The atmosphere of Mars is thin and cold. The average density is 0.7 percent the density of Earth, with average temperatures far below zero. The atmosphere consists almost entirely of carbon dioxide, with only a tiny amount of other elements and compounds. The atmosphere supports clouds of both water and carbon dioxide. The clouds sometimes produce snow. When Mars was young, its atmosphere was much thicker and warmer than it is today, allowing liquid water to flow and stand on the surface. During the last four billion years, though, most of that atmosphere has been lost. Much of it escaped into space, while some of the compounds in the atmosphere combined with others to form rocks at the Martian surface. Several spacecraft have studied the planet's atmosphere and continue to do so today.