A long time ago, billions of years ago, a great dance of attraction began in a giant cloud of gas and dust. Collisions, rotations, and explosions gave birth to our planets – each with its unique path. In this cosmic family, Earth and Venus were born side by side, similar in size, mass, and composition. They were like sisters growing up together in this tumultuous world.
However, as is often the case among relatives, their fates diverged. Earth became a green and blue oasis of life, while Venus became a scorching desert wrapped in thick acid clouds. What caused this stark difference? And how did the absence of a magnetic field on Venus turn it into a real hell? To answer these questions, we have to look into the planet’s past and understand what processes made it what it is today.
What the planet’s magnetic field is and how it is formed
To begin with, let’s talk about an amazing, invisible, tasteless, but tangible physical phenomenon – the magnetic field. On our planet, it is the result of complex processes in its depths. As the most recognized theory tells us, the field is formed by a dynamo effect arising from the movement of molten iron in the outer core. This motion generates electric currents, which in turn create a magnetic field. Because, as we learned in 1905, thanks to the work of Joseph Larmor, these generated particles arising in a conducting medium are a key element in its formation.
Later, in the 1940s, researchers led by Walter Elsius clarified that these currents arise from the complex interaction between convection currents in the liquid outer core and the Earth’s rotation. The molten iron in the core is constantly moving, creating a potential difference that generates electric currents. These processes produce a magnetic field that extends beyond the core, wrapping the entire planet.
This dynamo effect became the basis for the modern understanding of the geomagnetic field. Without the movement of the liquid metallic core and the electric currents it generates, the Earth would lose its protective barrier against the solar wind. This is why convection currents and electric currents are vital to maintaining the planet’s magnetic field.
The geomagnetic field has the shape of a dipole, like a magnet with two poles, north and south. Its action extends far beyond the atmosphere, forming the magnetosphere, a protective barrier shielding the planet from solar wind particles. Without this shield, charged particles would constantly bombard the atmosphere, gradually tearing off its upper layers.
Interestingly, the Earth’s magnetic field is not constant: its strength and the location of the poles change over time due to instability in the movement of liquid metal in the outer core. But it is thanks to it that our planet has retained water and a stable atmosphere, making it suitable for life. So why does Venus, despite its similarities to Earth, lack this important protection?
Venus, how do you lose your magnetic field?
The whole history is difficult for us to trace because we do not live on Venus and do not have access to its close study as we have on Earth. Therefore, despite all the high technology, mankind operates with theories, hypotheses, and in general logic to know such an interesting neighbor.
First, the weak magnetic field of Venus can be explained by its slow rotation and almost circular orbit. According to one modern theory, the intensity of the dipole magnetic field depends on the precession of the polar axis and the angular velocity of the planet’s rotation. Precession is a phenomenon where the polar axis of a planet makes a slow circular motion around a certain point, similar to the way a gyroscope or a wiggler wobbles during rotation.
This motion is caused by the gravitational influences of other bodies, such as the Sun or the Moon, and can affect the dynamics of internal processes in the planet’s core. In the case of Venus, these parameters are extremely small. However, measurements show that even taking these factors into account, its magnetic field is even weaker than the theory suggests.
Second, the problem may be the lack of convection currents in the core. The core of Venus is thought to be composed mostly of iron, similar to the Earth. However, the convection necessary for the formation of a magnetic field may be absent because the core is poorly cooled. This absence also affects tectonic processes.
There is no active plate tectonics on Venus, which on Earth plays an important role in the dissipation of heat from the planet’s depths. The absence of tectonics can be both a consequence of weak convection flows and additionally strengthen their lack, creating a closed geological environment. In addition, Venus lacks water, which on Earth acts as a “lubricant” for plate motion. Because of these factors, Venus’ crust probably remains too soft or static to support Earth-like mechanisms.
Third, the lack of tidal processes may also play a role. Unlike Earth, Venus has no large satellites that could cause tidal effects in the core and mantle. Also, its orbit is almost perfectly circular, which reduces the possibility of gravitational influence from the Sun. All of this reduces the likelihood of creating the necessary energy for core activity.
Interestingly, the absence of the magnetic field and the loss of water are difficult to consider separately. There is an assumption that it was the loss of water that led to the stopping of plate tectonics and, with it, to the weakening of convection in the core. On the other hand, there is a possibility that the loss of water was a consequence of the absence of a magnetic field, which failed to protect the atmosphere from the solar wind.
How did the lack of a magnetic field affect the atmosphere of Venus?
Without its shield, Venus was left virtually defenseless, and solar radiation split water molecules into hydrogen and oxygen (a process known as photodissociation). Due to the lack of a magnetic field, the light hydrogen atoms are lost in space and the heavy oxygen reacts with other chemical elements to form new compounds. It is by this chemistry that Venus has lost much of its water, which once probably existed on its surface in a liquid state.
Without water, which on Earth serves as a natural absorber of carbon dioxide, this gas has accumulated in the atmosphere of Venus in huge quantities. It now makes up more than 96% of its composition. The high concentration of CO₂ has caused an extreme greenhouse effect, raising surface temperatures to +465°C. The lack of a magnetic field also allows charged solar wind particles to interact with components of the atmosphere, creating chemically corrosive compounds. The pressure at the surface of Venus is 92 times that of Earth, which is equivalent to the pressure at a depth of nearly 900 meters underwater on our planet. As a result, Venus’ clouds are saturated with sulfuric acid, making its atmosphere even more hostile to life.
What this has meant for past missions and will mean for future explorations
The lack of its magnetic field on Venus has long been considered one of the main reasons for the loss of water and degradation of the atmosphere. However, studies conducted by the Venus Express (European Space Agency, ESA) have found that Venus has an induced magnetic field, which is formed as a result of the interaction of the planet’s ionosphere with the solar wind. This “magnetic shield” protects the upper atmosphere, but is much weaker than Earth’s magnetic field.
One of the most important discoveries of the mission was the observation of the phenomenon of magnetic reconnection in the Venus magnetola, a region where the solar wind interacts with the induced magnetic field. On May 15, 2006, Venus Express recorded a plasmoidal structure that shows similarities to processes that occur in Earth’s magnetosphere. This process, during which magnetic field energy is converted into kinetic energy, causes a plasma to be ejected from the magnetosphere into space while simultaneously channeling some of the energy back toward the night side of Venus’ atmosphere.
Such observations helped us realize that even without a strong magnetic field, Venus has a unique mechanism of energy exchanges that affect the loss of gases from its atmosphere. This discovery has important implications for future missions such as VERITAS and EnVision, which will focus on studying the evolution of Venus’ atmosphere and its geology. Understanding the induced magnetic field is also key to predicting how similar mechanisms might affect other planets, particularly exoplanets lacking their magnetic field.
So although the sisters took different paths, their story reminds us how a thin line can separate an oasis of life from a scorching desert.
