Lava polygon

In places a polygonal pattern breaks up the rock of the lava plains.

On Earth, polygonal ground is known from drying clay surfaces. It also forms through freeze–thaw cycles, and in lava. On Venus only lava can explain polygonal areas. We know only those polygonal areas on Venus that can be made out on radar images with about 100 m resolution. So they are usually larger than the polygonal ground known from Earth or Mars.

Giant polygons are 1–3 km across, and they often lie near coronae or shield fields. A polygonal area may be hundreds of kilometres across (see the plate). Polygonal patterns have been found in more than 200 places (Smrekar et al. 2002). Crossing fractures can also cut the surface into polygons (here, rectangles).

The simplest explanation of lava polygons is the slow cooling of a freshly emplaced lava surface. The contracting lava builds up uniform (isotropic) stresses. These are released in cracks that form a polygonal pattern. This is also how basalt columns form: long columns of basalt split apart along polygonal cracks.

On Venus this theory runs into a problem. Calculations show that polygons of the observed size would need a cooling lava layer at least 20 km thick. This is not a realistic assumption. So another explanation is needed.

Giant lava polygons may also form when stress builds up between the lower and upper parts of a lava layer. The cause would be growing heat from below, for example from a mantle diapir. Warming then creates compressive forces deep in the layer and tensile forces in its upper part. The tensile forces open extensional fractures. For this model to work, the geothermal gradient must be 3°C/km. Its value on Venus is unknown; on Earth it is 30°C/km.

Finally, the polygons may come from temperature changes caused by climate. Models show that volcanic eruptions that release volatiles (gases) into the dense atmosphere can change the surface temperature a great deal. A global lava flood 1–10 km thick would release enough water and sulphur dioxide to change the temperature by 90–200°C. The thickening cloud layer would cool the surface at first. Once the water and sulphur dioxide had dispersed, the clouds would thin, and the temperature would rise for a while. These temperature changes slowly reach below the surface.

15% of the polygons on Venus are made of compressional ridges, so they may have formed, for example, during climate warming. If a single global event created the polygons, they could serve as a stratigraphic age marker.