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Meteors that grazed Earth’s atmosphere could have brought microbial life from Earth to Venus, Harvard Astronomy department chair Avi Loeb and College student Amir Siraj ’22 hypothesized in a new research study, made available in preprint last week.

Earlier this month, researchers affiliated with MIT, Cardiff University, Harvard, and other institutions detected potential signs of life on Venus, in the form of phosphine gas in the planet’s atmosphere.

With corrosive droplets of sulfuric acid in its atmosphere and surface temperatures surpassing 800 degrees Fahrenheit, Venus appears an unlikely candidate for extraterrestrial life.

This led Loeb and Siraj to question whether any potential Venusian life might not really be alien after all; rather, it could have come from life on Earth.

“The bottom line is that you could have a transfer of life between Venus and the Earth. It's a process that was not considered before,” Loeb said. “People thought about what happens if a meteor hits the Earth, splatters the rock on the Earth, and then [sends] pieces of rock flying into space, and they hit Venus. That's a very different process, and it's not clear that life can survive such an impact.”

According to Loeb, this traditional idea of meteorite debris from Earth eventually reaching Venus leaves potential microbes exposed to more heat and radiation on their journey than they can withstand. Instead, Loeb and Siraj proposed the idea of meteors that graze Earth’s atmosphere.

“So then we came up with this idea of grazing, almost like a spoon taking the foam off of a cappuccino, and on top of the cappucino, these meteors can basically scoop up any microbes that exist in the atmosphere,” Siraj said.

A grazing meteor would enter the Earth’s atmosphere and then exit off into space, according to Loeb. The meteor would provide porous places for microbes to be shielded from harmful radiation. Since the meteor is only lightly brushing the atmosphere, it avoids excessive heating.

“If [an organism is] deep inside the rock, it's sort of protected. The rock serves as the spacesuit for these microbes,” Loeb said.

Loeb and Siraj estimated that, over the history of our solar system, at least 600,000 meteors may have passed through Earth’s atmosphere, and then landed on Venus within the next 100,000 years. The microbes they picked up on Earth could likely survive the journey to the neighboring planet.

Once the microbes reach Venus, they would likely live in the planet’s 35-mile-high cloud layer, which is cooler and less dense than the scorching surface of the planet.

“There, the temperature is similar to what we have here, in the lower atmosphere of the Earth,” Loeb said. “So, the clouds are habitable in principle, they have the right temperature and pressure. There might be microbes living inside the droplets.”

Although the meteor grazing idea appears promising, Siraj acknowledges that there is more work to be done. For this hypothesis to be accepted, Siraj said, researchers would need to study the abundance and variety of microbes in Earth’s atmosphere that could have been picked up by meteors.

Ultimately, confirmation of Loeb and Siraj’s theory will depend on the ongoing investigation into life on Venus.

“The best way to test this hypothesis will simply be to search for life on Venus, and if it is found, checking whether or not it shares chemical and biological structures and underpinnings with life on Earth,” Siraj said.