UNIVERSE: the solar system: Mercury and Venus

Discovery Space: Mercury and Venus

Mercury and Venus are relatively close to the sun, so both are scorching hot. Neither one has any moons. Other than that, they are very different. Venus—in contrast to Mercury—has a dense carbon dioxide atmosphere.

Mercury is the innermost and smallest planet of the solar system. Its surface is marked by giant cliffs and areas of angular terrain that extend over hundreds of miles. These features are the result of compressive stress within the crust. They may have developed after the igneous phase of the planet, when the planet cooled off and contracted.

Temperatures on the surface of Mercury reach as high as 800°F (430°C) and as low as about -270°F (-170°C). This extreme difference between high and low temperatures is due to Mercury’s thin gaseous atmosphere, which cannot store heat. Mercury’s proximity to the sun has made investigation difficult, because space probes need to withstand both the intense radiation from the sun and its gravitational pull.

In the 1970s, the Mariner 10 probe investigated Mercury as it flew past the planet. Today, NASA is preparing their space probe Messenger, which is due to orbit Mercury in 2011. The launch of the European-Japanese mission BepiColombo is planned for 2013.

Venus is the second planet from the sun and is nearly as large as the Earth. Venus glows brightly due to a dense creamy-colored cloud cover made up largely of sulfuric acid droplets, which reflects more than 70 percent of the incoming so- lar radiation. By comparison,
the Earth reflects only about 40 percent. The atmosphere of Venus contains carbon dioxide, nitrogen, and traces of sulfur dioxide, water, and other substances.

Venus has the densest atmosphere of all terrestrial planets: pressure on the ground is about 90 bar—the equivalent of oceanic pressure at a depth of 2,950 feet (900 m). Ground temperatures can reach 860T (460°C). The surface of Venus resembles a rocky desert, with giant plateaus, depressions, highlands, volcanoes, and craters.

Some interplanetary space probes have already brought back scientific data from Venus, and the planet has been studied by the European Venus Express since 2006.


The orbit of Mercury is neither circular nor elliptical; rather, it performs a rosette-like orbit. The deviation is small but measurable. Weaker deviations also occur among the other planets.

The orbital deviations cannot be fully explained by Newton’s theory of gravitation; however, Albert Einstein’s theory of general relativity provides an adequate explanation.