The solar system was formed about 4.6 billion years ago from a cloud of gas and dust (solar nebula) that slowly contracted under the force of gravity. The cloud mainly consisted of hydrogen (H) along with some helium (He) and smaller quantities of other chemical elements.
This contracting, rotating cloud flattened into a disc. Within the disc, the greatest concentration of matter was at the center. This formed the sun. Matter collecting in smaller clumps further out from the centre became the planets. The the sun and the planets grew by accretion of matter.
As this early stage of the sun grew, pressures and temperatures increased as a result of gravitational compression. Eventually the pressures and temperatures in the core of the forming sun became great enough for hydrogen nuclei to fuse together to form helium. This nuclear reaction released, and continues today to release, great amounts of energy. By this stage, the planets had attained almost all of their mass.
At the high temperatures of the inner region closer to the sun, Mercury, Venus, Earth, and Mars were too hot to keep hold of the volatile gases that dominated the solar nebula. Only high melting point compounds such as iron and silicates were stable. As a result, these terrestrial planets mainly consist of metallic cores and silicate outer regions with atmospheres either very thin or absent.
In the regions further from the sun, temperatures were low enough for gases to accumulate and be held by the gas giants Jupiter and Saturn. These consist mainly of hydrogen and helium.
The early Earth was very hot from gravitational compression, meteor impacts, and radioactive decay and was probably partially or completely molten. Denser metallic liquids sank to the center of the Earth and less dense silicate liquids rose to the top resulting in the Earth being differentiated into an iron core and a rocky silicate mantle.
The mantle of the Earth consists of silicate, rich in iron and magnesium. The outer crust consists of silicate with lower amounts of these elements. The silica rocks of the crust generally have a lower density and lower melting point than mantle rocks. Much of the continental crust, the most silica-rich and least dense kind, had been produced by 2.5 billion years ago.
The present Earth can be divided into the following geological layers:
- Lithosphere (crust and uppermost solid mantle): 0-60 km
- Lower mantle – 35-2,900 km
- Outer core – 2,900-5,100 km
- Inner core – 5,100-6,378 km
Now we can look at how the nature of the atmosphere at this early stage of the Earth’s development.