Star System Sol - Planet Venus: Earth's 'evil twin'
Above: the cloud-obscured orb of the planet Venus. The thick layer of clouds consists principally of an aerosol of sulphuric (sulfuric) acid! The sulphuric acid is formed by the action of sunlight on sulphur dioxide and subsequent chemical reactions. The sulphur dioxide is possibly emitted by volcanic eruptions. Cloud map courtesy of NASA.
Above: the surface of the planet Venus, as seen beneath the dense cloud layer. The surface is probably grey to black, but the yellow light filtering through the clouds gives it a yellow appearance. The large light-coloured feature is a highland area called Aphrodite Terra (what I like to call The Dragon!). (Image map courtesy of NASA/JPL).
type: large terrestrial (0.82 Earth masses)
Equatorial Radius: 6052 km (0.95 Earth radii)
Equatorial Diameter: 12 104 km
Orbit: rotation axis inclined 177 degrees to ecliptic, retrograde rotation (the Sun rises in the West and sets in the East), day is 243 Earth days in length. The day is longer than the year.
Atmosphere: about 96.5% carbon dioxide, 3.5% nitrogen, variable traces of sulphuric acid, 90-100 Earth atmospheres of pressure. A 20 km deep cloud layer extends from 45 to 65 km altitude. It rains sulphuric acid on Venus, but due to the immense surface temperatures, the sulphuric acid presumably decomposes before reaching the surface. Cloud-to-cloud lightning is frequent in the atmosphere.
Surface Temperature: average 460 degrees C; hottest planet in the Solar System due to a runaway greenhouse effect.
Magnetic Field: Venus has almost no intrinsic magnetic field. Interactions between the solar wind and the planet's ionosphere induces a weak magnetic field.
Natural Satellites: none.
Earth's evil twin?
Venus is the closest planet to the Earth (apart from the Moon which is a planet-sized satellite of the Earth). Venus is the closest planet, in the Solar System, to the Earth in terms of diameter and mass. Venus also has a dense atmosphere with clouds and lightning, but their the similarity ends! Venus' atmosphere consists almost entirely of carbon dioxide and is about 92 times as dense as the Earth's atmosphere. This very dense carbon dioxide atmosphere traps in heat from the Sun's rays, raising the surface temperature to incinerating temperatures. If you stepped onto the surface of Venus, then you would be squashed flat as a pancake and incinerated instantly! You probably wouldn't have time to even notice the sulphurous fumes and suffocating atmosphere.
Above: a color-coded map of Venus, red indicating high altitudes, blue low altitudes. This map gives us an idea of what Venus might look like without its punishing greenhouse effect, with oceans and vegetation and continents. Indeed, maybe it was like this once?
Above: a Pov-Ray simulation of the surface of Venus - dark volcanic rocks bathed in yellow-orange light.
Above: intercloud lightning.
Structure of Venus
Venus is thought to have a similar structure to the Earth - with a hot silica-rich mantle and an iron-rich core. The core is predicted to be just slightly larger than that of the Earth.
Above: a model of the predicted structure of Venus.
Venus has many channels etched into its surface, channels that were formed by flowing fluid. these channels meander in much the same way as river channels on Earth with meander loops getting cut-off to form oxbow lakes as the meanders moved over time. These channels are often obscured by later volcanic activity, such as massive lava flows and may run up and down slope as the rust was later buckled by tectonic activity.
Above: the longest known channel in the Solar System, a river channel 7000 km long (the ends have become obscured by later geological activity and so originally this channel was even longer!) showing a 600 km segment; the channel is about 1.8 km wide. This channel was probably etched by flowing lava and similar (but smaller) channels are common on Venus. Image courtesy of NASA/JPL-Caltech/Arizona State University; https://www.jpl.nasa.gov/images/venus-600-kilometer-segment-of-longest-channel-on-venus
Above: a sinuous channel on Venus. On Venus channels generally meander less than river channels on Earth. River channels meander because of the dynamics of flow, but the size (amplitude) of the meanders is largely dependent on the material making up the bed and walls of the channel. The amplitude of the meanders is a major determinant of sinuosity. Image courtesy of NASA/JPL-Caltech/Arizona State University; https://www.jpl.nasa.gov/images/venus-sinuous-channel
Venus is an unlikely place to find living things. The sulphuric acid is not the problem - there are bacteria on Earth which excrete and thrive in sulphuric acid. The immense pressure is not a problem - although it would splat you flat, there are creatures at the bottom's of the Earth's oceans who survive much greater pressures (1100 Earth atmospheres!). These creatures are not crushed by the immense pressure since they are adapted to these conditions and the pressure inside their cells is just as great, thus counterbalancing the external pressure upon them. They also have special enzymes, since certain chemical reactions (those involving a tiny increase in volume) are inhibited at such pressures. The problem is the immense temperature. Even if liquid water existed under pressure, the heat would tend to disrupt complex molecules. However, the clouds of Venus are cool and it is likely that certain bacteria can survive and grow within the clouds. We also must ask ourselves if conditions are so harsh deep down inside the planet's crust. However, even if a place can be found where bacteria-like organisms could thrive, this does not mean that they would have evolved there - the temperatures really do seem prohibitively high, but who knows?
Finally, we have to ask whether Venus was always this hostile to life. What happened, to make Earth's twin such a fierce world? Was Venus once thriving with life?