Robert Zemeckis’ performance capture system, which turns life-action into animation, gets quite a workout in Mars Needs Moms. Its characters, both human and Martian, tumble through space, slide down chutes, chase along corridors that turn every which way and dance around mountains and into caverns created out of huge mounds of trash. The process has also lost those weirdly inhuman faces that plagued The Polar Express, Zemeckis’ first foray into performance capture. In fact, so many things in this unique kind of animation, especially the designs and all its many details, keep getting better and better. So why does Mars Needs Moms feel so slight?