Tiny life forms tucked into debris from an asteroid hit could catapult to other planets – including Earth – and survive, a new Johns Hopkins University study finds.
The work demonstrates that a certain hardy bacterium easily withstands extreme pressure comparable to an ejection from Mars after an asteroid hit, as well as the inhospitable conditions it would face during the ensuing interplanetary journey.
The study, published today in PNAS Nexus, suggests that microorganisms can survive remarkably more extreme conditions than expected, and raises questions about origins of life. The work also has significant implications for planetary protection and space missions.
"Life might actually survive being ejected from one planet and moving to another," said senior author K.T. Ramesh. "This is a really big deal that changes the way you think about the question of how life begins and how life began on Earth."
Impact craters cover the surfaces of most bodies in the solar system. Mars, a planet that could harbor life, is one of the most cratered celestial bodies. We know asteroid strikes can launch material across space—and Martian meteorites have been found on Earth.
However, scientists have long wondered if life forms could also be launched from an asteroid impact. Tucked inside ejected debris, they might land on another planet—a theory called the lithopanspermia hypothesis.
Previous experiments to test the theory have been inconclusive, and targeted organisms widely found on Earth, rather than a life form that would suit the extreme environments of other planets.
To study how a microorganism would realistically handle the stress of a planetary ejection, the team devised a way to replicate the pressure and a singular biological model.
The team chose to test Deinococcus radiodurans, a desert bacterium found in the high deserts of Chile that is notorious for its ability to survive the most inhospitable, space-like conditions—everything from extreme cold and dryness to intense radiation. It has a thick shell and a remarkable ability to self-repair.
"We do not yet know if there is life on Mars, but if there is, it is likely to have similar abilities," Ramesh said.
The experiment simulated the pressure of an asteroid strike and ejection from Mars by sandwiching the microbe between metal plates and then firing a projectile at it from a gas gun. The projectile hit the plates at speeds up to 300 mph, generating 1 to 3 Gigapascals of pressure.
For perspective, the pressure at the bottom of the Mariana Trench, the deepest part of the Earth's oceans, is a tenth of a Gigapascal. Even the lowest pressure in this experiment is more than ten times that.
After shooting the microbes, the team determined whether they survived and examined the survivors' genetic material for clues to how they handled the pressure.
The bacteria proved very hard to kill. They survived nearly every test at 1.4 Gigapascal of pressure and 60% at 2.4 Gigapascals of pressure. The cells showed no signs of damage after the lower pressure hits, but after the higher pressure experiments, the team observed some ruptured membranes and internal damage.
"We expected it to be dead at that first pressure," said lead author Lily Zhao. "We started shooting it faster and faster. We kept trying to kill it, but it was really hard to kill."
In the end, what did die was the equipment. The steel configuration holding the plates fell apart before the bacteria did.
When asteroids hit Mars, ejected fragments experience a range of pressures, perhaps close to 5 Gigapascals, though some could see much higher. Here the microbe easily survived almost 3, much higher than previously thought possible.
"We have shown that it is possible for life to survive large-scale impact and ejection," Zhao said. "What that means is that life can potentially move between planets. Maybe we're Martians!"
The possibility of life spreading between planetary bodies has significant implications for planetary protection and space missions, the team said.
Space mission protocols evaluate the likelihood of life surviving on the target planet. When missions travel to planets that might sustain life, like Mars, there are tight restrictions and safety measures to prevent contaminating the planet with Earth life. And when a mission brings back materials from a planet, there are very strict measures to control the possible release of that life on Earth. Because this work demonstrates that materials from Mars might reach other bodies, particularly its two nearby moons that aren't currently restricted, the team said policies might need to be reassessed.
Phobos, in particular, orbits so close to Mars that any ejecta that gets there is probably exposed to much less pressure than what is required to get to Earth, the team said.
"We might need to be very careful about which planets we visit," Ramesh said.
The team next hopes to explore whether repeat asteroid impacts result in hardier bacterial populations—or whether bacteria adapt to this kind of stress. They'd also like to see if other organisms, including fungi, can survive these conditions.
Authors include: Cesar A. Perez-Fernandez, and Jocelyne DiRuggiero.
