Which process explains why the terrestrial planets do not have hydrogen and helium in their atmospheres?

Question

thermal escape (??)
condensation
out gassing
solar wind

Answer 1

Yes, I would guess those light gasses would exceed escape velocity due to heat.

Answer 2

Thanks much!!

Answer 3

yes, here

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCQQFjABahUKEwjI1qjtttzIAhVLGz4KHfxjBKY&url=http%3A%2F%2Fwww.atmos.washington.edu%2F~davidc%2Fpapers_mine%2FCatling2009_SciAm_AtmEscape_Preprint.pdf&usg=AFQjCNE-vlUw54iLGlGBqrCfCQJ1CQHcIw&sig2=09z2PlaTuIUXxwPV5ia5jQ

Answer 4

You are welcome.