Sometimes you have to stop what you’re doing and share a cool photo with your friends. Once a week I get an e-mail announcement with photo updates from the Mars Reconnaissance Orbiter (MRO), a NASA probe orbiting Mars since 2006. MRO’s main mission is to map the planet’s surface in detail with its high resolution camera; it’s also been studying Mars’ atmosphere, climate and geology.
The photo shows something I especially love about nature – how beautiful patterns evolve by natural happenstance. Here, boulders perched in an alcove within a crater wall tumbled down the slope until coming to rest near the crater’s floor. The long, dotted tracks help us picture each bounce as gravity did its magic and teased one boulder after another down the incline.
“Mass wasting” – the downhill movement of rocks and particles due to the force of gravity – is the technical term for what you’re seeing, and it’s as prevalent on Mars as on Earth. Landslides, avalanches and debris flows are familiar manifestations of mass wasting.
On Mars two of the most common forms of the process are rock landslides and dust and sand avalanches, also called “slope streaks”. Any numbers of things can trigger an avalanche. Erosion from Martian winds, subsurface water flows or vaporizing ice can steepen and widen a crater’s wall, undercutting the rim where the boulders have been in repose for millions of years. When the tipping point is reached, gravity’s tug can nudge them over the edge and down the slope.
Marsquakes and weakening of rocks from the day-night freeze-thaw cycle also play important roles in jimmying boulders loose and setting them in motion.
Last weekend I came across a delightful example of mass wasting along the north shore of Lake Superior. While crossing a pebble beach I was stopped in my tracks by a 4-foot-wide stream slicing across the strand as it hurried toward the lake. The current cut so quickly through the loose rock, pebbles along its banks tumbled continuously into the fast-moving water and were swept into the lake.