There are real and vital reasons why we should venture to Mars. It is the key to unlocking the secret of life in the universe. It is the challenge to adventure that will inspire millions of young people to enter science and engineering, and whose acceptance will reaffirm the nature of our society as a nation of pioneers. It is the door to an open future, a new frontier on a new world, a planet that can be settled, the beginning of humanity's career as a spacefaring species, with no limits to its resources or aspirations, as it continues to push outward into the infinite universe beyond.
The only meaningful counterargument against launching a humans [sic] to Mars initiative is the assertion that we cannot do it.
Is that really the only counterargument?
I will treat Zubrin's reasons here individually.
1. [A mission to Mars] is the key to unlocking the secret of life in the universe.
Why do we want to know the secret of life? Mostly, because we want to know our own origins. Why do we want to know our origins? I submit that it is because we suspect they will give us a clue to our purpose.
However, study of life origins to date have not given us any clue as to what our purpose might be. If anything, it has shown us that we have no purpose.
I would find it aesthetically beautiful to know how life emerged, how common it is, whether there are different chemical possibilities for life than ours. But my experience of aesthetic beauty does not make up for the very real suffering of others, and it is criminally negligent for me to use aesthetic beauty to distract myself from their very real suffering. Anaesthetic beauty, that.
2. It is the challenge to adventure that will inspire millions of young people to enter science and engineering
As with the moon race, this is probably true. But to what purpose? Is attracting more young people into science and engineering a good thing? The life of a scientist or engineer is frequently dull and unrewarding, not at all that promised by the grand adventure of a Mars mission. I could not comfortably usher teenagers into the kind of life lived by my friends who are actual aerospace engineers.
3. ...and whose acceptance will reaffirm the nature of our society as a nation of pioneers.
I thought we all agreed, after Vonnegut, that "pioneer" was a swear now, like "conquistador" and "rapist." Even if there's no one out there to be, um, "pioneered," geographical expansion is the most boring, primitive sort of exploration.
Also, Zubrin's appeal to in-group loyalty ("nation") must fall flat for anyone with a more global sense of empathy.
4. It is the door to an open future, a new frontier on a new world
In other words, a Mars mission will let us think about nice science-fiction fantasies, instead of the depressing reality of hunger and cancer and environmental destruction.
As to the emotional connotations of "pioneer" and "frontier," see my post on Political Metonymy.
5. ...a planet that can be settled, the beginning of humanity's career as a spacefaring species, with no limits to its resources or aspirations, as it continues to push outward into the infinite universe beyond.
Getting more resources and more space does not solve any of the interesting problems. We want geographical expansion for monkey reasons, not person reasons. Like longevity (see: The ____ Must Go On), expansion in physical space is one of those primitive goals that need not infect our thinking any longer, as we now know it cannot lead us to what we really care about.
Distraction is what we mean by finding meaning in our search for meaning, the canard of Camus.
The only reason to go to Mars is that we are lonely and bored. But if there is anything to be learned from the whole of human history, it is that nothing relieves loneliness or boredom. Adventure can, at best, distract us from it for a while, while we pass on our loneliness and boredom down into the future, and all around the galaxy.