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's old surface is heavily cratered like many moons. It is larger than most but smaller than Jupiter's Ganymede and Saturn's Titan; and much denser and more massive than any moon because it is made mostly of iron. Earth is the only planet more dense. Mercury rotates three times every two orbits around the Sun. Its orbit is elliptical, you might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set over the other horizon.

is similar to Earth in size and mass but has a different climate. Thick clouds composed of sulfuric acid droplets and closeness to the Sun make it the hottest planet. No life of any sort has ever been found. Many things about Venus remain unknown, including the cause of mysterious bursts of radio waves. Venus' highest mountain is Maxwell Montes. Other notable features include numerous mountains, coronas, impact craters, tessera, ridges, and lava flows.

is the third planet from the Sun. Sphere-shaped and composed mostly of rock, over 70 percent of the planet's surface is water. A relatively thin atmosphere composed of nitrogen and oxygen. Earth has a single large Moon that is about a quarter of its diameter and, from the planet's surface, is seen to have almost exactly the same angular size as the Sun. With its abundance of liquid water, Earth supports a large variety of life forms, including potentially intelligent species such as dolphins and humans.

rotates, making most of its surface visible. Dark and light sand and gravel create a blotted appearance for the red planet. Winds cause sand-tinted features on the Martian surface to shift over time. The north polar cap is made of water ice and dry ice, there are fan marks from old water flows, and huge volcanoes leftover from ancient times. Visible on Mars are large dust storms in light orange. A particularly large storm is pouring out of Hellas Basin, erupting into a huge planet wide cyclone that continues even today.

is the largest planet in the Solar System. Due to its rapid rotation the planet possesses a bulge around the equator, giving it an oblate appearance. The outer atmosphere is segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. One is the Great Red Spot, a giant storm that has been going on since the seventeenth century. Surrounding the planet is a faint ring system and a powerful magnetosphere. There are also at least 63 moons, including the four large ones discovered by Galileo Galilei in 1610.

is the only planet of the Solar System that is less dense than water. Like Jupiter it has a rocky core at the center, a liquid metallic hydrogen layer above that, and a molecular hydrogen layer above that. Traces of various ices are also present. Saturn has a very hot interior and radiates more energy into space than it receives from the Sun. The rings of Saturn average one kilometre in thickness and are composed of silica rock, iron oxide, and ice particles. In the northern hemisphere the planet appears blue, but deep in the clouds the natural gold returns. It is not known why these clouds are gold.

is faint and featureless when viewed in visible light. But enhanced images reveal the moons, rings, and clouds of this distant gas planet. Blue represents the deepest layers while the highest cloud features have a reddish tinge. Racing around the planet, high, bright clouds are seen to move substantially. Ring systems are common to the solar system's four giant planets. The main Uranian ring seems to vary in width and is brightest near the top. There are small Uranian moons beyond the ring system: Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda and Puck.

's atmosphere has long light coloured cirrus like clouds floating high in it. Shadows of these clouds can be seen on lower cloud decks. Most of Neptune's atmosphere is made of hydrogen and helium, which is invisible. Neptune's blue colour comes from smaller amounts of atmospheric methane, which absorbs red light. Neptune has the fastest winds in the Solar System, with gusts reaching 2000 kilometres per hour. It is thought that diamonds may be created in the dense hot conditions that exist under the clouds-tops of Neptune.

's horizon spans the foreground, gazing sunward across that distant and not yet explored world. Charon is a darkened, ghostly apparition with a luminous crescent against a starry background. Beyond Charon, the diminished Sun is immersed in a flattened cloud of zodiacal dust. Pluto's ruddy colours are based on existing astronomical observations while high atmospheric cirrus and dark plumes from surface vents resemble Neptune's large moon Triton. Craters suggest bombardment by Kuiper Belt objects.

sources include: NASA's Astronomy Picture of the Day & Wikipedia