 Casis is a high-resolution color stereo camera in Mars orbit, and is making remarkable images of the red planet. So, what mysteries has it revealed today? Recently, Casis observes some peculiar streaks that, after almost 20 years, have changed color. Casis is a fairly recent experiment, but Mars has always attracted our attention, and there have been a lot of previous missions. The color stereo camera is a Swiss-led experiment and was built at the University of Bern in Switzerland. It was selected to be part of the Trace Gas Orbiter mission. The instrument was launched on the 14th of March 2016 from Baikonur towards the red planet. After a seven-month journey through interplanetary space, 1.5 years was still needed to reach the final orbit. And on the 21st of April 2018, Casis started its prime mission. Since that time, Casis has collected and sent to Earth more than 25,000 remarkable images of the Martian landscape. So let's try to figure out what these chameleon-like structures are. 20 years ago, the Mars orbiter camera imaged the streaks, and they were dark. Now, at the time of Casis observations, they're bright. Searching the planet in detail, we can find more chameleon-like structures. But here, there's something different. These lines are also bright. But here, they lie on a smooth surface, not a steep slope. The structures look like someone has swept the surface with a broom. This could easily be the wind. But can wind really move material? If we look for more evidence for the action of wind, we should definitely look at dune structures. Like this. Here, there are long, diffuse tails coming out of these dunes. And in this example, we get an impression that dunes are actually climbing upslope. To find similar features on Earth, let's visit a dry and sandy desert. Dunes are produced by wind. In poorly inhabited areas with little vegetation, the dunes can become beautifully smooth. The winds in desert areas can be so fierce that you need to shelter from the moving sand. With a very strong wind, sand can even move upslope. That's exactly as we found in cassis images on Mars. So it seems that on Mars, despite the rarefied atmosphere, the wind is more than strong enough to displace material. The basaltic origin of dunes makes them appear dark blue in cassis images. But what are these bright tails behind them? If you put an obstacle into the wind, there's a calm zone created behind that obstacle. It seems that smaller, brighter dust settles down and deposits in that zone behind the dunes. Neither these stagnant areas, nor the wind streaks, are as long as our chameleon-like streaks. So it seems that winds are not, obviously, linked to these slope streaks. The processes forming them seem to be different. Slope streaks are connected to dust. And in certain regions of Mars, well, they're everywhere. The map shows that they're predominantly found in the equatorial regions. And wind and dust brings us back to dust devils. Off-piste skiers will recognize a relationship here to the production of avalanches. Even the weight of one skier can be sufficient to trigger an avalanche. One only needs to change the structure slightly and the snow load on a slope becomes bigger than its strength. The snow layer starts to slide down the slope, initiating the avalanche. A big avalanche takes a lot of material down the slope, creating a giant cloud and leaving a deep track behind it. But how do we search for similar effects on Mars? Hey Nick, why not look at some really high-resolution images from our high-rise camera? Hey Alfred, that's a great idea! High-rise images objects down to one meter in size from an altitude 300 kilometers above the surface of Mars. This dark line looks like the edge of a deep slope streak. And in these high-rise images, we can see a track that seems to have started a chain reaction by initiating the production of slope streaks. However, the influence of the dust devils on the production of slope streaks is more an exception than a rule. If slope streaks really are dust avalanches going down the dusty slopes and exposing some dark, underlying material, then any triggering event, such as a small meteorite impact or even an extra speck of dust, is enough to provoke an avalanche. But why do these dark streaks change color with age? To try to understand this, let's experiment. We need to prepare analogs of the Martian surface with different grain sizes and different material compaction. And then, let's look at the reflection properties and how they vary. Surprisingly, a compressed and fine-grained surface looks brighter than a surface made of large grains. It's possible that the harsh Martian environment acts to split newly exposed particles, producing fine-grained surfaces. Alternatively, fine-grained material might sediment out of the atmosphere to brighten the surface. Actually, we don't quite understand it yet. Although Mars is definitely a different world, sometimes Cassis can show us things that are very familiar to us on Earth. But before we go, while I was looking for evidence of slope streaks on Mars, I could also see striking seasonal changes. It looks like Cassis captured the same dunes in summer and in winter. And that looks like ice.