The sight of windswept desert dunes is evidently evocative, given that it has served as inspiration for everything from Frank Herbert's Dune franchise to indie games like Journey to the barren landscape of Tatooine. But aside from being great fodder for fictional adventure, these geological features are also fascinating windows into the dynamics and history of their host planets.
Take the tantalizing discovery of a new type of dune-like sand ripple on Mars, reported this Thursday in the journal Science. Called "wind-drag ripples," these novel formations are an intermediary between two rippled structures found on our own planet—large desert dunes and tiny, decimeter-scale sand waves. With crest-to-crest wavelengths measuring in the meters, wind-drag ripples are unlike anything observed on Earth, or elsewhere.
Led by Caltech planetary geologist Mathieu Lapôtre, the new research confirmed the existence of this third type of "bedform," the term for sedimentary structures shaped by air or water flow, with close-up ground observations of the Bagnold Dune Field provided by the Curiosity rover. Satellite imagery from the Mars Reconnaissance Orbiter was also used to study the ripples from an orbital vantage point.
"The meter-scale ripples are observed from orbit all over the planet, across a wide range of latitudes and elevations," Lapôtre told me via email. "However at the Bagnold Dunes, we were able to observe, for the first time, that two scales of active ripples (decimeter and meter-scale) were superimposed on top of dunes."
"It is the coexistence of these three scales of active bedforms that defied previous understanding of wind-formed ripples."
The main driver behind the new form is the low density Martian atmosphere, which allows the wavy patterns to develop in a similar manner to the water-worked ripples of Earth's riverbeds. Not only do they indicate that bedforms on Mars are more complex than previously thought, they can also be mined for clues about the planet's evolutionary history. Dunes and ripples are formed by sedimentary processes, which means ancient signatures of them can be preserved in Martian rock.
"Because the size of wind-drag ripples varies with the density of the atmosphere under which they formed, 'fossil' wind-drag ripples found in Martian sandstones can tell us about the ancient Martian environment," Lapôtre explained. "Thus, to document the early evolution of the Martian atmosphere, we need more rover observations of eolian sandstones that formed during, and thus recorded, climatic transitions."
And why limit ourselves to Mars missions? While the paper marks the first time wind-drag ripples have been identified, Lapôtre does not expect them to be unique to the Red Planet.
Indeed, they may be particularly common on other worlds with low atmospheric densities, such as Pluto or Comet 67P/Churyumov-Gerasimenko, which hosts the Rosetta orbiter.
"While ripple-looking features have been observed on the comet by Rosetta, the ground-resolution of the beautiful images provided by New Horizons are likely too coarse to resolve possible wind-drag ripples on Pluto," Lapôtre noted.
In other words, let's rush order the next generation of planetary rovers and landers so we can check out the prevalence and variety of alien dunes and ripples. It's what Frank Herbert would have wanted.