The Curiosity rover has sent back data on these multi-colored mineral veins it found on the surface of Mars — but the veins themselves are not the most interesting thing about the discovery.

What’s really interesting is what they tell us about ancient Mars, and the water situation there. The two-toned mineral veins (which Curiosity scientist Linda Kah compared to “ice-cream sandwiches: dark on both edges and white in the middle”) are pretty good evidence that liquid once flowed along the mountain base the vein was found at. While the white tone is made up of calcium sulfate that’s previously been analyzed nearby, NASA notes that the darker mineral stacked around it looks like what would be left behind if some kind of fluid had passed over that calcium sulfate long ago, just like we see on Earth.

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Of course, some kind of liquid flowing on Mars doesn’t necessarily mean the existence of anything like the water that we have here; in fact, a lot of the evidence points to the idea that flowing water on Mars would be more comparable to some sort of antifreeze brine than anything else. Still, it’s another clue to what ancient Mars was like.

Image: NASA/JPL-Caltech/MSSS.