Mars deceives. It mimics. It often taunts. And it frustrates.

Some sort of fluid -- acid brine? -- has catastrophically etched the surface over much of the planet, and for an apparently extended time. Some sort of fluid -- possibly more or less pure water -- has been carving gullies and channels (perhaps under a blanket of dust covered snow) on the slopes of craters as recently as yesterday. Or at least, that's what it looks like.

Yet Mars is very cold by our standards and we see extensive modification of the surface due to ices, heaving permafrosts and so forth -- some of this modification going on right now -- and the case for the rarity of liquids at or near the surface is a good one.

The problem is that there is much evidence suggesting that liquids are not as rare on Mars as may seem to be the case, likely weren't as rare in the past as they are now.

As for glacially slow erosion on Mars... in some areas, it is very slow. In others it is seemingly much faster. Accounting for this differential erosion is still one of the difficulties of understanding the Martian surface. Objectively, some parts of the surface are much fresher than others, but we don't have any reliable way, yet, to determine just how fresh those fresh areas are.

If water or an aqueous solution acts on Mars somewhat like lava and magma do on the Earth, extraordinary circumstances (such as a native "aquarium") could well be occurring right now in the Martian interior, but I'll make no claim as to how far beneath the surface such things may be.

If there is no sign anywhere of near-surface fluid, like there is apparently no sign anywhere of interior heat release at the surface, then it becomes harder and harder to sustain the notion of a biologically active Mars.