Mars’ and Mars’s are both acceptable possessive forms of Mars. Furthermore, the AP Stylebook requires that you use Mars’. E.g., “Mars’ craters have been thoroughly mapped.” In comparison, the Chicago Manual of Style states that you should use Mars’s. E.g., “Mars’s orbit period is 687 days.”
The following chart shows a breakdown of the different forms of the word Mars. Furthermore, since Mars is a proper noun and only one exists, there is no plural version.
| Singular | Mars |
| Plural | NA |
| Singular possessive | Mars’ / Mars’s |
| Plural possessive | NA |
As you can see, there are two forms of the singular possessive that are correct. The following sentences show the different forms in context.
- Singular: Mars is the fourth closest planet to the Sun.
- Singular possessive: Mars’ diameter is smaller than that of the Earth.
- Singular possessive: Mars’s atmosphere is thinner than our atmosphere.
You should use Mars’ if you write a text that follows the Associated Press Stylebook. However, when writing a text that follows the Chicago Manual of Style, you should use Mars’s.
In other writing that does not follow a specific style, you can choose which possessive version you use. However, just make sure to be consistent throughout your document.
We have covered the basics of using the possessive version of the planet Mars. Now, keep reading the rest of the page to discover more about each term.
Mars’
The term Mars’ is the singular possessive form of the planet Mars. That means that you can use it to mention things that belong to Mars.
Furthermore, the things that can belong to a planet include geographical features, size, and other characteristics.
- Mars’ poles have offered indications that they harbor frozen water.
In addition, the possessive version Mars’ is the most popular possessive in British English and worldwide. The only country where Mars’ comes second to Mars’s is the US, where the latter is slightly more frequent.
Mars’s
The term Mars’s is the singular possessive form of the planet Mars. Therefore, you can use it to discuss things belonging to Mars, usually geographical features or physical characteristics.
- Mars’s surface is mostly a reddish-brown color.
Furthermore, when following the Chicago Manual of Style, you should use Mars’sas the possessive form.
Mars’s is regarded as the “standard” singular possessive because, in modern English, the trend is to add an “s” when making singular possessives.
However, although Mars’s is more common in American English, the alternative Mars’ is more frequently in British English and worldwide.