Over time, that water has nearly all been lost. Figuring out how, when and why Venus lost its water helps planetary scientists like me understand what makes a planet habitable — or what can make a habitable planet transform into an uninhabitable world.
https://theconversation.com/venus-is-losing-water-faster-than-previously-thought-heres-what-that-could-mean-for-the-early-planets-habitability-229342
The process in which Venus lost most of its water is called “hydrodynamic escape.” When Venus got too hot, the hydrogen in its atmosphere left. (The linked article explains this using a metaphor of having too many blankets on your bed.)
However, Venus is still losing hydrogen, even though there is too little of it for hydrodynamic escape to work. So, logically, there must be another process at work: “HCO⁺ dissociative recombination,” in which individually positively charged atoms of hydrogen, carbon, and oxygen in the atmosphere react with negatively charged electrons. The process was first examined on Mars, and based on a reexamination of results from previous missions (Pioneer Venus 1 and Pioneer Venus 2), the same research group thinks it’s time to try it on Venus…perhaps a first step to seeing whether Venus had life at some point.
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