Have Phobos-Grunt land next to it
The publicity generated by saying that the Russian probe will examine
an "alien monolith" on a small Martian moon will be immense and a major
boon to interest in space by the general public.
Saying that it is a big natural boulder that happens to stand out on a
flat and dusty plain is just too boring and obvious and won't sell tickets.
And don't forget to add that Phobos-Grunt will be carrying living microbes
from Earth. Throw in something about delivering terran DNA to the alien
monolith as a gesture of intergalactic peace and understanding.