M
mikeemmert
Guest
Nobody's discussed presence/absence of an atmosphere. Maybe because Mercury doesn't have one. Any factor that eliminates Mercury won't do, because Mercury has been a planet since prehistoric times.<br /><br />Pluto does. Strangely, larger Ganymede and Callisto don't. Titan and Triton do. According to the N14/N15 ratios on Titan measured by Cassini, about 80% of Titan's atmosphere has evaporated. Unknown whether Xena has an atmosphere, but I bet right now it doesn't. It might never get close enough to the sun, since perihelion is farther than Neptune.<br /><br />Of course, Pluto's atmosphere is temporary. And comets have temporary atmospheres. Since Titan's atmosphere is evaporating, would you say it's also temporary?<br /><br />Temperature has an awful lot to do with this.<br /><br />As far as definition purposes goes, how thick would an atmosphere have to be? Our Moon has an atmosphere of argon. It is continuously generated by radioactive decay of potassium. When we reach the poles of the Moon, I wouldn't be particularly surprised if there's an enormous quantity of frozen argon there. The temperature's low enough. That might even be the factor that sets the Moon's atmospheric pressure.<br /><br />I have included moons in this discussion because at least two have atmospheres, and Enceladus appears to also have an atmosphere, although it might be temporary or transient.