Dune-like structures appear in the depths of Earth’s oceans, across its landscapes, and in the extremities of the solar system beyond. Dunes rise up under the thick dense atmosphere of Venus, and they have been found under the almost unimaginably ephemeral atmosphere of a comet.
Understanding how such similar bedforms (structures at the interface of a fluid and a movable solid) originate under such a wide range of environmental conditions is key to our comprehension of surface dynamics throughout the solar system. Because of the difficulty of observing the surfaces of other planets, researchers often study places on Earth where similar geologic processes occur. These places are called planetary analogues or analogue sites.
The fifth in a series of workshops focusing on planetary dunes brought together 65 terrestrial, marine, and planetary researchers, including students, from diverse backgrounds. Workshop participants agreed that inclusion of the terrestrial subaqueous research community, which studies underwater bedforms on Earth, in discussions about analogue sites was key to the workshop’s success.
Although the basic physics remains the same, fundamental environmental parameters may vary substantially between terrestrial dryland and subaqueous dune fields. Discussions included how the formation of the bedforms observed on comets may resemble Earth’s subaqueous processes more than dryland aeolian dynamics. Further discussions focused on how current in situ exploration of dune fields on the surface of Mars benefits from such terrestrial analogue developments and how these Mars exploration sites may provide a whole new set of geomorphic analogues for the rest of the solar system.
Along with modern analogue sites, the workshop also highlighted “fossilized dune fields”—sandstones that record the accumulation dynamics of ancient dune fields. Workshop participants discussed how ongoing exploration of these sites on Mars and Earth is providing new information about exotic environmental dynamics that have occurred in the past.
Participants agreed that aeolian processes seldom operate in isolation and cannot be fully understood outside the larger context of sedimentary systems. Discussion topics included sediment sources, transport, alteration, stabilization, and sinks, which occur through a long suite of diverse processes over the geologic and climatic history of the planetary body. Although each planetary system forms similar products, each operates within the confines of its own unique constraints and environment.
Attendees discussed the effect of prevailing wind directions and strengths in determining dune morphology. In general, winds that transport sediments in only one direction form barchans, or crescent-shaped dunes, whereas seasonally reversing transport winds form large, elongated longitudinal dunes. Other combinations of multidirectional winds form variations of these two extremes.
However, research presented at this workshop suggests that not only is the number of transport wind directions important, but so is the amount of variation that occurs within each prevailing wind direction. Participants agreed that a more complete understanding of these dynamics would enable us to interpret wind directions and dispersions from remote sensing images of planetary dune morphologies.
—Timothy Titus (email: [email protected]; @USGSAZ), Astrogeology Science Center, U.S. Geological Survey, Flagstaff, Ariz.; David M. Rubin, University of California, Santa Cruz; and Gerald Bryant, Physical Science Department, Dixie State University, St. George, Utah