NASA is all ears for new ways to explore Mars. Future missions could feature smart balloons, robotic rotorcraft, free-flying frisbees or even deep-drilling mechanized inchworms — there’s no shortage of ways to purge that planet of its secrets. Such ideas are among many conjured up by a mix of some 200 engineers, scientists and technologists from around the world meeting here July 18 to 20 at the Lunar and Planetary Institute.