The correct option is Condensation of water vapour released during degassing..
Explanation
The formation of the Earth's hydrosphere is intrinsically linked to the evolution of the early atmosphere and the cooling history of the planet. The primary mechanism involves the release of volatiles from the planetary interior.
Detailed Analysis:
- Melting of polar ice caps. is Incorrect: The melting of polar ice caps is a process associated with climatic fluctuations (such as interglacial periods) or contemporary global warming. It presupposes the existence of frozen water and does not explain the primordial origin of the water itself.
- Condensation of water vapour released during degassing. is Correct: During the early evolution of the Earth, the planet underwent a process called degassing, where gases and water vapour were released from the interior through intense volcanic activity. As the Earth cooled, this water vapour condensed into liquid droplets. This resulted in prolonged torrential rains that filled the geomorphological depressions on the Earth's crust, eventually forming the oceans.
- Impact of icy comets. is Incorrect: While the impact of icy comets and meteorites (specifically during the Late Heavy Bombardment) contributed to the Earth's total water volume, the fundamental process initiating the formation of the oceans is recognized as the condensation of water vapour from terrestrial degassing.
- Chemical reaction between hydrogen and oxygen in the atmosphere. is Incorrect: The primordial atmosphere was chemically reducing and lacked abundant free oxygen. Consequently, the oceans were not formed through a direct atmospheric combustion or reaction between hydrogen and oxygen, but through the physical phase change of water vapour released from the mantle.
Key Takeaway: The primary process responsible for the formation of Earth's oceans was degassing, followed by the condensation of water vapour and precipitation as the primordial Earth cooled.