There may already be aliens on Mars

http://en.wikipedia.org/wiki/Mars

ALMOST everywhere you go on Earth, you encounter alien species that were introduced – often inadvertently – by humans. Now it seems possible that we have done the same to Mars. Despite stringent rules designed to prevent contamination, Earth microbes may have reached the Red Planet.

Tags: 

Read more: There may already be aliens on Mars

Story added 5. July 2013, content source with full text you can find at link above.