PHYSICAL STATE OF THE VERY EARLY EARTH

The earliest surface environment of the:Earth is reconstructed in acco rdance with the planetary formation theory. Formation of an atmosphere is an inevitable consequence of Earth's formation. The atmosphere nea r the close of accretion is composed of 200 similar to 300 bars of H-2 and H2O, and several tens of bars of CO and CO,. Either by the blanke ting effect of the proto-atmosphere or heating by large planetesimal i mpacts a magma ocean is formed during accretion. We can distinguish th ree stages for the thermal evolution of the magma ocean and proto-crus t. Stage 0 is characterized by a super-liquidus (or completely molten) regime near the surface. At this stage the surface of the Earth is co vered by a super-liquidus magma ocean. No chemical differentiation is expected during this stage. Once the energy flux released by planet fo rmation decreases to the 200 W/m(2) level the super-liquidus magma oce an then disappears within a time interval of 1 m.y. This is the transi tion from stage 0 to 1. Stage 1 is characterized by a partially molten magma ocean. In the magma ocean consisting of 20 similar to 30% parti al melt, heat transport is controlled by melt-solid separation (a type of compositional convection) rather than thermal convection. Chemical differentiation of the mantle mainly occurs in this stage. Once the e nergy flux drops to the 160 W/m(2) level, more than 90% of water vapor in the proto-atmosphere condense to form the proto-oceans. Several te ns of bars of CO and CO2 remain in the atmosphere just after formation of the oceans. Water oceans are occasionally evaporated by large impa cts. After each such event, recondensation of the ocean takes several hundred years. Although the surface is covered by a chilled proto-crus t, it is short-lived because of extensive volcanic resurfacing activit y as well as meteorite impacts resurfacing. This stage ends when the e nergy flux drops to 0.1 similar to 1 W/m(2) level. The duration time o f stage 1 is estimated to be several hundred million years (the best e stimate is about 400 m.y.). Stage 2 is characterized by solid state co nvection. This stage continues to the present day. One of the most imp ortant change on the proto-Earth is the transition from stage 1 to 2, which occurs several hundred million years after the Earth formation. Long-lived crust is formed only after this transition.