Regolith

Photographs have been taken at four places on the hot surface of Venus. They show basaltic lava bedrock and debris in varying proportions. From Venera 9 to Venera 14, the share of rock grows and the share of debris shrinks.

On Earth we call the material that covers the surface either rock or soil (in everyday language, earth). “Soil” is a mixture of inorganic and organic matter. Without organic matter, it is called rock debris or regolith. It may be dust, sand, water-worn gravel, sharp-edged debris, or a mixture of these. The organic matter is humus. On a compost heap we can watch discarded food waste turn into fertile earth: soil, or rather the organic component of soil. This is the ingredient missing from all other bodies of the Solar System. This is why we call the granular material that covers the surface of Venus regolith, or rock debris, and not soil.

Regolith forms by chemical weathering, mechanical breakdown, erosion and deposition. It comes originally from bedrock exposed somewhere. It also comes from material that reaches the surface through condensation, impacts or volcanic eruptions. The zone above the bedrock of a planetary body is called the megaregolith. It may be several kilometres thick. There, for example, impacts large and small have been mixing the original crust, broken many times into breccia, for billions of years.

The regolith of Venus is special because it formed in the hottest environment, with the highest air pressure, in the Solar System. The environment would suggest that weathering is fast and intense. Observations show the opposite: it may be minimal, since radar images show hardly any trace of fine debris (dust, sand) (Dyar et al. 2020).

Venus also lacks a megaregolith, because its surface is young. Instead, it is covered by lava layers, the traces of intense volcanism. Because of the thick atmosphere, the small impacts that could pulverise and break up the surface do not reach it, nor do the micrometeorite impacts typical of the Moon. Deflation (blow-out by the wind) does not seem very effective either. All surface measurements so far have recorded only very light breezes (4 km/h).

Above all, and perhaps most importantly, the hot surface means that there is no water at all on the planet to help weathering or erosion. At such high pressure, explosive eruptions that produce ash and coarse debris are also unlikely – although traces thought to be pyroclastic deposits can be seen in a few places.

What remains is the debris made by impacts, including fine-grained impact material. The material of the rare sand dunes may also come from impacts. The most likely agent for moving material is the wind of the shock wave that sweeps across the surface at impact. Finally, according to Kreslavsky and Bondarenko (2017), one fact seals the fate of loose, easily moved sediments: on the hot surface, fine-grained materials soon fuse together.