ABSTRACT
The Earth is thought to be ∼4.54 × 109 years old (∼4.6 eons) (Jacobsen, 2003). One accepted view holds that it was derived from an accretion disk that resulted from gravitational collapse of interstellar matter. A major portion of the matter condensed to form the Sun, a star. Other components in the disk subsequently accreted to form planetesimals of various sizes. These in turn accreted to form our Earth and the other three inner planets of our solar system, namely, Mercury, Venus, and Mars. All four of these planets are rocky. As accretion of the Earth proceeded, its internal temperature could have risen suf ciently to result ultimately in separation of silicates and iron, leading to a differentiation into mantle and core. Alternatively, and more likely, a primordial rocky core could have been displaced by a liquid iron shell that surrounded it. Displacement of the rocky core would have been made possible if it fragmented as a result of nonhydrostatic pressures, causing the inner core to become surrounded by a hot, well-mixed mantle or rock material in a catastrophic process. Whichever process actually took place, much heat must have been released during this formational process, resulting in outgassing from the mantle to form a primordial atmosphere and, possibly, hydrosphere. It has been suggested recently that bombardment of the early Earth by giant comets that consisted of water ice and cosmic dust introduced much of the water on the Earth’s surface (see, for instance, Delsemme, 2001; Broad, 1997; Robert, 2001). All of this is thought to have occurred in a span of ∼108 years. Recent evidence suggests the presence of liquid water at the Earth’s surface as long ago as 4.3 eons before the present (BP) (Mojzsis et al., 2001).
