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Science & Exploration

Top five Venus mysteries Envision will solve

28/01/2025 328 views 3 likes
ESA / Science & Exploration / Space Science / Envision

Venus is a Solar System enigma. Similar in size to Earth and orbiting the Sun at a similar distance, it is remarkable how the two planets evolved so differently. While conditions on Earth allowed life to flourish, it seems that Venus may have experienced a dramatic climate change at some point in its history, leaving it with a toxic atmosphere, and an intensely hot, high-pressure surface.

Despite a fleet of spacecraft visiting the planet since the 1960s, many open questions remain. The European Space Agency’s Envision is heading to Venus in the 2030s to provide the most comprehensive study of the planet yet. It is set to investigate the planet’s many unsolved mysteries, from the makeup of its interior and possible volcanic activity, to intriguing processes in the atmosphere.

Here are the top five mysteries that Envision aims to solve:

1. How geologically active is Venus today?

2. How are Venus’s thick clouds sustained?

3. How does Venus lose its heat? 

4. Where did Venus’s water go?

5. How does the planet’s surface change over time?

Envision science: what will the mission observe?
Envision science: what will the mission observe?

1. How geologically active is Venus today?

Much of Venus’s surface is found to be young, covered by volcanic plains formed within the last 500 million to one billion years. One of Venus’s riddles is whether the surface is renewed in episodic global resurfacing events, or whether the resurfacing is happening more gradually over time.

Because Venus is similar in size and composition to Earth, it is assumed to have similar internal processes, such as mantle convection. This describes the way heat circulates in the thick layer between the planet’s central core and the surface crust, and can be likened to the rise and fall of material in a lava lamp. Indeed, surface features associated with volcanism such as shield volcanoes, domes, lava channels and coronae have already been detected, confirming this assumption.

Furthermore, previous missions to Venus, not least ESA’s Venus Express and NASA’s Magellan, spotted tantalising hints of active volcanism during the course of their missions. This included hotspots on the surface, volcanic features changing shape, and variations in volcanic gases in the atmosphere.

Envision’s suite of instruments, the mission duration, and its close orbit to the planet will provide higher resolution and repeatable observations for more reliable detection and mapping of surface and atmosphere changes. This will help constrain the type of any ongoing volcanism at Venus.

Envision’s radar imagery and elevation data will also help us understand how the surface has been shaped by tectonic activity. Past missions have documented steep slopes, landslides, rifts and ridges across the surface. But it was also found that Venus does not have plate tectonics like on Earth, where the planet’s crust is divided into rigid plates that glide over the mantle. On Earth, the interaction of these plates gives rise to different geologic features. For example, mountain ranges from colliding plates, or rifts and ridges from diverging plates leading to the formation of new crust.

However, some evidence was found that smaller regional surface blocks on Venus – so-called microplates – jostle and rotate. So Envision will use radar images and gravity measurements to look for clues as to how these features are created and move. It will also probe below the surface with ground-penetrating radar to reveal buried geological structures and their relation to surface activity.

Envision’s measurements will enable estimates of crustal thickness and provide clues as to the amount of volcanism or tectonic activity over time. Determining the location and nature of the boundary between the surface and mantle is key to understanding how Venus’s geological activity ‘works’.

Evidence for active volcanoes on Venus
Evidence for active volcanoes on Venus

2. How are Venus’s thick clouds sustained?

Modeling of Venus's atmosphere and climate has shown that the planet's thick cloud deck of sulphuric acid droplets can only be sustained by active volcanism. So where is the evidence?

Venus’s thick atmosphere is primarily composed of carbon dioxide, with small amounts of water vapour and sulphur dioxide. Volcanic activity on Venus can inject volcanic ash, sulphur dioxide, water vapour and other gases into the atmosphere, resulting in haze layers. At the same time, an influx of heat can cause changes in atmospheric buoyancy and circulation, leading to horizontal and upwards transport and the formation of atmospheric waves.

At an altitude of 50–70 km, temperatures drop from 450 °C at the planet’s surface down to 30–90 °C, and pressures are relatively lower. In this region, these gases undergo chemical reactions. In the upper part, ultraviolet radiation from the Sun drives photochemical reactions, breaking apart the sulphur dioxide and water molecules. The resulting molecules interact, forming sulphuric acid, which condenses into tiny droplets, creating thick, reflective clouds. Venus’s super-rotating winds, which circulate the planet much faster than the planet rotates, distribute the cloud droplets across the entire planet.

Global dynamics of Venus northern hemisphere
Global dynamics of Venus northern hemisphere

To put it another way, the thick cloud coverage would likely disappear over time if it were not replenished by volcanic activity. But scientists have not yet been able to directly link this variability to active volcanism on the surface. Envision will closely map and monitor key cloud ingredients to better understand the origin, transport and loss of these highly variable species. It will probe Venus’s atmosphere above, within, and below the planet’s cloud and haze layers, identifying how gases are formed and depleted, condensed and evaporated, and transported in the Venus system.

Importantly, Envision’s measurements will help separate out what is happening at the top of the atmosphere where molecules such as hydrogen and oxygen are lost to space, with what is happening close to the surface with respect to outgassing from the interior, and chemical reactions at the surface. Near-surface winds may also be able to lift dust and other particulates into the air.

