I suppose this thread is flickering out at the"first principle" level, but it sounds like there are still a few involved in discussing some of the details or scenarios derived thus far by scientific inquiry. And I'd like to reflect a little on one of those particulars. For a number of reasons I think it interesting to examine.
Back about 1980 I had opted out of a dissatisfactory job in industry and took some part time work at the nearby state university atmospheric science department. Auditing or taking the courses in the department was another lateral move in my graduate studies - and I was fortunate enough to join a class on planetary atmospheres, taught by a distinguished meteorologist who was studying Mars weather in real time in those days. We were not concerned directly with biology and life, but it was understood that life had an influence on the evolution of atmospheres: certainly here on Earth, based on sampling of sediments and ice cores, minerals and fossils. Life would also play a role theoretically, if we were to get a good observation of a planet revolving around another star where we could discern atmospheric properties. To make the explanation simple: 21% free oxygen (O2) is just not that natural an expectation when you throw rocks and ice together.
Somewhere in the midst of the course, a lecture was devoted to the famous Stanley Miller - Harold Urey experiment conducted at the University of Chicago in 1953. Miller, under his instructor's direction, the Nobel chemistry laureate Harold Urey, generated several amino acids in a heated flask with repeated electrical discharges. The idea was to simulate then understood conditions on the primitive earth, presuming an atmospheric mix of hydrogen, ammonia and methane....
It was a small lecture group of maybe a dozen and a half. So it was not too surprising that I got heard when I raised my hand.
Here we were getting data back from Mars and Venus which both had atmospheres 95% CO2 and ~5% N2. Titan, at 95 degrees Kelvin had some methane, but mostly nitrogen. CO2 would have all been dry ice... How come we were assuming that the Earth was different way back when? Wouldn't our atmosphere be largely CO2 judging from all we were learning in the last decade (1970s)?
I wish I could remember exactly what our instructor's answer was, but he certainly grasped the issue. I think he was willing to concede more CO2 atmosphere early on, but he had reservations. At the very least, white cliffs of Dover and other carbonate formations hadn't been there forever...The textbook, "Evolution of the Atmosphere", scanning now or reading ahead back then, seemed to have said very indirectly (e.g., volcanic emissions) that Earth without life would have an atmosphere much like Mars or Venus save that the atmospheric pressure would have likely been in between the extremes.
In effect, it looked like Miller and Urey had a way to explain glycine and other amino acids, some sugars - if there were big refineries back then similar to their lab experiments, with access to large quantities of methane, ammonia and hydrogen. But as far we knew, the atmosphere was mostly CO2 and N2. If there were ammonia and methane, it was produced in smaller quantities and broke down quickly over geologic time.
So was the experiment a failure?
Not exactly. Because it was an experiment that demonstrated a hypothesis: that building blocks of life could be derived from inorganic compounds mixed, heated or electrically charged (lightning) in a non biological environment. That part related to the so-called coming from nothing. And with many variations, including atmospheres and pressures more consistent with what we can discern from geology, the experiment has been repeated thousands of times over the decades. Some experiments were repeats to verify; others to change the environment. There have been experimenst with chemically riched early seas ( conducive), N2 and CO2 atmospheres (not very good incubators), environments to simulate undersea volcanic vents (good as well).
So the debate didn't end with people noticing the same discrepancy. There is a school of thought derived from Urey and Miller, but its critics are as likely to use similar methods to suggest different types of enclaves for early life: tidal pools, undersea volcanic vents, soil compounds...
But there were other problems that had to be addressed. What about exposure to UV radiation that would break down the compounds? What about all the processessing that has to go on to get to RNA and DNA? And then if you had another planet such as the ones which are being identified orbiting other stars (some young, some old), what are the odds?
Of course, this is a glass half empty or half full proposition. Going from the assumption that rodents spontaneous generated in trash piles to these type of deliberations is certainly headway, but not a demonstration of the solution.
Yet at the same time, looking at this problem from another angle, rather than atmospheric science, but something we might call the formation of planets, we can say that we actually can observe this part of the process: formation of planets.
That part of Genesis is on-going in the sky.
If you look in the right part of the sky such as the gaseous nebula close to Orion in the winter sky, one finds very young stars in which the formation of planets can be observed. Young in the sense that they might be only ten million years old, vs. the age of the sun, more like 4.5 billion years old. Other stars within or surrounding the galaxy might date all the way back to 13 billion years ago - but that is a discussion certainly off topic. Many of these very young stars still have the rings of gas and dust surrounding them that were thought to have formed early in the solar system's history. These were surmised before astronomical observatories had the capability to detect them. In 1980 that part of the story was hypothesis too. But they are there, and in some cases now, the planets embedded in them can be seen in the process of formation.
At that stage, where gas, dust and rocks are falling on a planet, I suspect that discussions about atmospheric gas are largely a moot point. This is a state of perpetual explosion and intense heat. The rubble has got to clear away and things have to cool down. But then when the dust all clears and the internal heat dies down, it will eventually be possible to observe or catalog planets around other stars that might be very young, as old as the earth or older. If a planet were to have absorption lines in the infra red for water vapor, ozone, methane ( since they are easier to observe than say an identifier for free oxygen), we might have some very intriguing questions to entertain after that.
This website would not be named for JWs if it did not deal some with prophecy. So let us engage to a limited extent. Identification of atmospheric conditions in extra-solar planets has already begun with the easy pickings - the planets very close to their suns. I believe it will continue to the planets with placement in thermal conditions similar to earth's and that we will have identifications of their atmospheres within a decade or so. Whether they show H2O, O3 and CH4? Well, that's where I bow out on prophecy. That's what I want to find out the conventional way.