Upper atmosphere and ionosphere The mesosphere of Venus extends from 65 km to 120 km in height, and the thermosphere begins at around 120, eventually reaching the upper limit of the atmosphere (exosphere) at about 220 to 350 km.  The exosphere is the altitude at which the atmosphere becomes collisionless. The mesosphere of Venus can be divided into two layers: the lower one between 62–73 km  and the upper one between 73–95 km.  In the first layer the temperature is nearly constant at 230 K (−43°C). This layer coincides with the upper cloud deck. In the second layer temperature starts to decrease again reaching about 165 K (−108°C) at the altitude of 95 km, where mesopause begins.  It is the coldest part of the Venusian dayside atmosphere.  In the dayside mesopause, which serves as a boundary between the mesophere and thermosphere and is located between 95–120 km, temperature grows up to a constant—about 300–400 K (27–127°C)—value prevalent in the thermosphere. 
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