As long as we are a single-planet species, we are vulnerable to extinction by a planetwide catastrophe, natural or self-induced. Once we become a multiplanet species, our chances to live long and prosper will take a huge leap skyward.

The planet Mars -- crimson and bright, filling our telescopes with vague intimations of almost-familiar landforms -- has long formed a celestial tabula rasa on which we have inscribed our planeto-logical theories, utopian fantasies, and fears of alien invasion or ecological ruin.

Before we go there and set up greenhouses, dance clubs, and falafel stands, let's make sure that, in some subtle form that could be harmed by the human hubbub, life does not already exist there. If not, then by all means build cities, plant forests and fill lakes and streams with trout -- bring life to Mars and Mars to life. We'll then be the Martians we've been dreaming about for all these years.

As long as we are a single-planet species, we are vulnerable to extinction by a planetwide catastrophe, natural or self-induced. Once we become a multiplanet species, our chances to live long and prosper will take a huge leap skyward.