The atmosphere of the Earth plays an essential role in sustaining ecosystems. The atmospheric layer closest to the Earth's surface is the troposhere, which contains water vapor and clouds (these may also be considered part of the hydrosphere), as well as other gases, that envelop the Earth. This layer of gases is held in place by gravity, and is the most important layer of the atmosphere for the natural environment, since this layer spans the lowest seven kilometers above the surface, where most of the landforms and biota are present. Dry air consists of 78% nitrogen, 21% oxygen, 1% argon and other minor constituents such ashelium and carbon dioxide.