Context: Mars is the fourth planet from the Sun, and the first planet beyond Earths orbit (Figure 1.1). Mars is a quite different from Earth and yet more similar than any other planet. Mars is smaller, colder, drier, and appears to have no life, but volcanoes are common to both planets and Mars has many. Mars is easy to observe, so Mars has been studied more thoroughly than any other extraterrestrial planet. Space probes, rovers, and orbiting satellites have all yielded information to planetary geologists. Although no humans have ever set foot on Mars, both NASA and the European Space Agency have set goals of sending people to Mars sometime between 2030 and 2040. This image of Mars, taken by the Hubble Space Telescope in October, 2005, shows the planets red color, a small ice cap on the south pole, and a dust storm. Viewed from Earth, Mars is reddish in color. The ancient Greeks and Romans named the planet after the god of war. The surface is not red from blood but from large amounts of iron oxide in the soil. The Martian atmosphere is very thin relative to Earths and has much lower atmospheric pressure. Although the atmosphere is made up mostly of carbon dioxide, the planet has only a weak greenhouse effect, so temperatures are only slightly higher than if the planet had no atmosphere. Mars has mountains, canyons, and other features similar to Earth. Some of these surface features are amazing for their size! Olympus Mons is a shield volcano, similar to the volcanoes that make up the Hawaiian Islands. But Olympus Mons is also the largest mountain in the solar system (Figure 1.2). Mars also has the largest canyon in the solar system, Valles Marineris (Figure 1.3). It was previously believed that water cannot stay in liquid form on Mars because the atmospheric pressure is too low. However, there is a lot of water in the form of ice and even prominent ice caps (Figure 1.4). Scientists also think Olympus Mons is about 27 km (16.7 miles/88,580 ft) above the Martian sur- face, more than three times taller than Mount Everest. The volcanos base is about the size of the state of Arizona. Valles Marineris is 4,000 km (2,500 mi) long, as long as Europe is wide, and one-fifth the circumference of Mars. The canyon is 7 km (4.3 mi) deep. By comparison, the Grand Canyon on Earth is only 446 km (277 mi) long and about 2 km (1.2 mi) deep. that there is a lot of ice present just under the Martian surface. This ice can melt when volcanoes erupt, and water can flow across the surface. In late 2015, NASA confirmed the presence of water on Mars. Scientists think that water once flowed over the Martian surface because there are surface features that look like water-eroded canyons. The presence of water on Mars suggests that it might have been possible for life to exist on Mars in the past. Mars has two very small moons that are irregular rocky bodies (Figure 1.5). Phobos and Deimos are named after characters in Greek mythology the two sons of Ares, who followed their father into war. Ares is equivalent to the Roman god Mars. Mars has two small moons, Phobos (left) and Deimos (right). Both were discovered in 1877 and are thought to be captured asteroids. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL: 
 Question: water cannot stay in liquid form on mars because the
Output: atmospheric pressure is too low.