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

Journal name:
Nature Geoscience
Volume:
8,
Pages:
15–19
Year published:
DOI:
doi:10.1038/ngeo2323
Received
Accepted
Published online

Linear sand dunes cover the equatorial latitudes of Saturn’s moon Titan and are shaped by global wind patterns1, 2, 3. These dunes are thought to reflect present-day diurnal, tidal and seasonal winds1, 3, 4, 5, 6, but climate models have failed to reproduce observed dune morphologies with these wind patterns4, 6. Dunes diagnostic of a specific wind7, 8 or formative timescale9, 10, 11 have remained elusive3, 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° 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 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 lakes13—implies a dune-forming climate on equatorial Titan that is analogous to Earth.

At a glance

Figures

  1. Comparison of sand dune patterns in the solar system.
    Figure 1: Comparison of sand dune patterns in the solar system.

    ad, Well-organized dune patterns on Earth (a), Mars (b), Titan (c) and Venus (d). e, Comparison between defect density ρ (given by ρ = N/L, where L is the total crest line length and N is the number of defect pairs26) and spacing shows well-organized patterns plot along a trend indicating patterns in equilibrium with formative conditions30. Degraded patterns fall off the trend with shorter crest lengths for measured spacing30 (Supplementary Section 1 and Supplementary Fig. 2). Error bars are less than the symbol size. Supplementary Table 2 contains all image locations and north is up for all images.

  2. Despeckled Cassini radar images of barchan, star and reoriented dunes.
    Figure 2: Despeckled Cassini radar images of barchan, star and reoriented dunes.

    White arrows highlight type dune examples in each panel. a, Star dunes isolated and along linear crests in Belet Dune Field. b,e, Barchan dunes with elongate southern horns and reoriented crest lines indicating the influence of a second wind in Shangri-La Dune Field (b) and Senkyo Dune Field (e). c, Star dunes forming along linear dunes and as isolated dunes in Fensal Dune Field. d, Barchanoid and reoriented crest lines in Belet Dune Field. Supplementary Figs 17 and 18 show a comparison between despeckled radar and radar.

  3. Modelled dune reorientation timescales of Titan/'s dunes.
    Figure 3: Modelled dune reorientation timescales of Titan’s dunes.

    a, Timescales required to reorient crest lines from 23° off steady state. Modelled solutions shown for the final 5° of reorientation. Reorientation in this asymptotic model is considered to be within 5° of steady state, which represents the error in determining the orientation for the smallest crest lines mapped in this study. b, Rose diagram showing 23° offset between linear dunes ( , n = 229) and reoriented crest lines ( , n = 319) mapped in c,d. Colours correspond to digitized crest lines in d and distinguish among the linear dunes (green) and reoriented dunes (yellow). c, Despeckled Cassini SAR image showing reoriented crest lines and linear dunes in Shangri-La Dune Field. d, Digitized crest lines of reoriented and linear dunes shown in c.

References

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Affiliations

  1. Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843, USA

    • Ryan C. Ewing
  2. Department of Astronomy, Cornell University, Ithaca, New York 14853, USA

    • Alex G. Hayes
  3. Laboratoire AIM, Université Paris-Diderot CEA-SACLAY, 91191 Gif sur Yvette, France

    • Antoine Lucas

Contributions

R.C.E. and A.G.H. contributed to the design, research, analysis and writing of the study. A.L. contributed the de-noised Titan data and manuscript editing.

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The authors declare no competing financial interests.

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