4.5 BYA the kerbol system was formed with large chunks of debris, this would eventually coalesse to form the planets: Moho,Eve,Kerbin,Duna, and Jool. However another body was also forming the Dwarf Planet known as the Mun. However this was no ordinary dwarf planet for early in its developement the Mun formed a thick atmosphere with lakes,oceans,rivers,and possibly inteligent life(Hint: Munoliths). One day the Mun passed very close to the young kerbin within 6,000 kilometers of its surface during this capture it is believed that the Mun may have had a smaller Mun thrown off during this capture. Eventually the Mun settled into its 11,400 kilometer orbit and all was well until 5.2 MYA it is believed Kerbol suffered a massive Solar Flare that caused mass extinctions on Kerbin and Wiping the Mun clean of its atmosphere. Now the Mun is a dead rock with reminders of what it once was these include: Arches, Canyons, and River deltas.