Saltation Sensor

On any planet with an atmosphere and granular material on the surface, wind-driven grain movement is a fundamental process of sediment transport and deposition, and landscape transformation. Wind moves the finest particles ("dust") in suspension, resulting in dust devils and dust storms observed on Earth and Mars. Somewhat coarser grains ("sand") cannot become suspended and instead are driven downwind in a series of hops across the surface called "saltation." Saltating sand accumulates into drifts, ripples, and the dunes seen on Earth, Mars and Titan. These saltating sand grains, because of their coarseness and the speed with which they are driven across the surface, have significant potential to abrade materials they impact against. This can have effects not only on geomorphology, but is also a serious engineering hazard for both manned and robotic planetary explorers. Saltation has proven to be a difficult process to study and understand, partly because of its dynamic character. The first fundamental question about saltation involves determining the minimum wind strength required to mobilize sand, under the various conditions on Earth, Mars or Titan. A second fundamental question involves the actual flux of saltating grains (their speed and number density), and how this depends on the grain and atmosphere characteristics, gravity, wind speed, and height above the ground. Most state-of-the-art sensors for measuring saltating grain movements are unsuitable for robotic missions, requiring complex servicing to extract measurements. The most viable candidate for remote application, the "Sensit" instrument (commonly used in terrestrial field studies), only yields the grain impact rate and an estimate of the typical energy impacting the sensor over an interval at its single sensing height (i.e., with no vertical resolution on the flux). Impacts from small, high-speed grains cannot be distinguished from lower-speed impacts of larger grains. We have developed a science traceability matrix establishing what is required to advance our general understanding of saltation in planetary environments. None of the existing instrumentation can fulfill these measurement requirements, especially considering the accommodation requirements for planetary instrumentation. We propose a technical approach to an instrument that would meet these measurement requirements and be well-suited to accommodation on a wide variety of planetary landers, rovers or aircraft. We propose to adapt capacitive ultrasonic transducers (as used in our Martian Sonic Anemometer) to instead passively detect grain impacts. We have completed a preliminary proof-of-concept showing that we can extract information separately about impact energy and impact momentum from the signal generated during each impact. We have also demonstrated a conceptual approach to add position sensing to such a grain impact transducer to yield also each impact location. We propose to mature these transducer designs to optimize their ability to determine separately impact energy, momentum and position and to ensure durability of the sensors in the erosive saltating sand flux. We also propose developing the signal processing instrument back-end to handle the data stream from an array of such impact sensing transducers, and digest it to efficient and economically-sized information in light of limited spacecraft downlink availability. We anticipate no planetary protection hurdles for this heat- and disinfectant-tolerant instrument. The saltation sensor is currently at TRL ~2, but we expect to mature it up to TRL ~5 by the end of this work, poising it well to be proposed to space flight opportunities.