Venus has a violent past

Earthquakes and volcanic eruptions caused by continental drift, and an ocean that is no longer there

At one time, perhaps hundreds of millions of years ago, the ground on Venus shook violently. Earthquakes and volcanic eruptions were the order of the day, activity that results from the collisions of tectonic plates floating in an aqueous ocean. In fact, it would have been a process analogous to the continental drift on Earth which led to the rocky upland areas of our twin in the Solar System. This is the fascinating hypothesis that is taking shape now that the ESA probe Venus Express has completed the first map of the southern hemisphere of Venus. In the light of this discovery, that small point shining in the night sky seems to be even more like our planet, not only in terms of size and solid composition, but also in terms of its geological past.

The map of Venus (pictured below) was obtained by combining thousands of individual images taken by VIRTIS (the Visible and Infrared Thermal Imaging Spectrometer), an infrared camera largely designed and produced in Italy by researchers from the INAF and Galileo Avionica on behalf of the Italian Space Agency. The blanket of clouds that covers Venus prevents normal optical instruments from photographing the surface of the planet, whereas VIRTIS is able to ‘pierce’ the deadly and irreversible greenhouse effect that makes the planet so hot and inhospitable to life.

The data collected by the spectrograph allow us to study the chemical composition of the rocks much more accurately than radar observation systems allowed in the past. The first analyses of the data suggest that the mountainous formations of the Phoebe and Alpha Regio plateaus were the result of adjoining plates colliding.

Using the intensity of the infrared emissions, a measure of the ground temperature, it is possible to note on the map that the upland areas are paler in colour and older than the rest of the surface. On the Earth these rocky formations are usually made of granite, which is formed when ancient basalt rocks intrude the ground as the result of continental movements. Granite is formed when water combines with the basalt, and then comes to the surface by volcanic eruptions.

“If there is granite on Venus, it would mean that there was once an ocean and proves the presence of plate tectonics,” said Nils Müller, from the Joint Planetary Interior Physics Research Group of the University of Münster, who led the mapping. “The only way to be certain that these are in fact continental upland areas is to send a probe up there.” If there ever was water on Venus, today however it is no longer there. There could still be volcanic activity on Venus, although Venus Express has not registered seismic shocks so far.