Venus is ‘quaking’: Clues to a seismically active planet

To advance our knowledge of seismic activity on Venus, an international team led by Dr Iris van Zelst and funded by the International Space Science Institute, embarked on a scientific quest. Their mission? To determine how we could predict and detect seismic activity on Venus, and to make a strong case that the ‘quest for quakes on Venus’ should be a scientific priority on the next generation of missions.

Venus, sometimes known as our sister planet or ‘twin’, is our closest planetary neighbour (at its closest approach) and the second planet from the Sun.

Earth and Venus are sometimes called twins because they are similar in size (Venus is almost as big as Earth) and composition, having formed in the same region of the inner solar system. Despite these similarities, Venus evolved very differently to Earth and there are very significant differences between the two planets. Venus might be better labelled as Earth’s ‘evil twin’.

Venus has no surface water and is a scorching, toxic and geologically active planet. It has the densest atmosphere of all the terrestrial planets in our solar system, composed primarily of carbon dioxide. This acts as a greenhouse gas, trapping in heat and driving surface temperatures to a blistering 465 degrees Celsius, hot enough to melt lead and making Venus the hottest planet in our solar system. The atmosphere also contains clouds of highly corrosive sulfuric acid, wrapping Venus in a thick, acidic haze.

Venus also rotates very slowly, and in the opposite direction to most other planets, including Earth. A single day on Venus (one full spin) takes about 234 Earth days, while a year on Venus (one trip around the Sun) is only about 225 Earth days, meaning that a day on Venus is longer than a year! 

There have been a number of successful missions to Venus (including gravity-assist flybys), and while we have learned a great deal about the planet, many mysteries still remain.

Over the next decade, there are plans for missions and probes to be sent to Venus to understand more about the planet, how it formed, its activity and climate. However, none of these missions are specifically designed to explore whether Venus experiences quakes.

There is growing evidence that Venus is volcanically active today, which suggests that the planet might also be experiencing quakes. On Earth, seismic activity provides a window into the planet’s otherwise inaccessible interior. The same could be true for Venus: detecting and analysing ‘venusquakes’ could help us to understand more about what’s happening inside Venus, in the same way that seismic studies have done for Mars and the Moon. 

No clear seismic data has yet been recorded on Venus, and so scientists have had to rely on indirect evidence. But with increasing signs of geological activity, and new technologies to detect seismic signals from orbit or within the atmosphere, there is a growing case for sending seismology-focused missions to Venus.

To explore this possibility, an international team - experts in seismology, geology and planetary science - led by Dr Iris van Zelst, set out to estimate how seismically active Venus might be. The team investigated the depth at which these quakes could occur, and how seismic activity could be detected during future missions.

The team estimated that Venus could experience between a few hundred to over 17,000 venusquakes per year (at magnitude 4 or higher), depending on how active different regions of Venus are.

To understand where quakes are most likely to occur on Venus, the team combined computer models with existing data on the planet’s surface shape and gravity. These models then helped them to estimate the thermal structure beneath the surface, a key factor in determining where venusquakes happen.

To do this, they focused on the depth where temperatures reach around 600°C, known as the 600°C ‘isotherm’. This depth is important because in shallower, cooler parts of the planet, rocks are brittle enough to crack and produce quakes. However, in deeper, hotter regions rocks are too soft to break, and instead slowly deform without causing seismic shaking.

By mapping how deep this 600°C boundary lies, the team were able to estimate the thickness of Venus’ seismogenic layer, the zone within the outer shell of a planet where quakes can actually occur. Their results suggest that this layer varies from about 2 to 35km thick across different regions of the planet. 

This study presents the first global map of the depth of Venus’ quake-prone layer, offering a more accurate picture of the planet’s seismic potential which can help to inform future missions.

After mapping where quakes are most likely to occur, the team then assessed different ways of actually detecting venusquakes - from sensors on the ground or high-altitude balloons, to imagers on orbiting spacecraft. They concluded that orbit-based airglow imagers currently offer the best chance of detecting seismic signals.

Airglow imagers are instruments that monitor the faint natural light (airglow) emitted by a planet’s atmosphere (the image above shows airglow emitted from Earth). When a quake occurs it sends waves of energy up into the upper atmosphere, disturbing these thin layers and causing small, detectable changes in the airglow.

Spacecraft equipped with airglow imagers can detect these changes from orbit, which means that they can cover much larger areas than instruments on the ground. They also avoid the need to survive Venus’ extreme surface conditions and don’t need to remain fixed in one spot - making them a practical and promising tool for seismic detection.

This work set out to use existing data and models to predict where quakes are most likely to occur on Venus and how they might be detected. In doing so, the team have provided compelling evidence that Venus is seismically active and highlights the planet’s significant potential for quakes. By identifying where and how venusquakes might happen – and evaluating which detection methods are most effective – the team have laid important groundwork for future exploration. Their findings are already directly informing the design of concept mission proposals, most recently supporting the VIVA (Venus’ Interior, Volcanism and Atmosphere) mission concept submitted to the European Space Agency.

Hopefully, by studying seismic activity on Venus we can unlock vital clues about the planet’s interior, its geological past, and why it evolved so differently from Earth. The teams’ findings strengthen the case for making seismic detection a priority in the next generation of missions – paving the way for the first direct measurements of seismic activity on Earth’s sister planet.