It may sometimes be called Earth’s twin, but the relationship between our home and its nearest planetary neighbour is more Jekyll and Hyde. “Early on we believe Venus had oceans and quite possibly life,” says David Grinspoon of the University of Denver in Colorado. That makes sense; Venus is the same size and composition as Earth, and gets roughly the same amount of sunlight. It is technically inside the solar system’s habitable zone where liquid water can exist. So how did it become so inhospitable?
We are still not entirely sure. That seems odd, considering we have been sending probes to Venus since the dawn of the space age. But our attempts to peer at the planet have been thwarted by its opaque sulphuric acid clouds, impenetrable to early orbiters. Of the craft we have sent to investigate the surface, less than half survived the trip, the rest collapsing under the punishing pressures Venus’s atmosphere generates. The few survivors didn’t last long. Between them, they amassed less than a single day of ground observations.
Advertisement
What they saw could have been painted by Salvador Dali: a dim, desolate wasteland pitted by endless sulphuric acid rain and scoured by syrupy winds whose speeds are dictated by the time – fast at dawn and dusk but slowing in the heat of the day. If the choking, largely carbon dioxide atmosphere doesn’t kill you, the heat – a lead-melting 460 °C – surely will.
These scenes support the standard explanation for Venus’s predicament: the planet is just a bit too close to the sun. This caused any existing water to boil into a thick atmosphere that trapped heat, leading to a runaway greenhouse effect and today’s hellish conditions.
But observations from the Venus Express orbiter cast this simple theory into doubt. In 2007, it spotted hydrogen and oxygen ions – what was left of the planet’s water – streaming off the planet. The cause was the solar wind, a stream of charged particles that sails right through Venus’s inconsequential magnetic field. Its passage also triggers regular plasma explosions that rip huge chunks off the planet’s atmosphere.
Under this constant assault, how can Venus sustain any atmosphere at all, not to mention such a heavy one? The most likely candidate is sulphur and carbon dioxide released by volcanoes at the surface, says Colin Wilson at the University of Oxford. As yet, no one has found active volcanism on the planet. But the evidence is piling up. Venus Express also discovered that volcanic flows make up 80 per cent of the planet’s surface. Some may be just tens of thousands of years old.
“Under this constant assault, how can Venus sustain any atmosphere at all, not to mention such a heavy one? ”
More certainty about Venus’s past would help us rule out similar dead-end planets in our search for Earth-like worlds around other stars. We might also find out if a similar fate is in store for us. According to the accepted theory, says Grinspoon, Earth’s climate will begin to resemble Venus’s in about 2 billion years as the sun ages and slowly heats up.
But what if the models are wrong? What if Earth is actually closer to the edge of the habitable zone than we assumed? And what if some unknown variable could tip us over the brink of destruction much sooner? “It looks like it’s quite possible for an Earth-like planet to lose all of its water to space,” says Glyn Collinson of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “And that’s terrifying.” Such questions have prompted a flurry of proposals for a return to Venus, hoping to find out whether it was always destined to be an uninhabitable wasteland.
Is it, though? Even today, this is an open question: in the cloud decks 70 kilometres above Venus’s infernal surface, the weather gets downright balmy – plenty of sunshine and water, and Earth-like pressures and temperatures. Thanks to these conditions, says Grinspoon, “there is possibly life in the clouds”.
To explore would require some kind of atmospheric rover. Aerospace giant Northrop Grumman has designed an that could bob around the planet for a year, sniffing for signs of life. that could carry scientists to these clouds – perhaps even doubling as a test bed for a Mars mission. They may not be twins, but Venus and Earth share a family resemblance.
Read more: The 6 greatest mysteries of the solar system
(Image: NASA/JPL)
This article appeared in print under the headline “What happened to Venus?”
