>The problem with this is its never ending. A hundred years from now with colonies on Mars someone is going to say the same thing about that little niche over there on Mars that hasn't been explored to their satisfaction.

That's right. However, the more we explore a planet without uncovering any signs of life, the more confidently we can say there probably isn't life there. I'm personally not holding my breath for finding evidence of life on Mars or any other body in our solar system for that matter. There's no evidence yet that I find very convincing.

But it's a large leap from that to claiming with any certainty that there IS no life on any body in our solar system other than Earth. It may seem like we've explored our solar system quite a bit, but we really haven't - not in the detail that would be required to rule out the existence of microbes. To use a common analogy, that's like taking a bucket of water out of a large lake, looking at it with a magnifying glass, and claiming there's no fish living there.

>As much as we want it to be life isn't common.

I'd agree that life as we know it is definitely not common, in the sense that it probably occurs on a small minority of all rocky bodies (most of which don't have any water or atmosphere, so it's hard to imagine any life arising or existing for any geological length of time there). We don't yet know exactly how uncommon it is. Does it arise on 25% of terrestrial planets or moons with liquid water in the habitable zone? 1%? 0.00001% Does it arise in 25% of star systems with terrestrial planets in the habitable zone? 1%? 0.00001%? We don't have sufficient information to rule out any of these possibilities, as far as I know. We can't even analyze the atmospheres, much less the surfaces and soil chemistries, of any extrasolar planets in much detail (or at all, in the case of soil chemistries).

I understand the motivation to tentatively conclude in the negative, so as not to be disappointed. I have a Ph.D. in chemistry and it's still difficult for me to imagine a probable series of steps to life on Earth, so it wouldn't surprise me if less than 1 in 1000 star systems had life. But it's ok (and indeed more accurate) to say that we just don't know how common life is yet.

I don’t think we can say yet whether it’s probable that any life is out there, or even close by. There could be microbes or even multicellular organisms on Titan and we wouldn’t necessarily know it yet. There could be intelligent non-technological organisms in 25% of the systems within 50 light years for all we know.

However, I agree with your assessment that sending probes returning evidence of life is likely going to take a LONG time, especially if searches in our own solar system come up dry. I’m hopeful that the Starshot or similar ventures will reach the nearest stars sometime in the next few centuries (MAYBE decades), but it could well be millennia before any definitive evidence of life on extrasolar planets is communicated back to us, if ever.

Smells are typically transmitted by vapors (gases), which lack the intermolecular forces that are responsible for cohesion, and cause water and other similar liquids to gather into droplets in zero-g environments. What would happen is that the gas responsible for the smell would diffuse away from the object equally in all directions.

I'm actually not sure how true it is that smells are stronger above an object than below it on Earth. It's hard to smell a mug of coffee from below because it isn't open at the bottom, for obvious reasons. And our nostrils are on the underside of our nose, so it might be a bit harder to smell scents coming from above our nose. However, if I held a piece of stinky cheese above my head and turned my nose up to sniff it, I'm pretty sure I could.

Edit: on large scales, gases do indeed gather into nebulae and sometimes even gas giants or stars (or solar systems or galaxies), but this is due to gravity rather than cohesion, and to be a significant force this requires much much more mass than the tiny amount of scent molecules that one would find radiating from an object that would fit in the ISS.

Well… sort of. While encoding proteins is arguably the most important and certainly the most visible function of the genome, there are parts that code for RNA that does not get translated into protein. These and other non-coding segments actually make up the majority of the human genome, and many of them play important roles. Though it is true that almost all those roles support the expression or regulation of proteins in some indirect way.

Also, a gene moving to a different locus can actually make a big difference, because the way it is expressed and regulated can change, even if it codes for the same protein.