Peter Smith, professor of planetary sciences, University of Arizona.
Memorable moment: On July 4, 1997, Pathfinder landed safely on Chryse Planitia near the outwash plains of Ares Valles. The first set of images came down in a scrambled order and we began piecing them together, aligning boulder and horizon shots until an amazing scene was revealed, with a row of large stones nearly blocking a distant view of a twin peaked hill.
Biggest surprise: The Phoenix mission’s discovery of an abundance of perchlorate in the arctic soil. Perchlorate is a powerful oxidant and combusts organics at the high temperature of the Viking and Phoenix ovens. The combustion products would be perceived as water vapor, carbon dioxide, and some chloro-hydrocarbons that can be mistaken for cleaning agents. Viking saw these signatures, leading to the conclusion that the Martian soil is sterile. The truth awaits Curiosity, with its ability to measure organics without heating.
Human landing: My guess is we’ll have humans there in 2031.
Ruslan Kuzmin, leading research scientist, Laboratory of Comparative Planetology, Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow.
Memorable moment: My first encounter with Mars took place in 1956 as a middle school student, when I attended a summer camp near Moscow. There, I paid attention to the red star, which twinkled beautifully in the evening sky. That led me to take courses in astronomy, read science fiction books about Mars, and study maps of the planet made from telescopic observations. My favorite book was The Martian Chronicles by Ray Bradbury.
Biggest surprise: The images of Mars taken by Mariner 9 in 1971. The pictures of a dry riverbed like the Nirgal Vallis and the huge canyons and volcanoes just staggered me. It became clear that Mars has many Earth-like landscapes. Besides the mystery of whether there ever was primitive life, another important question is what climatic conditions existed on Mars in the geologic period between 4 billion and 3.6 billion years ago?
Human landing: No earlier than 2039 or 2040.
Mary Roach, author of Packing for Mars.
Memorable moment: Sometime in 2009, I visited the meteorite collection at the Johnson Space Center in Houston. Mike Zolensky, a man with the enviable job title Curator of Cosmic Dust, handed me a Mars meteorite as heavy as a bowling ball. I stood there making an expression I’m sure I’d never before had occasion to make. The meteorite wasn’t especially beautiful. Give me a chunk of asphalt and some shoe polish and I can make you a simulated Mars meteorite. What I can’t possibly simulate for you is the feeling of holding a 20-pound divot of Mars in your hands.