Andrew Gunn , Douglas J Jerolmack 

Sand dunes arise wherever loose sediment is mobilized by winds that exceed threshold speeds, and grains are sufficiently strong to survive collisions. The ubiquity of dunes in our solar system is remarkable and confounding; their occurrence under conditions of thin atmospheres, and/or friable materials, challenges our understanding of sediment transport mechanics. Current threshold theories lose meaning and diverge from one another when extrapolated to some planetary bodies, because they neglect physical processes that become relevant under such exotic conditions. Here we draw on results in contact, rarified gas, statistical and adhesion mechanics to present more complete theories for the ‘fluid’ and ‘impact’ thresholds of aeolian transport. Our theoretical predictions compare well with all available experimental threshold observations, and shed light on the contentious issues of sediment mineralogy on Titan and the high threshold for dune activity on Mars. This work will aid in interpreting planetary atmospheric dynamics from observed dunes, and determining what observations are required for future space missions.