The reason Venus missed the chance to be a water-filled planet might boil down to chemical reactions in the planet’s atmosphere allowing hydrogen to flee.
Venus and Earth have much in common – roughly the same size, density and rocky composition.
But Venus boasts an environment completely inhospitable to life as we know it: in short, average surface temperatures of 465 °C and clouds of sulfuric acid that rain down and evaporates almost instantly due to the incredible heat.
So extreme is Venus, that Soviet probes sent to the planet in 1984 melted after sending home a set of black and white images of the planet’s rocky surface.
Despite this violent environment, it’s long been theorised that Venus may have once hosted liquid water on its surface and scientists from University of Colorado Boulder think they know why.
In a new study published in the journal Nature used computer modelling to simulate how hydrogen behaves in Venus’s atmosphere.
Hydrogen atoms can be found in Venus’s upper atmosphere together with carbon and oxygen as the ‘formyl cation’ (HCO+), which effectively results from water and carbon dioxide.
HCO+, which contains an atom of hydrogen, carbon and oxygen, can be found in cosmic molecular clouds and planetary atmospheres. It can form through several chemical reactions, including ones involving water.
In the Venusian atmosphere, these cations can bond with free electrons in the leading to ‘dissociative recombination’ where HCO+ is split into a hydrogen atom and carbon monoxide molecule.
Hydrogen is incredibly light and its ability to escape atmospheres has previously been theorised by the study’s lead authors Michael Chaffin and Eryn Cangi from CU Boulder’s Laboratory for Atmospheric and Space Physics in their analysis of Mars’s history.
They speculate that dissociative recombination in Venus’ upper atmosphere leads to the loss of hydrogen to space, depriving the planet of one of the required elements for water formation.
Hydrogen escape was also a potential consequence of major changes in Earth’s atmospheric behaviour nearly 600 million years ago.
An absence of instruments
“Venus has 100,000 times less water than the Earth, even though it’s basically the same size and mass,” says Chaffin.
But if the behaviour of HCO+ in the atmosphere is the key to understanding water on Venus, there is a major snag that Chaffin and Cangi need to overcome.
While their analysis says the planet-wide drought could only be explained by Venus holding huge amounts of HCO+ in the atmosphere, the molecule has never been measured by the few probes that have gone exploring the planet.
“HCO+ should actually be among the most abundant ions in the Venus atmosphere,” Chaffin says.
With instruments on board previous expeditions to Earth’s nearest neighbour being unable to measure HCO+, the CU Boulder team is hopeful new missions will help confirm their theory.
NASA’s DAVINCI mission will specifically analyse the chemistry of Venus’ atmosphere when it launches at the end of this decade. By 2031, the 1m-wide probe will be lowered into the planet’s thick clouds where it will measure the gases within and photograph the planet’s surface when it emerges.
Like its Soviet predecessors, it’s not expected to last long. It will descent for an hour, and could last for an extra 17 minutes on the planet’s surface.
