Texas-size asteroids make for exciting summer blockbusters, but when it comes to long-term damage, they’re not the most menacing threat out there. Lurking at the edge of our galaxy are giant molecular dust clouds — agglomerations of hydrogen gas, small organic molecules and minerals — roughly 150 light-years across. If our solar system hit one, it would take 100,000 years to pop out on the other side.
During that time, the dust would accumulate in the atmosphere and block out nearly all light from the sun. The oceans would freeze over and terrestrial plants would die off, leading to a near-total extinction of life, says Alexander Pavlov, a NASA-affiliated astrobiologist at the University of Arizona. The good news? We probably won’t hit one for at least 40 million years.
Look closer to home for a less catastrophic disaster-starter, Pavlov says. Underwater volcanoes could spew hydrogen sulfide and carbon dioxide into the atmosphere, which many scientists think caused the worst mass extinction in history 250 million years ago, when 90 percent of all marine species and 70 percent of land animals died. But there’s an upside even to this: Bacteria and other microorganisms thrive in these conditions, so there’s a good chance of life hitting the restart button.