Tying this all back to the CRISM spectral study I mentioned at the beginning of this post, Nuñez and his colleagues found that most of the gullies in their study were "spectrally indistinct" from their surroundings. On Mars, this is actually a common problem: It doesn't take much dust cover to make everything look spectrally like dust. In places where CRISM detected not-dusty minerals, they found that the spectral signature of the gullies matched the source material upslope. This would also be expected: Stuff that falls down a slope has to come from somewhere upslope. CRISM didn't detect anything like hydrated salts, silica, sulfates, or carbonates—minerals that form from the evaporation of water and/or the interaction of water with rock. If gullies formed from debris flows, as their morphology suggests, there wouldn't need to be much water involved in the process. On Earth, debris flows can consist of as little as ~15% water by volume, while the rest of the flow is comprised of sediment. In a hyperarid climate like that on Mars, a sudden influx of even a small amount of water could be enough to trigger debris flow activity. With such small amounts of water required for debris flows, we might not expect to see any abundant mineralogical evidence of water. The CRISM study results, as its authors point out in their paper, do not preclude gully formation by water. The results suggest there was not a lot of water involved, which is consistent with what we would expect from the morphology of the gullies.

The take-home point from all of this is that gullies on Mars are a complex system, likely evolving through multiple mechanisms over time. Not all gullies on the planet necessarily formed the same way—they don't on Earth—but certain observations like the latitudinal restriction and the orientation shift with latitude suggest some overarching control on how they formed.

It's important to note that this discussion only applies to alcove-channel-apron gullies—other mass movement processes observed on Mars, such as recurring slope lineae (RSL) or slope streaks, are morphologically distinct and thus potentially formed differently. NASA/JPL created this nifty chart to show how all the different types of mass movements observed on Mars thus far differ from one another: