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A defining characteristic of our Solar System is the heavily cratered surfaces found on many planets and moons. These circular scars record a long history of violent collisions with asteroids and comets. On some celestial bodies, these impact craters are the most prominent features of the landscape, dominating the terrain. While these massive impacts have fundamentally shaped the physical history of the Solar System, they also play a critical role in the potential story of life.
On Earth, we know that at least one enormous impact is famously linked to a mass extinction event that ended the age of the dinosaurs. However, impacts may also serve a positive function in the evolution of life. While one asteroid strike wiped out the dinosaurs, other impacts might theoretically spread life between different planets. This concept is not entirely new. The idea that life can travel from one world to another dates back to ancient Greece and the philosopher Anaxagoras. This hypothesis is known as panspermia. Although it is not a mainstream scientific theory, it has persisted for centuries. Recent discoveries have provided some support for this idea. Scientists now understand that the basic chemical building blocks of life are more common in the universe than was once believed.
New research now provides stronger evidence for a key component of the panspermia process. A recent study demonstrates that certain hardy microorganisms, known as extremophiles, could potentially survive being ejected from the surface of Mars by a massive asteroid strike. Not only could these microbes endure the incredible pressures of the initial impact, but they might also survive the long, hazardous journey through the vacuum of space between planets. This transfer could occur if the microbes become embedded in rocky debris blasted off the Martian surface.
The research is titled "Extremophile survives the transient pressures associated with impact-induced ejection from Mars." It was published in the journal PNAS Nexus. The lead author is Lily Zhao, a graduate student in Mechanical Engineering at Johns Hopkins University. The authors explain that large-scale impacts are ubiquitous throughout the solar system. Consequently, the likelihood of survival of organisms after such an event plays a key role in planetary protection, the search for extraterrestrial life, and the assessment of the panspermia hypothesis. They asked a direct and critical question: "Impacts generate very high stresses for short times, resulting in extreme pressures and high rates of loading. Can microorganisms survive such extreme conditions?"
To find an answer, the research team selected a specific extremophile for testing: Deinococcus radiodurans. This bacterium is famous for its ability to survive the dangerous conditions of deep space. It has been studied extensively by scientists around the world. D. radiodurans is the most radiation-resistant life form known to exist. It can also survive extreme cold, severe dehydration, the vacuum of space, and even strong acids. Because it resists so many different types of environmental dangers, it is sometimes referred to as a polyextremophile.
In laboratory experiments, the researchers subjected samples of D. radiodurans to extremely high pressures for very short periods. This experimental setup was carefully designed to mimic the intense shockwave of an asteroid impact. They then measured precisely how many of the bacteria survived the ordeal. They also studied how the survivors repaired cellular damage and how they reacted to the stress at a molecular level.
The RNA from the surviving bacteria was extracted and analyzed to understand their biological response. The data showed that as the pressure increased, so did the biological stress on the organisms. However, survival rates in some tests were remarkably high, defying earlier expectations.
"We demonstrated that the extremophile D. radiodurans has remarkably high survivability and viability after being subjected to pressures of up to 3 GPa," the authors report. One Gigapascal (GPa) is approximately 10,000 times the normal atmospheric pressure at Earth's surface. "As the pressure increases, D. radiodurans exhibited indicators of increased biological stress, as determined by the transcriptional analysis of impacted samples."
"Our results suggested that microorganisms can survive much more extreme conditions than previously thought, potentially surviving conditions that result in the formation of ejecta that can move across planetary systems," the researchers concluded.
"Life might actually survive being ejected from one planet and moving to another," said senior author K.T. Ramesh, an engineer who studies material behavior in extreme conditions. "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."
The team also examined the bacterial cells after the impact to look for physical damage. They used a powerful microscope called a Transmission Electron Microscope (TEM) to see the cells clearly. They compared a sample that was not shocked to samples subjected to 1.4 GPa and 2.4 GPa of pressure. They found "structural and morphological changes that result from these transient pressures at the higher pressures."
Cells subjected to 1.4 GPa of pressure showed a similar structure to the control sample, indicating they remained largely intact. However, cells subjected to 2.4 GPa showed clear internal damage and distinct cell wall damage. The key finding, however, is that D. radiodurans can withstand extremely high, though brief, pressures with a minimal overall effect on survival.
Lead author Lily Zhao expressed surprise at the results during the research process. "We expected it to be dead at that first pressure," she said in a press release. "We started shooting it faster and faster. We kept trying to kill it, but it was really hard to kill." In fact, the laboratory equipment failed under the stress before all the bacteria were killed.
Calculations suggest that impacts on Mars could subject Martian rocks to pressures up to 5 GPa or higher, depending on various factors such as the size and speed of the asteroid. The fact that D. radiodurans survived up to 3 GPa is encouraging for the panspermia hypothesis. It suggests that if life existed on Mars, it could have been launched into space and potentially reached Earth.
"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 implications of this research extend far beyond the theory of panspermia. The ability of D. radiodurans to survive extreme pressure suggests a pathway where similar Earth microbes could accidentally survive a trip to Mars on one of our spacecraft. This possibility introduces a significant risk for future exploration.
"We might need to be very careful about which planets we visit," Ramesh cautioned, referring to the need for strict planetary protection protocols to prevent contaminating other worlds with Earth life. Such contamination could compromise our ability to detect native life forms.
The authors summarized the broad importance of their work, noting that it reshapes our understanding of biological limits. "These findings have important implications for our understanding of the extreme limits of life, planetary protection, the design of space missions, and the possibility of the dispersal of life throughout solar systems." As we continue to explore the cosmos, understanding the resilience of life becomes increasingly vital.