The moon has an atmosphere, but it's negligible. There's no indication that it ever had any sort of serious atmosphere for any serious length of time – solar radiation would have stripped away anything that didn't just freeze out onto the surface. The gravity isn't strong enough to hold an atmosphere thick enough to allow e.g. liquid water on the surface – certainly not for the length of time required to allow life to evolve, become multicellular, develop intelligence, tool use and technology, and start building structures. Granted, that's using our one and only example – Earth – for the speed of evolution, but there is no reason to believe that the moon ever evolved any form of life capable of building structures – and the structures we can see are not so wildly unusual as to lack any natural explanation.Greyth wrote:Indeed, lunar regolith itself is an effective radiation shield at a depth of a few feet. As for lunar mounds and rills being caused by lava... possibly... as for scale they come in all sizes. I would have thought it worth mounting a lunar expedition just to find out if our best guess concerning their origin is correct It has been postulated that Luna once had an atmosphere and if that is true then she may have supported life, so it is not totally inconceivable that they are artefacts/structures.
It's not beyond the bounds of imagination to create stories where visiting aliens build structures on the moon – it's the central plot device at the start of 2001 – but equally we can imagine aliens building all sorts of stuff anywhere we want, from the heart of the sun to underneath the Pentagon. Crucially though there's absolutely no reason to believe they ever have. The surface of the moon and Mars and all the planets and other bodies in the solar system – Earth included – contain mysteries, puzzles and surprises, but nothing that screams, or even hints, at any non-natural processes (at least, not ones that don't involve humans operating broadly within currently understood historical and archaeological parameters).
Because something is currently inexplicable, or even just currently subject to scientific debate, doesn't mean that we should postulate outré explanations and fill the gaps with huge conceptual leaps. Occam's razor is just a rule of thumb but it's a bloody good one. All sorts of things now comfortably within our understanding – things which we routinely exploit to carry out everyday wonders – used to be regarded as "forever beyond our mortal ken". All sorts of other "amazing" things – the "face(s) on Mars", for example, or the whole "moon landings were faked!" fallacy – dissolve when examined more closely. "Aliens", like "gods" or "magic", can be pulled out to explain anything at all – except they don't actually explain it: all they are is a big box marked "Don't ask – it's forever beyond you", into which people try to dump things they don't want examined or which they'd prefer to be left "wonderful", i.e. unexplained. Science constantly drags stuff out of that box and nails it firmly down in the real world (which is why some people tend to get hostile towards scientists).