In 1978, NASA's Pioneer Venus (aka. Pioneer 12) mission reached Venus ("Earth's Sister") and found indications that Venus may have once had oceans on its surface. Since then, several missions have been sent to Venus and collected data on its surface and atmosphere. From this, a picture has emerged of how Venus made the transition from being an “Earth-like” planet to the hot and hellish place it is today.
It all started about 700 million years ago when a massive resurfacing event triggered a runaway Greenhouse Effect that caused Venus's atmosphere to become incredibly dense and hot. This means that for 2 to 3 billion years after Venus formed, the planet could have maintained a habitable environment. According to a recent study, that would be long enough for life to emerge on “Earth's Sister”.
The study was presented yesterday (Sept. 20th) at the 2019 Joint Meeting of the European Planetary Science Congress (EPSC-DPS), which took place from Sept. 15th to 20th in Geneva, Switzerland. It was here that Michael Way and Anthony Del Genio of NASA's Goddard Institute for Space Science (GISS) shared a new take on Venus's climatic history, which could have implications in the search for habitable exoplanets.
For the sake of their study, Dr. Way and Dr. Del Genio created a series of five simulations that considered what the Venus environment would be like based on different levels of water coverage. This consisted of adapting a 3D general circulation model that took into account changing atmospheric compositions and the gradual increase in solar radiation as the Sun became warmer over the course of its lifetime.
In three of the five scenarios, Way and Del Genio assumed that the topography of Venus was much the same as it is today, the ocean ranged from a minimum depth of 10 m (~ 30 ft) to a maximum of about 310 m (~ 1000 ft) and a small amount of water was locked in the soil. They also considered a scenario with Earth's topography and a 310-meter ocean, and another where Venus was completely covered in an ocean 158 m (~ 500ft).
In the end, all five simulations indicated the same thing: that Venus would have been able to maintain stable temperatures – from a low of 20 ° C (68 ° F) to a high of 50 ° C (122 ° F) – for about three billion years. We're not in for a series of events that caused 80% of the planet's surface to be resurfaced (which ice to the outgassing of CO² contained within the crust), it might even be habitable today. As Way explained it:
“Our hypothesis is that Venus may have had a stable climate for billions of years. It is possible that the near-global resurfacing event is responsible for its transformation from the Earth-like climate to the hellish hot-house we see today.
It all started about 4.2 billion years ago, and a few hundred million years after Venus formed and just started a period of rapid cooling. At this point, assuming Venus underwent a similar process as Earth, the atmosphere would be dominated by carbon dioxide. This would have been slowly absorbed by silicate rocks to form carbonates that were then locked into the planet's crust.
By about 715 million years ago, according to the Way and Del Genio study, the atmosphere would be similar to what Earth is like today – composed predominantly of nitrogen gas with trace amounts of CO² and methane. These conditions could have remained stable up until the present times were it not for a massive outgassing event.
The cause of this remains a mystery; however, scientists believe it was due to a geological event that caused 80% of the planet to resurface. This could have involved large amounts of magma bubbling up and releasing massive amounts of CO² into the atmosphere. The magma would then be solidified before reaching the surface, thus creating a barrier that prevented the CO² from being reabsorbed. As Way explained:
“Something happened on Venus where a huge amount of gas was released into the atmosphere and was re-absorbed by the rocks. On Earth we have some examples of large-scale outgassing, for an instance of the creation of the Siberian Traps 500 million years ago that is linked to a mass extinction, but nothing on this scale. It completely transformed Venus. ”
This would explain how Venus' atmosphere was thickened to the point where it was over 90 times as dense as Earth's (92 bar compared to 1 bar). Combined with high concentrations of CO², this would have led to a Greenhouse Effect runaway that would explain how the planet became the hellish place we know today, with surface temperatures averaging 462 ° C (864 ° F).
This flies in the face of conventional habitat notations, which state that Venus' orbit places it beyond the inner edge of our Sun's habitable zone (HZ). Within this "Venus Zone", according to conventional wisdom, the planet absorbs too much solar radiation to ever be able to maintain liquid water on its surface. But as Way indicated, their simulations all indicated otherwise:
“Venus currently has almost twice the solar radiation that we have at Earth. However, in all the scenarios we have modeled, we have found that Venus could still support surface temperatures amenable to liquid water. ”
These findings are in line with a similar study that Way and Del Genio conducted in 2016 with colleagues from the NASA Goddard Space Flight Center, the Planetary Science Institute (PSI), Uppsala University and Columbia University. For this study, their team created a suite of 3D climate simulations using data from the Magellan mission that examined how the presence of the ocean on ancient Venus would affect its habitability.
From this, they determined that if Venus had a rotational period slower than about 16 Earth days, its climate would have remained habitable until 715 million years ago. However, there are still two major unknowns that need to be addressed before scientists can say with confidence that Venus was habitable until very recently.
First, scientists will need to determine how quickly Venus cooled and if it was able to condense liquid water on its surface in the first place. Second, it remains unknown whether the global resurfacing event that led to the Venus' transition was a single event or merely part of a series that had been taking place for billions of years.
"We need more missions to study Venus and get a more detailed understanding of its history and evolution," Way said. “However, our models show that there is a real possibility that Venus could have been habitable and radically different from the Venus we see today. This opens up all kinds of implications for exoplanets found in what is called the 'Venus Zone', which may in fact host liquid water and temperate climates. ”
Think of it… had Venus not undergone a massive resurfacing event (or a series of them), humanity would only need to look next-door for proof of extra-terrestrial life. For that matter, had Mars not lost its magnetosphere 4.2 billion years ago, it could have produced a life of its own that would still be around today.
These findings are likely to be encouraging to those who believe that Venus should be terraformed someday. Knowing that the planet once had a stable climate, and could maintain it despite its orbit, effectively means that any environmental engineering we would there would stick.
That means that Venus could someday be made into a balmy world mostly covered with oceans with several large continents and extensive archipelagos. Sound Like Any Place You Know?
Further Reading: Europlanet Society EPSC