On August 7, 1996, a chunk of rock made front-page news. It was a meteorite from Mars that was believed to contain fossils of one-celled life forms. Although that particular claim is still the subject of much debate, scientists are still intrigued by the possibility that microbes from Mars may have once seeded the Earth. In this Science Update, you'll hear about an unusual experiment that could help provide the answer.
Bacteria ballistics. I'm Bob Hirshon and this is Science Update.
Wayne Nicholson has a gun that shoots bullets laced with bacteria. He's not a secret agent; he's a microbiologist at the University of Arizona. He and planentary scientist Jay Milosh are trying to figure out if microbes from Mars ever could have reached the Earth.
He says that if microbes ever existed on the Martian surface, they might have hitched a ride to Earth on a meteorite: a piece of Mars' crust that would have been knocked loose by the impact of an asteroid. To see if bacteria could even survive this initial impact, he and Milosh shot pellets seeded with bacteria into hardened clay.
We've been able to replicate the acceleration forces that would be generated from an impact-mediated launch from Mars. And we've been finding that the bacteria survive quite nicely.
Next, they'll borrow a giant gas gun from NASA, and fire it at a granite wall filled with bacterial spores.
We're going to catch the fragments that come flying off the target and see if they've undergone shock damage, and we'll also cultivate those particles to see if any of the passenger bacteria have survived.
If they do, it'll support the idea that life can travel from one planet to another. For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
You may or may not remember the hoopla surrounding ALH84001, the meteorite that became an international celebrity in 1996 and gave new meaning to the term "rock star." This meteorite contained tiny fossils that appeared to have been created by bacteria. Some announced it as nothing less than proof that life existed on Mars, at least in the distant past.
For some scientists, the Mars meteorite had a slightly different meaning: it gave credence to a hypothesis called panspermia. Panspermia answers the tricky question of how life first began on Earth by suggesting that it was seeded with organic molecules and microorganisms from outer space, which piggybacked on comets, asteroids, and meteors that collided with our young planet.
Of course, panspermia presupposes that life was already chugging along elsewhere in the universe—otherwise, how would it end up on comets and meteorites? And it also supposes that life would survive being knocked off another planet (by another comet or asteroid impact, for example) and hurtled into space. Certainly a human being wouldn't live through that, but what about bacteria? That's what Nicholson's experiments are trying to find out.
Nicholson's team can't use the actual Martian bacteria to answer the question, since after all, nobody really knows if those bacteria even existed. Instead, they use a common species of bacteria called Bacillus subtilis, which is known for surviving in harsh conditions. (The assumption is that if these guys couldn't survive a cataclysmic collision, other bacteria probably wouldn't either.)
Their goal is to see if the bacteria can survive the rapid acceleration forces that would be needed to knock them off the surface of Mars and into outer space. Shooting bullets into the wall basically tests this in reverse: the bullets are fired at a high speed, and then slammed quickly to a halt by the wall. Fortunately, in physics, acceleration and deceleration are the same thing, so the fact that these bacteria survived the deceleration indicates that they would also survive an equivalent acceleration.
But there are other things to survive besides the acceleration: when a comet or meteor slams into a planet, it also creates tremendous heat and pressure. Those are two things that will be studied when they start the experiments with NASA's giant gas gun.
Now try and answer these questions:
- What is the panspermia hypothesis?
- What question are Nicholson and his colleagues trying to answer?
- What are the advantages and limitations of Nicholson's experiments?
- Can you think of other questions that would need to be answered in order to support the panspermia hypothesis?
This article, Panspermia: Spreading Life Through the Universe at SETI.org, summarizes the history and philosophy of the panspermia hypothesis.
NASA's Jet Propulsion Lab has a site called Mars Meteorites, dedicated to tracking and analyzing chunks of Mars that are found on Earth.
Astrobiology, a site run by NASA's Ames Research Center, is devoted to the search for signs of life in outer space, past or present.