Research on the atmosphere began in the ancient world. The fifth-century Greek philosopher Empedocles tried to explain the different elements (earth, air, fire and water) and to account for the formation of the earth, oceans, sun and moon and atmosphere. The thermosphere meaning “heat sphere” is the outer layer of the atmosphere, separated from the mesosphere by the mesopause. Within the thermosphere temperatures rise continually to well beyond 1000°C. The thermosphere is the biggest of all the layers of the earth’s atmosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between 200 and 240 mi. The few molecules that are present in the thermosphere receive extraordinary amounts of energy from the Sun, causing the layer to warm to such high temperatures. The lower part of the thermosphere, from 80 to 550 km above the Earth’s surface, contains the ionosphere. Beyond the ionosphere extending out to perhaps 10,000 km is the exosphere or outer thermosphere, which gradually merges into space.
How Was the Thermosphere Discovered?
In the 17th Century, Galileo wanted to understand the element air. He was able to show that it had weight and was not just empty space. Galileo also invented the thermometer. With this instrument and Evangelista Torricelli’s barometer, it was possible to measure surface air temperature and pressure for the first time. In the following century kites, and later unmanned balloons, took these instruments higher into the atmosphere, marking the beginning of atmospheric science. The data that were collected from these measurements could be used to explain and predict weather patterns on earth. They also began to tell scientists about the different layers of the atmosphere and the variations in temperature and density that are characteristic of those atmospheric levels.
In the early 1900s, the radiosonde, an unmanned balloon able to reach high altitudes, revealed a significant change in temperatures beginning about 18 kilometers above the surface of the earth. This discovery led researchers to describe divisions of the atmosphere according to temperature, noting a drop in temperature in each successive layer. One scientist, Leon Teisserenc de Bort, identified two layers, the troposphere and stratosphere. Later, the outer layers of the mesosphere and thermosphere were also identified.
In the 18th century the Russian scientist Mikhail Vasilyevich Lomonosov discovered that Venus has an atmosphere much like Earth’s. In 1761 Lomonosov observed a transit of Venus, a rare event in which the planet passes directly in front of the sun. Lomonosov wanted to calculate the diameter of Venus but he noticed that the edge of the planet appeared fuzzy, not distinct. He recognized a halo of clouds circling the planet, similar to Earth’s atmosphere.
In the 1850s John Tyndall invented thermopile technology, which allowed him to measure the absorptive powers of atmospheric gases. Through these measurements he was able to explain infrared radiation and confirm that the earth’s atmosphere has a Greenhouse Effect. The unique chemical components of the gases that make up the atmosphere were identified by scientists such as John Dalton in the early 19th century.
Modern research on the thermosphere and other aspects of the structure of the atmosphere also advanced with rocket exploration. The first rocket probes of the upper atmosphere were launched in the late 1940s. The launch of Sputnik, the first satellite to orbit the earth, occurred in 1957. With further technological advances such as satellite imaging, atmospheric scientists working in the 1970s were able to make more precise calculations of the gases in the thermosphere. The composition of this layer of the atmosphere and the behavior of the ionosphere were the main subjects of research in this period.