Venus is the second planet from the sun and Earth’s inner neighbour. The two planets are virtually identical in size and composition but these are very different worlds. An unbroken blanket of dense clouds permanently envelops Venus. Underneath lies a gloomy, lifeless, dry world with a scorching surface, hotter than that of any other planet. Radar has penetrated the clouds and revealed a landscape dominated by volcanism.
Orbit. Venus’s orbital path is the least elliptical of all planets. It is almost a perfect circle so there is little difference between the planet’s aphelion and perihelion distances. Venus takes 224,7 Earth days to complete one orbit. As it orbits the Sun, Venus spins extremely slowly on its axis – slower than any other planet. It takes 243 Earth days for just one spin, which means that a Venusian day is longer than a Venusian year. However, the time between one sunrise and the next on Venus is 117 Earth days. This is because the planet is traveling along the orbit as it spins, and so any one spot on the surface faces the Sun every 117 Earth days. Venus’s slow spin is also in the opposite direction from most other planets. Venus does not have seasons as it moves through its orbit. This is because of its almost circular path and the planet’s small axial tilt. Venus’s orbit lies inside that of the Earth, and about every 19 months Venus moves ahead of Earth on its inside track and passes between our planet and the Sun. Venus is within 100 times the distance of the moon.
Structure. Venus is one of the four terrestrial planets and the most similar of the group to Earth. It is a dense, rocky world just smaller than Earth and with less mass. Venus’s Earth-like size and density leads scientists to believe thats its internal structure, its core dimensions, and the thickness of its mantle are also similar to Earth’s. So, Venus’s metal core is thought to have a solid inner part and a molten outer part, like Earth’s core. In contrast to Earth, Venus has no detectable magnetic field. The planet spins extremely slowly compared to Earth, far to slowly to produce the circulation of the molten core that is needed to generate a magnetic field. Venus’s internal heat – generated early in the planet’s history and from radioactive decay in the mantle – is lost through the crust by conduction and volcanism. Heat melts the subsurface mantle material, and magma is released onto the surface.
Atmosphere. Venus’s carbon dioxide rich atmosphere stretches up from the ground for about 50 km. A deck of clouds with three distinct layers lies within the atmosphere. The lowest layer is the densest and contains large droplets of sulphuric acid. The middle layer contains fewer droplets, and the top layer has small droplets. Close to the planet’s surface, the atmosphere moves very slowly and turns with the planet’s spin. Higher up, in the cloudy part of the atmosphere, fierce winds blow westwards. The clouds speed round Venus once every four Earth days. The clouds reflect the majority of sunlight reaching Venus back into space, and so this is an overcast, orange-coloured world. Venus’s equator receives more solar heat than its polar regions. Yet, the surface temperature at the equator and the poles varies by only a few degrees from 464℃, as do the day and night temperatures. The initial difference generates cloud-top winds that transfer the heat to the polar regions in one large circulation cell. As a result Venus has no weather.
Greenhouse effect. Venus’s thick cloud layers trap heat and help produce the planet’s high surface temperature in the same way that glass traps heat in a greenhouse. Only 20 percent of sunlight reaches the surface. Once there, it warms up the rock. Heat in the form of infrared radiation is then released, but it cannot escape and adds to the warming process.
Tectonic features. Venus could be expected to have global features similar to those on Earth, but it differs in one key respect: it does not have moving plates. This means that its surface tends to move up or down rather than sideways. Yet, Venus displays many familiar Earth-like features formed by a range of tectonic processes, as well as some unfamiliar ones, such as arachnoids. Venus has hundreds of volcanoes, from large, shallow-sloped shield volcanoes such as Maat Mons, to small nameless domes. About 85 per cent of the planetary surface is low-lying volcanic plains consisting of vast areas of flood lava. There has been volcanic activity as recently as about 500 million years ago, and it is possible that some of the volcanoes could be active. Other features are a result of the crust pulling apart or compressing. There are troughs, rifts, and chasms, as well a mountain belts such as Maxwell Montes, ridges, and rugged highland regions. Venus’s highest mountains and biggest volcanoes are comparable in size to largest on Earth, but overall this planet has less variation in height.
Impact craters. Although many hundreds of impact craters have been identified on Venus, this total is low compared to that for the Moon and Mercury. There were more craters in the past, but they were wiped out by resurfacing due to volcanic activity about 500 million years ago. Venus’s craters have some characteristics not seen elsewhere in the Solar System, because its dense atmosphere and high temperature affect the incoming impactor and crater ejecta. Ejecta can, for example, be blown by winds and form fluid-like flows. And some potential impactors are to small to reach the surface intact. They break up in the atmosphere, and either a resultant shock wave pulverizes the surface or a blanket of fine material formed by the break-up produces a dark hallo mark before a crater forms. Wind has also modified the surface, creating wind streaks and what may be sand dunes.
Average distance from the Sun: 108.2 million km
Surface temperature: 464℃
Number of moons: 0
Observation: Venus is the brightest planet in Earth sky and surpassed in brightness only by the Sun and the Moon. It is maximum magnitude is -4,7. It is seen in the early morning or early evening sky.
Rotation period: 243 Earth days
Orbital period (length of year): 224.7 Earth days
Mass (Earth = 1): 0.82
Gravity at equator (Earth = 1): 0.9
Volume (Earth = 1): 0.86
Universe: The Definitive Visual Guide by Martin J. Rees (Editor). Published in 2007