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.)

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    I'm asking which one is correct, while that question was asking WHEN? The first part of the question you stated was not answered, as the answerer provided only dates. – kabahaly Jan 25 '14 at 1:22
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    Voting to close. This question is answered by any adequate punctuation guide. – Pitarou Jan 25 '14 at 3:21
  • @kabahaly: I think "correct" is a meaningless concept in this context. Choose your style guide, or make your own decision. Or go with the majority usage - full details of which are provided by the runaway top answer to the question you've rejected. – FumbleFingers Jan 25 '14 at 4:54
  • Perhaps this answer upvoted 55 times will prove to be more helpful to the OP. It was reading such clearly explained answers such as this one by JSBձոգչ that helped improve my understanding of "grammar rules". – Mari-Lou A Jan 25 '14 at 6:39
  • @Pitarou When is something 'adequate'? Until you check a second source and find that it contradicts what you thought was a closed Q.? :) – Kris Jan 25 '14 at 6:45

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".

  • I think that because Mars is the name of an ancient mythological god, it would follow the same tradition as Jesus', Moses' and Aeneas'. (I agree with you about the spoken possessive.) – anongoodnurse Jan 25 '14 at 1:52
  • I'm aware that there are some biblical exceptions, but I'm not sure that they apply to all gods. The manual of Scientific Style does have a specific rule that if a proper name ends in an 'eez' sound, such as Achilles, Ares, or Hercules, then it would take just an apostrophe. That would cover quite a few of the ancient gods and heroes. The test would be whether it is Zeus's or Zeus'. I wonder what Dr. Seuss's view is. – Mike Jan 25 '14 at 2:04
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    All that matters is how it is is said: speech is primary, not writing. When you know how it is said, you will know how it should be spelt. And Mike, you have it right. The supposed thing about old biblical names is a complete misunderstanding, and wrong. There are NO EXCEPTIONS to the rule: if you pronounce an extra “iz” sound, you must write an apostrophe s, and if do not, you do not. For goodness’ sake, what could be easier? :) – tchrist Jan 25 '14 at 3:20
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    There's a recent post that asks if in fact, any (inanimate) object's anything is grammatical at all, or should it always use the of form instead. english.stackexchange.com/q/147922/14666 – Kris Jan 25 '14 at 6:49

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