Going With the Grains

By John Marshall, Senior Research Scientist

My research focuses on the basic physics of granular materials. These materials make up the vast dune fields of Earth, Mars, and Titan; they are an important component of regoliths and soils on many asteroids, moons, and rocky planets, as well as the debris rings of the giant gas planets. They are also the mediator of global cooling from impact and volcanic dust plumes. And of course, they are the basic pieces of material that ultimately form all rocky bodies born out of the primordial nebula.

How are these materials moved around? How do they aggregate or break apart? What do their shapes and surface textures tell us about the history of planetary surfaces? What role do these materials play in the evolution of planetary surfaces? These are some of the fundamental questions for planetology and astrobiology.

To answer these questions, I’ve been conducting a long-term research program that combines theoretical modeling, field observations, laboratory experiments (with benchtop apparatus and full-scale Mars, Titan, and Venus wind tunnels), Venus simulation chambers, zero gravity experiments, and planetary missions.

Mars has been a special focus. There are two mysteries: Why are there dunes on Mars? They have to be formed of sand, yet the sand should have long ago destroyed itself by abrasion given the speeds at which it must travel on Mars (this is the Sagan “kamikaze” paradox). And how do the global martian dust storms occur? Wind tunnel experiments suggest that impossibly high wind speeds are needed! To get to grips with these mysteries, I’ve conducted Mars Wind Tunnel experiments at NASA Ames, dust devil probing in the Arizona desert, and recently was part of the microscope imaging team for the Mars Phoenix lander.  Currently I’m conducting experiments in the lab to determine the survivability of sand in a martian dune.