Venus vs Earth: Why Did the "Twin" Become a Hell?

A Sister Planet Turned Hostile

Venus has a diameter and mass very close to those of our planet. Yet behind this apparent resemblance hides an atmospheric and geological hell. Its thick layer of CO₂ (carbon dioxide) and its clouds of H₂SO₄ (sulfuric acid) cause a devastating greenhouse effect, raising the average surface temperature to nearly 465 °C, a heat sufficient to melt lead.

Common Origins, Opposite Fates

Although Venus and Earth share similar size and mass, their evolution has diverged radically. Initially, Venus could have had oceans and a more temperate climate, comparable to that of the primitive Earth. However, its proximity to the Sun, about 108 million kilometers compared to 150 million for Earth, directed its fate towards climatic runaway. The more intense solar heat favored the evaporation of water, releasing large quantities of water vapor into the atmosphere. This additional greenhouse gas trapped more heat, accentuating warming and triggering a vicious cycle that led to the near-complete desiccation of the planet.

N.B.:
The idea that Venus effectively had liquid oceans for a long period remains debated by climate modelers. Some scenarios suggest instead an "uncontrolled greenhouse effect from the origin," without a stable oceanic phase, due to the already more intense luminosity of the young Sun at this distance. The future DAVINCI, VERITAS, and EnVision missions aim precisely to decide between these hypotheses.

The First Indications of a Hostile World

The first astronomical observations quickly revealed the exceptional nature of Venus. Its intense brightness in the night sky, which earned it the nickname "morning star," actually masks a surface perpetually veiled by a thick cloud layer. It was only with the advent of space probes that scientists discovered the extent of the extreme conditions reigning on its surface, forever transforming our vision of this planet once imagined as a possible tropical paradise.

A Crushing and Acidic Atmosphere

Venus's atmosphere is composed of more than 96% carbon dioxide, with a surface pressure about 92 times that of Earth, equivalent to a dive to 900 meters under the ocean. Under these conditions, even metals such as lead melt on the surface. The Soviet Venera missions were the first to transmit images from this burning ground, before succumbing in less than two hours to the heat and pressure.

A Unique Atmospheric Composition in the Solar System

Venusian air is a true poison for living beings. Besides the largely dominant carbon dioxide, the atmosphere contains about 3.5% nitrogen and traces of sulfur dioxide, argon, and water vapor. The sulfuric acid clouds that envelop the planet form a continuous layer between 45 and 70 km in altitude, making any direct observation of the surface impossible from space. This chemical composition results notably from the absence of oceans that could have dissolved the CO₂, unlike what happened on Earth.

Note: since 2020, a major scientific controversy has surrounded the detection of traces of phosphine (PH₃) and, more recently, ammonia in the Venusian clouds, two gases considered on Earth as potential biosignatures. The initial detection by the JCMT and ALMA telescopes has been strongly contested (possible pollution by sulfur dioxide, instrumental uncertainties), and several reanalyses have given contradictory results. The debate remains open and motivates the design of the EnVision probe and the complementary mission concept VERVE, dedicated to the search for biosignature gases in the cloud layers.

The Technological Challenges of Exploration

The exploration of Venus has represented a considerable technical challenge for space engineers. The Soviet probes of the Venera program, launched from the 1960s, had to be designed with thermal shields and innovative cooling systems to resist, if only for a few moments, the extreme conditions of the surface. Venera 9 and 10 successfully transmitted the first photographs of the Venusian soil in 1975, revealing a rocky landscape under yellowish light, before being destroyed by the ambient heat.

Retrograde Rotation: A Dynamic Anomaly

Unlike most planets, Venus rotates in the opposite direction to its revolution around the Sun, a phenomenon known as retrograde rotation (retrograde rotation). Its sidereal day lasts 243 Earth days, longer than its Venusian year of 225 days. This slow rotation and the absence of a significant intrinsic magnetic field (intrinsic magnetic field generated by a liquid rotating core) expose the upper atmosphere more to solar winds, which contributes to the progressive loss of its light gases.

Theories on the Origin of this Reversed Rotation

The origin of Venus's retrograde rotation has intrigued scientists for decades. Several hypotheses have been put forward to explain this anomaly. One hypothesis suggests that a major collision with a large celestial body, occurring early in the Solar System's history, could have reversed the planet's rotation. Another theory, now favored by part of the scientific community, suggests that gravitational interactions between atmospheric solar tides, solid-body tides, and core-mantle friction effects would have gradually slowed then reversed its rotation over hundreds of millions of years, without the need for a giant impact.

Consequences of a Slow Rotation

Venus's extremely slow rotation has major repercussions on its climate and geology. The absence of significant Coriolis force prevents the formation of complex weather systems like those observed on Earth or Jupiter. The winds in the upper atmosphere, although violent (up to 350 km/h), are mainly due to a phenomenon of super-rotation where the atmosphere rotates much faster than the planet itself. This mechanism, still poorly understood, contributes to uniformizing temperatures between the equator and the poles.

