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Venus: a very unique 'twin'

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Venus was once thought to be Earth’s twin because of their similarity in size, mass and proximity to the Sun: it is only slightly smaller than Earth (by 650 km (404 mi) at the equator), has a mass which is 85% of Earth’s, and is 72% of an AU away from Sun. It’s also a rocky planet just like Earth. However this is where the similarities end. For example it is much hotter than Earth, with a mean surface temperature of 462°C (863°F) - that's a mere 447°C (848°F) more than what we’re used to here on Earth! It's this high temperature that has won Venus the title of hottest planet in the Solar System, despite being further away from the Sun than Mercury. And there are other differences relating to surface pressure, its landscape and its movement in space.

How did Venus get so hot?

It is thought that once upon a time (some billions of years ago) Venus had a lot of water, however the consensus is now that the water never liquefied. Water existed as vapour and started the runway greenhouse effect that created the hot Venus we know today. The water vapour locked in heat which in turn helped release carbon dioxide (CO2) from rocks, and this created a positive feedback. Most of the water in the atmosphere has since disappeared due to a process called ultraviolet dissociation (UV photons dissociate water molecules, separating the individual atoms, in effect breaking water molecules into their separate components and allowing the hydrogen atoms to escape into space). Venus now has a dense atmosphere consisting of 96% CO2 which is stopping heat from escaping. Some heat from the Sun does get through the cloud cover and then gets trapped. This process has created a planetary ‘oven’. The dense atmosphere also has a high cover of sulphuric acid clouds, making optical observations of Venus’s surface practically impossible. However probes on the surface and radar imaging have given us a glimpse of Venus’s surface.

Venus (radar composite image)

Venus (radar composite image).

The Venusian surface

Venera 7 was the first probe to successfully land on Venus and to send back images of the very interesting Venusian surface. Though we share similar features such as volcanoes and craters, Venus has more and does it better! The Venusian landscape is mostly made of volcanic plains. It has more volcanoes than any of the planets in the Solar System, and 167 of them are more than 100 km (62 mi) wide. The impact craters are impressive too with none of them being less than 3 km (2 mi) wide. Venus also has a high mountain, Maxwell Montes, which is taller than Everest by a couple of kilometres (roughly 1.2 mi) but only half the size of Olympus Mons on Mars.

As well as having a terrain that is not great for a hike, the surface pressure (the pressure exerted by the mass of the atmosphere on the surface of the planet) also adds to the difficulty as it is 90 times that of Earth. It’s the equivalent of being almost 1 km (0.62 mi) under sea level on Earth, except that you would need a marine research submersible to reach and survive such depth.

Venus in space

Again here Venus is very different to Earth. It doesn’t have a moon. It has a very small tilt of only 3 degrees, compared to Earth’s tilt of 23.5 degrees, which means that there are no seasons on Venus. Its orbit around the Sun is inclined relative to Earth’s orbit making transits rare. Venus spins on its axis clockwise whereas Earth, and most planets in the Solar System, spin counter-clockwise. It also spins very slowly. Earth looks like an Olympian ice skater compared to Venus, with a spin speed of 1,669.8 km/h (1,037.6 mph) to Venus’s 6.52 km/h (4.05 mph). In fact Venus is the slowest rotating planet in the Solar System. This means that a Venusian day is longer than a Venusian year, which is 224.7 Earth days … that would be one very long Monday!

Observing Venus

Observed from Earth, Venus has a path resembling a pentagram and has an apparent magnitude of -4.6. It is called the Evening or Morning Star, though of course it is not a star but being so close to the Sun inside of Earth’s orbit it appears very bright just after sunset or before sunrise. Occasionally it transits the Sun’s surface as seen from Earth. Venus’s transits are paired with an interval of 8 years between them. However, the next pair of transits will occur in 2117 and 2125… a fairly long time from now!

Considering all of the above, it’s pretty clear that Venus is not a potential contender for viable human colonisation. While we still have a lot to learn about Venus, there are no missions planned to go back there in the near future. NASA and Roscosmos may partner up for a Venera-D mission, but the project it still in the study stage. NASA also had two other potential missions (DAVINCI and VERITAS) in the pipeline, but they seem to have been scrapped for now.

For the time being we’ll just have to enjoy the evening star through a telescope, and let it remain a place of wonder and mystery.

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