Sean C. Solomon

Coupling between Climate Change and Tectonics on Venus

Planets have considerable internal energy, and the processes of energy transport and loss drive long-term changes. Understanding the differences in evolution among the planets of our solar system poses a considerable challenge. A particularly interesting puzzle has been Venus, in size and in bulk composition the planet most similar to Earth. One important difference in the two planets is their climates. The dense carbon dioxide atmosphere and global cloud cover have turned Venus into an efficient greenhouse, trapping solar radiation and rendering the modern surface temperature a searing 740 K. Another difference is in their volcanic and tectonic history. Lacking Earth-like plate tectonics, Venus shows evidence for widespread resurfacing of the planet about 500 million years ago as well as globally coherent episodes of deformation that appear to have been concentrated over short intervals of the preserved geological history. Recent work of mine, carried out with collaborators Mark Bullock and David Grinspoon at the Southwest Research Institute, suggests that these two aspects of the evolution of Venus may be strongly coupled.

Bullock and Grinspoon have shown that the climate of Venus is unstable with respect to a very large volcanic eruption, because such an event would release into the atmosphere significant quantities of water and sulfur dioxide from the erupting lavas. Water and sulfur dioxide are greenhouse gases that modify the radiative balance in the atmosphere; their abundance also affects strongly the distribution of sulfuric acid-water aerosols that make up the Venus clouds. There are important sinks for both species: photodissociation of water molecules and loss of hydrogen to space, and chemical reaction of sulfur dioxide with surface and near-surface minerals. Climate models incorporating all of these processes indicate that for a sufficiently large eruption the excursions in surface temperature accompanying changes to the greenhouse and to the global cloud cover can exceed ±100 K and can extend over periods from tens to hundreds of millions of years.

Such temperature changes over these timescales diffuse deep into the interior of a planet. Differential temperature changes of this magnitude expand or contract surface and near-surface material by amounts sufficient to fracture rock. Climate change on Venus is therefore capable of affecting the deformation of the surface and the global distribution of tectonic features.

We tested this idea by examining the climate change that would have followed the emplacement of the largest distinct episode of widespread volcanism known to have occurred on Venus: the eruption of the ridged plains. These plains, making up 60-65% of the present surface, are characterized by pervasive wrinkle ridges (Fig. 9), produced by horizontal shortening of the lithosphere. The volume of lavas that erupted to form the plains, on the basis of area and relief of buried topography, was at least 2 x 10⁸ km³, as much as an order of magnitude greater than even the largest of the major igneous provinces on Earth. Widespread ridge formation evidently occurred less than 100 million years after the plains were emplaced, on the grounds that only about 1% of impact craters on these plains have been deformed by the wrinkle ridges. The change in surface horizontal stress predicted to accompany the climate change that would follow eruption of the ridged plains lavas is of the correct sign and magnitude to account for a narrow interval of time between plains emplacement and wrinkle ridge formation (Fig. 10), as observed.

Many further tests are in progress. These include exploring the sensitivity of climate models to changes in key parameters, incorporating a fuller description of the known volcanic flux history of the planet, and exploring whether other apparently global episodes of coherent contractional or extensional deformation might also be the result of stresses induced by climate change. What is already apparent is that climate change on Venus has been much more pronounced than the "global warming" that heats political debates on this planet, and that the coupling between climatic and tectonic change may be a major contributor to the distinct evolutionary tracks of Venus and Earth.

Fig. 9. A radar image of ridged plains is shown for Rusalka Planitia, Venus. The image is about 155 km wide. Northeast-southwest contraction has produced the radar-bright, northwest-southeast trending wrinkle ridges, which postdate plains emplacement

Fig. 10. The predicted evolution of surface horizontal stress resulting from the climate change that accompanied and followed emplacement of ridged plains lavas is depicted here. The compressive stresses that accumulated in the first 100 million years of the model would have been sufficient to produce widespread faulting manifested in the formation of wrinkle ridges.