THE CARNEGIE INTERNATIONAL is the oldest contemporary art exhibition in North America—second in age worldwide only to the Venice Biennale—so it seems apt that its fifty-fifth incarnation is ambitious in scope and duration. Not only does this year’s installment, “Life on Mars,” take over almost the entire square footage of Pittsburgh’s Carnegie Museum of Art, but it also has an unusually long run of eight months, through January 2009. Scope and duration are not virtues in themselves, however, and here they may in fact accentuate many of the problems critics regularly find with such large-scale