VENUS and Mars are the closest we will ever come to controlled experiments for Earth. Take three rocky planets, two roughly the same mass, one about one-tenth of that mass. Add water. Make one of the big planets a little closer to your star and the small one a little further out. Now wait 4 billion years and observe.
This is a golden age of comparative planetology. Spacecraft within our solar system, combined with new awareness of exoplanets, are bringing the diversity of planets into focus, which also gives us a听new understanding of our home听world. As a comparative planetologist, I am particularly fascinated by this local trio of rocky worlds with substantial atmospheres. We can鈥檛 test our ideas about planetary evolution with laboratory experiments, so听we are fortunate to have two close neighbours that have a听lot in听common with Earth, as well as听significant differences.
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Our age is fraught because our newly spacefaring civilisation is changing our planet in unplanned ways. An urgent priority today is to understand the human role in climate change. Fortunately, studies of our neighbouring planets can help.
Venus has a nearly pure carbon听dioxide atmosphere with a mass听almost 100听times greater than Earth鈥檚. Historically, attempts to determine the climate of Venus were closely tied to our growing understanding of greenhouse warming on Earth. Now we know听Venus is the epitome of听an听intense atmospheric greenhouse, with a surface temperature of nearly 480掳C.
While we aren鈥檛 close to triggering a Venus-style runaway greenhouse effect on Earth, we are in danger of tipping the balance of听our climate away from the safe zone humans have enjoyed for more than 10,000 years. By testing our climate models on Venus, we听learn how well they predict conditions far from the present-day circumstances of Earth.
For example, one of the weakest points of our current models is predicting the effects of clouds on future climate. Clouds are tricky because of the complex feedbacks between radiation and the physics of droplets. Depending on the details of a particular cloud, they can either cool or warm a planet. By studying phenomena like the clouds of Venus and their effects on the planet鈥檚 extreme climate, we听push our models to the limit and discover hidden weaknesses. This improves our models for the all-important task of predicting Earth鈥檚 changing climate.
Mars has the opposite problem. Its atmosphere is also nearly pure CO鈧�, but lower gravity means most of its air has been lost over time, and what is left is far too thin to have a substantial greenhouse effect. That and its greater distance from the sun have doomed Mars to be a frozen desert. The fact that our models accurately predict the surface climate of the Red Planet provides another extreme test validating our climate tools.
Mars has helped warn us of other potential climate dangers. The theory of nuclear winter was worked out by planetary scientists studying the dramatic Martian global dust storms. They saw what听could happen to a planet completely shrouded in dust particles and realised they could apply the same physics to the extreme case of Earth enshrouded in the smoke from nuclear fires. With studies such as these, Earth鈥檚听sibling worlds become labs听for听understanding听鈥� and circumventing听鈥� the dangerous, unintended consequences of our increasingly powerful technology.
Let us hope that, helped by the knowledge we are getting from studying our closest planetary neighbours and the perspective space exploration provides, we can also gain the wisdom to avoid precipitating future disasters and learn how to sustain the unique biosphere of our home world.
David Grinspoon is senior scientist at the Planetary Science Institute in Arizona. He will be appearing at the sixth in Armenia
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