The correct option is A barren, rocky and hot object with a thin atmosphere of hydrogen and helium..
Explanation
The formation of the Earth occurred approximately 4.6 billion years ago through the process of accretion within the solar nebula. During this primordial stage, the physical and atmospheric conditions of the planet were dictated by intense heat from planetesimal collisions, gravitational compression, and radioactive decay.
Detailed Analysis:
- Barren, Rocky, and Hot: The primordial Earth was a hot, barren, and rocky object. Due to the high temperatures, the surface was largely molten, often described as a magma ocean. It was devoid of life and liquid water in this initial phase.
- Primordial Atmosphere: The early atmosphere was thin and composed primarily of light gases, specifically Hydrogen and Helium. These gases were captured from the solar nebula. However, due to the weak gravitational hold of the Earth relative to giant planets and the intense solar wind from the young Sun, this primordial atmosphere was eventually stripped away.
- Comparison with Other Options:
- It was not a cold, frozen ball (Option a) because the accretion energy and radioactive heating kept the planet extremely hot.
- It was not water-covered with a nitrogen atmosphere (Option c) at this stage. The secondary atmosphere (rich in water vapor, nitrogen, and carbon dioxide) formed later through volcanic degassing, and liquid water accumulated only after the planet cooled sufficiently.
- It was not a gas giant (Option d). Earth is a terrestrial planet; it did not possess the immense mass required to accrete and retain a massive envelope of gases like Jupiter or Saturn.
Key Takeaway:
Primordial Earth was a hot, rocky body with a fleeting atmosphere of Hydrogen and Helium, distinct from its current state of a water-rich planet with a Nitrogen-Oxygen atmosphere.