Context: Mercury is the smallest planet. It has no moon. The planet is also closest to the Sun and appears in Figure 25.7. As Figure 25.8 shows, the surface of Mercury is covered with craters, like Earths Moon. The presence of impact craters that are so old means that Mercury hasnt changed much geologically for billions of years. With only a trace of an atmosphere, it has no weather to wear down the ancient craters. Because Mercury is so close to the Sun, it is difficult to observe from Earth, even with a telescope. The Mariner 10 spacecraft did a flyby of Mercury in 19741975, which was the best data from the planet for decades. In 2004, the MESSENGER mission left Earth. On its way to Mercury it did one flyby of Earth, two of Venus and three of Mercury. In March 2011, MESSENGER became the first spacecraft to enter an orbit around Mercury. During its year-long mission, the craft will map the planets surface and conduct other studies. One of these images can be seen in Figure 25.9. Mercury is named for the Roman messenger god. Mercury was a messenger because he could run extremely fast. The Greeks gave the planet this name because Mercury moves very quickly in its orbit around the Sun. Mercury orbits the Sun in just 88 Earth days. Mercury has a very short year, but it also has very long days. Mercury rotates slowly on its axis, turning exactly three times for every two times it orbits the Sun. Therefore, each day on Mercury is 58 Earth days long. Mercury is very close to the Sun, so it can get very hot. Mercury also has virtually no atmosphere. As the planet rotates very slowly, the temperature varies tremendously. In direct sunlight, the surface can be as hot as 427C (801F). On the dark side, the surface can be as cold as 183C (297F)! The coldest temperatures may be on the insides of craters. Most of Mercury is extremely dry. Scientists think that there may be a small amount of water, in the form of ice, at the planets poles. The poles never receive direct sunlight. Figure 25.10 shows a diagram of Mercurys interior. Mercury is one of the densest planets. Scientists think that the interior contains a large core made mostly of melted iron. Mercurys core takes up about 42% of the planets volume. Mercurys highly cratered surface is evidence that Mercury is not geologically active. Named after the Roman goddess of love, Venus is the only planet named after a female. Venus is sometimes called Earths sister planet. But just how similar is Venus to Earth? Venus is our nearest neighbor. Venus is most like Earth in size. Viewed through a telescope, Venus looks smooth and featureless. The planet is covered by a thick layer of clouds. You can see the clouds in pictures of Venus, such as Figure 25.11. We make maps of the surface using radar, because the thick clouds wont allow us to take photographs of the surface of Venus. Figure 25.12 shows the topographical features of Venus. The image was produced by the Magellan probe on a flyby. Radar waves sent by the spacecraft reveal mountains, valleys, vast lava plains, and canyons. Like Mercury, Venus does not have a moon. Clouds on Earth are made of water vapor. Venuss clouds are a lot less pleasant. They are made of carbon dioxide, sulfur dioxide and large amounts of corrosive sulfuric acid! The atmosphere of Venus is so thick that the pressure on the surface of Venus is very high. In fact, it is 90 times greater than the pressure at Earths surface! The thick atmosphere causes a strong greenhouse effect. As a result, Venus is the hottest planet. Even though it is farther from the Sun, Venus is much hotter even than Mercury. Temperatures at the surface reach 465C (860F). Thats hot enough to melt lead! Venus has more volcanoes than any other planet. There are between 100,000 and one million volcanoes on Venus! Most of the volcanoes are now inactive. There are also a large number of craters. This means 
 Question: Which process explains why Venus is very hot?
Output: greenhouse effect