Thanks to evidence provided by missions like NASA’s Magellan spacecraft, scientists have theorized that Venus likely experienced a catastrophic resurfacing event about 500 million years ago (give or take 200 Mya). This is believed to be the reason why Venus is such a hellish place today, with an atmosphere that is 92 times as dense as Earth’s, predominantly composed of carbon dioxide (CO2), and temperatures hot enough to melt lead.
The question of what Venus was like before this event took place – particularly, whether or not it had oceans – has been the subject of debate ever since. While many believe that Venus’s surface was covered in large bodies of water, a recent study has contradicted this claim. Using a state-of-the-art climate model, a team of French researchers has developed an alternative scenario of how Venus evolved to become what it is today.
The research was conducted by a team scientists from the Observatoire Astronomique de l’Université de Genève, the Laboratoire d’astrophysique de Bordeaux, the Centre National de la Recherche Scientifique (CNRS) , and University of Versailles-Saint Quentin-en-Yvelines (UVSQ). The paper that describes their findings, titled “Day–night cloud asymmetry prevents early oceans on Venus but not on Earth,” was published in the Oct. 13th issue of Nature.
Artist’s impression illustrating the lack of water on Venus. Credit and ©: Manchu
For over a century, scientists have speculated whether or not its surface was covered in oceans. At first, the dense clouds that obscure the surface were thought to be rainclouds, which fueled speculation that Venus’ surface was covered in oceans. By the 1960s, this notion was dispelled as multiple Soviet, and NASA missions conducted flybys of the planet (and even attempted to land on the surface) that demonstrated just how hot and hellish the planet is.
According to current planet formation models, Venus formed from the protoplanetary disk that orbited the Sun 4.5 billion years ago. Like the other rocky planets (Mercury, Earth, and Mars), the accretion process left Venus covered in magma for much of its early history. Over time, the surface slowly cooled and solidified to the point that water would condense in the atmospheres and rainfall could occur.
This process gave rise to the oceans on both Earth and Mars and is believed to have played an indispensable role in the emergence of life on Earth ca. 3.7 billion years ago. Whereas Mars failed to hold onto the water that once flowed across its surface, evidence of its watery past is retained in the form of flow channels, sedimentary deposits, and clays – all features that form in the presence of water.
While Venus was also very different, the existence of surface water has remained an unresolved question. For this reason, five missions surveyed Venus’ atmosphere between 1994 and 2010 – NASA’s Magellan, Cassini–Huygens, and MESSENGER missions, the ESA’s Venus Express, and JAXA’s Akatsuki. Other missions contributed by gathering data from Venus during flybys and gravity assists, such as the NASA’s Parker Solar Probe, the ESA/JAXA BepiColombo, and the NASA/ESA Solar Orbiter.
Artist’s impression of Venus Express performing aerobraking maneuvers in the planet’s atmosphere in June and July 2014. Credit: ESA/C. Carreau
As Martin Turbet, a postdoctoral researcher at the Observatoire Astronomique de l’Université de Genève and the lead author on the study, explained to Universe Today via email:
“There are basically two main scenarios that have been considered so far. In the first one, Venus had liquid water oceans on the surface, and this global resurfacing (which could have started well before 500 Mya) coincided with the complete evaporation of the oceans. In the second, which is strongly supported by our new results, the ‘oceans’ would have always been vaporized (that is, always in vapor form) and would have gradually escaped from the atmosphere over the course of the evolution of Venus.”
When modeling the ancient climates of all of the rocky planets in the Solar System, scientists are careful to consider how ancient atmospheres interacted with Solar radiation. During the era in question – ca. 3.7 billion years ago (Gya), the Sun was 30% fainter than it is now, which allowed atmospheres on Earth and Mars to cool to the point where oceans formed on their surfaces.
For the sake of their study, the team simulated the climates of Earth and Venus at the very beginning of their evolution (ca. more than 4 Gya) when their surfaces were still molten. This consisted of creating advanced 3D models of the atmospheres, similar to those used by Earth scientists to simulate Earth’s current climate and future evolution. From this, the team studied how Earth and Venus evolved over time and whether oceans could form in the process.
Cloud structure in the Venusian atmosphere in 2016, revealed by observations in the two ultraviolet bands by Akatsuki. Credit: Kevin M. Gill
They found that temperatures on Venus would not have been cool enough for water vapor to condense before the catastrophic resurfacing event. Such a fall in temperatures, said Turbet, would only have been possible if the surface was shielded by sufficient cloud cover:
“Our new results reveal that the clouds played a major role to prevent the formation of early oceans on Venus. In our simulations, clouds form mainly on the night side of the planet. The absence of clouds on the day side significantly reduces the albedo (that is, increases the absorptivity) of the planet; the presence of (high-altitude) clouds in the night side significantly increases the greenhouse effect. The two effects combined produce a strong warming of the atmosphere of early Venus, which prevented the formation of oceans.”
These findings go against recent research led by Jun Wang (2014), a geophysicist with the University of Chicago, and physical scientist Michael Way (2019) of the NASA Goddard Institute for Space Sciences (GISS). Both studies showed that clouds would mostly form on the dayside of Venus, which would produce an intense cooling and stabilize temperatures to the point that water would condense to have rain.
However, the climate model used by Turbet and his colleagues indicated that clouds were more likely to have formed on the night side of Venus, where they wouldn’t be able to shield the surface. “We show that the change of behavior of clouds (which varies according to the stage of the evolution of Venus considered) – from the dayside to the night side – had dramatic consequences on the past evolution of Venus,” said Turbet.
According to this climate model, cloud cover on Venus would have helped maintain high surface temperatures by causing a greenhouse effect that trapped heat in the planet’s dense atmosphere. These high temperatures prevented any rainfall, thereby ensuring that oceans could never form on Venus’ surface. As Turbet summarized it:
“In recent years, many scientific studies have focused on trying to understand the sequence of events that led to the disappearance of the oceans on Venus. Our results show that it may be even more important to understand the earlier evolution of Venus, to identify whether or not oceans ever formed on the surface of Venus.”
These results not only challenge the idea that Venus once had oceans but also the belief that Earth’s “sister planet” might have supported life once. Fortunately, there are several proposed missions to explore Venus’ atmosphere and surface in the next decade, including the ESA’s EnVision, NASA’s Venus Emissivity, Radio Science, InSAR, Topography & Spectroscopy (VERITAS), and Deep Atmosphere of Venus Investigation of Noble Gases, Chemistry, and Imaging (DAVINCI+), and the Russian Venera-D mission.
These missions, which are scheduled to launch between 2026 and the early 2030s, will investigate these theoretical results further. With any luck, they will divulge information that will permanently resolve these enduring mysteries about Venus’ past.