to 245 degrees Kelvin—a high that is 28 degrees below the freezing point of water. 
    With a radius about 53 percent and a mass 11 percent Earth’s, Mars’s gravity has been sufficient to retain a respectable, if rather thin, atmosphere. Surface pressure is about 2 percent that of Earth 
(on Earth the atmospheric pressure is equal to Mars’s at about 132,000 feet—over four times the height of Mount Everest). The atmosphere on Mars is composed mainly of carbon dioxide with small amounts of nitrogen and argon and a few tenths of a percent of oxygen. 
    Mars has two small moons: Phobos and Deimos, 21 and 13 kilometers in diameter respec- tively. Irregularly shaped, they