What is Gravity?
Earth holds its atmosphere due to the force of gravity. Gravity pulls the gases of the atmosphere towards the surface of the Earth, preventing them from escaping into space. The Earth's gravity is strong enough to hold gases like nitrogen, oxygen, carbon dioxide, and others close to its surface.
Yes, a sky would still exist even without gravity. Gravity is what holds our atmosphere close to Earth, but even without it, the atmosphere would still exist and interact with light to create the appearance of a sky.
The Earth's gravitational attraction prevents gasses from escaping into outer space.
The rocky or terrestrial planets have less atmosphere (but more breathable atmosphere) than the gas giants have, for two reasons. First, they are smaller, and therefore have weaker gravitational fields, which makes it easier for gas to leak away into space, and secondly, they are closer to the sun and therefore hotter, and the additional heat causes gas to expand, thus also contributing to its escape into space.
Mercury has a surface pressure that is near enough zero (or a vacuum), the planet holds no real atmosphere due to its small size. There are some trace gases at the surface, but not what one would call a proper atmosphere.
The gravitational force between the Earth and the atmosphere holds the atmosphere close, just like it holds you close to the Earth. (ground)
The force that holds the atmosphere to the Earth is gravity. Gravity is the force of attraction between all objects and it keeps the gases in our atmosphere close to the Earth's surface.
Gravity keeps 99 percent of the total mass of the atmosphere within 32 km of Earth's surface. This force prevents the gases in the atmosphere from escaping into space and holds them close to the Earth's surface.
The air is close to the surface of the earth because gravity holds it there
the gravitational force holds its atmosphere together
Where a planet or moon has an atmosphere, it is the gravity of the moon or planet that holds the atmosphere on the planet or moon.
Gravity holds the gases of the atmosphere near Earth's surface. The force of gravity pulls the gases towards the Earth, preventing them from escaping into space.
Venus, due to its close proximity to the sun and because of its thick carbon Dioxide atmosphere, which holds the heat more effectively (Mercury has no atmosphere). Surface temperatures on Venus average 735 Kelvin or 460 °C.
It has a strong greenhouse effect and its atmosphere is so thick it holds in heat in such a way that it is hotter than Mercury, even though Mercury is closer it the sun, because Mercury has no atmosphere. It is also, near the surface, very high in sulfuric acid vapor.
Gravity.
Mercury has a surface pressure that is near enough zero (or a vacuum), the planet holds no atmosphere due to its small size. Its small size (and therefore mass) means that it has a low gravitational pull, not strong enough to hold gases close to its surface to create an atmosphere.
Earth holds its atmosphere due to the force of gravity. Gravity pulls the gases of the atmosphere towards the surface of the Earth, preventing them from escaping into space. The Earth's gravity is strong enough to hold gases like nitrogen, oxygen, carbon dioxide, and others close to its surface.