How life arose on Earth remains a mystery, though many theories have been proposed. Now a new study by Japanese scientists has reinvigorated the discussion around panspermia: The idea that life may have reached Earth from Mars.
The panspermia hypothesis suggests life may have arisen on another planet, with bacteria travelling through space, hitching a ride on a piece of rock or other means, eventually making its long-distance journey to Earth. Mars is a particularly appealing source, as studies suggest it was once potentially habitable with a large hemispheric ocean.
However, the biggest challenge has been determining if bacteria could survive the harsh interplanetary — or even intergalactic — journey.
To answer that question, a group of Japanese scientists, in participation with the Japanese space agency, JAXA, conducted an experiment on the International Space Station.
In the new study, published Wednesday in the journal Frontiers of Microbiology, researchers found, with some shielding, some bacteria could survive harsh ultraviolet radiation in space for up to 10 years.
For their experiment, the team used Deinococcal bacteria, well-known for tolerating large amounts of radiation. They placed dried aggregates (think of them as a collection of bacteria) varying in thickness (in the sub-millimetre range) in exposure panels outside the space station for one, two and three years beginning in 2015.
Early results in 2017 suggested the top layer of aggregates died but ultimately provided a kind of protective shield for the underlying bacteria that continued to live. Still, it was unclear whether that sub-layer would survive beyond one year.
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The new three-year experiment found they could. Aggregates larger than 0.5 mm all survived below the top layer.
Researchers hypothesized that a colony larger than one millimetre could survive up to eight years in space. If the colony was further shielded by a rock — perhaps ejected after something slammed into a planet such as Mars — its lifespan could extend up to 10 years.
Akihiko Yamagishi, a professor at Tokyo University in the department of pharmacy and life sciences who was principal investigator of the Tanpopo mission designed to test the durability of microorganisms on the ISS, said one of the important findings is that microbes could indeed survive the voyage from Mars to Earth.
“It increases the probability of the process, [making it] much higher,” Yamagishi said in an interview.
“Some think that life is very rare and happened only once in the universe, while others think that life can happen on every suitable planet. If panspermia is possible, life must exist much more often than we previously thought.”
There are two important factors, he believes: Mars and Earth come relatively close together in their orbits every two years, which would allow time for transfer of bacteria; and the RNA World theory.
The theory hypothesizes that Earth was once composed of self-replicating ribonucleic acids (RNA) before deoxyribonucleic acid (DNA) and other proteins took hold. Yamagishi believes that RNA could have once existed on Mars before conditions for life arose on Earth and potentially travelled towards Earth bringing along RNA which began to seed our planet.
Not ‘ironclad proof’
This isn’t the first experiment to see whether bacteria could survive in space.
In past experiments, where microbes were mixed with clay, sugar or other elements, the bacteria died. However, this is the most promising finding to date supporting the panspermia hypothesis.
While some research suggests bacteria could survive a trip embedded in rock, this is the first of its kind to suggest they could survive without that kind of aid, what the researchers term “massapanspermia.”
However, it’s not an open and shut case.
“Actually proving that it could happen is another thing, so I wouldn’t say that this is ironclad proof,” said Mike Reid, a professor at the University of Toronto’s Dunlap Institute for Astronomy and Astrophysics who wasn’t involved in the Japanese study. “It’s certainly leading in that direction.”
Does Reid believe life could have made its way from Mars to Earth?
“If you’d asked me 20 years ago, I would have said no, of course not. But now, it’s a little hard to say,” he said. “I think we won’t be able to answer that question until we’ve had a really thorough look at the surface of Mars … did it ever have life … and was it like us?”
The answer to that question could come in the form of NASA’s Perseverance mission to Mars that launched on July 30. One of the main goals of the state-of-the-art rover is to look for past signs of life on the red planet, taking samples to be returned to Earth at a later date.
While promising, the Japanese research team acknowledged that, while their research strengthens the case for panspermia, other factors need to be considered, such as whether bacteria could survive the descent through Earth’s atmosphere.