Mars Mars is the fourth terrestrial planet. It is smaller in size than Earth and Venus, but larger than Mercury. Of the planets, Mars is the most hospitable to visiting humans, as it has an atmosphere, though it has no oxygen and is very thin. In the early days of the solar system, Mars has had liquid water on its surface, which is proved by the existence of various minerals that require liquid water to form. The current atmosphere of Mars is too thin and cold for liquid water to exist for long periods of times. Still, there is evidence of terrain shaped by flowing liquids, which are probably temporary flows or floods caused by the melting of ice in the ground. Mars has seasons like the Earth, and ice caps at the poles that grow and shrink over the course of the year. They consist of water ice and CO2 ice, the latter varying more with the seasons. One year on Mars is a bit less than two Earth years. Mars has two moons, Phobos and Deimos. They are very small compared to Mars, and are likely to be asteroids captured by Mars's gravity in the distant past. Giant planets and their moons The outer four planets of the solar system are known as the giant planets, because of their large size (compared to the terrestrial planets). The giant planets of our solar system can be further divided into two categories: gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune). The gas giants consist mainly of hydrogen (up to 90%) and helium, while the ice giants are mostly "ices" such as water and ammonia, with only some 20% of hydrogen. Jupiter Jupiter is by far the largest of the planets. Its mass is 320 Earth masses, and it's three times heavier than the second largest planet, Saturn. Compared to the Sun, it is still small. The Sun is over a thousand times heavier. Jupiter consists probably of a rocky core maybe the size of Earth, deep inside the planet, surrounded by a layer of mostly hydrogen and helium over fifty thousand kilometers thick. The top 5000 kilometers or so form a gaseous atmosphere, under this the pressure is high enough for the gases to turn into liquids, and even deeper, around the core, the liquid hydrogen is compressed so much that it becomes "metallic" in its electrical properties. The Galilean moons Jupiter has dozens of moons orbiting it. Most of these are small, and are probably captured asteroids, but there are four particularly large and interesting moons. These were first discovered by Galileo within his first year of telescope observations and are collectively now known as the Galilean moons. Io Io is strikingly yellow in colour. It is the most volcanically active body in the solar system, due to strong tidal effects of Jupiter, which deform and heat up the crust. Europa Europa's surface is covered in ice. The surface is very smooth, and shows long reddish crack lines. Under the ice may be a liquid ocean or a layer of icy slush, surrounding a rocky core. The ice surface shows evidence of a process similar to Earth's plate tectonics. The surface has very little cratering. An old surface would have accumulated many more craters from asteroid and meteor impacts, so some process must renew the surface fast enough to erase the cratering record. The possibility of a liquid ocean makes Europa very interesting for the search of life outside Earth. All life as we know it on Earth requires liquid water to thrive, so we look for signs of water first, when we look for life. The reddish areas at the cracks in the ice are interesting targets for future space missions, because the reddish material is likely to have brought up from under the ice by water flowing up to the surface through the cracks. Ganymede Ganymede is the largest moon of Jupiter. It consists of silicate rock and water ice. Its surface contains two types of terrains: dark regions, saturated with impact craters and dated to four billion years ago, and lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient. Ganymede is the only moon in the Solar System known to posses a magnetosphere. Scientists believe that there is an ocean 200km below the ice surface. Callisto Callisto's surface looks rocky and heavily cratered. The moon seems to consist of a mixture of mineral and icy material. The cratering of the surface shows that no active processes have changed the surface in a long time. .