More than four billion years ago, Venus had enough water to cover its surface with an ocean 3 km deep. Today, the planet only has enough for this ocean to be 3 cm deep.
Scientists have been able to account for a lot of the water Venus lost in this time — but not all of it. Now, a team of scientists in the U.S. may have made a crucial advance.
The team’s findings, reported in a paper in Nature, could plug a long-standing gap between the amount of water scientists expected Venus to have lost in the last 4.5 billion years and how much satellite observations say the planet has actually lost, which is a lot more.
“We have a pretty cool thing here,” co-author Eryn Cangi, a planetary scientist at the University of Colorado Boulder, told The Hindu. “And it was time to release it into the community and see what they make of it.”
“The bigger picture is about planetary habitability, and more specifically the history of water on Venus compared to the earth,” Emmanuel Marcq, a planetary scientist at France’s Laboratoire Atmosphères, Milieux, Observations Spatiales, who was not involved in the study, said.
Following the water trail
There are two reasons why Venus lost its water. The first is its hellish atmosphere — a result of its carbon dioxide-rich composition, which causes a strong greenhouse effect. The planet’s surface is hotter than water’s boiling point, simmering at 450 degrees C. So water can only exist as vapor in Venus’ atmosphere.
Second, water was a victim of the planet’s proximity to the Sun. The Sun’s heat and ultraviolet radiation combined to shred water molecules into their constituent hydrogen and oxygen atoms in Venus’s ionosphere — the upper region of the atmosphere, where charged atoms, molecules, and their electrons zoom around at high speeds.
However, we don’t know the rates at which these processes happened. “There’s a couple different theories about how [water levels] changed over time,” Dr. Cangi said.
The two theories broadly blame thermal and non-thermal processes for the water loss.
The thermal process refers to hydrodynamic escape. As the Sun heated Venus’s outer atmosphere, it expanded, allowing hydrogen gas to leak to space. This escape lasted until the outer atmosphere sufficiently cooled, by about 2.5 billion years ago.
Dr. Cangi and her colleagues’ research focused on how water loss occurs in the present day, specifically via a non-thermal process.
They focused on hydrogen atoms escaping Venus to space. Water levels drop as a result because the oxygen atoms left behind have fewer hydrogen atoms with which to form water.
However, estimates of the water-loss rate before Dr. Cangi’s study suggested the planet had more water than what satellite observations indicated.
Dr. Cangi and her colleagues reported that the discrepancy vanished when they accounted for a chemical reaction that, according to a statement accompanying the paper, the scientific community has overlooked for more than five decades.
The key findings
Dr. Cangi first encountered the formyl cation (HCO+) — a positively charged molecule — during her PhD days, when she was studying water loss in Mars’ atmosphere.
Scientists have known for a while that HCO+ molecules drive hydrogen escape on Mars. According to Dr. Cangi, the Venusian and the Martian upper atmospheres are similar, so she and her colleagues decided to model the same underlying reactions in Venus’ ionosphere.
On Venus, the team found that a particular reaction, called the HCO+ dissociative recombination reaction (DR) occurs in bulk at an altitude of about 125 km, above the clouds made of sulphuric acid.
HCO+ is created when a carbon monoxide molecule (CO) loses an electron while absorbing an hydrogen atom. DR is the reverse reaction: HCO+ absorbs an electron and breaks up into CO and an hydrogen atom. These energetic hydrogen atoms then escape into space.
The team built models to simulate the influence of this reaction on the upper atmosphere, and found that it accelerated water decline once the hydrodynamic escape of hydrogen gas ended.
Specifically, the researchers found HCO+ DR could have doubled the rate at which Venus lost water by hydrogen escape.
This means if Venus had oceans in the past, they could have lasted longer than expected — because the faster rate of hydrogen escape means the planet could have lost more water in the same amount of time.
Further, the model predicted that the amount of water on Venus would have stayed roughly the same from nearly 2 billion years ago. This is because, as a non-thermal process, the HCO+ DR reaction would’ve gone on indefinitely and drained all the water. (The thermal process was time-bound because the upper atmosphere returned to thermal equilibrium). Yet Venus still has some water today.
According to Dr. Marcq, one way water could have been replenished was by comet impacts.
The missing molecule
However, we have no proof that HCO+ ions existed in Venus’s atmosphere in the first place — let alone proof that they participated in the HCO+ DR process.
The authors wrote in their paper that past space missions had neglected looking for HCO+ ions, and that orbiters sent to Venus couldn’t decipher the chemical signatures of HCO+ DR from afar. These missions instead paid attention to other important atmospheric chemical reactions that scientists were interested in. According to Dr. Cangi, there would have had to be a connection between HCO+ DR and water loss on Venus for scientists to have shown interest.
This said, she said the team’s analysis of data collected by the NASA Pioneer Venus orbiter (launched 1978) contained some indirect evidence of HCO+ DR.
“By looking at the other molecules that are important in the chemistry to form it, we saw that those are present in an amount that would imply [HCO+] should be there,” Dr. Cangi said.
Future Venus missions
Dr. Marcq referred to aNature Astronomy paper published in April in which scientists reported finding a signature of carbon ions escaping Venus in data collected by the BepiColombo spacecraft. “At least qualitatively, it seems to support the [HCO+ DR] model,” Dr. Marcq said. The quantitative evidence remains wanting.
Dr. Cangi implored scientists working on future Venus missions to look for HCO+ in the planet’s upper atmosphere. She referred to NASA’s MAVEN mission to Mars as an example of a mission dedicated to probing the upper atmosphere. “If we had a similar mission to Venus, I think we could learn a lot.”
Most upcoming Venus missions are focused on the lower atmosphere instead.
“The fact that Venus is [100,000-times] drier than the earth is … an anomaly that deserves an explanation,” Dr. Marcq said. “Is Venus abnormally dry? Is the earth abnormally wet? Depending on which one is the exception, the implications for planetary habitability are different.”
Karthik Vinod is an intern with The Hindu.
