By Frank D. Roylance
Panspermia is the name scientists give to the notion that life could spread among the planets aboard meteorites - hitchhikers on rocks blasted off one planet and hurled through space until they fall to seed new life on another.
One problem with this theory: traveling organisms would have to survive the seemingly lethal heat and impact of their arrival on their new home.
The destruction of the space shuttle Columbia three years ago is providing new evidence that life - even fairly complicated life - can survive the searing heat of an uncontrolled fall through the Earth's atmosphere.
In November, scientists from NASA's Ames Research Center in Mountain View, Calif., reported that part of a colony of tiny worms called nematodes was found alive in five canisters recovered from Columbia wreckage that fell in Texas near San Augustine and Bronson.
Now another team, led by biologist Robert J.C. McLean of Texas State University in San Marcos, will report in the journal Icarus that it found living samples of a slow-growing, heat-tolerant bacterium that also survived Columbia's fiery destruction.
Both turned up in parts of biology experiments that rode with other science equipment in lockers on Columbia's mid-deck.
Their survival is one link in a chain of evidence needed to prove the plausibility of panspermia, said Bruce Runnegar, science director at NASA's Astrobiology Institute. "But it's a very encouraging result. ... I think all reasonable scientists regard the idea as plausible."
Space slime from Mars might be a good candidate for interplanetary transport, scientists say. "Mars rocks are falling on Earth as we speak. Some have actually been observed," Runnegar said. "Integrate that over billions of years, and you have an enormous number of repeated experiments.
"That doesn't say it's ever happened," he said. "But very strange things have happened in nature that take a long time to demonstrate scientifically."
Columbia and its seven astronauts were lost Feb. 1, 2003, as the spacecraft returned from a 16-day scientific mission.
An investigation concluded that foam insulation broke away from the shuttle's external fuel tank during liftoff. It punched a hole in the heat-resistant panels on the leading edge of the space plane's left wing.
The damage went unnoticed for most of the mission. During re-entry, however, gases heated by friction to 3,000 degrees Fahrenheit penetrated the wing. The heat damage caused the craft to break apart when it was 61 miles above the Earth and traveling at 12,500 mph - more than 16 times the speed of sound.
In addition to its crew, Columbia carried 80 scientific experiments, including a variety of living organisms ranging from bacteria to rats.
Shuttle debris rained down across hundreds of square miles of Texas and Louisiana, in pieces that were eventually collected and returned to Houston. Several months later, a few scientists began to get back what remained of their experiments.
The Ames Research Center team examined the recovered canisters and found evidence of melting, suggesting that the exterior reached temperatures above 1,000 degrees Fahrenheit.
Inside, partly melted plastic components suggested temperatures as high as 500 F. But only a portion of the petri dishes containing the nematodes reached their melting point of 147 F to 176 F.
Other experiments have shown that nematodes are quickly killed at temperatures above 104 F. "These observations suggest that, though the exterior of the canisters became quite hot, heat transfer toward the center of each canister was inefficient," the scientists reported.
That fits well with theories about the potential survival of living organisms in meteors entering the Earth's atmosphere.
"It's also fairly clear that these can be lifted off Mars and transported to us without too much internal heating," NASA's Runnegar said. And while their exteriors are scorched by passage through the Earth's atmosphere, "thermal inertia" - the slow penetration of that heat into the interior of the meteorites - allows them to stay cool on the inside.
The fall to Earth, he said, "is so fast compared with the propagation of heat into the interior, that it is plausible that organisms would survive."
McLean estimated that a meteorite would zip through the atmosphere in five seconds, compared with several minutes for the Columbia debris.
"And the force of impact [of the Columbia wreckage] was a lot less," he said.
The scientists at Ames estimated their canisters' deceleration at impact was about 2,500 Gs - 2,500 times the force of Earth's gravity. But nematodes commonly withstand up to 10,000 Gs during laboratory work involving centrifuge machines.
The Texas State experiment involved three species of microbe, sealed inside plastic containers that rode inside aluminum cases. The goal of McLean and his students was to see which species out-competed the others in microgravity, and whether the outcome was different than on Earth.
The work has implications for long-duration space flights, he said. Planners need to know whether health hazards might arise if pathogenic bacteria on board a flight to Mars, for example, behave and multiply in unexpected ways.
The team's scorched aluminum container crashed onto a Texas parking lot. When team members got it back in May, they opened the container and tried to grow and measure the populations of microbial passengers - while preventing contamination with new bacteria.
They found that all three species had been wiped out during re-entry. But a fourth turned up in their place. DNA tests identified it as Microbispora sp., a heat-tolerant soil bacterium.
Because the containers' seals were intact, and the experiment was run under sterile conditions, McLean believes the stowaway organism was an airborne contaminant that got into the container during assembly before the launch.
But making the journey from Mars to Earth involves much more than being ejected from one planet and landing safely on the next. There's also a long trip --- perhaps hundreds of thousands of years - through the vacuum and radiation of interplanetary space.
To survive radiation, a microbe would have to be deep within the rock. "Rocks are pretty good radiation shields, particularly for the kinds of radiation or energetic particles likely to be in solar-system space," Runnegar said.
But the cold and vacuum of space aren't necessarily enemies of traveling microbes, said Texas State's McLean. "They're likely to be freeze-dried. That's how we preserve bacteria," he said.
While the Columbia accident didn't duplicate a meteorite's fall to Earth, Mclean said, it was a much-closer approximation of those conditions than anything else done to date, and lends "a fair amount of support" to panspermia theorists.
"Ultimately we have to do the experiments," he said, but "I would hope that we on this Earth are not alone, and that this might be a mechanism of how small life forms can spread from one planet to another."