Winter Lake Circles

I noticed these circles in the snow on a small lake (Canvasback Lake) in our community in Idaho in December of 2005. Since that time I seem to find these circles on lakes and ponds just about everywhere I look (after freezing and snowfall). What caused them, and why did I not see them previously? My theory is in 2005 the space aliens that make crop circles in England decided to confound Americans during the winter when there were no crops to circle in England. Not finding crops in the U.S. during that time of year, they decided to do their thing on lakes. Recently I spotted “crop circles” on a pond on the Ohio University campus and asked a physicist friend to explain that phenomenon. He had a look at the circles and responded that physics does not deal with the paranormal. Have an alternative theory? I sure would like to hear it!