Venus may have had liquid water for 2-3 billion years until a dramatic transformation starting over 700 million years ago resurfaced around 80 per cent of our neighbouring planet, according to a recent study by NASA. The study which was presented at EPSC-DPS Joint Meeting 2019, gives a new view of the climatic history of Venus and may have implications for the habitability of exoplanets in similar orbits.
Around 40 years ago, NASA’s Pioneer Venus mission had found some fascinating hints that Earth’s ‘twisted sister’ planet may have once had water worth that of a shallow ocean, according to the study provided by Michael Way of NASA’s Goddard Institute for Space Science.
To check if Venus might have ever had a stable climate which was capable of supporting liquid water, Way and his colleague Anthony Del Genio created a series of five simulations assuming different levels of water coverage.
In all the five cases, the researchers found that our neighbouring planet was able to maintain stable temperatures ranging from a minimum of about 20 degrees Celsius to a maximum of about 50 degrees Celsius for approximately three billion years.
A temperate climate might even have been maintained on Venus even today had there not been a series of events that led to the release, or ‘outgassing’, of carbon dioxide which were stored in the rocks of the planet nearly 700-750 million years ago.
“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 an Earth-like climate to the hellish hot-house we see today,” Way said in a statement.
Three of the five scenarios studied by the two NASA researchers assumed that the topography of Venus as seen today and considered a deep ocean averaging 310 metres, a shallow layer of water averaging 10 metres and a small amount of water locked in the soil. To compare, they also included a scenario with the topography of the Earth and a 310-metre ocean and, finally, a world completely covered by an ocean of 158 metres depth.
For simulating the environmental conditions at 4.2 billion years ago, 715 million years ago and today, the researchers adapted a 3D general circulation model to account for the increase in solar radiation because our Sun has warmed up over its lifetime, as well as for changing atmospheric compositions.
Although a lot of researchers believe that Venus is beyond the inner boundary of the habitable zone of the Solar System and it is very close to the Sun to support water in liquid form, the new study suggests otherwise.
“Venus currently has almost twice the solar radiation that we have at Earth. However, in all the scenarios we have modelled, we have found that Venus could still support surface temperatures amenable for liquid water,” Way said.
Approximately 4.2 billion years ago, soon after its formation, Venus would have completed a period of rapid cooling and its atmosphere would have been dominated by carbon-dioxide. If the planet evolved in an Earth-like way over the next 3 billion years, the carbon dioxide would have been drawn down by silicate rocks and locked into the surface, the study said.
By the second epoch modelled at 715 million years ago, the atmosphere would likely have been dominated by nitrogen with trace amounts of carbon dioxide and methane – similar to today’s Earth – and these conditions could have remained stable until the present times.
According to the researchers, the cause of the outgassing which led to Venus’s dramatic transformation is a mystery, although it probably may be linked to the planet’s volcanic activity. One possibility is that large amounts of magma bubbled up, releasing carbon dioxide from molten rocks into the atmosphere.
The magma solidified before reaching the surface and this created a barrier which meant that the gas could not get reabsorbed. The presence of large amounts of carbon dioxide triggered a runaway greenhouse effect, that has resulted in the scorching 462 degree average temperatures found on Venus today, according to the researchers.
“Something happened on Venus where a huge amount of gas was released into the atmosphere and couldn’t be re-absorbed by the rocks. On Earth we have some examples of large-scale outgassing, for instance the creation of the Siberian Traps 500 million years ago which is linked to a mass extinction, but nothing on this scale. It completely transformed Venus,” Way said.