Of the planets in the solar system Mercury, nearest to the Sun, has almost no atmosphere. The next three planets, Venus, Earth, and Mars, lost whatever gases were present at their formation and the atmosphere they now possess arise from gases related from their interiors and reactions that these have undergone. The atmosphere of each planet is unique. The outer planets have extremely deep atmospheres made up mostly of hydrogen and helium, little changed from the original composition at the time of formation.
The Earth's atmosphere is a thin shell of gases surrounding the globe. The thinness becomes clear when we realize that the part of the atmosphere in which all human activities take place adds less than 0.3% to all the radius of the Earth. Its unique chemistry, including compounds such as oxygen (molecular oxygen or dioxygen) and carbon dioxide that support the processes upon which all forms of life depend, distinguishes this atmosphere from that of other planets in the solar system.
The mixing ratios of the major gases remain relatively constant up to an altitude of about 80 km. The constancy is because the kinetic energy of these molecules is sufficient to overcome any gravitational forces that would lead to settling. Because the mixing ratios of the major components are constant, the average molar mass, Ma can be calculated.
Above about 80 km altitude, the concentrations of the major species do begin to change significantly due to photochemical processes that cause dinitrogen and especially dioxygen to dissociate.
In contrast to the major species, the composition of some trace gases is not constant in the atmosphere near Earth's surface due to the various sources and removal processes that operate in different regions, on both horizontal and vertical scales.
0 comments:
Post a Comment