Geology of Venus - Surface Processes

Surface Processes

Water is almost nonexistent on Venus, and thus the only erosive process to be found (apart from thermal erosion by lava flows) is the interaction produced by the atmosphere with the surface. This interaction is present in the ejecta of impact craters expelled onto the surface of Venus. The material ejected during a meteorite impact is lifted to the upper atmosphere, where winds transport the material toward the west. As the material is deposited on the surface, it forms parabola-shaped patterns. This type of deposit can be established on top of various geologic features or lava flows. Therefore, these deposits are the youngest structures on the planet. Images from Magellan reveal the existence of more than 60 of these parabola-shaped deposits that are associated with crater impacts.

The ejection material, transported by the wind, is responsible for the process of renovation of the surface at speeds, according to the measurements of the Venera soundings, of approximately one metre per second. Given the density of the lower Venusian atmosphere, the winds are more than sufficient to provoke the erosion of the surface and the transportation of fine-grained material. In the regions covered by ejection deposits one may find wind lines, dunes, and yardangs. The wind lines are formed when the wind blows ejection material and volcano ash, depositing it on top of topographic obstacles such as domes. As a consequence, the leeward sides of domes are exposed to the impact of small grains that remove the surface cap. Such processes expose the material beneath, which has a different roughness, and thus different characteristics under radar, compared to formed sediment.

The dunes are formed by the depositing of particulates that are the size of grains of sand and have wavy shapes. Yardangs are formed when the wind-transported material carves the fragile deposits and produces deep furrows.

The line-shaped patterns of wind associated with impact craters follow a trajectory in the direction of the equator. This tendency suggests the presence of a system of circulation of Hadley cells between medium latitudes and the equator. Magellan radar data confirm the existence of strong winds that blow toward the east in the upper surface of Venus, and meridional winds on the surface.

Meteor impacts on Venus have occurred for the last hundreds of millions of years. The superposition of lava flows can be noted. Radar reflection from the oldest lava flows, covered by the newest flows, present distinct intensities. The oldest flows reflect less than the plains that surround the flows. Data from Magellan show that the most recent flows are similar to ʻaʻa and pāhoehoe. However, the oldest lava flows are darker and look like deposits in arid regions of the Earth that have suffered meteor impacts.

Chemical and mechanical erosion of the old lava flows is caused by reactions of the surface with the atmosphere in the presence of carbon dioxide and sulfur dioxide (see carbonate-silicate cycle for details). These two gases are the planet's first and third most abundant gases, respectively; the second most abundant gas is inert nitrogen. The reactions probably include the deterioration of silicates by carbon dioxide to produce carbonates and quartz, as well as the deterioration of silicates by sulfur dioxide to produce anhydrate calcium sulfate and carbon dioxide.

One of the most interesting characteristics of radar images is the diminishing of reflection at high altitudes, exhibiting extremely low values beyond a radius of 6,054 kilometres (3,762 mi). This change is related to the diminishing of emission and temperature at high altitudes.

There are various hypotheses for the unusual characteristics of Venus' surface. One idea is that the surface consists of loose ground with spherical hollows that produce an efficient reflection of radar. Another idea is that the surface is not smooth and is covered by material that has an extremely high dielectric constant. Yet another theory says that the layer one metre above the surface is formed by sheets of a conductive material such as pyrite. Last, a recent model supposes the existence of a small proportion of ferroelectric mineral.

Ferroelectric minerals exhibit a unique property at high temperatures: the dielectric constant increases abruptly, yet as the temperature increases further, the dielectric constant returns to its normal values. The minerals that could explain this behaviour on the surface of Venus are perovskite and pyrochlores.

Despite these theories, the existence of ferroelectric minerals on Venus has not been confirmed. Only in situ exploration will lead to an explanation of such unresolved enigmas.