It should be no surprise that Venus is dry. It is famous for its hellish conditions, with dense sulfur clouds, acid rain, atmospheric pressures comparable to that of a 900-meter-deep lake, and a surface temperature high enough to melt lead. But the lack of water is not only due to the lack of rain and oceans: there is no ice or water vapor either. Like Earth, Venus lies within the goldilocks zone of our Solar System, so it would have had a lot of water when it first formed. So where did all the water on Venus go?
Venus is an extremely dry planet, although this was not always the case. At some point in its history an uncontrolled greenhouse effect began that ended in its current extreme state. Most models agree that this process would have removed most of the original water, but that some should still remain. And yet, observations show us that there is practically no water. Planetary scientists at the University of Colorado Boulder believe they have found an explanation: a molecule called HCO+ high in Venus' atmosphere may be responsible. Unfortunately, they may have to wait for future missions to Venus before they can confirm this.
Until the mid-20th century, Venus was thought to be Earth's twin. Both planets are about the same size and mass, and both are within the sun's habitable zone, the region where temperatures can exist warm enough to melt ice, but not so hot that water turns into vapor. It was long assumed that beneath its bright white cloud cover, Venus must have a climate similar to that of Earth. Science fiction authors even wrote stories about visitors from Venus who explored verdant jungles and encountered exotic civilizations. But the truth is a lot harder: Venus is an extreme place, with sulfuric acid rains, crushing atmospheric pressure, and a surface temperature high enough to melt lead. But it was not always like this.
The general assumption among astronomers and planetary scientists is that both Earth and Venus began life with similar amounts of water. But something happened that released huge amounts of carbon dioxide into its atmosphere, causing an extreme runaway greenhouse effect. The high temperatures melted the ice and evaporated the liquid water, filling the atmosphere with water vapor. Much of this hot vapor would eventually fly off into space, drying out the planet, but some should remain. The puzzle is that standard models predict that there is much more water vapor left than is actually there. So what happened?
According to a study, led by Dr. Eryn Cangi and Dr. Mike Chafin, both of the Laboratory of Atmospheric and Space Physics (LASP), the answer may be a molecule called HCO+. In their previous work studying the atmosphere of Mars, they discovered a process by which this molecule can remove water from planetary atmospheres. In their new paper, they suggest that the same process could be at work on Venus. The only drawback is that this molecule has never been detected in the atmosphere of Venus.
Unfortunately, there is little evidence to confirm this theory. HCO+ has never been detected in the atmosphere of Venus. However, Cangi and Chafin point out that this is because no one has ever looked for it and none of the missions sent so far to Venus were equipped with instruments that could detect it. However, they are optimistic for future missions.
“One of the surprising conclusions of this work is that HCO+ should be among the most abundant ions in the atmosphere of Venus,” says Chaffin.
“There haven't been many missions to Venus,” adds Cangi. “But the newly planned missions will leverage decades of collective experience and a burgeoning interest in Venus to explore the extremes of planetary atmospheres, evolution and habitability.”
The planetary science community has become increasingly interested in Venus and several future missions are planned to study it in more detail. NASA's planned Deep Atmosphere Venus Imaging, Chemistry and Noble Gas Investigation (DAVINCI) mission is one example. DAVINCI will drop a probe to the surface, which will study the atmosphere at different altitudes as it falls. Unfortunately for Cangi and Chafin, it's not specifically designed to look for HCO+, but it may reveal other clues to confirm or refute their theory. But they remain optimistic that additional missions will be sent in the future carrying the necessary instruments they can use to test their work.
For more information, visit CU Boulder's announcement at https://www.colorado.edu/today/2024/05/06/venus-has-almost-no-water-new-study-may-reveal-why
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