Understanding the Relationship between Gravity and Planet Atmospheres

The question often arises: why do planets like Mercury or Venus, which lack or have thin atmospheres, have higher gravity than Earth, which has a comparatively dense atmosphere? Conversely, why doesn't Earth have higher gravity despite its atmospheric presence?

The Role of Mass and Density

When discussing the gravity of a planet, the primary factor to consider is its mass and density. Gravity is primarily determined by the mass of the planet, and density is a related but distinct property that affects its gravitational pull at the surface. An atmosphere, while it does have mass, contributes very little to the overall gravitational pull, especially in comparison to the mass of the planet itself.

Comparing Venus, Earth, and Mercury

Venus, often cited as an example, does indeed have a very thick atmosphere. Its atmosphere is about 92 times denser than Earth’s. Despite this, Venus has a surface gravity of 8.87 m/s2, which is slightly higher than Earth's 9.81 m/s2. This is because Venus and Earth have similar masses and therefore similar surface gravity, despite their differing atmospheric densities.

Mercury, on the other hand, has a much lower surface gravity of 3.7 m/s2. Despite having a much thinner atmosphere, its lower mass results in a weaker gravitational pull. This highlights the importance of planetary mass over atmospheric density when determining a planet's gravity.

Gravitational Impact of Atmospheres

It is a common misconception that a planet's atmosphere significantly affects its gravitational pull. In fact, the mass of the atmosphere contributes only marginally to the overall gravity. For Earth, the atmospheric mass accounts for approximately one millionth of the total gravitational pull. This means that even if Earth lost its atmosphere completely, the change in gravitational pull would be minimal.

A Real-World Example: Venus

Venus is a prime example of a planet with a massive atmosphere. Its atmosphere is composed primarily of carbon dioxide and has a surface pressure over 90 times that of Earth's. While this atmosphere is incredibly dense and could collapse a battle tank, its contribution to Venus's gravitational pull is not significant. Venus's surface gravity is slightly lower than Earth's, but this is mainly due to Venus's smaller mass rather than its atmosphere.

Conclusion

Gravity is fundamentally a measure of a planet's mass and its density. While an atmosphere does have its own mass, its contribution to gravity is so minimal that it is almost negligible. The thick atmosphere of planets like Venus or the thin atmosphere of Mercury do not significantly impact their overall gravitational pull.

Therefore, while Earth's atmosphere contributes to the overall mass of the planet, it does not significantly alter the surface gravity. Both Mercury and Venus, despite having much denser or thinner atmospheres, have gravitational pulls that are similar to, or slightly lower than, Earth's, because of their respective masses.