Mars is the fourth planet from the Sun. It was formed approximately 4.5 billion years ago, is a terrestrial planet and is the second smallest of the Solar System's planets with a diameter of 6,779 km (4,212 mi). A Martian solar day (sol) is 24.5 hours and a Martian solar year is 1.88 Earth years (687 Earth days). Mars has two small and irregular natural satellites: Phobos and Deimos. Carbon dioxide is substantially present in Mars's polar ice caps and thin atmosphere. It has the highest mountain in the solar system, Olympus Mons, and the largest canyon, Valles Marineris. There are large annual temperature swings on the surface, between −78.5 °C (−109.3 °F) and 5.7 °C (42.3 °F) – similar to Earth's seasons. Due to its geological history, the possibility of past or present life on Mars remains of great scientific interest. Mars has been explored by uncrewed spacecraft and rovers, and is an attractive target for future human exploration missions. (This article is part of a featured topic: Solar System.)