Sand dune patterns on Titan controlled by long-term climate cycles

Linear sand dunes cover the equatorial latitudes of Saturn's moon Titan and are shaped by global wind patterns(1-3). These dunes are thought to reflect present-day diurnal, tidal and seasonal winds(1,3-6), but climate models have failed to reproduce observed dune morphologies with these wind patterns(4,6). Dunes diagnostic of a specific wind(7,8) or formative timescale(9-11) have remained elusive(3,5,12). Here we analyse radar imagery from NASA's Cassini spacecraft and identify barchan, star and reoriented dunes in sediment-limited regions of Titan's equatorial dune fields that diverge by 23 degrees on average from the orientation of linear dunes. These morphologies imply shifts in wind direction and sediment availability. Using a numerical model, we estimate that the observed reorientation of dune crests to a change in wind direction would have taken around 3,000 Saturn years (1 Saturn year similar to 29.4 Earth years) or longer-a timescale that exceeds diurnal, seasonal or tidal cycles. We propose that shifts in winds and sediment availability are the product of long-term climate cycles associated with variations in Saturn's orbit. Orbitally controlled landscape evolution-also proposed to explain the distribution of Titan's polar lakes(13)-implies a dune-forming climate on equatorial Titan that is analogous to Earth.