A Surface Reshaped by Volcanism

Radar data, particularly from the Magellan mission (in orbit around Venus from 1990 to 1994), revealed that Venus's surface is dotted with vast lava plains, volcanic domes, and structures called coronae (circular geological structure created by crustal deformation). These formations testify to intense volcanism that completely resurfaced the planet about 500 million years ago — an estimate derived from impact crater counting, still debated as to whether it was abrupt or more gradual.

Characteristic Geological Formations of Venus

Venus's surface presents a diversity of landforms that testify to intense and varied geological activity. Besides the vast lava plains that cover about 80% of the surface, there are shield volcanoes reaching several hundred kilometers in diameter, lava fields extending over thousands of kilometers, and unique tectonic structures such as tesserae — highly deformed and fractured terrains. The coronae, these large circular structures 100 to 1000 km in diameter, would result from the rise of mantle plumes that deformed the crust before collapsing.

Active Volcanism Now Confirmed by Magellan Data

Long hypothetical, the current volcanic activity of Venus has had direct geological evidence since 2023. By comparing two Magellan radar images taken eight months apart in 1991, planetary scientist Robert Herrick (University of Alaska Fairbanks) and Scott Hensley (JPL) identified a volcanic vent at Maat Mons that had changed shape and doubled in surface area, accompanied by a probable lava flow — the first direct evidence of an eruption on Venus, published in the journal Science. In 2024, an Italian team led by A. Sulcanese extended this research to other regions and identified two additional lava flows, associated with the volcanoes Sif Mons and the Niobe Planitia region, also formed during the Magellan mission in the early 1990s. These discoveries, combined with thermal models of the planet's interior, suggest that Venus remains volcanically active today, with global eruption rates estimated between 0.1 and 10 km³ per year. The VERITAS and EnVision missions, equipped with high-resolution radars, should allow direct detection of new ongoing eruptions.

Water Loss and Irreversible Greenhouse Effect

In the past, Venus could have had surface water. But proximity to the Sun favored accelerated evaporation, followed by photodissociation of water molecules under ultraviolet radiation. Hydrogen, being lighter, escaped into space, leaving an atmosphere dominated by CO₂. This process led to an uncontrolled greenhouse effect, a scenario often cited as a climate warning for Earth.

The Mechanism of Greenhouse Runaway

Venus's history illustrates the concept of runaway greenhouse effect. According to the most widespread hypothesis, the planet could have, in its youth, maintained liquid water on its surface for a limited period. But the progressive increase in solar luminosity, coupled with the natural accumulation of CO₂, would have warmed the planet to a critical threshold. At this stage, the evaporation of water produced water vapor that intensified the greenhouse effect, causing further evaporation. This vicious cycle led to the near-total disappearance of the oceans, a process that made the planet permanently uninhabitable.

The Importance of Studying Venus for Terrestrial Climatology

Venus is today considered a natural laboratory for understanding the limits of the greenhouse effect and extreme climate risks. If Venus's history does not foretell an identical fate for Earth — our planet being farther from the Sun — it reminds us that critical thresholds exist in any climate system. The climate models used to predict the evolution of terrestrial warming are partly inspired by the lessons learned from the study of Venus, highlighting the importance of limiting greenhouse gas emissions to avoid any difficult-to-reverse drift.

Lessons for Earth's Future

Venus's example is regularly cited by climatologists as an illustration of the potential consequences of uncontrolled global warming, even if the mechanisms at play and the time scales differ significantly from the current terrestrial situation. Understanding the mechanisms that led Venus to its current state helps define the planetary boundaries not to be crossed and to refine comparative climate models of terrestrial planets.

The Venusian Decade: The Return of Probes

After more than thirty years without a dedicated US mission since Magellan, several projects are to return to study Venus in the coming years, forming what planetary scientists call the "Venus decade." NASA is preparing two complementary missions selected in 2021 under its Discovery program: DAVINCI (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging), which will combine an orbiter and an atmospheric descent probe targeting the Alpha Regio region, and VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy), an orbiter designed to map the surface in high resolution using an interferometric radar. For its part, the European Space Agency is developing EnVision, an orbiter combining radar, spectrometers, and subsurface radar sounding, with launch scheduled no earlier than 2031 on an Ariane 6 rocket.

N.B.:
These schedules remain uncertain. VERITAS, already postponed from 2027 to a window (no earlier than 2031), as well as DAVINCI, have seen their funding weakened by NASA's budget decisions for fiscal year 2026, while the budget allocated to the planetary sciences division remains below the project's initial requests. EnVision partly depends on a radar instrument (VenSAR) initially provided by NASA, which could force ESA to revise its design. The private company Rocket Lab, in partnership with MIT, is also considering a private atmospheric probe mission, which would be the first to target Venus's atmosphere directly since the Pioneer Venus probes of 1978.

Comparative Table of Physical Parameters of Venus and Earth