Planet Earth

Abstract Except for major short-term perturbations in surface environments caused by a declining flux of impactors, equable conditions for prebiotic evolution could have existed as early as 4.4 GA. The earth is about 4.6 Ga old. At that remote time, known as the Hadean era (fig. 13.1), its surface was very hot as a result of the accretion process, which, according to recent hypotheses, took about a hundred million years: Its temperature, according to recent models, was about 1,500°K. Thus, the surface was molten. The iron-group elements (Fe, Ni, and Co) melted and passed through the lighter silicate molten rocks down beneath the crust in a process known as the iron catastrophe (R. F. Fox, 1988). Gradually the surface, rich in silicates, cooled down as the accretion energy input decreased. Solid rocks started to emerge, forming a thin scum, and a steam atmosphere began to condense and rain down to form the primordial oceans. Surface temperatures at or below l00°C could have developed as many as 4.4 Ga ago (Chang, 1993, 1994).are 3.8-3.9 billion years old, and there is no geological evidence of prebiotic organic chemical processes taking place on the earth’s surface prior to this time. Most of the very old rocks on Earth were transformed geologically by plate tectonics. Moreover, the early craters formed by impactors disappeared through erosion processes. Fortunately, despite the destruction of much of the geological record of primordial Earth, some of this evidence has been preserved and shown to be relevant to the study of the origin of life.