Smith is skeptical that microorganisms are growing or dividing at such high altitudes, because it's so cold and dry up there. But he says that microorganisms may be "persisting", or lingering and not being killed. "Nobody's been able to measure how long microorganisms can stay in the stratosphere. There's works that still needs to be done."
"Virtually all terrestrial and marine surfaces have microorganisms associated with them that can get disattached from the surfaces by wind or other physical disturbances," wrote Aarhus University assistant professor Tina Santl-Temkiv, who has studied microorganisms in hailstones, in an e-mail to Seeker.
"[They] can reach higher levels of troposphere, above around one kilometer, can stay suspended in air for around a week and can travel thousands of kilometers, riding on wind currents. Eventually, they get deposited back to the ground wither through the formation of rain or simply due to gravity."
If Earth's atmosphere is shown to be a great spot for life to divide, however, it could have implications for locations such as Venus. Back in the 1960s, astronomer and science popularizer Carl Sagan suggested that the upper atmosphere of Venus could harbor the descendants of organisms that could have evolved on the surface of the planet when it was cooler.
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Even though today the surface can crush and cook unprotected spacecraft, 50 kilometers (31 miles) above is more temperate. Moreover, researchers have found an intriguing substance that blocks ultraviolet light in Venus' clouds. Life hasn't yet been ruled out as a possibility.
"Venus and Earth were similar for 3 billion years [of their evolution] and perhaps as recently as up to about half a billion years ago," said Dr. Lynn Rothschild, a NASA astrobiologist and synthetic biologist that is on Smith's research team. She said this includes liquid oceans, similar atmosphere, and probably the same sorts of minerals and organic compounds as well.
But Venus would be a difficult prospect if the life returns to the surface. The sun got more luminous as the solar system aged, evaporating the water from Venus' oceans. The water vapor, now in the atmosphere, contributed to giving Venus a hellish greenhouse effect on its surface.
It seems that life is hardy, but we don't know if it's tough enough to survive living high above a planetary surface. If it does, however, that could mean that even missions that sample a planet's atmosphere could have to worry about protections against hurting possible life. We'll have to see what these new experiments yield, though, before reaching any conclusions.
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