They came from Mars

Christopher Carr thinks Martians invaded Earth a few billion years ago. If the research scientist from Massachusetts Institute of Technology is right, these Martians were tiny, and they came on rocks instead of spaceships. Their journey would have begun with the explosive BANG! of a meteorite smashing the red planet and sending boulders hurtling into space. Sheltered within the boulders, microbial Martians could have survived the frigid, irradiating darkness of space, as well as the fiery entry into Earth’s atmosphere and subsequent crash landing.

It may sound far-fetched, Carr acknowledges, but it’s not impossible. And the theory has been gaining support in recent years.

Mars and Earth have exchanged nearly a billion tons of rock over time. Now Carr wants to see if they’ve exchanged life as well. That’s why his research team at MIT is building a DNA-detecting machine for possible use on a 2018 Mars rover. If they can get the instrument rover-ready, its findings could knock down half of what we think we know about life on Earth. Finding DNA on Mars would mean the planet held life — maybe still does — and that we’re probably related to it.

Even more tantalizingly, most rocks travel from Mars to Earth; it’s harder to knock stuff off of Earth because of its stronger gravity and thick atmosphere. So finding DNA on Mars could mean that life on Mars spawned life on Earth.

“It’s an interesting thing to try,” said Steven Squyres, a Cornell University planetary scientist and lead scientist for NASA’s Mars Exploration Rover Project.

Although no one has ever looked for DNA on Mars before, NASA has spent decades exploring whether life ever took hold there. Past methods approached the search broadly: assuming Mars’ life would have evolved independently from life on Earth, scientists sought molecules that could be general to all life. So instead of looking for DNA — the molecule that defines life here — they’ve looked for geology and climates conducive to life and for molecules like methane that can be formed from decaying organic matter.

The problem, according to Carr, is that these approaches are inconclusive. Markers like methane could indicate life, or they could be a result of geologic processes.

Seth Shostak, lead astronomer at the SETI Institute, which looks for evidence of extraterrestrial life, thinks it’s too soon to narrow the search down to DNA. “I admire their audacity, but it’s a very restrictive set of assumptions” that completely overlooks the possibility that life arose independently on Mars, he said.

Norman Pace, a microbial ecologist from the University of Colorado at Boulder, also supports a broad approach. “We haven’t even found organic material on Mars,” he pointed out. And Tori Hoehler, an exobiology research scientist at NASA’s Ames Research Center, believes in an even wider search. He thinks the expeditions should first determine whether Mars was ever habitable, and then look for organic molecules. If they find organics, he said, only then should they check for DNA.

While Carr acknowledges the value of the broader studies Pace and Hoehler advocate, he still sees a potential benefit in getting more specific. “We can’t really envision what a second genesis would look like, but we can envision what something that’s related to us would look like,” he said. “We should do the most straightforward thing first.”

Looking for DNA on Mars wouldn’t be straightforward without the possibility of a shared ancestry; the odds of DNA evolving on two separate planets are infinitesimal. Natalia Artemieva, a senior scientist at the Planetary Science Institute in Arizona, believes a common descent is plausible. She says that chemists and astrophysicists have confirmed that asteroids can knock Mars rocks into Earth, and evidence is mounting that microbes can survive the journey.

The idea that Earth’s life may have arrived on rocks from outer space is several centuries old. But it experienced a revival in 1996, when scientists found structures inside a Mars meteorite that looked like nanobacterial remains. Subsequent research demonstrated that bacterial spores can survive conditions in space for up to six years and that living inside the rocks shelters them from radiation and heat extremes. Laboratory experiments from the last two years also concluded that the rocks’ inhabitants can survive the shock and heat of impact, allowing some bacteria to land safely on another planet. Thus, survival at all stages of the transfer process — ejection, the cold vacuum of space and the heat and shock of re-entry — appears to be possible.

“There seems to be little doubt that spores of microbes could survive the thousands of years a typical ‘journey’ by a rock from Mars to Earth would take,” says Shostak.

But that doesn’t mean NASA will decide to put Carr’s instrument on the next rover. It’s a typical debate in space exploration, said Hoehler: given limited time, money and weight, what are the most important instruments to send?

Before NASA has to decide, however, Carr’s team must finish building the instrument. It will need to be tiny, durable and reliable. They’ll need to consider how the equipment will be affected by conditions on Mars and how to distinguish between the DNA of Martian microbes and Earthly contaminants.

It may sound crazy. But what if Mars does have DNA-based life, Carr asks, and it takes twenty or thirty years to find it, simply because we insist on searching in a roundabout fashion? “We would feel pretty silly,” he says.