Implications of dune pattern analysis for Titan’s surface history

Abstract

Analyzing dune parameters such as dune width and spacing can be useful in determining the reaction of dunes to changes in atmospheric and sedimentary conditions currently and in the recent geologic past. Dune parameters, dune width and spacing, were measured for linear dunes in regions across Saturn's moon Titan from images T21, 23, 28, 44 and 48 collected by Synthetic Aperture RADAR aboard the Cassini spacecraft in order to reconstruct the surface history of Titan. Dunes in the five study swaths are all linear in form, but lack superimposed or flanking dunes. They have a mean width of 1.3 km and mean crest spacing of 2.7 km, wider and farther apart on average than similar terrestrial dunes in the Namib and Agneitir Sand Seas though larger linear dunes exist on Earth. Because of the lack of superimposed and flanking dunes and their size, Titan's dunes are classified as very large simple linear dunes. The large size, spacing and uniform morphology are all indicators that Titan's dunes are very mature and long-lived features. The ratio of dune width to spacing for Titan's dunes is similar to that found in terrestrial dunes in that dune spacing tends to be twice dune width. In addition to being similar in size, this is further evidence that terrestrial dunes can be used as analogues for Titan's dunes and vice versa and that the essential dune-forming processes are the same on both bodies. Dune width and spacing decrease northward, which is attributed to, but not limited to, increased maturity of dune fields to the south or increased sediment stabilization to the north. Sediment stabilization may be caused by Titan's asymmetric seasons and a net transport of moisture from south to north. The majority of dunes have spacings consistent with an upper limit of 2 to 4 km established by the atmospheric boundary layer, further evidence they are mature. Dunes are more widely spaced in the south are evidence they have been growing toward a steady state for a longer period of time than those in the north. Titan's large linear dunes have long reconstitution times. This is in part due to the fact that winds sufficient for saltation are reached only near the Titan equinox every 14 Earth years. Based on rates for similar terrestrial dunes the reconstitution time for Titan's dune is 600,000 Earth years or more, and therefore substantial changes in dune form should not be observable over Cassini's lifetime. Cumulative probability plots of dune parameters measured at different locations across Titan indicate there is a single population of dunes on Titan. This suggests that, unlike analogous dunes in the Namib and Agneitir Sand Seas, dune-forming conditions that currently exist on Titan are either the only dune-friendly conditions in the moon's history, or the current conditions have been stable and active long enough to erase any evidence of past conditions.

Long‐Wavelength Sinuosity of Linear Dunes on Earth and Titan and the Effect of Underlying Topography

“…Unlike in the Kalahari, variation in dune orientation occurs at a relatively local scale. In some instances (Figure b), such as in the far northwest of the studied area, these deviations are clearly adjacent to radar‐bright gaps in the dune sands, and this effect is both well‐reported (Ewing et al, ; Lorenz et al, ; Radebaugh et al, ; Radebaugh et al, ; Savage et al, ), and likely to be due to deflection of dunes around obstacles (i.e., mountains or hills) standing proud of the dunefield. But in other areas, localized variance in dune trendline is not obviously associated with obstacles, and thus, we propose that this is likely the result of underlying topography at a scale not observed by Cassini SAR and radar altimetry data.…”

Titan's atmosphere and climate

“…The observed 1-3 km spacing [Lorenz et al, 2006b;Radebaugh et al, 2008;Savage et al, 2014] is approximately the same height as the boundary layer indicating that the dunes are mature and have therefore stopped growing [Griffith et al, 2008;Lorenz et al, 2010;. Dune orientation and morphology are determined by wind patterns; given the difficulties in measuring Titan's winds remotely, the dunes provide some of our only constraints on the wind speed and direction at the surface.…”

“…Titan's dunes are eastward propagating longitudinal dunes ∼100 m tall, 1 km wide, and on average 30–50 km long [ Lorenz et al , ; Radebaugh et al , ; Barnes et al , ; Neish et al , ; Le Gall et al , ]. The observed 1–3 km spacing [ Lorenz et al , ; Radebaugh et al , ; Savage et al , ] is approximately the same height as the boundary layer indicating that the dunes are mature and have therefore stopped growing [ Griffith et al , ; Lorenz et al , ; Lorenz , ]. Dune orientation and morphology are determined by wind patterns; given the difficulties in measuring Titan's winds remotely, the dunes provide some of our only constraints on the wind speed and direction at the surface.…”

“…As with the channels observed at the Huygens landing site, it seems that many of the observed surface features may form during brief, intense periods rather than under the prevailing conditions. There is evidence that the dunes are active and therefore record the relatively recent climate conditions [ Barnes et al , ; Neish et al , ; Lorenz , ; Savage et al , ]. Lorenz [] estimated a reorientation timescale of ∼50,000 years, which is a similar timescale to climate forcing so the dunes could potentially be out of equilibrium with the present winds [ Lorenz , ; Ewing et al , ].…”

Introduction to the Planetary Dunes special issue, and the aeolian career of Ronald Greeley

“…c o m / l o c a t e / i c a r u s represent the largest visible surface reservoir of hydrocarbons on Titan. Savage et al (2014) report on analyzing 7000 measurements of the width and spacing of dunes obtained from five study sites on Titan; the average width is 1.3 km and the average crest spacing is 2.7 km, comparable to large dunes on Earth, and cumulative probability plots of the measurements indicate that the dunes on Titan are of a single population.…”