Imagine viewing a web page with a reference to a landing site on Mars - and being able to use an add-on to you browser to be taken directly to that site, in a 'Martian' mapping application of your choosing, or on a site like Google Mars. Or being able to search, easily, for web sites referring to the same site, without seeing places on Earth with co-incidentally similar coordinates.
Those are just two of the things a "mars" microformat, based on the existing [geo] version, might do for you.
Here's some work-in-progress:
- luna - the same sort of thing, for The Moon