If a rock fell on Mars, would you hear it?

Very early one morning, while camping in a river gorge in central Australia, I woke to the sound of a boulder crashing down into the gorge. I remember a cracking sound, followed by a booming long echo as sound waves rolled their way past me along the steep, meandering bedrock walls.

Excitement doesn’t begin to describe my reaction! I had to locate this geomorphic event that happened within my earshot. Although I drove along the sandy river bed for several kilometers,  I never did find it. Apparently, sound travels a long way along canyons on Earth.

The movement of boulders is an active geomorphic process that very little is known about. Movement may be triggered by vibrations from fresh meteor impacts nearby, or they may be started by impact from crater ejecta.

Recently, there have been some spectacular images returned from the Moon and Mars that show various styles of rolling, bouncing, hopping boulders, which plough through the ubiquitous dust-rich regolith. They got me wondering. What would a crashing, smashing rolling boulder on Mars sound like?

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Left: The Apollo 17 astronauts used this strategy as an easy way to sample high outcrops without having to climb.
Right: Closeup of LROC image showing boulders and tracks (arrow) that have rolled down the slope of Tsiolkovskiy Crater. Credit: NASA/GSFC/Arizona State University

The science of sound in our solar system.

Sound is perceived when our ears detect the vibrations of molecules in the atmosphere. Research has shown that sounds on Mars would be very different to those on Earth. While the majority of Earth’s atmosphere is composed of nitrogen and oxygen, Mars’ atmosphere is mostly made up of carbon dioxide and it is very thin.

Sound does not propagate well in a thin atmosphere. In addition, the molecular structure of carbon dioxide makes it a great absorber of sound. To hear noise clearly on Mars you may need to be <10 m from its source. In other words, if you were close enough to hear a megaclast crashing down a slope on Mars; it may be the very last sound you would ever hear!

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Authored by: Dr. Mary Bourke, Assistant Professor, TCD Geography

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