Thermosphere and ionosphere The outermost layer of the Uranian atmosphere, extending for thousands of kilometres, is the thermosphere , which has a uniform temperature around 800 to 850 K.   This is much higher than, for instance, the 420 K observed in the thermosphere of Saturn.  The heat sources necessary to sustain such high temperatures are not understood, since neither solar FUV / EUV radiation nor auroral activity can provide the necessary energy.   The weak cooling efficiency due to the depletion of hydrocarbons in the stratosphere may contribute to this phenomenon.  In addition to molecular hydrogen , the thermosphere contains a large proportion of free hydrogen atoms ,  while helium is thought to be absent here, because it separates diffusively at lower altitudes.  The thermosphere and upper part of the stratosphere contain a large concentration of ions and electrons , forming the ionosphere of Uranus. 
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This note was uploaded on 12/15/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.