I'm a pretty fluent English speaker, but this has been bothering me for a long time... It all started when I saw people, for example, saying Mars' atmosphere, according to my knowledge, the only time you have an apostrophe without an "s" is when said word is plural, so in this case it would be Mars's or not Mars'. Am I correct? (I'm using the word Mars merely as an example, this applies to other singular words that ends with an "s" as well.)
Most style books say that either would be technically correct, but some, such as the Chicago Manual of Style, would prefer Mars's in this case since the second s is usually pronounced. Phonetically, you would say "Marzez atmosphere", rather than "Marz atmosphere".
Ultimately, it is a style choice, and if you googled Mars's moons/atmosphere or Bruno Mars's debut album, you'd see that people have gone both ways on the subject. It's probably easiest just to avoid the possessive and say the "atmosphere of Mars".