The surface of Venus has been seen by three methods:

1. Landings by a handful of Soviet spacecraft between 1975 and 1982

Copyrighted imageCredit: Used with permission
The first ever picture from the surface of Venus, by Venera 9 in 1975. The image is distorted by the camera optics. The horizon is visible in the top left and top right corners. Part of the lander can be seen near the lower edge. [Image: NASA]

2. Mapping the height of the surface by downward-looking radar from orbit
This is known as radar altimetry.

Copyrighted imageCredit: Used with permission
A colour-coded surface heights of one hemisphere of Venus (blue lowest, red and white highest). At the NASA Jet Propulsion Laboratory site you can find a similar, animated view of the planet.

3. Constructing an image by means of obliquely looking radar
This process is known as imaging radar.

Copyrighted imageCredit: Used with permission
A 350 km wide region of Venus, marked by volcanoes, lava flows and fractures [Image: NASA-JPL Photojournal]

Surprisingly, the distribution of impact craters across the surface of Venus is random, suggesting that the entire surface is all pretty much the same age, estimated at about half a billion years. If so, it seems that the whole globe was resurfaced at that time in a global orgy of volcanism, in which case the volcanoes and lava flows we can see today must be survivors from the waning phase of that episode, and are unlikely to be active today.

This behaviour represents a major difference between Venus and the Earth. Both planets are hot inside because of radioactivity. Earth gets rid of its heat continually, by 'plate tectonics': hot new crust is created at 'mid-ocean ridges' and old cold crust is drawn back into the interior at 'subduction zones' such as beneath the Andes.

A possible explanation for Venus's situation is that its internal heat is stored for hundred of millions of years until the old, cold surface breaks apart and is flooded by magma spilling out from the overheated interior. Then things calm down until the global catastrophe is repeated.

A rival theory that can explain the distribution of impact craters is that Venus's volcanism is on the wane but has continued, off and on, to the present day, creating random patches of lava a few hundred km across at sporadic intervals.

Take a closer look at aspects of Venus terrain by taking a virtual journey across the surface:

Volcanoes
Lava flows
Craters
Coronae
Fractures
Mountain belts

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