Either those circles are just from center-pivot irrigation, or the aliens have arrived en masse.
Actually, these strange circles on Google Maps are exactly that: the result of circle irrigation, a method some farmers use to water crops. The method works rotating sprinkling equipment in a big circle. Better coverage that way, I guess.
From space, these large circles can appear pretty extraordinary, and if you didn’t know what you were looking at, well — wild speculation ensues. The above images are from the Google Maps view of Earth, Texas, by the way, and you can check it out yourself here. There’s so many of them.