Not all Martian sand dunes are located in craters. This image from NASA 2001 Mars Odyssey spacecraft shows dunes located on the plains of Terra Sirenum. Orbit Number: 67736 Latitude: -71.7973 Longitude: 216.088 Instrument: VIS Captured: 2017-03-22 07:46 https://photojournal.jpl.nasa.gov/catalog/PIA21662