Ripples in the sand are a beauty to behold. Sometimes their sinuous curves can be spied beneath a tinkling stream, and other times you feel them under your feet as the currents in the sea create ridges and hollows in the sand.
But what controls the size and shape of a ripple? Is it the size of the grains, the depth of the water, or perhaps the strength of the flow? For geologists the question matters because they use fossilised ripples to try to better understand past environments or interpret conditions on other planets.
Using a large database of ripple data scientists have now pinned down exactly which factors determine how ripples form, and devised an equation that can describe current ripples on any planet. Their results, published in Geology, show that the gloopier the fluid making the ripples, the larger they will be; dinky 15cm underwater ripples on Earth would become mammoth 2.3m wavelength ripples if produced in a thick brine on Mars, or 70cm waves under a methane flow on Titan for example.
Gravity also matters: freshwater ripples on Mars can be expected to be 14% larger than those on Earth, due to the smaller gravitational force. And grain size also plays a role, with larger grains tending to produce larger ripples.
Importantly, the scientists also show that sand dunes don’t fit the same equation, suggesting that different physical processes are at play in the formation of dunes. But armed with the new information, planetary geologists will now be able to better interpret environments on other planets and perhaps work out why water dried up on Mars.
