How can we know if there is life on any planet (close or far) if we can only look for it using our senses? Forget Mars, is it just possible that there is life on earth that we simply cannot know of because we do not have the required senses to perceive it.
Physicalism is essential to science. What sense could we (and our instruments) lack that still exists in the physical world? There is no doubt that there are species we have yet to identify and categorize, but those species will still be made of the same matter that comprises the rest of the universe, detectable by the same means.
The only major obstacle I see to knowing about some form of life on Earth is location -- we can't scan the entire ocean floor, nor can we penetrate into the mantle.
