So similar yet so different – the story of Earth and Venus
To understand the dynamic processes of extraterrestrial planets, scientists need to understand the properties of their surfaces. The overall picture of Venus was taken by the Magellan mission in 1989–1994. It involved mountain plateaus and big volcanoes alike. Although the surface of Venus is primarily basaltic, like some parts of Earth, the slopes of shield volcanoes are lower and look more like long flows of basaltic lava.
The Magellan mission also confirmed the results of previous studies such as the lack of impact craters on Venus. This leads scientists to think that the crust of the planet is a lot younger than the one of the Moon or Mars. Why should this be the case? “The main reason is that the atmosphere of Venus is much denser than the one on Earth. This leads to stronger friction thanks to tidal deformation from the Sun. As Venus is rotating, it is getting deformed slowly and the gravity of the Sun is forming Venus into sort of an ellipsoid instead of a globe. Both Venus and Earth used to rotate with the same frequency, and both are slowing down as a result of their comparable mass and atmosphere. However, as the atmosphere of Venus is much denser, it is slowing Venus down much faster compared to Earth. Let´s suppose both planets had the same plate tectonics, the so-called inner convection of the material, which facilitated the distribution of the heat accumulated inside of the planet by the decay of radioactive elements. The original rotation of both planets formed a strong moment of inertia which functioned much like when you spin a flywheel. As long as it is spinning, it is stable and stands on its own but the moment it starts to slow down, it starts to vibrate and eventually falls down and stops spinning. The moment of inertia thus facilitates the stability of planets in space as long as they are spinning fast enough. If they slow down, their axis starts to lose its stability and spin in space. That is exactly what happened to Venus. As its rotation started to slow down because of the tidal braking, the rotational axis became unstable, and the north rotational pole started spinning to the south and eventually got fully to the south pole. That is why Venus is rotating slowly against the direction of all other planets,” explains the author of the study Gunther Kletetschka.
How is the rotation of Venus related to plate tectonics? “If a planet rotates with a significant rotational moment of inertia (like Earth), its inner convection is symmetrical around its rotational axis, and that facilitates the plate tectonics as we know it on Earth. However, if the rotation of a planet slows down enough for the planet to lose its rotational stability, its inner convection also loses its rotational symmetry and thus the plate tectonics stops. We suppose this is what happened to Venus around 800 million years ago according to calculations,” describes Gunther Kletetschka.
The surface of Venus is thus not as mobile as the terrestrial one. So far, no evidence of current tectonic activity on Venus has been found. Things used to be different though. The significant mobility of lithospheric plates on Venus is proved to this day by the massive thickening of the planet´s crust in some places, for example, the studied plane Lakshmi Planum.
A new study led by Gunther Kletetschka compares the geology of selected parts of Earth and Venus based on their gravitational models and their anomalies in such areas. The goal of the research was to fill a gap in research data, on the tectonics of Venus's surface. The gravitational model for Venus came from the Magellan mission, the one for Earth was from the GOCE mission that collected data in 2009–2013. Some problems arose with the resolution of these data. For the models to be comparable, scientists had to lower the resolution of the Earth model by several orders of magnitude.
Scientists have compared the areas of the Indo-Eurasian contact zone in the Himalayas area on Earth and a contact zone in the Lakshmi Planum area on Venus. Even though, previous studies have suggested that Lakshmi surrenders to its north part, current data show otherwise and compare this area to the passive border of the tectonic plate around the Indian peninsula. Other parameters investigate the deformations in the contact zone. They suggest that deformational processes in diverging plates are even milder on Venus than on Earth.