On average, the troposphere extends from the ground to about 10 kilometers (six miles) high, ranging from about six kilometers (four miles) at the poles to more than 16 kilometers (10 miles) at the Equator. Troposphere The troposphere is the lowest atmospheric layer. The boundaries between atmospheric layers are not clearly defined, and change depending on latitude and season. Another layer, called the ionosphere, extends from the mesosphere to the exosphere. From the ground toward the sky, the layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Earth’s atmosphere has a layered structure. Solar heat, on the other hand, is necessary for all life on Earth. Ultraviolet radiation is harmful to living things, and is what causes sunburns. The atmosphere acts as a gigantic filter, keeping out most ultraviolet radiation while letting in the sun’s warming rays. The oxygen in today’s atmosphere probably took millions of years to accumulate. Later, more complex forms of plant life added more oxygen to the atmosphere. Photosynthesis is the process a plant or other autotroph uses to make food and oxygen from carbon dioxide and water.
Free oxygen may have been added to the atmosphere by primitive organisms, probably bacteria, during photosynthesis. Free oxygen consists of oxygen molecules not attached to another element, like carbon (to form carbon dioxide) or hydrogen (to form water). At that time, there would have been little or no free oxygen surrounding Earth. Scientists say many of the gases in our atmosphere were ejected into the air by early volcanoes. The atmosphere- air-is much thinner at high altitudes.
The bottom 30 kilometers (19 miles) of the atmosphere contains about 98 percent of its mass. The atmosphere is so spread out that we barely not ice it, yet its weight is equal to a layer of water more than 10 meters (34 feet) deep covering the entire planet. Other planets and moons have very different atmospheres, and some have no atmospheres at all. Water vapor and dust are also part of Earth’s atmosphere. Nitrogen and oxygen account for 99 percent of the gases in dry air, with argon, carbon dioxide, helium, neon, and other gases making up minute port ions. We live at the bottom of an invisible ocean called the atmosphere, a layer of gases surrounding our planet.
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