Space-traveling microbes? An unusual experiment shocked skeptics.

Scientists have discovered that a hardy microbe can endure pressures strong enough to pulverize rock, strengthening the case that life might survive the impact of an asteroid blasting it off a planet.

In a series of experiments at Johns Hopkins University, Lily Zhao fired tiny samples of a microorganism with a room-sized gas gun. The gun drove a steel plate into a thin, carefully prepared layer of bacteria at up to 2.4 gigapascals — tens of thousands of times Earth's atmosphere at sea level. The purpose was to simulate the highest pressure a microorganism might face on its space journey: The initial launch. 

Instead of total extermination, Zhao, a mechanical engineering doctoral student, found life — lots of it, in fact. After her initial test run, she cultured a regular sample, as well as the shocked sample so she could compare them side by side. 

"I really didn't know what to expect," she told Mashable. "I was like, 'Did I mislabel something or mix things up? Did I get the control and the shot sample confused?' I was quite hesitant because it was such a high survival — like 95 or 97 percent survival."

The research, funded by NASA and published in the journal PNAS Nexus, examines a key part of the long-debated lithopanspermia hypothesis — the notion that alien life might migrate between worlds sealed inside rocks knocked loose by asteroids or comets. Though no one knows whether this has happened, scientists have identified at least 400 meteorites on Earth that originated from Mars.

Even at the highest pressure the setup could reach before the steel hardware began to break, survival stayed around 60 percent.

Zhao's faculty supervisor, K.T. Ramesh, said his interest in the problem grew out of involvement in a National Academies study that asked whether microbes could move from Mars to one of its close potato-shaped moons, Phobos.

"We ended up saying the probability was very low, but we also ended up saying there really wasn't any good data on what microbes could survive," Ramesh, a mechanical engineering professor, told Mashable. "So I thought, 'Well, somebody should get that data.'"

Hopkins microbiologist Jocelyne DiRuggiero chose the superbug for the experiment. She selected Deinococcus radiodurans — or, "D. rad" — for its resistance to extreme radiation, dehydration, cold, and other factors.  Those kinds of adaptations would be relevant for anything trying to persevere in space conditions. The so-called extremophile has even been found living in Chile's Atacama Desert, one of the driest and most radiated places on Earth.

Extremophiles and space

Earlier experiments by other groups had tried to test microbial survival from asteroid-like impacts, but the data were often sparse and hard to interpret, the researchers said. Some studies shot pellets containing microbes into sand or rock. But when a fraction survived, no one knew exactly what pressures those specific cells had experienced because their positions inside the target were unknown.

The Hopkins team set out to control that key variable. Zhao grew the cells in a liquid broth, then filtered them onto a thin membrane to create a uniform layer. She sandwiched that membrane between two ultra-flat steel plates, then used the gas gun to slam a third plate into the stack.

Machining and polishing the plates to the required flatness took weeks. On a firing day, Zhao spent eight to nine hours setting up the gun, then moved to a biology lab after each shot to put the shocked cells back into liquid culture and watch them regrow. A single experiment could take a few weeks of preparation for just a few microseconds of data.

DiRuggiero didn't have high hopes for what would remain.

"I'm like, 'There is no way,'" she said of the plan. "'Shooting a bullet at a microorganism? This thing is going to explode.'" 

From a physics standpoint, the pressures are extreme, even for non-living materials. Ramesh noted that water — which makes up much of any cell — begins to respond strongly around two gigapascals, changing its volume and forming ices.

Working with detailed modeling, DiRuggiero realized the worst damage didn't happen when the cells were squeezed. The real trouble came when the pressure suddenly let up.

Damage to the microbes

Among the surviving cells, some of their outer lining received damage, allowing DNA and proteins to get hurt. The cells temporarily dropped their normal routine — feeding, growing, and dividing — and switched into repair mode. Within a couple of hours, though, they had already begun to look like their old selves. The real surprise was in something basic: how the physical structure of a single cell could hold up under such violence in the first place. 

"I should know better by now that microorganisms are absolutely amazing. They are colonizing every possible environment on Earth. We found them at the bottom of the ocean. We found them in Antarctica sea ice. We found them in acidic mud pools," DiRuggiero said. "If we find any life elsewhere in the solar system — or outside of the solar system — it most likely is going to be microorganisms."

But for lithopanspermia to cross from possible on paper to something that actually happens, life would have to survive much more than the ejection from its home turf. An inhabited rock would have to withstand the deep freeze of space, drying out, space radiation, perhaps millions of years of travel, and then the heat of reentering another world before it lands. For years, Ramesh considered that chain of events to offer incredibly remote odds.

While the new results don't prove life moves between planets and moons, it has changed how he thinks about the possibility.

"I've gone from saying, 'This is just extraordinarily unlikely, and we shouldn't worry about it,' to saying, 'Well, OK, this is possible,'" he said. 

Planetary protection and contamination

The study also touches a live nerve in planetary protection — the effort to avoid accidentally seeding other planets with Earth life. Space agencies already scrub spacecraft within reason before sending them on their missions, but a few resilient hangers-on almost always remain. 

Scientists have particularly wondered what that means for Mars. If bacteria, fungi, or other microscopic life were to survive a clean room on Earth, it doesn't guarantee those stragglers will actually grow once they get to the Red Planet. But dead microbes still leave traces of DNA, which could complicate future attempts to discern a native Martian from our own contamination.

Planetary protection policies classify some worlds as needing strict spacecraft cleanliness to prevent contamination. Results like these could influence which bodies space agencies deem vulnerable. Phobos, in Ramesh's view, probably should be added to that list.

In the meantime, the work underscores how tough even simple, tiny life can be. For Ramesh, who has studied the mechanics of asteroid cratering for more than 15 years, the results have convinced him that fresh craters might actually be good places to look for life. Craters have cracks, perhaps allowing water to flow through them. 

"Maybe they're not as good at sterilizing life as I thought," he said.