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Check out tonight’s sky and look for the bright red dot.
That’s Mars, just 35.8 million miles away—the Red Planet’s closest approach to Earth in 15 years.
But there’s a possible consequence to the closeness: “swapping spit.”
That's the colloquial term; the scientific name is “panspermia.”
“The transport of life between Earth and Mars,” says Ken Farley of the Jet Propulsion Laboratory in Pasadena, California. “And it doesn’t seem all that implausible.”
Though only a theory, panspermia is a monumental proposition. If true, then finding life on Mars comes with a catch.
Here’s why: About four billion years ago, asteroids regularly slammed into the solar system’s inner planets (Mercury, Venus, Earth, and Mars). They still do, just not as often.
An ancient asteroid, smashing into Earth with the impact of a million atom bombs, would blast terrestrial rocks into outer space.
Those rocks might have housed hidden microbial life—“extremeophiles,” single-celled organisms able to endure both the blast and the cosmos.
All quite conceivable, suggests Jean-Luc Margot, professor and chair of the Department of Earth, Planetary and Space Sciences at UCLA: “Some microbes are fairly resilient in space.”
Maybe a few of those microbe-filled rocks, after millions or billions of years, eventually crashed on Mars.
That too is reasonable. “We have meteorites from Mars here on Earth,” says Margot, “and there may be rocks from Earth that landed on Mars.”
If so, then any surviving extremeophiles within the rocks—sheltered and in virtual suspended animation—perhaps awakened; those microbes, native to Earth, became Martians.
In other words: Martians R Us.
“A rock from Earth could have colonized Mars,” says Cynthia Phillips, a JPL planetary geologist.
And a probe digging up a microbe on Mars might only be a case of cross-contamination, “not an independent sign of life,” she says.
What's more, determining whether the microbe was Martian or an Earth imposter “would be very difficult.”
The reverse could also be true.
Primitive life could have started on Mars, rode a rock to an uninhabited Earth, found a niche, and flourished. “Maybe we’re all Martians,” speculates Phillips.
Cross-contamination, however, isn’t an issue everywhere in the solar system.
Take Jupiter’s moon, Europa.
Scientists believe a salt water sea roils below Europa’s surface, bigger than all the oceans of Earth combined. Within that water might be life, probably primitive, analogous to terrestrial bacteria.
Europa, ten times further out than Mars, is about 400 million miles from Earth.
That’s too far for panspermia to take place, claims Phillips.
“It’s basically impossible to get a rock from Europa to Earth,” she says. Find life there, and odds are “overwhelming that it started independently.”
Scientists will know more after the flybys of NASA's Europa Clipper; the robotic probe launches in the 2020's. A tantalizing world, safe from spit-swap, beckons.
