Inhospitable planet only beautiful from far, far away

Venus is one of the most beautiful sights in the sky.

That brilliant, white spark in the morning or evening star got it referred to as Phosphorus "The Morning Star" and Hesperus "The Evening Star". The beauty of the planet got it to be named after Venus, the Goddess of Love.

However, we now know that Venus is one of the most hostile places in the Solar System. Under a deep layer of cloud is a torrid surface hot enough to melt lead and tin and the atmospheric pressure is around 90 times the atmospheric pressure on the surface of the Earth.

The survival record for a lander sent to the surface is around 20 minutes. Radar images of the surface show undulating terrain with many volcanoes and lava flows. Some of the volcanoes are old and probably extinct, but there are others that appear to be active.

Some of the volcanoes are flat domes, others resemble pancakes and some are long fissures that are erupting lava. To understand volcanoes on Venus and on our world we need to go back the two planets' early history.

Around 4.5 billion years ago, Earth and Venus were balls of hot, molten rock. Over time the heavy materials such as iron and nickel and rocks containing them sank towards the middle and the lightest stuff, a scum of silica (sand) and aluminium minerals accumulated on top.

The Earth also had a surface layer of water. It looks as though Venus never got cool enough for water to accumulate. The water plays an important role in plate tectonics. As far as we have found so far, Venus shows little sign of plate motions. There is, however, a heaving and cracking of the surface as magma moves around inside the planet.

On our world we have two main kinds of volcano. One has steep cones and erupts explosively. Mount St. Helens and Vesuvius (the volcano that buried Pompeii) are examples. Mount Krakatoa was another. In 1883, it exploded in one of the biggest explosions in recorded history. The volcano completely destroyed itself. A "Son of Krakatoa" is now slowly building.

The other kind of volcano forms a much flatter hill, erupting much more gently, producing huge lava flows in eruptions that can continue for decades. These are known as shield volcanoes. The Hawaiian Islands were formed in this way. A new island is forming but has not yet emerged above the sea.

The explosive volcanoes form above subduction zones, where one tectonic plate is pushed down under another. Seawater and silica from the surface gets carried down, where it melts and combines with the molten rock producing a viscous, sticky lava pervaded with highly-compressed superheated steam. The sticky lava plugs the volcanic vent. The steam pressure builds until the volcano explodes, showering lava powder (ash), superheated gases and lava over the surrounding land.

Then, gradually the volcanic cone starts to rebuild. The lava forming the shield volcanoes comes from deep down, and contains little silica and no water. It runs freely and can cover large distances before solidifying.

Many of the volcanoes on Venus are huge shield volcanoes. This would be expected, with no plate tectonics and no water getting added to the molten rock. Some of the volcanoes show evidence of viscous lava. Probably, over time, the silica-rich material on the surface got buried deeper and deeper under lava flows, until it joined some of the underground magma, making it viscous.

However, without the superheated steam, the eruptions would usually not be explosive. With the high surface pressure and temperature the lava would remain runny for longer.

Venus is a fascinating world, like ours in some ways, but in other ways bizarre and hostile. Manned visits are unlikely.

•••

• Jupiter has disappeared in the sunset glow, leaving Venus shining brightly.

•Mars lies high in the south. Saturn is very low in the sunrise glow.

•The Moon will reach its first quarter on March 28.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.