Venus is sometimes regarded as Earth's sister planet, since in some ways they are very similar: Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass). Both have few craters implying relatively young surfaces. Both their densities and chemical compositions are similar. Because of these similarities, it was thought that Venus might be very earthlike and might even have life. However, more detailed study of Venus shows those in many ways it is drastically different from Earth.
Venus' interior is assumed to be very similar to that of Earth, holding an iron core about 3000 km (1900 mi.) in radius and a molten rocky mantle making up the majority of the planet. Recent results from the Magellan spacecraft suggest that Venus' crust is stronger and thicker than previously assumed.
Venus is covered by thick, rapidly spinning clouds that trap surface heat, creating a scorched greenhouse-like world with temperatures hot enough to melt lead, and extremely high pressure. These clouds reflect sunlight while trapping in heat. Because Venus reflects so much sunlight, it is usually the brightest planet in the sky. While there are strong (350 kph) winds at the cloud tops winds at the surface are very slow, a few kilometers per hour at the most.
Venus probably once had large amounts of water like Earth but it all boiled away. Venus is now quite dry. Earth would have suffered the same fate had it been just a little closer to the Sun.
Venus slowly rotates on its axis once every 243 Earth days, while orbiting the Sun every 225 days. The periods of Venus' rotation and of its orbit are synchronized so that it always presents the same facade toward Earth when the two planets are at their closest approach. Whether this is a resonance effect or just a coincidence is not known.
Venus is the only planet in our solar system that rotates "backwards"--from its surface, to an observer of Venus the Sun would seem to rise in the west and set in the east. Venus has no magnetic field, perhaps because of its slow rotation. Furthermore, it has no satellites and thereby hangs a tale. But the solar wind rushing by Venus creates a pseudo-field around the planet.
