Mostly for the same reason you stay close to the surface of the planet - gravity!
Because it just does.
Earth is the only known planet with an oxygenated atmosphere.
The planet you're referring to is Earth. It has a rocky surface and its atmosphere is composed primarily of nitrogen (78%) and oxygen (21%). Earth also has one natural moon, which is simply called the Moon.
None. Only Earth has such characteristics. Venus, Saturn, Uranus, and Neptune all have surface gravity similar to that of Earth, but have very different atmospheres and temperatures. Venus is much hotter with a crushing carbon dioxide atmosphere while the others have very cold hydrogen-helium atmospheres. Mars has the most Earth-like temperatures, but is still not very close. The warmest places on Mars are about as cold as the coldest places on Earth. The atmosphere is very thin and made mostly of carbon dioxide. Surface gravity is less than half that on Earth. The most Earth-like atmosphere belongs not to a planet, but to Titan, one of the moons of Saturn. Titan's atmosphere is mostly nitrogen and methane (Earth's is nitrogen and oxygen). Titan is also the only other objects in the solar system with liquid on it surface, though it is liquid methane rather than water. Titan is extremely cold and has much weaker gravity than Earth does. Earth is the only planet in the solar system to have bodies of water on its surface.
It is none other than our own planet Earth, which has nitrogen and oxygen in its atmosphere.
The gravitational pull keeps the Earth's atmosphere close to the surface...
Due to gravitational pull to the earth's atmosphere.
The gravitational force between the Earth and the atmosphere holds the atmosphere close, just like it holds you close to the Earth. (ground)
Because it just does.
layer close to the surface is the top part of the mantle
layer close to the surface is the top part of the mantle
layer close to the surface is the top part of the mantle
layer close to the surface is the top part of the mantle
earths surface
layer close to the surface is the top part of the mantle
layer close to the surface is the top part of the mantle
The surface of Mercury is a bit like that of the moon. There is no real atmosphere as the planet is too close to the sun and is not massive enough. It is pitted with billions of craters, from meteor impacts. Since there is no atmosphere, there is no weather to errode the features away over time caused by the impacts.