Scientists May Have Solved Mystery of Venus' Lost Water

Our planetary neighbor Venus is thought to have once had water, like Earth, but how it became the hellish world it is today has remained a mystery to scientists for decades.

Now, however, researchers suggest that Venus may have lost its water via a chemical reaction called HCO+ dissociative recombination, according to a new study in the journal Nature.

This new theory may solve some discrepancies in previous hypotheses of how Venus dried out, the researchers say.

Venus is the second planet from our sun, and it is often called Earth's "sister planet" because of its similar size and proximity. Venus has a thick atmosphere composed mainly of carbon dioxide, with clouds of sulfuric acid, making it opaque in visible light. Its atmosphere is much denser and hotter than Earth's, with the atmospheric pressure at its surface about 92 times that of Earth's—equivalent to being about 3,000 feet underwater on our planet. The terrain is mostly a barren, rocky landscape covered in a layer of sulfuric dust, and there are thousands of volcanoes, some of which may still be active.

"Venus is not a nice place. Metal space-exploration landers melt in minutes. Mean surface temperature is 867 degrees F. This is because 1) it is closer to the sun than Earth, so warmer, and 2) because it has a super-greenhouse atmosphere composed of 96 percent CO2 (carbon dioxide). Life would literally cook on the surface now and be reduced to tar," Martin van Kranendonk, an astrobiology and geology professor at Curtin University, told Newsweek. "And, of course, there is no water on Venus, which is host to all life on Earth."

It is hypothesized that Venus may have had a substantial amount of water early in its history, possibly in the form of oceans or large bodies of surface water. Over time, as solar radiation increased and geological activity released more carbon dioxide, Venus might have undergone a runaway greenhouse effect. This would have led to the evaporation of any surface water, an increase in atmospheric pressure and subsequent extreme temperatures, transforming the planet into the inhospitable place it is today.

According to the research, Venus may have lost all its evaporated water from its atmosphere via a chemical reaction called HCO+ dissociative recombination. This is a process by which HCO+ ions combine with electrons to form carbon monoxide and hydrogen atoms, which are then lost to space.

Previous theories suggested that Venus' water was lost via a process called hydrodynamic outflow, which describes how gas escapes from the atmosphere of a planet.

However, this process could not have removed enough water to result in the dry conditions we see on Venus today. HCO+ dissociative recombination, on the other hand, would mean water was lost at twice the rate predicted in hydrodynamic outflow. It would also explain any discrepancies in data from previous Venus spacecraft instruments.

"This process nearly doubles the Venus H escape rate and, consequently, doubles the amount of present-day volcanic water outgassing and/or impactor infall required to maintain a steady-state atmospheric water abundance. These higher loss rates resolve long-standing difficulties in simultaneously explaining the measured abundance and isotope ratio of Venusian water and would enable faster desiccation in the wake of speculative late ocean scenarios," the researchers wrote in the paper.

Further missions to Venus are needed to measure whether HCO+ dissociative recombination is truly the way Venus lost most of its water.

The researchers wrote, "Design limitations prevented past Venus missions from measuring both HCO+ and the escaping hydrogen produced by its recombination; future spacecraft measurements are imperative."

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