re: 5xJupiter mass
"The most likely scenario for a Mars sized satellite might be a capture. Unlikely but possible with almost infinite opportunities and no more unlikely than our own moon."
Yes. I can't begin to imagine what the atmosphere would be like on such a 'moon'.
Speaking of our own moon... imagine if that 'Mars sized' planet/asteroid/comet had not colided with Earth. Would Earth have had less or more water? Less mass? Just wondering.
Ahh, Nick offers some ideas here.
Typing and reading at the same time...