Gravity
Explanation
The atmosphere consists of a mixture of gases, which are composed of molecules that possess mass. Like all matter with mass, these gas molecules are subject to the force of gravitational attraction exerted by the Earth.
Detailed Analysis:
- Magnetic field Magnetic field: Incorrect. The Earth's magnetic field (magnetosphere) plays a crucial role in shielding the atmosphere from the solar wind, which could otherwise strip it away. However, it acts primarily on charged particles and is not the force responsible for binding the neutral gas molecules of the atmosphere to the Earth.
- Gravity: Correct. Gravity is the attractive force exerted by the Earth on all objects with mass, including gas molecules. This gravitational pull counteracts the thermal velocity of the gas molecules, preventing them from escaping into outer space and thereby holding the atmosphere around the planet.
- Atmospheric pressure Atmospheric pressure: Incorrect. Atmospheric pressure is the result or effect of the weight of the air pressing down on the Earth's surface. It is caused by gravity acting on the air column; it is not the force that holds the atmosphere in place.
- Solar wind Solar wind: Incorrect. The solar wind consists of a stream of charged particles released from the upper atmosphere of the Sun. It exerts a pressure that tends to erode or strip away planetary atmospheres rather than hold them.
Key Takeaway:
Gravity is the primary force that retains a planet's atmosphere by pulling gas molecules toward the surface, preventing their escape into space.