds – attractive procreative ingredients for the origin of life Some postulate that all the water on the Earth’s surface could have been delivered from comets without the need for outgassing of water vapour from the mantle However, if the 2D/1H composition of the three comets measured so far is typical of these objects, the oceans could not have been produced solely by cometary water (Figure 4) To explain the data whilst still accepting the cometary origin of the hydrosphere, terrestrial water requires an additional component with a D/1H much less than that of average seawater The weight of present evidence suggests that the likeliest source of this water is intrinsic planetary outgassing However, it could be that prebiotic reactions on the early Earth leading to the origin of life required a significant contribution from comets, and several models predict that these contributions could have been large Liquid water, a source of energy, and organic raw materials are presumed to be necessary to initiate biological processes Depending on the compositions of the earliest retained (secondary) atmosphere and hydrosphere, prebiotic molecules could have been synthesized in the atmosphere, formed at hydrothermal vents, or delivered by comets The oceans were an early feature of the Earth The oldest known rocks of sedimentary origin are >3.8 Ga ferruginous quartzites and banded iron formations from southern West Greenland and unequivocally support the presence of surface seas at that time Rare zircons from the Jack Hills region of Western Australia are as old as 4.38 Ga and contain oxygen isotopic compositions that are consistent with the presence of substantial liquid water at or near the Earth’s surface and interacting with the crust within 150 Ma of the origin of the Moon