If it has a powerful magnetic field, 3/4th G should be more than enough to hold onto an atmosphere. A magnetic field has a lot more to do with keeping an atmosphere than raw gravity, because it keeps it from being stripped away by solar wind. Look at mars, for an example - Its magnetic field died long ago, and the sun slowly eroded the atmosphere away, but its gravity is only a third of earths. Titan, on the other hand, has only 1/6th earth gravity, but has a healthy atmosphere because it's protected by Saturns magnetic field. Also, argon, being heavier, would make for a denser atmosphere, so you could achieve earth-like pressures even with the lower gravity (if you want to.)
As for the composition itself, neon and argon are just as inert as our nitrogen, so whatever life there is is probably going to be oxygen-breathing. Oxygen is the most reactive element in the universe, so it only makes sense to use it in your biochemistry. That being said, if they were to visit earth they'd probably suffer oxygen toxicity, since they're used to breathing a very rarefied mixture.
As for the lights, sure, why not? It might actually be easier for them to develop the technology - Gas discharge lamps simply excite Argon/Neon/Xenon/Ect until it starts to glow. We have to contain the gas in a bulb because if we don't it'll diffuse into our nitrogen (and non-Argon/Neon) atmosphere. On the world you're talking about, something as simple as a static spark would have an orange or violet glow. I'd imagine them having some very pretty thunderstorms.
Side benefit: Any kind of Taser or Laser weapon that has a mundane invisible beam on earth would be very vibrant and colorful on their world.
argon is heavy 74% is a catastrophic level of greenhouse gas... your planet would be a desert ball of fire, on earth 0,04% of greenhouse gases on the atmosphere are just enough to put the world at great risk, and our greenhouse gases are way lighter than argon. – Charon – 2016-08-15T19:18:01.613
3@渡し守シャロン Argon isn't a greenhouse gas; heat goes through the certain wavelengths that trap heat. Are you saying because the atmosphere would be so dense that it would reek havoc on the planet's surface? – Twig – 2016-08-15T22:42:14.300
I'm not sure this can work, for reasons mentioned in the accepted answer to your other question. Life on Earth relies on the nitrogen cycle, which is itself based on Earth's atmosphere being three-quarters nitrogen and how that nitrogen reacts chemically with life. If you replace most of that nitrogen with one of the noble gases like argon or neon (known for their lack of reactivity), I'm not sure you can have any life, at least not anything we'd recognize.
– Palarran – 2016-08-15T23:20:39.987@Palarran the life on the planet might evolve to use the nitrogen more efficiently. In any case, "atmospheric life" is relatively new, most of the time the life evolved in water, so it is the concentration of nitrogen in water which matters. – user31264 – 2016-08-16T00:00:32.300
@Palarran How much of the atmosphere should be nitrogen in order to sustain life; would it have to be the main component of the atmosphere? – Twig – 2016-08-16T00:29:39.763
@Twig I'm not sure anybody can give you an exact figure, and I'm not an expert on this. All I can do is point out that Earth (the only planet known to support life) has an atmosphere of 75-80% nitrogen, and that plant life depends on that nitrogen to thrive; agricultural fertilizer is mostly nitrogen, with some other materials. Your proposed atmosphere is replacing most of that nitrogen (a fairly reactive gas) with neon or argon (both notable for their lack of reactivity). Maybe a much denser atmosphere could help, by providing sufficient nitrogen despite the lower concentration? – Palarran – 2016-08-16T04:43:39.970
@Palarran In that case, an argon atmosphere could work as it's denser then nitrogen at 1.78 grams per liter (compared to 1.25 grams per liter). Perhaps nitrogen could be abundant in the soil but never go to the atmosphere, just stored by plant/bacteria-like organisms; those organisms could get consumed by higher organisms, witch release it back into the soil for more nitrogen-storing organisms or plants to use. – Twig – 2016-08-16T13:59:56.163
Greenhouse gas effects can probably be ignored, as far as story-telling goes; you can just move the planet's orbit closer-to or further-from the star to compensate. – Nat – 2017-01-18T05:44:56.443
As for water vapor, you probably can't pick it like that. The maximum amount of water vapor the atmosphere can hold is determined by thermodynamics; once you hit that limit, it starts to precipitate (rain, etc.). This limit varies heavily with temperature; water's far more volatile when it's warm. – Nat – 2017-01-18T05:50:24.097
What's the total pressure? Percentages don't mean much unless you know what they're percentages of. – Logan R. Kearsley – 2017-07-06T22:50:04.537