The Earth shrugs and the ground shifts. Houses crumble and children scream. This planet can be a dangerous place to live.
On October 29, 2008 a magnitude 6.4 earthquake stuck near Quetta, Pakistan, killing hundreds and leaving tens of thousands homeless in a remote, mountainous area already ravaged by conflict and oppression. This event was both tragic and ordinary. The planet we live on does this kind of thing all the time. Indeed, Earth seems to be constructed almost perfectly to create such calamities, with an outer shell, the surface on which we live, just thick enough to be rigid but breakable. It is broken in about a dozen pieces which slide around, riding around on the convecting cauldron of Earth’s hot mantle, pulling apart and smashing together.
The people of Southwestern Pakistan live where the Indian plate is slowly, inexorably colliding with the Eurasian plate, buckling and wrinkling the obstinate Earth in twitches and shudders which, to us insects riding on these breaking plates, can be catastrophic.
Earth is a minefield of lethal hazards. Is this instability simply a fact of planetary existence? No. Most solid planets have stable plates and relatively quiescent surfaces. So why are we cursed to inhabit a planet with the strange combination of factors that makes life here so risky?
Actually, this is the only kind of planet we could come from. Take Mars. Compared to Earth, the Red planet looks dead even from space –relatively uniform in color and rusted over because there is nothing new there under the sun. Mars does have a sun-stirred atmosphere and dramatic seasons, but the wind-whipped, frost-heaved surface overlies a dead crust – way too cold and thick to break into competing plates. On Mars you would not have to worry about Marsquakes or volcanoes. And it is no coincidence that there do not appear to be Martians who can worry about such things.
Because ultimately it is these same tragic flaws that make Earth’s surface so fit for life. We inhabit the interface between two giant heat engines: a churning, hot interior flexes the faults and feeds the fertile flows which continually renew Earth’s nutrient rich surface, while venting volcanic gases that refresh our atmosphere. This atmosphere sometimes rages into violent storms that splinter or flood cities, but it also maintains our comfortable climate and its solar-powered weather feeds and waters us.
Other than the external threat of asteroid and comet strikes – a common hazard throughout the solar system, all of Earth’s natural hazards go hand in hand with its habitability. Even wildfires are a manifestation of Earth’s fertility. On Mars, even if there was anything organic to burn, fire would not be a threat because the stale atmosphere is in near-perfect equilibrium. Here on Earth plant life pushes the oxygen level up toward unsafe levels. Fire is the atmosphere fighting back. This same disequilibrium, tamed by enzymes within our cells, fuels the slow fires of our animal metabolism.
Looking around the solar system we do not find another surface with this dangerous combination of meteorological and geological hazards. Jupiter’s Moon Io is so geologically active its surface changes every time we look, but there is no atmosphere to speak of. Saturn’s moon Titan may come the closest to possessing both active geology and meteorology. Not surprisingly, Titan is high on our list for future astrobiological exploration.
Our mysterious neighbor Venus is shrouded in clouds and possesses a greenhouse climate so severe it is hard for even our machines to explore. As a result, we still have no idea how frequent Venusquakes are. This is a huge gap in our knowledge – arguably the largest gap in our picture of comparative planetology. Determining whether or not Venus is currently active is a prime objective for future missions.
Tsunamis, earthquakes, tornados, hurricanes, blizzards, avalanches, lightning, fires and floods ; these destructive elements are also the symptoms of Earth’s productive, creative energy flows. We may even want to add the right combination of natural disasters to our list of “bioindicators” on distant planets. Only dangerous places will ultimately reward our searches for alien biology. As we explore the universe, we should seek other places that are comfortable for life – but not too comfortable.