May be the underlying ice melting and refreezing like on Europa ... http://i.space.com/images/i/000/029/810/i02/europa-thick-ice-crust.jpg http://i.space.com/i

May be the underlying ice melting and refreezing like on Europa ... http://i.space.com/images/i/000/029/810/i02/europa-thick-ice-crust.jpg http://i.space.com/images/i/000/029/810/i02/europa-thick-ice... ... due to occasional asteroid strike?