Mars is a terrestrial (mostly silicate rocks and metals) planet with a thin atmosphere and surface features reminiscent of both lunar impact craters and Earth's volcanoes, valleys, deserts, and polar ice caps. Mars' rotation period and seasonal cycles are similar to Earth's; the tilt of the axis creates the seasons. Mars is home to the second highest known mountain in the Solar System (the first highest on the planet), Mount Olympus, and one of the largest canyons, Valles Marineris. The smooth Borealis Basin in the Northern Hemisphere covers 40% of the planet and may be a feature formed after a giant collision.