Shifting Sands of Far-Off Lands

Martian dunes, captured by NASA's Mars Reconnaissance OrbiterWhat started out to be a workaday chore—replacing a broken motor in an exhibit—panned out to be a voyage of discovery to the shifting sands of another world. This is an occupational hazard when working at a place like Chabot Space & Science Center….

The motor in question powers a fan in an exhibit built to demonstrate the physical processes of duning—the fluid transport and deposition of solid particulates into collections and patterns. The fan blows up a constant micro-gale within the exhibit enclosure, and visitors get to play Mother Nature by turning a handle and redirecting the wind. Meanwhile, a mass of tiny white glass beads is constantly whipped up into a fair recreation of a sand storm on planet Arakis….

After the chore of installing the new motor, I rewarded myself by enjoying the exhibit a bit. I piled up all of the sand on one side of the tank to see how the fan would redistribute it; I sent the wind from different directions, watching how the freshly blown grains were scattered across the pristine black undersurface; I placed all of the pyrite rocks, which serve as wind obstacles, in one pile. It was a lot of fun.

One thing I noticed that I hadn't paid much attention to in the past was how the dune actually moved, or migrated. Maybe I hadn't watched long enough before, or maybe it was easier to witness because I had stacked the deck by mounding the sand all in one corner, but it was fascinating to see the process.

On the windward side of the giant dune, the scouring wind picked up the sand and carried it racing to the top—slowly peeling away the front face of the dune. As soon as the sand-laden wind reached the crest and took a sudden turn downward, it was slowed a bit, becoming less able to support the sand grains, which then fell out onto the leeward side of the dune in a sandy-wind version of precipitation. The buildup of sand on the lee side eventually formed small avalanches that slid down the face in little dry floods.

In this fashion, the dune moved along, slowly being erased on its windward side and formed on the lee.

Almost coincidentally, a few days later I read a report from NASA about sand dunes on Mars. In some areas, dunes have been observed to migrate over time, while on others the patterns have remained stock-still—some of them for perhaps thousands of years, or longer.

So I had successfully created the right conditions for a migrating sand dune. What about static dunes? Well—I had noticed already that some of the pyrite rock obstacles that I placed in the sand stream formed small dunes in the wind-shadows of their leeward sides. The rocks weren't moving, and so the dunes they were nurturing and protecting remained in place.

Some of the static dune ripples observed in Meridiani Planum—where the rover Opportunity is exploring—have been explained as possibly being protected by the presence of "blueberries": tiny nodules of gray hematite that have eroded out of Martian rocks, but which themselves are erosion-resistant, and too large (1-3 millimeters) to be carried by the wind. The blueberries, as the explanation goes, embed in the sand and form a protective "armor" layer for the dune ripples, which remain safe and still in their lee.

Where else do we find dunes, other than Earth? Well, you need wind of sufficient strength and sand of sufficiently small size, for starters. We don't know about dunes on Venus; Venus has a thick enough atmosphere, but the winds may be too sluggish to whip up much of a sand storm. The only other object with a thick enough atmosphere and a solid surface is Saturn's moon Titan—and in fact we have pictures of Titanian dunes taken by Cassini.

Now I'm feeling that old itch to make another trip to my favorite place in the Solar System, Death Valley, to explore the macroscopic dunes of Stovepipe Wells . I'll send a postcard….

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