Water is life. This precious resource was crucial to the evolution of life on Earth. In the distant past, fellow rocky planets Mars and Venus had plenty of water too, but they are now literally worlds apart. Venus is a pressurized acid hell and Mars is a frigid, dry desert.
Now, researchers have taken a closer look at their loss of water into space. Mars' moisture is regularly stripped from its atmosphere and lost to the depths of space. On the other hand, Venus seems to be holding on to most of its original water.
As reported in Science, researchers have analyzed data from NASA’s MAVEN spacecraft and found evidence that the water in Mars’ atmosphere is transported high into the atmosphere, where water molecules are destroyed by other ions present in the air. The oxygen in the water reacts with the ions, and the freed hydrogen is lost to space.
The team was able to track this behavior seasonally to show that water abundance peaked during the southern summer. And more strikingly, dust storms like the one that killed NASA's rover Opportunity increase the atmospheric loss.
“The seasonal and dust storm-mediated delivery of water to the upper atmosphere could have played a substantial role in the evolution of the Martian climate from its warm and wet state billions of years ago to the cold and dry planet we observe today,” wrote the author in the paper. “Mars has likely lost enough H2O to cover the planet’s surface with an ocean tens to hundreds of meters deep, and loss rates must have been higher in the past.”
On Venus, the situation seems to be reversed. Moa Persson from the Institute for Space Physics, IRF, and Umeå University has just completed her thesis on the subject. Using data from the European Space Agency’s Venus Express, she was able to estimate how much water Venus has lost over the last 4 billion years. The amount of hydrogen and oxygen lost to space depends on the intensity of sunlight and the solar wind. According to Persson’s research, Venus is losing water to space but it is less than previously thought.
Future missions to Venus might characterize the evolution of its atmosphere even better and maybe confirm or deny the complex issue of its phosphine.