The Early Earth Our star, the Sun, formed about 4570 million years ago when a fragment of a molecular cloud collapsed into a protoplanetary disk. The trigger that led to collapse of the rotating and turbulent cloud of gas and dust is not known: it may have been a shockwave from a nearby supernova. Planets form in the protoplanetary disk Within the disk, dust clumped into bigger and bigger bodies, eventually forming planets. The dust grains were mostly silicates: minerals made of iron, magnesium, silicon and oxygen, with lesser amounts of calcium, aluminium, sodium and potassium. The grains might be covered with layers of organic molecules, and in cooler regions of the nebula, coated by ices, including water ice. These are the original materials that clumped together to form the Earth. All traces of the original materials that came together to form the Earth have now been obliterated by impact bombardment and geological processing. The earliest history of our Earth must therefore be inferred from many different sources, including those outside our planet. Evolution of the young Earth Once the Earth had aggregated, internal heat from radioactive decay, combined with gravitational energy and collisional energy from planetesimal bombardment kept the planet molten. As the Earth cooled, reduction reactions within the convecting system resulted in production of a metal-rich core and a silicate-rich crust-mantle structure. The last major event in the history of the proto-Earth was the formation of the Moon, probably in a dramatic collision with some unknown body. How to study the early Earth The rocks which are accessible for direct study at the Earth's crust are therefore not representative of the original material that accreted from the solar nebula. In order to understand the precursors of the Earth, the only relevant materials available for study in the laboratory are meteorites. Sometime in the future it may be possible to study ancient material in comets by means of space missions, either in-situ or with samples returned to terrestrial laboratories.

High-resolution images of volcanic activity on Jupiter's moon Io act like a time machine, giving a view of what conditions may have been like on the young Earth. This image was obtained with the Galileo spacecraft.