Monday, 6 February 2012

Venus – Hell Planet

Venus Data
Mass: 4.87 x 1024 kg or 0.82 of Earth’s
Diameter: 12 104 km or 0.95 of Earth’s
Surface gravity: 0.90 gees
Axial tilt: 177.4°
Mean surface temperature: 480 Celsius
Rotation period: 243.02 days or 0.66 years
Orbital period: 224.70 days or 0.62 years
Inclination of orbit to ecliptic: 3.4°
Orbital eccentricity: 0.007
Distance from the Sun: 0.72–0.73 AU
Sunlight strength: 1.9 times Earth’s
Satellites: 0

The Solar System’s second planet is Venus, which moves in an almost circular orbit at an average distance of 0.72 AU from the Sun. With a size, mass and density comparable to those of the Earth, Venus has often been thought of as our planet’s twin. But in virtually every other way the two planets are about as similar as Heaven and Hell – with Venus a good substitute for Hell. Like Mercury, Venus has no moon. But it does have a substantial atmosphere – a choking shroud of almost pure carbon dioxide. So dense is this waterless sky that it pushes down on the surface at a pressure 92 times that on Earth – equivalent to that at the bottom of a lake 900 metres deep. A global greenhouse e¤ect keeps the surface temperature twice as hot as a domestic oven. And, high above the surface, thick clouds of battery acid permanently block Venus’ terrain from optical telescopes. Only radar can penetrate the clouds. The results have revealed a geologically young surface, less than 500 million years old, replete with volcanic features but with relatively few impact craters, compared with the Moon, Mars and Mercury.


Only slightly smaller than the Earth, Venus is easily massive enough to exert a gravity capable of clinging to an atmosphere. But thick clouds cover 100 per cent of Venus’ surface. Early astronomers, unable to penetrate the planet’s permanent blanket, had only their imaginations with which to paint this world. They envisaged a lush, fertile swamp-planet, full of strange creatures perhaps resembling alien dinosaurs. They were completely wrong.

Probes have revealed that the chief constituent of Venus’ atmosphere is the poisonous gas carbon dioxide, 96.5 per cent by volume. The rest is mainly nitrogen, and there are minute traces of sulphur dioxide, argon, hydrogen and water vapour. This is a substantially different composition from the nitrogen–oxygen-rich skies of Earth. Perhaps the most impressive trait of Venus’ dry atmosphere, though, is its incredible density. It harbours enough gas to make almost 100 atmospheres as thin as Earth’s. And the weight of that gas is formidable. At the surface, the atmosphere exerts a pressure of more than 90 kilograms on every square centimetre. You’d need a spaceship built to withstand submarine environments just to survive there.

Step onto the surface in a spacesuit and you’d be crushed instantly. And, as if that were not enough, the ship would have to be exceedingly well insulated too, to shield its occupants from the devastating temperatures outside. Despite the fact that Venus is much further from the Sun than Mercury, Venus has the hottest surface in the entire Solar System. On average the temperature is around 480 Celsius – enough to melt tin, zinc and lead. The reason for this is the carbon dioxide. Carbon dioxide is a so-called greenhouse gas. It lets in sunlight, but is opaque to the infrared radiation that comprises heat. Venus has become a heat trap.

Venus’ clouds are also unlike Earth’s. They are composed mainly of concentrated sulphuric acid, often used in batteries. The highest cloud layers float 65 kilometres above the surface – more than four times higher than on Earth. These clouds scoot around at speeds of more than 350 kilometres per hour, driven by the planet’s slow rotation. It takes Venus a leisurely 243 days to spin once on its axis. As the sun-facing hemisphere warms up, a high-pressure region develops there, which creates winds that blow clouds around the planet in only four days. At first, astronomers thought that the planet itself was rotating at this speed. But when they glimpsed the surface with radar, they discovered its true rotation period – and more besides.

Beneath the Clouds

Most of Venus’ surface consists of rolling planes and lowlands – the 80 per cent of the terrain that has a relief of less than 1 kilometre. Highlands, where the altitude rises more than 2 kilometres above the mean planetary radius, make up the remaining 20 per cent.

