Expert: Jayendra Upadhye - 4/17/2004

Question
How can several of the larger moons of the outer planets retain an atmosphere when they are no larger than Mercury, which has none?

Thanks

Answer
Hi James..
Temperature..Temperature..it is all about that in Astronomy!
You will notice that right from studies involving the Big Bang ..(decoupling of matter and energy depends on temperature of the expanding universe..) to the absorption/emission ratios of planets ..it is all about temperature.
   In the case of the said moons, the heat of the sun as recieved in that area of space is not enough to blast away the gases from the weak gravity these moons have.
   That is clearly not the case with mercury which has lost whatever nascent atmosphere it may have had..(actually it does have some you know..).
   Eventually when the sun expands and becomes a red giant, the gas giants like jupiter and saturen etc will like mercury loose most of their atmospheres to attrition due to the rising temperatures in their environs.
Jayen