Marvin the Martian has invaded Penny Boston’s office. Icons of the cartoon character hang from every available portion of shelf space. She says he’s helping her design a way for astronauts to live on the Red Planet – and she’s practicing in New Mexico caves. Boston, a scientist at New Mexico Tech, has studied Mars for the past 30 years. For the last 15 of them, she has also become one of the nation’s top experts on caves. Why not combine the two, she figured. Her idea: build inflatable living environments inside caves.