In a written statement and telephone conversations with a National Transportation Safety Board investigator, the pilot reported that as he entered the vicinity of the destination airport, he decided to land as the passengers needed to use the airport's facilities. Using the visual clues of sprigs of grass, he determined that the runway had a layer of snow on the surface several inches in depth. After making a stabilized approach he touched down in the snow and the airplane suddenly stopped, nosing over inverted. After egressing the airplane, the pilot noted that the snow was in actuality over 1-foot deep. The pilot reported no preimpact mechanical malfunctions or failures with the airframe or engine. He stated that because he could not confirm the snow cover, he should not have attempted to land at that airport. The wing spar sustained damage during the accident sequence.