VENUS

Named after the Roman goddess of Beauty and Love, the Hottest Planet Venus is the second closest to the Sun. It orbits the Sun at 0.72 times the Earth’s distance, or 108.2 million km. Because of its location relative to the Earth, Venus is always seen close to the Sun, at less than 48? angular distance from it. The planet appears either as a morning ‘star’ or as an evening ‘star’. The first successful space mission to Venus was in 1962. Until then, all that was known about Venus came from Earth-based investigations.

The distance of Venus to the Earth varies from 41million km to 257 million km. As seen from the Earth, Venus’s apparent diameter varies from 10 to 64.5 arcsec (Venus is in the opposite side of the sun). However, we see the full disk illuminated by the Sun only at Superior conjunction. The planet has phases, like the Moon. They were first seen by Galileo in 1610. Venus is the brightest object in the sky, after the Sun and the Moon, but the planet’s brightness is very variable. Venus is the brightest between maximum elongation and inferior conjunction. At inferior conjunction (Position in which Venus lies between the Earth and the Sun), which happens about every 19months, when Venus’s size is the largest, the disk is completely dark at visible wavelengths. The brightness of Venus is due not only to its close proximity to the Sun (Venus receives twice more solar light as the Earth) and to the Earth but also to the high reflecting power of the clouds that reflect 75% of the solar light.

The rotation of Venus is nearly synchronous. The period of rotation of Venus around the Sun is 224.7 days, while the planet’s rotational period is 243 days (Earth days), which is extremely slow, and in the retrograde direction (i.e. opposite to the direction of the movement of the rotation of the planet around the Sun). Because of the combined effects of the slow retrograde rotation of Venus and of its orbital motion around the sun, the solar day (time between two sunrises) on Venus corresponds to 117 Earth days. Another curious fact is that the Sun appears to rise in the west and set in the east.

The eccentricity of the orbit and the inclination of the planetary rotation axis are both very small. Therefore no major seasonal changes are expected. In fact, Venus is the planet with the most circular orbit. It is also the most Earth-like of the terrestrial planets in terms of size and bulk properties. The planetary radius at the equator is 6051.5 km, which corresponds to 95% of the Earth’s radius. Furthermore, Venus’s mean density (5.24 g cm−3) and surface gravity (8.6 m s−2) are comparable with those of the Earth. However, we find many more differences in Venus from the Earth than was believed years ago. In particular, Venus has a very high surface temperature of 740 K (hot enough to melt lead) and a very dense atmosphere (93 times that on Earth). The main atmospheric constituent is carbon dioxide, instead of nitrogen for the Earth. The clouds are made of sulfuric acid and not pure water as on Earth. Besides, the surface of Venus is essentially of volcanic origin and does not present evidence for lateral plate tectonics in recent history. Furthermore, Venus has no intrinsic magnetic field. In addition, Venus does not possess any satellites.

 Questions to ponder:

• Why can't we see Venus at night when it is an inner planet?
• Mercury is closer to the sun than Venus. Still, how can Venus be the hottest planet?