Physical conditions on the early Earth

Abstract

The formation of the Earth as a planet was a large stochastic process in which the rapid assembly of asteroidal-to-Mars-sized bodies was followed by a more extended period of growth through collisions of these objects, facilitated by the gravitational perturbations associated with Jupiter. The Earth's inventory of water and organic molecules may have come from diverse sources, not more than 10% roughly from comets, the rest from asteroidal precursors to chondritic bodies and possibly objects near Earth's orbit for which no representative class of meteorites exists today in laboratory collections. The final assembly of the Earth included a catastrophic impact with a Mars-sized body, ejecting mantle and crustal material to form the Moon, and also devolatilizing part of the Earth. A magma ocean and steam atmosphere (possibly with silica vapour) existed briefly in this period, but terrestrial surface waters were below the critical point within 100 million years after Earth's formation, and liquid water existed continuously on the surface within a few hundred million years. Organic material delivered by comets and asteroids would have survived, in part, this violent early period, but frequent impacts of remaining debris probably prevented the continuous habitability of the Earth for one to several hundred million years. Planetary analogues to or records of this early time when life began include Io (heat flow), Titan (organic chemistry) and Venus (remnant early granites).

Figure 1

Example of the formation of terrestrial planets from a series of asteroidal-to-Mars-sized bodies using a symplector integrator simulation as described in the text. Six snapshots in time show the eccentricity versus semi-major axis of 1885 objects that collide, coalesce and grow under the perturbing influence of Jupiter and of their own growing gravitational fields. The size of each body is proportional to its linear diameter as it grows, and the amount of iron it contains is shown in black. Colours allow the eye to track mixing across regions of the solar system, and can also be considered a rough indication of the amount of water assumed present in these objects at the beginning, and in the bodies as they collide and grow, with the water mass fraction scale shown at the bottom. The final system is not precisely our own terrestrial planet system, but similar, and outcomes vary dramatically as boundary conditions are changed. From Raymond et al. (2006).

Figure 2

Schematic change in cratering rate over time in the Earth–Moon system, with major events based on dating of Apollo samples labelled. Vertical lines indicate the ages of the oldest terrestrial zircon, and whole rock, samples. Modified from a variety of sources and originally published in Lunine (2005).

Figure 3

Speculative integrated early history of the Earth covering the first 700 million years from the formation to the tail-off of the impactor flux. During this time, the Earth and Moon form, the Earth acquires its water and carbon content, oceans and hydrothermal vent systems appear, and life begins. The hydrosphere-destroying megaimpacts give way to sterilizing impacts and then, at the end of the timeline, non-globally lethal impacts that might have created habitable hydrothermal systems.