Astronomers have long been intrigued by the idea of panspermia, or the theory that organisms can travel around the solar system on comets and debris from meteor strikes. Now, advances in computer technology have made it possible to simulate the journeys of such objects and map out where they might have traveled. In what may be the most extended simulation yet, a group of researchers led by Rachel Worth of Penn State University has calculated how many Earth rocks large enough to shelter life have been ejected by asteroid impacts over the past 3.5 billion years.
According to their study, published in the journal Astrobiology, the most famous of these impacts–which occurred near Chicxulub, Mexico some 65 million years ago–may have launched some chunks of debris as far as Europa, a satellite of Jupiter that boasts a liquid ocean covered by a crust of ice. A good deal more might have made it the shorter distance (an average of 140 million miles) to Mars. Their conclusions suggest that if extraterrestrial life is discovered on the planets or their moons in future, scientists may have to trace its roots back to Earth. According to the researchers: “Any life found there cannot be assumed to be of independent origin.”
The researchers began their study by estimating the number of rocks larger than three meters that had been ejected from the Earth in major impacts over billions of years. Three meters, they believe, is the minimum size necessary to protect microbes from Sun’s radiation over a journey that could have lasted some 10 million years. While many of these rocks remained in Earth’s orbit, and others were pulled towards the Sun or left the solar system entirely, a relatively small but significant number traveled all the way to Mars and Jupiter’s moons, where they could have sustained life. According to Worth, around 360,000 large rocks reached Mars in historical asteroid impacts, and around six made their way all the way to Europa.
The asteroid that struck near Chicxulub some 65 million years ago is believed to have triggered a massive wave of volcanic eruptions and wildfires across the planet. The resulting dust and smoke may have choked out most life forms, including the giant dinosaurs that had roamed Earth for some 135 million years before disappearing around that same time. The new study holds that the Chicxulub impact also launched 70 billion kilograms of rock into space, of which researchers say 20,000 kilograms could have traveled as far as Europa. They estimate that there’s a better than 50-50 chance that a rock big enough to shelter life arrived at Jupiter’s icy moon. As for the Red Planet, Worth told BBC News that “I’d be surprised if life hasn’t gotten to Mars.” If extraterrestrial life is discovered on the planets or their moons in future, she and her colleagues argue, its roots may have to be traced back to Earth, as “any life found there cannot be assumed to be of independent origin.”
Even if an organism did survive the epic journey, however, it would have to find a hospitable climate. While Europa, Mars and Titan have all likely held water, the thick ice crust on Europa’s lake may have been difficult to penetrate. And while Mars has shown little evidence of flowing water during the last 3.5 billion years (the most likely time for Earth life to have arrived), the reverse journey of Martian material to Earth seems much more likely. According to Worth: “Billions [of meteorites] have fallen on Earth from Mars since the dawn of our planetary system. It is even possible that life on Earth originated on Mars.”
The theory that a giant impact from an asteroid or comet led to the extinction of the dinosaurs has provoked much controversy over the past several decades. It began with the discovery of a layer of clay on Earth’s surface enriched with iridium, a rare metal on Earth but relatively common in space rocks. In 1980, the father-and-son team Luis and Walter Alvarez, along with Frank Asaro and Helen Michel, proposed that the “iridium anomaly” indicated that an extraterrestrial collision caused the demise of the dinosaurs (known as the end-Cretaceous or Cretaceous-Tertiary event, or the K-T boundary). At the time, the conventional wisdom was that the extinction must have occurred gradually, and their theory met with widespread resistance. Then in 1991, researchers confirmed that the giant crater at Chicxulub was formed around the time of the K-T boundary. Much more recent findings, published this past February in the journal Science, indicate that these events occurred no more than 33,000 years apart. Such evidence, along with the iridium anomaly, has convinced many scientists of the likelihood of the asteroid theory, though debate still continues.