The enigma of Venus

Venus has historically been regarded as a“ twin planet ” to the Earth as amongst the planets, it is closest to the Earth in mass, density, size and in distance from the Sun. However it has, by terrestrial standards, extraordinary crustal features and a geo-logical history that bears little resemblance to that of the Earth. In addition, it does not possess a satellite and has a retrograde rotation with a period of 243 days.

The planet clearly warrants closer study particularly as the differences between these twin planets emphasize the problems of building crusts or discovering habi-table planets in other planetary systems.The density of Venus (5. 24 g/c m³) is about 5% less than that of the Earth(5.514 g/cm³). This difference is mostly due to the slightly lower internal pressures as the planetary radius is 320 km less than that of the Earth. But the uncompressed density of both planets is very close (Earth 3.96 g/cm3; Venus 3.9 g/cm³). The similar density of Venus to the Earth and the presence of a basaltic crust on the planet are the basis for assuming a broadly similar composition and internal structure.

From analogy with the Earth, a metallic core is expected, but the lack of data for the moment of inertia precludes estimates of its size. Although various bulk planetary compositions have appeared in the literature, these are model dependent with the status of inspired guesses. Because of thedensity similarity, there is probably little real difference in the major element 181 composition (Si, Ti, Al, Fe, Mg, Ca) of Venus compared to that of the Earth. Until more data are available, the best guide to the bulk composition of Venus is that of the Earth.

The abundances of the heat-producing elements, as discussed later, are probably similar in both planets. The MgO contents of the venusian basalts are around 10% and the Mg# is about 70. This indicates that mantle temperatures are similar to those of the Earth and are not hot enough in Venus to produce MgO-rich lavas analogous to terrestrial magnesium-rich komatiites (MgO ≈ 18%). An important caveat in all such comparisons is that Venus was resurfaced at about 750 Myr and accordingly surface geochemical measurements on Venus reflect igneous activity from that event or subsequently.

Measured surface temperatures, cooler at higher altitudes, range from 453 °C to 473 °C, hot enough to melt not only lead but also tellurium (m.p. 449.5 °C). The surface pressure of 95 bar is equivalent to the pressure on the Earth beneath about 1 km of seawater. The crustal thickness (the best estimate is about 30 km) appears to be relatively uniform in contrast to the varying crustal thicknesses observed on the Earth, Moon or Mars so that the venusian crust is similar in this respect, albeit thicker, to the uniform oceanic crust of the Earth. A uniform crustal thickness of 30 km represents approximately 1.5% of the volume of the planet.

However there are major differences between the crust of Venus and the Earth. Venus lacks the bimodal distribution of elevations of the terrestrial crust, while the venusian crust is strong and, in contrast to the Earth, displays a positive correlation between gravity and topography with gravity values reaching 70 mgal.

Compared with the Earth, the rocks are much stronger, a consequence of the effective absence of water.