 Horowitz was a member of both the National Academy of Sciences and the American Academy of Arts and Sciences.In 1965 he began work with the Jet Propulsion Laboratory in Pasadena, serving for five years as chief of JPL’s bioscience section and as a member of the science teams for the Mariner and Viking missions to Mars.From 1977 to 1980, he was chairman of the biology division at Caltech.