Beyond that, these alluvial fan rocks already seem to be lithifying into a concrete-like rock (conglomerate or breccia). This can survive essentially forever – tens of millions of years, at least – especially without active tectonics. It can be buried, yes, of course – like your gully example – but just as in Death Valley there really isn’t enough dust around to do that on Mars. And I suppose it’s possible that it goes through periods where it’s covered with silt or sand or whatever for a while, and then uncovered. The alluvial conglomerate is hard enough to not be eroded beyond recognition from basic wind-powered erosion (best exemplified by e.g. Arches National Park, which formed the way it did because it’s all relatively loose/weak sandstone).

Nobody in their right mind would do any sort of geology research, or just basic geologic mapping, without visiting the area of study in person to check things out – there are always surprises in the field and it’s the field observations that are the very core of geology.

This is basic science at, well, its most basic ;) It’s a hugely important finding, though – this is fundamental, groundbreaking stuff in the planetary geology field. This is like the age of enlightenment for planetary geology right now, basic low hanging fruit being discovered left and right.

That said, if I were not a geologist and thus not inclined to being so pedantic about geology, prior to this latest discovery one could have theoretically argued that the alluvial fans on mars are not alluvial but are wind-formed. Maggie covered it here, though – we now know that the pebbles forming the alluvial fans are too large to have been moved by wind.

For parts of the martian surface that haven’t been seen up close and personal by rovers, it hasn’t historically always been easy to tell what we’re looking at in terms of geomorphology or basic geology. You can compare aerial photos of mars to aerial photos of earth and assume things that look the same are the same, but you can’t say for sure without getting up close to see if the details are actually the same.

This discovery – just a handful of photographs – is huge for this reason. It means that we can say with much more certainty, now, that features we see on the martian surface are essentially the same as what we see on earth. Discoveries like this will be extrapolated to the rest of the surface and will result in reasonably accurate geologic maps, which up until this point were comprised of pretty much 100% speculation.

Water, methanol, ethanol, mercury- not many others. And of those, water should be by far the most common.

e.t.a. Lava wouldn’t produce the same effects.

Update: Apparently Curiosity does have a microscope on its robotic arm. I don’t know how powerful its magnification is though.

I guess that the cold temperatures imply that is was likely not liquid chocolate flowing there, but maybe there are other possibilities?