At first glance, Jupiter’s moon seems totally unsuitable for dune formation, but dune-like features have been spotted on its surface. And researchers now think they know how they came about.

Here on earth, and certainly also in the Netherlands, it is a familiar sight: the dune landscape. Those dunes are formed by the wind; strong winds blow the sand into a heap. And in this way, sometimes meters high hills can be created. We now know that those dunes are not a typical terrestrial phenomenon; for example, Mars also has dunes. And years ago, when researchers looked at images captured by the spacecraft Galileo of the surface of Jupiter’s moon Io, they also saw tall and sprawling knolls suspiciously reminiscent of dunes. It caused some surprise, because it was actually unthinkable that dunes were formed on Io – which is also referred to as the ‘pizzamane’ because of the orange and red spots on the surface; the winds on the moon were much too weak for that.

Weak winds

“Although the winds on Io blow much faster than on Earth (at speeds of up to 1000 kilometers per hour), their strength is much more limited than on Earth,” said study researcher George McDonald. Scientias.nl† The winds on Io are even 100,000 times weaker than the winds on Earth. “That’s because Io’s atmosphere is very thin,” McDonald said. He explains by means of an analogy that such a thin atmosphere results in weak winds. “Imagine someone throwing a beach ball at you with tremendous speed. And then imagine someone throwing a rugby ball at you at a very slow speed. Although the beach ball moves much faster, the impact you receive from it is much more limited because its density is much smaller.” And so it is on Io: in the thin atmosphere winds can reach a high speed, but never exert much force on sand grains, for example.

Alternative way of training

And with that, dunes on Io seemed off the track. But in a new study, scientists have now found a way for dunes to form on the pizza moon, too. A key role is played by the volcanic activity on Io. Several studies have shown that volcanoes greatly affect the moon’s surface and have even reshaped it repeatedly, very quickly. The result is a surface consisting of solidified lava flows, sand, still liquid lava flows and ‘snow’ of sulfur dioxide. And that interesting mix, McDonald and colleagues now argue, could lead to a situation where the grains of sand are whipped up and swept into a heap.

How about that?

“On Earth, when lava moves through the snow, depending on the type of lava, it’s possible for it to move within the snowpack, rather than on top of it,” McDonald explains. “The same is probably happening on Io; lava flows move through the sulfur dioxide snow there.” The heat from the lava causes the underground snow to evaporate and the pressure under the surface rises due to the resulting vapors. When the vapors find their way out, they enter Io’s atmosphere at an extremely high speed, carrying and displacing the surface sand grains they encounter along the way. The fact that Io’s atmosphere is very thin helps. “Because Io’s atmosphere is so thin, the vapor can easily have a million times higher air pressure than the surrounding atmosphere. What happens can therefore be compared to a situation in which you open the doors of an airplane high in the air (where our atmosphere is much thinner). All that very dense vapor rushes very quickly into the surrounding atmosphere.” And so both the speed and density of the underground vapors that make their way into Io’s atmosphere are great enough to move sand grains on Io and allow the formation of decent dunes.

moving sand

And if you sent a probe to Io to capture high-resolution images of the surface, you could even witness the sand grains being displaced by the subterranean interaction between lava and snow. “We’ve obviously already observed Io with the Voyager and Galileo probes,” McDonald said. “But their image resolution was way too low.” In order to confirm that dunes can be created in the manner described by McDonald and colleagues, we actually have to go back to Io. “However, future probes would then have to orbit Jupiter and then repeatedly fly past Io,” McDonald said. “Because the radiation that probes orbiting Io are exposed to prevents them from circling Io for long.”

So while it wouldn’t be that easy to have a probe study the dunes on Io, it’s really the only way we can study the alleged dunes and their formation more closely at the moment. “This is the only way we can directly observe the movements of the sand or at least make very accurate maps of the surface, in order to get a better grip on the heights and slopes of these hills.”

Short term change

If future research shows that Io does indeed have dunes created by the underground interaction between lava and snow, it could change our view of the moon significantly. “The presence of dunes would suggest that Io’s surface is even more dynamic than most people think. We have known for some time that volcanic eruptions occur regularly and that the entire surface is reshaped about every million years. But dunes on Earth can change direction, grow or move over thousands of years. So long before the entire moon is resurfaced by new magma, complex changes can occur due to the movement of sand.”

In addition, the presence of dunes on Io would once again underscore that dunes are anything but terrestrial phenomena. “Dunes increasingly appear to be one of the most common phenomena on celestial bodies (planets and moons),” McDonald said. “That might have been an exaggeration as little as five years ago, but since then we’ve seen dune-like phenomena on comets (Comet 67P/Churyumov–Gerasimenko), possible evidence for dunes on Pluto and now Io. We always thought you needed a relatively thick atmosphere, like on Mars and Earth, to move sand and form dunes. But it increasingly seems that dunes can be formed even in thin atmospheres.”