Superimposed on this landscape, by far the most common features on Venus are its volcanoes. There are no obvious signs that the planet is still active, but it might well be – and certainly was in the very recent geological past. There are large shield volcanoes like the Hawaiian islands, volcanic domes, extensive lava flows and volcanic craters called calderas. Calderas are created when magma chambers beneath the crust run out of magma (molten rock), and the surface above caves in and collapses. Among the most interesting volcanic features are those that have no analogue on Earth: the pancake domes are an example. They are flat-topped, circular patches of lava that oozed out of the crust, solidified and cracked as they cooled. So-called coronae are also volcanic in nature. These are regions where rising subterranean magma has pushed up the surface and cracked it
but not penetrated it – they look somewhat like blisters. Volcanoes are everywhere on Venus, not just concentrated in lines as they are on Earth. This means that Venus’ surface is not broken up into plates like Earth’s, with volcanoes populating the plate boundaries. Nevertheless the crust does show signs of cracking and stretching that indicates some lateral motion – tectonic activity.

Venus also has its share of impact craters, but they number less than 1000 in total and so are not as common as on Mercury. The reason is that the planet’s surface is geologically young. Volcanism has destroyed all but the earliest craters – none from the heavy bombardment has survived to this day. Those craters that exist now were formed only in the last 10 per cent of the planet’s history. Also, most of these craters are large: none is  smaller than 3 kilometres, and few are smaller than 25 kilometres. The atmosphere is to blame for this. It is so thick that all but the largest meteorites are either completely destroyed or broken up into smaller bits before they hit the hostile surface.

Evolution of Venus

So why is Venus so hostile? After all, its physical dimensions, mass and density are very comparable to those of our own world. It even formed in a part of the Solar Nebula not too dissimilar from that where Earth formed. And because of this it is quite likely that Venus originally had a lot more water than it does now. Calculations show that the planet may have had enough water to form a global ocean 25 metres deep. This is still a lot less than Earth’s own watery oasis, but it is very much greater than the amount of water left on Venus today. Crucially, however, Venus is a lot closer to the Sun than Earth is. More than anything else, this proximity is the reason for the planet’s gradual evolution into the Hell we see today.

The newly formed Venus was very volcanic, as were all of the new terrestrial planets. Global episodes of volcanism vented huge quantities of water vapour, carbon dioxide and other gases to form the planet’s atmosphere. Because Venus is so much closer to the Sun than Earth, it was almost certainly always too hot for any of its water to fall as rain – or at least to pool on the surface. Thus, from the very start, Venus lacked oceans and most likely even seas. This would deal a fatal blow to the planet’s prospects for hospitability, because carbon dioxide is soluble in liquid water. On Earth, where it was cool enough for water to pool on the surface, this poisonous gas got dissolved in our oceans, and was thus removed from
the atmosphere. But because Venus never had any substantial oceans, if any, it was unable to cleanse its atmosphere of the carbon dioxide vented by volcanoes. So the gas remained in the atmosphere in great quantities, trapping the planet’s heat, and the planet grew hotter. The heat in turn baked more and more water vapour and carbon dioxide out of the soil and rocks. The Sun’s deadly ultraviolet rays destroyed the water vapour (Venus lacks a protective ozone layer), the leftover carbon dioxide piled up, and the planet grew hotter still… This sad state of affairs is known as a runaway greenhouse effect. Earth was lucky. If it had been just 5 per cent closer to the Sun, it too would have gone the same way, and I would not be here now telling you this.

Aside from Venus’ atmosphere, very little is known about the planet’s early history. Its oldest impact craters are less than 800 million years old. And on average the surface is 300–500 million years younger even than that. The crust has been completely recycled since it first formed billions of years ago, the handiwork of planet-wide volcanism. But, despite the lack of clues to the past, one other aspect of the planet’s history seems certain, and evidence for it comes from the planet’s strange, slow rotation. Venus not only spins very leisurely, but it does so backwards compared with most of the other planets. It is hard to see how this state of affairs could have arisen naturally at birth. More than likely, the unlucky world was hit very early in its formation by a gigantic planetoid of comparable size, and knocked over nearly 180 degrees.

Both Mercury and Venus have suffered enormous impacts. Soon it will become apparent that quite a few other planets are the product of chance cosmic encounters, too. In fact the Earth is one of them – as we shall see next.

Source :
Mark A. Garlick. The Story Of The Solar System. University Press: Cambridge. 2002.



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