A planet's density can be determined by dividing its mass by its volume. The result is how much material it has within the space the planet occupies. It is important to note that density does not equal size. Planets could range in sizes, but if they are made of the same materials, their density will be close. A planet many times the size of Earth could have equal density as a small, low-mass planet. Density is generally expressed in gram over centimeter to the third power or 1,000 kilograms/meter3.
Water has a density of one. Planets similar to Earth will have densities from four to five. These are generally planets with rocks on the surface such as the moon and Mars. Silicate rock has a density of three, while iron has a density of nearly eight. A planet's density will reflect what types of materials are most dominant within. The planets much larger than Earth and Venus such as Jupiter, Saturn and Uranus are mostly liquid worlds with thick atmospheres of gas. These planets have densities ranging from 0.7 to 1.7 Gases such as hydrogen and helium have densities much lower than water. These planets are believed to be mainly made up of ammonia, hydrogen, helium and methane with a rock core.
Venus is referred to as Earth's "sister" planet because of shared traits. Its diameter is only 650 km less than Earth's. However, surface conditions are drastically different because of the dense carbon dioxide atmosphere. It is theorized that Venus has a liquid or partially liquid core and its smaller size would mean less pressure within its core than Earth's. Because Venus has a much drier mantle, it does not demonstrate the plate tectonics that Earth does. The result is less heat loss, hindering planetary cooling and perhaps explaining the lack of an internally generated magnetic field.
Beginning with the Soviet Venera 3 probe in 1966, Venus has been studied by NASA and the European Space Agency. Flights by unmanned probes indicate that Venus has several times more volcanoes than Earth; it has an estimated 170 giant volcanoes that are more than 100km across. Venus' surface is estimated to be about 500 million years old.
The pressure on the surface is about 92 times that of Earth, similar to the depth at nearly 1 kilometer under the ocean. The atmosphere causes an extreme greenhouse effect, creating surface temperatures exceeding 460 degrees centigrade. Though Venus is nearly twice as far away from the sun as Mercury, it is nearly twice as hot.