When the first samples from Mars return to Earth, scientists should be on the lookout for ancient dormant bacteria, according to a new study.
In a first-of-its-kind study, a research team including Northwestern University’s Brian Hoffman and Ajay Sharma found that ancient bacteria could survive near the surface of Mars much longer than previously thought. And – when bacteria are buried and thus protected from galactic cosmic radiation and solar protons – they can survive much longer.
These findings raise the possibility that if life ever did develop on Mars, its biological remains may be revealed by future missions, including ExoMars (rover Rosalind Franklin) and the Mars Life Explorer, which will carry drills to extract material from 2 meters under the surface.
And because scientists have shown that certain strains of bacteria can survive despite the harsh Martian environment, future astronauts and space tourists could unwittingly contaminate Mars with their own hitchhiking bacteria.
The paper will be published Tuesday (October 25) in the journal Astrobiology.
“Our model organisms serve as receptors for both Mars forward contamination and Earth back contamination, both of which must be avoided,” said Michael Daly, a professor of pathology at the Uniformed Services University of the Health Sciences. (USU) and a member of the National Academies Committee on Planetary Protection, who led the study. “Importantly, these findings also have biodefense implications because the threat of biological agents, such as Anthrax, remains a concern for military and homeland defense.”
“We concluded that terrestrial contamination on Mars would be essentially permanent — on timescales of thousands of years,” said Hoffman, senior co-author of the study. “This could complicate scientific efforts to search for life on Mars. Likewise, if microbes evolved on Mars, they could survive to this day. This means that returned samples from Mars could contaminate Earth.”
Hoffman is the Charles E. and Emma H. Morrison Professor of Chemistry and Professor of Molecular Life Sciences at Northwestern’s Weinberg College of Arts and Sciences. He is also a member of the Chemistry of Life Processes Institute.
The environment on Mars is harsh and unforgiving. Arid and icy conditions, averaging -80 degrees Fahrenheit (-63 degrees Celsius) in mid-latitudes, make the Red Planet seem inhospitable to life. Even worse: Mars is also constantly bombarded by intense galactic cosmic rays and solar protons.
To investigate whether or not life could survive under these conditions, Daly, Hoffman and their colleagues first determined the survival limits of microbial life from ionizing radiation. They then exposed six types of Earth bacteria and fungi to a simulated Martian surface – which is frozen and dry – and treated them with gamma rays or protons (to mimic radiation in space).
“There is no flowing water or significant water in the Martian atmosphere, so cells and spores will dry out,” Hoffman said. “It is also known that the surface temperature on Mars is roughly similar to dry ice, so it is indeed deeply frozen.”
Ultimately, the researchers determined that some terrestrial microorganisms could potentially survive on Mars over geologic timescales of hundreds of millions of years. In fact, the researchers discovered that a powerful microbe, Deinococcus radiodurans (affectionately known as “Conan the Bacterium”), is particularly suited to survive the harsh conditions of Mars. In the new experiments, Conan the Bacterium survived astronomical amounts of radiation in the icy, waterless environment—far superior to Bacillus spores, which can survive on Earth for millions of years.
To test the effects of radiation, the team exposed samples to large doses of gamma and proton radiation—typical of what Mars receives in the near-subsurface—and much smaller doses, which would occur if a microorganism were deeply buried.
Next, Hoffman’s team at Northwestern used an advanced spectroscopy technique to measure the accumulation of manganese antioxidants in the cells of the irradiated microorganisms. According to Hoffman, the amount of radiation dose that a microorganism or its spores can survive is related to the amount of manganese antioxidants it contains. Therefore, more manganese antioxidants means more resistance to radiation – and longer survival.
In previous studies, previous researchers found that Conan bacteria, when suspended in liquid, can survive 25,000 radiative units (or “grays”), which equates to about 1.2 million years just below the surface of Mars. However, the new study found that when the rich bacterium is dried, frozen and buried deep – something that would be typical in a Martian environment – it could outlast 140,000 grays of radiation. This dose is 28,000 times greater than what would kill a human.
Although Conan the Bacterium could only survive for a few hours on the surface while bathed in UV light, its lifespan is dramatically improved when shaded or just below the Martian surface. Buried just 10 centimeters below the surface of Mars, the Conan bacterium’s survival period increases to 1.5 million years. And, when buried 10 meters down, the pumpkin-colored bacterium could survive for 280 million years.
Looking at future missions
This amazing feat of survival is due in part to the bacterium’s genomic structure, the researchers found. Long suspected, the researchers discovered that the Conan bacterium’s chromosomes and plasmids bind together, keeping them in perfect alignment and ready for repair after intense radiation.
This means that if a microbe, similar to the Conan bacterium, evolved at a time when water last flowed on Mars, then its living remains could still lie dormant in the deep subsurface.
“Although D. radiodurans buried in the Martian subsoil could not have survived in a dormant state for the estimated 2 to 2.5 billion years since flowing water on Mars disappeared, such Martian environments are regularly altered and melted by meteor impacts,” Daly said. “We suggest that periodic melting could allow intermittent repopulation and dispersal. Also, if there ever was life on Mars, even if there is no viable life on Mars now, macromolecules and their viruses would survive much, much longer. This raises the possibility that, if life ever develops on Mars, it will be revealed in future missions.”
The study, “Effects of Desiccation and Freezing on Microbial Ionizing Radiation Survival: Considerations for Returning Samples to Mars,” was supported by the Defense Threat Reduction Agency (grant number HDTRA1620354) and the National Institutes of Health (grant number GM111097).
Effects of desiccation and freezing on microbial ionizing radiation survival: Considerations for sample return to Mars, Astrobiology
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