Surviving the Final Frontier
Gerda Horneck of DLR German Aerospace Center (shown above) credits the longevity of spores to the well-protected DNA within them.
It came from outer space. Life, that is.
This concept has drifted around the universe of space science since at least as long ago as 1864, when William Thomson Kelvin told the Royal Society of Edinburgh "The hypothesis that life originated on this earth through moss-grown fragments from the ruins of another world may seem wild and visionary; all I maintain is that it is not unscientific." He repeated the assertion in 1871 at the Forty-First Meeting of the British Association for the Advancement of Science, using the less colorful term "seed-bearing meteoritic stones."
In 1903, in the German journal Umschau, Svante Arrhenius removed the meteors from the equation. Instead, he wrote, individual spores wafted throughout space, colonizing any hospitable planet they lit on. Arrhenius named the theory panspermia.
"Spores," says Gerda Horneck, of DLR German Aerospace Center in Köln, "can withstand a variety of different hostile conditions: heat, radiation, desiccation, chemical substances, such as alcohol, acetone and others. They have an extremely long shelf-life. This is because the sensitive material, the DNA, is especially packed and protected in the spores."
As tough as bacterial spores are, however, they cannot survive direct exposure to solar ultraviolet (UV) radiation, Horneck writes in Origins of Life and Evolution of the Biosphere, 2001. But while Arhennius's panspermia is out, Kelvin's fanciful "moss-grown fragments" may be back in, after a fashion.
Horneck assessed the protective effect of meteorlike matter in an experiment on three flights of the Russian FOTON satellite in 1994, 1997 and 1999. FOTON carried an appliance called BIOPAN. Once in space, the BIOPAN lid flips open, like the top of a waffle iron, exposing experiments inside to the cold vacuum of space, and, when BIOPAN is in the sun, to ultraviolet and other radiation with no intervening atmosphere. FOTON rotates, so BIOPAN passes in and out of the sun both during rotation and during each 90 minute orbit.
In an earlier experiment on the Long Duration Exposure Facility, flown by NASA from 1984 to 1990, Horneck found that even after six years in space, more than two-thirds of bacterial spores sprouted back on Earth. But those spores were protected by a thin aluminum cover as well as chemical protectants. Would dirt do?
Spores, like the one shown above, are sensitive to UV rays that damage their DNA.
Credit: S. Pankratz
Horneck and her colleagues embedded spores from the common bacterium Bacillus subtilis in a variety of materials: clay, red sandstone, grit from the meteorite Millbillillie, simulated Martian soil and sand from the Martian meteorite Zagami. Some spores were laid in layers of the dust, others mixed and stored in artificial meteorites a centimeter on a side, still others exposed directly to space or shaded by a layer of dust. They remained exposed in BIOPAN for up to two weeks.
"In the selection of the rock or soil samples, we got advice from experts working with meteorites and geologists interested in Mars research," Horneck says. "Some of the material (clay) was used in previous experiments and all others were used for the first time."
Only one in a million spores exposed to space or merely shaded survived. Hard UV directly damages DNA, causing chemical crosslinking and changes in bases, Horneck says.
But spores spared exposure to UV and other light-that is, stored in the dark-fared well, with between 50% and 97% survival, Horneck writes.
Horneck tried two methods of protecting spores with various soils and sands. In the first, she made a sort of layer cake, alternating layers of spores with layers of soil or clay, etc. In the second, she mixed spores and soils in about the ratio found in earthbound soils-a hundred million cells per gram. In both cases, the spores were in direct contact with the soil grains.
Horneck assessed the protective effect of meteorlike matter in an experiment on three flights of the Russian FOTON satellite (shown above) in 1994, 1997 and 1999.
These spores survived as well as spores stored in the dark. On one flight, 100 percent of the spores exposed in such artificial meteorites survived.
Horneck's results lend more support to the modern variant of Lord Kelvin's moss-grown rock idea. When large objects impact a planet, there's a sort of splash zone in a ring abound the impact. Large chunks of planet may shoot into space. Impacts this large were common late in the formation of the solar system. But even as recently as about 65 million years ago a meteor hit Mexico, ejecting so much material into the atmosphere that it's suspected of causing the extinction of the dinosaurs.
"This concept, called also litho-panspermia, is based on the scenario that by an impact of a very large kilometer-sized meteor or comet, material is ejected from a planet and can reach escape velocity," Horneck explains. "There are certain areas at the rim of the impact crater, called the spallation zone, where by reflection of the shock wave the temperatures do not exceed 100 degrees Celsius" [212 degrees Fahrenheit, the boiling point of water].
