What is the plural of “Mars”?

What is the plural of Mars? Is it Marses or Mars' (note the apostrophe)? Or something else?

EDIT: I'm not sure where I got the idea of Mars' from. I suppose I was thinking it might be a zero plural where the word doesn't change, like fish, and just threw in the apostrophe like certain other oddities do, e.g., 1960's, pulling 5 g's (g-force).

This came up because a certain attack helicopter configuration is called the Mars, and I need to refer to multiple Marses and multiple Neptunes, but at least the latter is clearly plural.

• I agree that "Marses" would be acceptable, but I would avoid it as far as possible - refer to "Mars helicopters", for instance. You won't be able to avoid "Marses" entirely, but your readers will thank you for keeping it to a minimum. – MT_Head Sep 30 '15 at 23:01
• Sure, but the trouble is using the same phrase too much. Already in my novel, I've used the words "helicopters", "helis", "choppers", "aircraft", "rotorcraft", "Mantises", and "Marses" so much already. I need as many options as possible to dilute, for lack of a better term, the number of reuses. Plus in many action sequences I have to distinguish between Marses and Neptunes, and 'Marses' and 'Mars + something' is the only way. 'Mars + something' becomes much more wordy much more fast than Marses. – DrZ214 Oct 1 '15 at 0:48
• A Google search for <"the Mars are" -"on Mars" -"and Mars" -"from Mars" -"of Mars"> (I left the filters in by accident after a preliminary search; they are still very useful in removing inanities like "journeys to the Mars are now possible" for some reason) shows that the invariant plural is used for aircraft (The Mars are also equipped to deliver Thermo-Gel) and the family name (The Mars [Bruno et al] Are A Talented Family). This, combined with sumelic's answer, shows that attempts to apply rules-of-thumb to the behaviour of proper nouns are possibly not all that wise. – Edwin Ashworth Oct 1 '15 at 2:07
• Isn't it obvious? One Mars - two Mars bars. – user86291 Oct 1 '15 at 4:38
• POB: primarily opinion based [answers are inevitable]. Here, the behaviour of proper nouns is notoriously idiosyncratic, and trying to apply a 'rule' may well lead to a non-idiomatic result. – Edwin Ashworth Oct 1 '15 at 23:27

As MT_Head points out in the comments, you may be able to avoid using a plural form by resorting to phrases such as ""Mars helicopter configurations" or "Mars configurations." But to answer the question that you actually posed:

Marses?

Even though it sounds a bit funny, Marses is certainly one valid plural form. This follows the default English rules for forming a plural, which have us add /ɪz/ ("izz"), spelled "es" (or just "s" if there is already a silent e) after the sounds /s/, /z/, /tʃ/ ("ch"), /dʒ/ ("j"), /ʃ/ ("sh") or /ʒ/ ("zh"). By convention, we fall back to these default rules when pluralizing proper nouns with no commonly used plural forms. (Source: the "Daily Writing Tips" online post "Plurals of Proper Names" by Mark Nichol. There is also another question on this site that seems particularly relevant: Pluralization of names)

Here are the examples I've found so far online of Marses being used as the plural of Mars. (All books cited were accessed through Google Books.)

For the family name "Mars":

For the god Mars:

• "Like so many little Marses, / With their tilters at their a–s," from "Helter Skelter," a poem by Jonathan Swift, collected in "The works of the English poets, from Chaucer to Cowper," edited and with prefaces by Samuel Johnson.
• "The ancient patriarchs, afore the flood, [...] become stout Marses, and beget young Cupids." From "The Alchemist", by Ben Jonson, collected in The Works of Ben Johnson, with notes and a biographical memoir by William Gifford.

For the planet Mars:

• "It explores relationships among the Marses seen with the naked eye, through the telescopic lens, and with the mind's eye as well as the various Marses that have been perceived–or created–by scientists, writers, and visual artists[...]" from Imagining Mars: A Literary History, by Robert Crossley.
• "They had themselves already dreamed Marses more radical than any she could believe in, Marses that were truly independent, egalitarian, just and joyous." From Green Mars, by Kim Stanley Robinson.

Mars?

Thanks to Edwin Ashworth, it's been brought to my attention that in some contexts, the invariant plural Mars, with no apostrophe, has also been used.

In particular, this has been used when referring to the "Martin Mars" flying boats/water bombers/tankers/seaplanes. I'll try to find more citations and examples of this, but this might actually turn out to be the most relevant situation for you, as it's an example from the context of aircraft.

At first I wondered if there might be some convention to never use a distinct plural form of an aircraft name, but from what I can tell there isn't: a pluralized form of the aircraft Martin P5M Marlin, "Marlins", is used on the site of the National Naval Aviation Museum (located in Florida in the United States of America).

So it's possible that people simply thought "Marses" sounded wrong in this case.

Why it doesn't have an apostrophe

It could not be Mars' because a single apostrophe is never added to a word to form a plural; this is only ever used for forming possessives.

An apostrophe plus s ('s) is also generally only used to form possessives, although besides this it is sometimes used to form plurals in highly restricted circumstances: after a numeral, a single letter of the alphabet, or an abbreviation ending in a capital letter or a period. (I didn't quite cover everything; here's Wikipedia on apostrophized plurals.) In any case, Mars is none of these, so Mars's is also not correct as a plural form.

Other options?

If you go with the (highly inadvisable) Latin plural, Wiktionary says it would be Martes (which would be pronounced in an English language context as /ˈmɑrtiːz/ (the same as "Marty's"). I'm just listing this for fun.

• Why is the Latin plural highly inadvisable? I think it's rather nice. – Cerberus Sep 30 '15 at 23:14
• @Cerberus: Because then you either have to use the inconsistent "Martes and Neptunes," or you have to use the technically incorrect "Martes and Neptuni" (technically incorrect because "Neptuni" is properly the plural of "Neptunus" rather than of "Neptune). – sumelic Sep 30 '15 at 23:16
• Ah. I am at least glad that you have thought about it. – Cerberus Sep 30 '15 at 23:21
• Mars is modelled on say Winters ==> Winters, used quite frequently by eg Mr X Winters and family on the internet {eg here}, probably because Winterses sounds outlandish outside Tolkien. // We went to three works do's last year. – Edwin Ashworth Oct 1 '15 at 1:00
• I think you've made your point... – Mari-Lou A Oct 1 '15 at 4:38

I'm not sure where I got the idea of Mars' from.

Early in the last century, perhaps.

It did once used to be acceptable (and earlier still, quite the done thing) to use an apostrophe when pluralising proper names, especially if they end in s, and so Mars's. You may have seen enough Jones's etc. to half-remember it and come up with Mars' instead of Mars's.

The form is beyond old-fashioned though, to the point where it is probably not really acceptable today. Go with Marses.

Or dodge the issue and go with "Mars configurations".

• I was just about to ask, well what about the Jones's? I'm old; I was taught by people raised at that time. Joneses to me is yo-nes-is. One out of five Google hits (100K vs 400k) for "keeping up with the [Jones family]" includes Jones's. It's no wonder why I always 'seem' to get this wrong. – Mazura Nov 7 '15 at 23:25