By correlating measurements relating to surface signatures of active volcanism with vertical profiles of temperature, pressure and the amount of volcanic gasses and sulphuric acid droplets at various locations in the atmosphere, Envision will paint a clearer picture of the processes driving the short- and long-term variabilities seen in the atmosphere. This will also include details on local cloud variability as well as global circulation patterns.

Furthermore, Envision will be able to monitor the mysterious ultraviolet absorber of solar radiation in the cloud deck, which gives rise to the observed pattern of dark and bright markings at the cloud tops. While likely linked to iron, chlorine and sulphur, it is unclear how and whether the cloud-level abundance of the absorber is the result of material upwelling from below, meteorite dust from above, or if it depends on chemical reactions between upwelling and downwelling species.

3. How does Venus lose its heat?

Planets are constantly losing heat left over from their formation 4.6 billion years ago, through radioactive decay of elements, mantle convection, volcanism and plate tectonics, for example – as is the case for Earth. Given Venus’s similarities to Earth in terms of size and composition, we expect Venus to have formed and cooled in a similar way. But the fact that Venus has a noxious carbon dioxide-rich atmosphere and conditions suggestive of a runaway greenhouse effect, imply a much different evolution.

Understanding how the planet cooled after its formation when its surface was still a vast ocean of magma, and defining the nature of active surface processes today, is key to unlocking Venus’s climatic evolution.

Envision will study how Venus releases and distributes heat today through a variety of investigations. It will analyse the various kinds of volcanic features and flows, and where the surface has become deformed by tectonic activity. It will measure differences in surface rock composition and changes in elevation, which is linked to crustal thickness. And it will determine the flexibility of the lithosphere – the rigid upper part of the crust and mantle, which in turn are linked to heat flow in the mantle.

Topography and gravity data will also help determine the presence of mantle plumes – large upwellings of magma and heat from deep in the planet’s interior. This process is responsible for the formation of volcanic islands on Earth such as Hawaii.

Detailed gravity mapping will enable a better definition of the boundaries between the planet’s core, mantle and surface. In addition, by looking at the way Envision’s orbit around Venus is gently tugged by the gravity of the planet, we will be able to infer details about the planet’s core, that is, whether it is liquid or solid, as well as its size.

The multi-instrument approach to studying the planet’s interior will provide greater insight into the heat-loss processes in action today. This will help unravel the complexity of the interplay between the planet’s interior, surface and atmosphere.

Envision: Understanding why Earth's closest neighbour is so different
Envision: Understanding why Earth's closest neighbour is so different

4. Where did Venus’s water go?

Did Venus once have vast oceans like Earth, and could it have been habitable? If it did have oceans, how and when did water occur on the planet beyond the initial delivery from comets and asteroids? And, when and why did the planet lose its water?

The most variable atmospheric ingredients on Venus are sulphur dioxide and water vapour. These components are thought to be enriched by active volcanic processes, transported by atmospheric circulation, cycled through chemical reactions, and lost through interactions with the surface or by atmospheric escape. 

Indeed, Venus Express made the first ever detection of atmospheric water loss at Venus, showing that its constituent hydrogen and oxygen atoms are split by solar radiation and swept away by the constant stream of particles blown out by the Sun, the solar wind. The results implied that Venus has been losing a large quantity of water to space over billions of years.

But whether or not Venus hosted oceans in the past is highly debated. Envision will attempt to read the planet’s history by looking at the chemical signatures of different rock types, since certain rocks, such as granites, point to formation in the presence of water. 

It will also observe the amount of water vapour above and below the cloud deck and how they vary with time. This will be used to investigate whether thermal hotspots on the surface – associated with active volcanism – also leads to increased amounts of atmospheric water vapour close to the surface.

Water vapour also plays an important role in the transfer of heat and energy through the atmosphere, which helps to maintain the planet’s massive greenhouse effect and drives its atmospheric super-rotation (a phenomenon whereby the atmosphere rotates faster than the planet itself). By monitoring the behaviour of water vapour, together with other dominant atmospheric gases, Envision will aim to unravel the mysteries around Venus’s intriguing climate evolution.

Interaction between Venus and the solar wind
Interaction between Venus and the solar wind

5. How does the planet’s surface change over time?

Aside from volcanic and tectonic activity, Venus’s surface undergoes changes linked to wind erosion, as evidenced in dune fields and wind streaks, and gravity-driven movements such as landslides or avalanches.

The planet’s hot, dense and highly oxidising atmospheric conditions are also likely to cause intense chemical weathering of surface materials. With a surface pressure of 90 bar and a temperature of more than 450 °C, the atmosphere is in a so-called ‘supercritical state’. That means that despite the high surface pressure – equivalent to that at a depth of around 80 m in Earth’s oceans – the air flows like a gas.

Observations of the distribution, scales and composition of surface features will contribute to our understanding of how Venus’s surface has been affected by changes in the planet's interior and atmosphere. Envision’s repeated radar, ultraviolet and infrared imaging at high resolution will help us understand how the planet’s geological, dynamic and chemical processes are at work.

Envision is set to provide the most complete view of Venus yet, bringing us closer to solving these intriguing open questions about our mysteriously different neighbour planet.

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