Proto-Earth

According to the homogeneous model, the metal-containing materials (in particular iron and nickel) and the silicate-containing material of the primeval solar cloud condensed out at about the same time. The proto-Earth thus formed was composed of a mixture of these two types of matter, which differed greatly in their densities. At that time, the Earth s temperature was probably only a few hundred degrees, and... 

Fig. 2.3 According to the homogeneous accretion model (a), iron-containing material (black) and silicate-containing material (colorless) condensed out at the same time, i.e., the proto-Earth consisted of a mixture of the two. The concentration of iron in the Earth s core took place later. According to the heterogeneous model (b), the iron condensed out of the primeval solar nebula first, while the silicates later formed a crust around the heavy core. From Jeanloz (1983)...

A vital event in the further development of the Earth was its collision with a smaller planet, possibly as big as Mars. It is assumed that this gigantic collision took place between four and four and a half billion years ago (Sleep et al., 2001), and that it also resulted in the birth of our moon (Luna), which was formed from partially vaporized matter from the Earth. It is likely that not all of the proto-Earth was melted by the energy set free in the collision, but that sections of it remained in their original form. However, more exact information is not yet available. 

A similar niobium deficit to that on Earth was found on the moon, although the latter s lower mass would preclude the existence of pressures high enough to lead to an absorption of niobium by the FeNi core. It is thus very likely that the moon was formed from material derived from the heavenly body which collided with the Earth and from the proto-Earth s silicate-rich cmst around 4.4 billion years ago. 

Earth, relative to average solar system (chondrites). However, the tungsten isotopic difference between early metals and the silicate Earth on its own does not provide constraints on timing. One needs to know the atomic abundance of Hf at the start of the solar system (or the ( Hf/ Hf)Bssn the bulk solar system initial ) and the composition of the chondritic reservoirs from which most metal and silicate reservoirs were segregated. In other words, it is essential to know to what extent the extra in the silicate Earth relative to iron meteorites accumulated in the accreted chondritic precursor materials or proto-Earth with an HfAV 1 prior to core formation, and to what extent it reflects an accelerated change in isotopic composition because of the high HfAV ( 15) in the silicate Earth. 

Wanke H. and Dreibus G. (1986) Geochemical evidence for formation of the Moon by impact induced fission of the proto-Earth. In Origin of the Moon (eds. W. K. Hartmann,... 

As nebular fractionations have produced a variety of chondritic meteorites, it is necessary to study the whole spectrum of nebular fractionations in order to see if the proto-earth material has been subjected to the same processes as those recorded in the chondritic meteorites. As the Earth makes up more than 50% of the inner solar system, any nebular fractionation that affected the Earth s composition must have been a major process in the inner solar system. We will begin the discussion of nebular fractionations in meteorites and in the Earth with refractory elements and continue with increasingly more volatile components as outlined in Table 2. 

For most other elements there is no difference between the isotopic composition of carbonaceous chondrites and the Earth. As of early 2000s, only two exceptions, chromium and titanium, are known for these two elements very small differences in the isotopic composition between carbonaceous chondrites and the Earth were found. Bulk carbonaceous chondrites have isotope anomalies in chromium and titanium. Isotopically unusual material may have been mixed to the CC-source after proto-earth material has accumulated to larger objects. 

Sasaki S. and Nakazawa K. (1990) Did a primary solar-type atmosphere exist around the proto-Earth Icarus 85, 21-42. 

So when does the first evidence of improbable, information-containing, metabolic replication occur in the fossil record The Earth is 4,500 million years old, as judged by several corroborating radionuclide studies of the oldest rocks on the planet show. Meteoric bombardment of the proto-Earth continued heavily until 4,000 MY A, probably precluding life during this period. The majority of the oldest rocks on Earth are 3,500 million years old, and the earliest microfossils are from 3,000+ MYA, hence we only have a window of about 500 million years from the end of the meteoric bombardment to the first signs of microbial life. This means we are either very lucky, or life is a high-on certainty ... 

The source of water for the formation of the hydrosphere is problematical. Some meteorites contain up to 20% water in bonded hydroxyl (OH) groups, while bombardment of the proto-Earth by comets rich in water vapour is another possible source. Whatever the origin, once the Earth s surface cooled to 100°C, water vapour, degassing from the mantle, was able to condense. Mineralogical evidence suggests water was present on the Earth s surface by 4.4 billion years... 

The latest of these involved an oblique collision between the proto-Earth and a body the size of Mars, at about 30 Ma after the formation of the solar system. This impact generated a huge amount of thermal energy so that a significant portion of the Earth was vaporized. This vapor coalesced around the Earth and cooled to form the Moon. A consequence of this impact is that a significant proportion of the Earth s mass would have been molten, creating what has become known as a magma ocean. 

The standard model for the formation of the Moon is the Giant Impact hypothesis (Stevenson, 1987). This model proposes that a planetesimal, the mass of Mars (about 15% the mass of the Earth), collided with the proto-Earth, at a time after core formation. The impact generated a huge amount of thermal energy so that the impactor and part of the Earth were vaporized, and some of this material coalesced... 

In the case of the inner planets, there was no dissipation of those gaseous compounds that either condensed on (depeding on temperature and pressure conditions) or chemically reacted with the solid and liquid phases. Under the conditions of the proto-Earth water accumulated, an event of major importance. It was this accumulation that made the formation of the biosphere possible, the evolution of which led to the appearance of a thinking species, man. 

Sasaki S, Nakazawa K (1988) Origin of isotopic fractionation of terrestrial Xe hydrodynamic fractionation during escape of the primordial H2-He atmosphere. Earth Planet Sci Lett 89 323-334 Sasaki S, Nakazawa K (1990) Did a primary solar-type atmosphere exist around the proto-Earth Icarus 85 21-42... 

---