Introduction

Venus is an intriguing place.  It is our closest planetary neighbour.  It some ways it is like the Earth. It has a similar density, size and mass.  It is a similar distance from the sun.  Unlike the Earth, there is no water and the greenhouse effect takes surface temperatures to a scorching 450°C (842°F) day and night. 

Exploration

This planet is a punishing place to explore.  It is enveloped in a dense carbon dioxide atmosphere.  Carbon dioxide contributed to a greenhouse effect that warms the planet. Corrosive clouds contain sulphuric acid. The surface pressure is 90 times that of Earth, the surface temperate is about 450°C (842°F). Moderate winds brace the surface. At altitude winds circulate the planet every four days at an average speed of 500 km/hour (311 mph).

Radar Studies

We can not see through the atmosphere using a conventional telescope.  We can not make a controlled descent to the surface because of the high winds.  When our spacecraft reaches the surface it fails to function after a few hours because of the harsh conditions.

Our explorations rely upon the ability for radar to penetrate to the surface. In 1961 NASA began to study Venus from the Goldstone Observatory using radar.  Late,r between August 1990 and September 1994, the Magellan probe circled the planet and used radar to map the surface.  

The Surface

The surface of Venus is relatively flat and barren.  Bare rocks cover three quarters of the surface.  There are extensive lowlands.  Four fifths of the topography lies within 1 kilometre (0.6 mi) of the median radius. There are large inclined plains.  Some ten percent of the surface is mountainous. 

The Impact Craters

Unlike the moon, Mercury or Mars the surface of Venus has comparatively few impact craters.  Whereas small meteors burn up in the dense atmosphere large meteors ought to leave their mark on the surface.  The rarity of meteorite craters suggests that the surface of Venus is relatively young. Geologists suggests an age of 300 – 500 million years old. Their interpretation is that there must have been a major reforming event that wiped out the earlier impact scars.

The lack of craters is a puzzle.  Some scientists use it as evidence that the surface of Venus is relatively young.  Their view is that the surface underwent a major makeover about 300 – 500 years ago.

Vulcanism

The surface of Venus is dominated by volcanic activity. 

Some four fifths of the planet is covered with volcanic lava plains, dotted with more than a hundred large isolated shield volcanoes. These are relatively flat volcanoes that reach an average height of 1.5 kilometres (0.9 miles) and extend over hundreds of kilometres in area. 

Smaller volcanoes less than 20 kilometres (12 mi) in diameter are widespread.  There are hundreds of thousands, perhaps millions of these smaller volcanoes on Venus.

Some volcanic features found on Venus do not have counterparts on Earth.

There are huge ring shaped structures called coronae. These are 100 – 300 kilometres (60 – 180 miles) in diameter and rise several hundred metres above the surface. They are believed to form when hot material from the mantle pushes the crust upwards into a dome shape. When the dome collapses the molten lava cools and leaks out the sides.

Strange patterns form where the magma extrudes to the surface. Sometimes these resemble cobwebs.  Novae form when large quantities of magma extrudes onto the surface to form radiating ridges and trenches. Arachnoids form concentric ovals surrounding complex network of radial fractures similar to the novae.

Very large lava flows are common. Over 200 channels have been identified. These can be up to several hundred kilometres long and tens of kilometres wide.

Source: Wikipedia An arachnoid feature on the surface of Venus

Tectonic Activity

There is no conventional plate tectonics on Venus.  There is evidence of neither continents nor great mountain ranges. The planet lacks an asthenosphere.  This is the layer of low viscosity material that lubricates the whole process of continental drift on Earth.  

Compared with the Earth, the surface of Venus is more akin to the skin on a hot bowl of custard.  Deformations in the surface of Venus are directly related to the convective movements within the planet.

The planet is fissured and folded reflecting the underlying convection currents.

Source: Wikipedia Computer generated image of lava flow on Venus

Surface processes

Venus has a dry surface so there is no opportunity for water based erosion.

The Venusian wind is erosive.  Scour marks and sand dunes have been found.  

A particularly unusual feature forms behind the impact craters. Material thrown up by a meteor impact reaches the upper atmosphere where winds transport it to the west. When material reaches the surface it forms parabolic shaped patterns. Over sixty of these structures have been found.

Heat Loss

The reason for the major makeover lies in the interior of the planet. Venus struggles to remove heat from the interior.  Although the exact structure is not understood the planet is believed to contain a radio-active core, as does the Earth. This core generates considerable heat.  

The Earth generates a magnetic field and has an active plate tectonic system.  These processes are not present on Venus.  

On Venus the heat builds up in the interior until the mantle is hot enough to force its way to the surface.  The subsequent resurfacing event covers most, or all, of the planet with lava, until the mantle is cool enough that the process starts over.

Venus is an intriguing place. It gives us an insight into a place dominated by global warming.

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