Martian Gullies Not Water-Based

It's pretty well known that Mars once had plenty of liquid water, and still does, though it doesn't sit around on the surface. But science is strange sometimes. Sometimes, the obvious answer isn't correct. Take the numerous Martian gullies. They look like dry creekbeds, and a lot of scientists sort of assumed that hey, these features that look remarkably like a feature we see on Earth probably formed the same way. They were taken as proof that liquid water existed on the surface of Mars in recent history, and that seemed like the end of the story.

But this is science, and in science, the story never ends. Someone dug deeper, did more research, and concluded that the gullies formed through the constant freezing and thawing of carbon dioxide frost, not liquid water. Using spectroscopy from the HiRISE space probe, scientists discovered no evidence that the gullies had deposited any sort of clay or other mineral associated with running water, and that any clay associated with the gullies were ancient and had been exposed during the formation of the gullies. I take this as a reminder that science is never easy, and that even if the answer seems obvious, you should always check to make sure.