The fourth planet from the sun is frigid and nearly airless, treacherous and distant, but somehow alluring. Its desertlike terrain looks as if it were scooped from the American Southwest or the African Sahara. It is a mystery waiting to be solved. When Mars swung close to the Earth this summer, as close as it has been in 60,000 years, thousands of curious stargazers searched out the planet’s uniquely reddish glow and pondered what it might be like to visit.