And that makes categorizing the often eye-pleasing ripples across vast expanses of sand – both on Earth and on Mars – a lot easier.

You have probably walked on the beach or in a desert and marveled at the beautiful waves beneath your feet. These ‘sand waves’ are created when the wind whips sand grains into the air, which then settle in undulating patterns. However, researchers have now discovered that the way the grains are arranged obeys a “mathematical law.” And that may also help explain the origin of some recently discovered enigmatic alien sand structures.

sand waves

Eye-pleasing wrinkles can form on many sandy plains. But these are not all the same. They come in different shapes and sizes. “Mega ripples (megapples in English, ed.) you usually find them in sandy deserts, but also occasionally on beaches,” says researcher Klaus Kroy in conversation with Scientias.nl. “They stand out because they are relatively large (think about a few decimetres) and are somewhere between beach ripples (of just centimeters) and dunes (varying from ten to 100 meters). In addition, they mainly consist of coarse grains, alternating with a few fine grains. This is also the reason why we don’t see them in the middle of a dune area, where the sand is fairly uniform.”

Mega ripples in sandy deserts (bottom). Researchers study the composition of coarse and fine grains (top). Image: Hezi Yizhaq and Klaus Kroy

In the new study researchers decided to take a closer look at these mega ripples to reveal more about their precise composition. And that’s important. We do not only find such sand ripples on Earth, they have also been spotted on Mars. And by learning more about how sand waves form and form, researchers also hope to discover how these enigmatic alien sand structures came to be.

mathematical law

Researchers studied several mega ripples and compared the sizes of the sand grains. The researchers find out that the way in which the coarsest grains and the finer grains are arranged obeys a kind of mathematical law. If you divide the diameter of the coarsest grain by the diameter of the smallest grain, you get a surprisingly consistent number. In other words, the researchers discovered a ‘universal, characteristic grain size ratio’. This uniform feature had gone unnoticed until now. “This hidden order is due to the way the grains spin around,” Kroy explains. “As winds rage across the sand, the fine grains kick the coarser grains—which are too heavy to go with the wind—in small steps. This process creates the mega ripples, which sometimes resemble the humps of camels.”

Order

It is therefore quite surprising that there is a hitherto hidden mathematical law behind sand waves. “Especially when you consider the many different mega ripples we measured,” Kroy says. “They all look different. For example, they consist of different types of sand, often different materials and are formed by different winds.” Thanks to the discovery, researchers can now better determine to which category of newly discovered sand waves belong and by which specific transport process they were formed.

Mars

In addition, the discovery may also help better explain the formation of some mysterious sand waves recently discovered on Mars. “Mars has a much thinner atmosphere than Earth,” Kroy said. “As a result, everything moves less there. So while sands on Earth and Mars are nearly indistinguishable at first glance, experts note that Mars has other sand waves. These are similar, but not quite identical, to those on Earth. The question is therefore how they come about. Much is still up for debate. But with our ‘universal characteristic grain size ratio’ at hand, we can at least determine with certainty whether a sand wave is actually a mega ripple; if you find the predicted number it will be mega ripple and otherwise not. This can then help us better understand recent wind and climate conditions on Mars.”

Implications

The study has several implications, according to Kroy. “First of all, this is basic science,” he says. “We just want to understand how sand waves form on Earth and on other celestial bodies. If we then discover that they harbor certain specific sand waves, we may also be able to learn more about the prevailing atmospheric conditions. The same goes for old, or even fossilized sand deposits on Earth. If we are sure that they originated from ancient mega ripples, for example, we can also better understand the circumstances under which they originated.”

In short, the researchers hope that their discovery will help us learn more about past weather and climate conditions on Earth while at the same time expanding our knowledge of atmospheric conditions on Mars. “We want to better understand and classify sand waves,” Kroy says. “And hopefully then we can explain some of the more difficult – and confusing – examples.”