MYSTERIES OF THE UNIVERSE

A new study reckons the answer lies high in the Venusian atmosphere

Reported by David Crookes

© NASA

Venus is an especially interesting planet for many reasons, but there’s a particular thirst among astronomers to discover why this world is so incredibly dry. It’s a hot planet with a mean temperature of 462 degrees Celsius (863 degrees Fahrenheit), which makes it hotter than Mercury, so a largely bone-dry desert landscape may well be expected. Yet scientists think oceans once flowed on Venus in a similar vein to the stretches of water on Earth. But what happened to it all?

Dr Eryn Cangi, a research scientist at the University of Colorado Boulder, is keen to get to the bottom of the mystery. On the one hand, she appreciates that we largely know what happened to the surface water on Venus, of which there is now none. On the other hand, she is keen to work out what happened to the smaller amount of water that was left over in the atmosphere – the bits she compares to the drops in a bottle after it’s been emptied.

To make headway, she drew upon her PhD, which focused on Mars. Through that effort, she says she developed a computational model that is flexible and adaptable to more than one type of planet, and this has allowed her to make a breakthrough with Venus that goes a long way towards explaining how and why the situation arose. “We noticed some interesting results when we modelled the Mars atmosphere, and we decided to apply the model to Venus, which has a similar atmospheric composition,” Cangi says of the work carried out by her team. “By understanding Venus and Mars and their history, we can understand how rocky planets may go from cold and uninhabitable like Mars to scorching and uninhabitable like Venus. It may help discover what’s necessary to form and maintain a habitable world like Earth.”

Cangi’s study began around 18 months ago, and it hinged around a key fact. “Venus is similar in size and mass to Earth, and it formed from the same stuff, so we might expect Venus to be similar, making it worthy of the name ‘Earth’s twin’,” she explains. The suspicion is that Venus had as much water as Earth billions of years ago because minimal amounts of water vapour exist in the planet’s clouds, showing it once had a functioning water cycle. But at some point in the past, likely within Venus’ first billion years or so, the early ocean evaporated under intense sunlight, ultraviolet radiation broke water vapour molecules apart and hydrogen escaped into space in a process called hydrodynamic escape. Venus developed a carbon dioxide atmosphere 90 times as thick as Earth’s and a greenhouse effect with an intensity not seen on any other planet in the Solar System.