RACE TO VENUS
Scientists want to return to Venus, so they can try to find out why it morphed from a pleasant planet into a fiery and in hospitable hell hole
Our planet’s toxic twin has been long ignored. But now space agencies around the world are plotting a return...
We have a toxic twin. Venus is the closest planet to Earth, both in size and often in distance, yet the surface conditions couldn’t be more different. One planet is home to abundant life; the other is hellishly hot, choked by an atmosphere of carbon dioxide that creates a surface pressure equivalent to being almost one kilometre underwater on Earth.
However, things weren’t always this way. Once upon a time, Venus might have had a similar climate to Earth, complete with water oceans and plate tectonics.
Finding out what went wrong with Venus is the question behind a fresh surge in missions to explore the planet. It’s an endeavour that promises to shed new light on how planets become habitable, and could even guide our search for life elsewhere in the cosmos.
Over the past 20 years, exploration of Venus has fallen out of favour. Missions to Mars, Jupiter, Saturn and Pluto have dominated the headlines, and poor old Venus has become something of a forgotten planet. But this wasn’t always the case. In fact, in the early days of space exploration, Venus was our first target…
EARLY EXPLORERS
In 1962, NASA’s Mariner 2 spacecraft flew past Venus, becoming the first space probe to encounter another planet. Five years later, the Soviet Venera 4 probe entered the Venusian atmosphere, becoming the first to enter the atmosphere of another world. The same year, NASA’s Mariner 5 set off on the space agency’s second successful flyby mission. The exploration of Venus was in full swing.
What followed was a series of missions, some failures but mostly successes, to find out more about this planet that, at first glance, appeared so similar to our own. But from the 1980s onwards, the pace slowed down considerably. NASA’s last dedicated mission to Venus was the Magellan spacecraft, which launched in 1989. The reason for this drop-off? As the data started to come back from our twin planet, astronomers interpreted the high temperatures, suffocating atmosphere and impact craters they saw on the surface as evidence that Venus was biologically and geologically dead – and therefore of limited interest to scientists searching for extraterrestrial life or Earthlike geology. The pristine condition of most of Venus’s impact craters, for instance, indicated a comparatively young surface, which suggested that some kind of global, volcanic event in the planet’s history had completely resurfaced the planet, resulting in a dramatic reduction in geological activity.
This idea, however, is still up for debate. “Since then, a lot of people have done [computer] modelling that indicates that this is a very unlikely interpretation,” says Dr Sue Smrekar, a planetary geophysicist at NASA’s Jet Propulsion Lab, and self-confessed ‘Venusophile’. Instead of one huge event, says Smrekar, “you could have ‘steady state’ [smaller and reoccurring] processes of volcanism to produce the impact crater record.” Settling this debate, and discovering the true story of Venus’s history, is the motivation behind a proposed NASA mission that’s being led by Smrekar, called VERITAS.
“FOUR AND A HALF BILLION YEARS AGO, THINGS LOOKED DIFFERENT. YOU’D SEE WATER AND A PLEASANT CLIMATE ON MARS, EARTH AND VENUS”
SAME PLANET, DIFFERENT CLIMATE
Today, the average surface temperature on Venus is 462°C. But the planet wasn’t always such a hot mess. When the Solar System was in its early stages, four and a half billion years ago, things looked different. “You’d see water and a pleasant climate on Mars, Earth, and Venus, most likely,” says Dr Richard Ghail at Royal Holloway, University of London, who is lead scientist of a proposed European Space Agency (ESA) mission to Venus, called EnVision.
Two billion years later, it was a different story. Mars was basically dead and Earth was a frozen snowball, says Ghail. Earth was active as a planet, in the geological sense, but frozen solid, resembling how Jupiter’s moon Europa looks today.
“Venus probably looked like a hot version of the Earth,” says Ghail. “It still had oceans but they were evaporating… it was starting to get really unpleasant.” At this point, “you’d think all three of these planets were doomed biologically. And yet, Earth came out of that and into this new phase where life appeared,” he adds.
Understanding Venus’s geological history will be crucial to piecing together the contrasting fortunes of the two planets. While Venus is not known to be geologically active today, its past patterns of volcanic activity will be a vital clue in helping us find out more about the planet. The amount of volcanism could be linked to the amount of toxic sulphur dioxide in Venus’s atmosphere, for example, which is a key reason why it is uninhabitable. “Ultimately, we want to understand why Venus and Earth are different,” says Smrekar.
The surface of Venus has not been mapped since NASA’s 1989 Magellan mission. “We now have better topography maps for Pluto than we do for Venus, so it’s time for an update,” says Smrekar.
This is where VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) comes in. The aim of this mission, which is currently being considered for funding by the agency’s Discovery Program (a series of lower-cost missions to explore the Solar System), is to use radar and measurements of the planet’s thermal properties to produce