That's cool enough to allow spores to survive ejection, the first stage of interplanetary travel. The second stage Horneck addressed in her research. "There are some studies simulating the reentry process," Horneck says of the last stage of an interplanetary trip. "It seems, that spores can also survive this in the inner part of the meteorite. Entering the atmosphere goes very fast, and so only the outer layer is heated."
Once in space, the BIOPAN (above) lid flips open, like the top of a waffle iron, exposing experiments inside to the cold vacuum of space, and, when BIOPAN is in the sun, to ultraviolet and other radiation with no intervening atmosphere.
"What we're talking about is life originating essentially on a planet, and asking can that life survive travel from one planet to another planet. In my opinion, for a spore, it's quite likely," says Rocco Mancinelli, of the NASA Ames Research Center.
But spore-forming bacteria are not the only organisms that could survive. Mancinelli studies archaea and Synechococcus cyanobacteria (photosynthetic bacteria) that live in very salty environments. He also flew experiments on BIOPAN, similar to Horneck's. "The only thing I did not do is I did not mix the organisms with any soil or anything at all. They were just dried onto the 7 mm quartz disks, placed in a holder, put in BIOPAN, shot up into earth orbit and BIOPAN opened and they were exposed," he says.
Most of Mancinelli's salt-loving organisms, or halophiles, survived space when stored in the dark.Except for radiation, the main threats of space are the lack of water and extremes of temperature. Mancinelli reasoned that even though they do not form spores, halophiles might survive because they routinely survive drying. "The particular organisms that I chose to fly were collected from an essentially dry crystal of salt. One was a pure sodium chloride crystal, where I got the archaean. The other one was a gypsum halide crystal, which is where I got the Synechococcus."
But surprisingly, about 25 percent of the archaeans and about 35-40 percent of the cyanobacteria survived full exposure to the sun.
So spores and even desiccated cells survive space and sunlight. But can they survive long enough to travel between, say, Mars and Earth?
The LDEF mission showed spores surviving six years, and six years is a realistic time frame for travel between Earth and Mars, the other planet in our solar system most likely to harbor life, Mancinelli and Horneck agree. But for longer trips, all scientists can do is calculate.
"If the time becomes too long, then the cosmic radiation that penetrates the rock would finally be lethal," Horneck says. "We have calculated (in the Mileikowsky paper in Icarus (2000) that in order to protect spores for 1 million years against cosmic radiation, a 1-meter-thick layer of the meteorite is necessary."
Radiation from the Sun is not the only hazard microbes would face during long-term space travel. Meteoric material itself would emit some radiation over very long periods of time. "This is one of the things that I really think is limiting," Mancinelli says, "radiation from naturally occurring minerals within the meteor and within the organism itself. Through geological time these decay and give off enough radiation that it could chop up the DNA and destroy it."
Horneck, Mancinelli and their colleagues have several experiments planned.
Horneck's team has more BIOPAN experiments planned, she says. "My colleague, Petra Rettberg, has a further experiment for BIOPAN, called MarsTox." MarsTox will use special filters to simulate the radiation reaching the surface of Mars. The experiment will gauge the protective effect, or lack of protection, of several simulated Mars soils. [Editor's note: Unfortunately, the FOTON rocket that was to carry the BIOPAN MarsTox experiment into orbit failed at launch, and the experiment was lost.]
Both Horneck and Mancinelli have plans for experiments on the International Space Station in 2004.
"She's going to fly spores-she does spores-under various different conditions, including mixing them with simulated meteorite material," Mancinelli says. "And I'm going to fly a variety of different kinds of organisms that are non-spores that I think might withstand the stress of space environment. I'm trying to figure out what's the breadth of life that might survive exposure to the space environment and thus be transferred from planet to planet."
Related Web Pages
Protection of Bacterial Spores in Space, a Contribution to the Discussion on Panspermia
Horneck, et. al, Origins of Life and Evolution of the Biosphere 31: 527-547, 2001.
"Did Life on Earth come from Mars?" By Robert Irion DISCOVER Vol. 22 No. 8 (August 2001).
"On The Secular Cooling Of The Earth" By Lord Kelvin (William Thomson) Excerpt. Transactions of the Royal Society of Edinburgh, Vol. XX111 (1864), pp. 167-169.
Natural Transfer of Viable Microbes in Space 1. From Mars to Earth and Earth to Mars Mileikowsky, et. al,.Icarus 145, 391-427 (2000).