<P> Other bodies within the Solar System have extremely thin atmospheres not in equilibrium . These include the Moon (sodium gas), Mercury (sodium gas), Europa (oxygen), Io (sulfur), and Enceladus (water vapor). </P> <P> The first exoplanet whose atmospheric composition was determined is HD 209458b, a gas giant with a close orbit around a star in the constellation Pegasus . Its atmosphere is heated to temperatures over 1,000 K, and is steadily escaping into space . Hydrogen, oxygen, carbon and sulfur have been detected in the planet's inflated atmosphere . </P> <P> Earth's atmosphere consists of a number of layers that differ in properties such as composition, temperature and pressure . The lowest layer is the troposphere, which extends from the surface to the bottom of the stratosphere . Three quarters of the atmosphere's mass resides within the troposphere, and is the layer within which the Earth's terrestrial weather develops . The depth of this layer varies between 17 km at the equator to 7 km at the poles . The stratosphere, extending from the top of the troposphere to the bottom of the mesosphere, contains the ozone layer . The ozone layer ranges in altitude between 15 and 35 km, and is where most of the ultraviolet radiation from the Sun is absorbed . The top of the mesosphere, ranges from 50 to 85 km, and is the layer wherein most meteors burn up . The thermosphere extends from 85 km to the base of the exosphere at 690 km and contains the ionosphere, a region where the atmosphere is ionised by incoming solar radiation . The ionosphere increases in thickness and moves closer to the Earth during daylight and rises at night allowing certain frequencies of radio communication a greater range . The Kármán line, located within the thermosphere at an altitude of 100 km, is commonly used to define the boundary between Earth's atmosphere and outer space . The exosphere begins variously from about 690 to 1,000 km above the surface, where it interacts with the planet's magnetosphere . Each of the layers has a different lapse rate, defining the rate of change in temperature with height . </P> <P> Other astronomical bodies such as these listed have known atmospheres . </P>

What term do we use for the lowest layer of the atmosphere