Could We Really All Be Martians?

Considering the likelihood of an exchange of microbes between Earth and Mars.

Sunset on Mars, as seen by the Curiosity rover in Gale Crater. Could this be our ancestral home? (NASA/JPL-Caltech/MSSS/Texas A&M Univ.)
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In a new paper published in the journal Astrobiology, Alfonso Davila from the NASA Ames Research Center analyzes the likelihood that microbial life was transferred between Earth and Mars, and what that might mean for efforts to detect life on the Red Planet. The topic is especially timely as NASA’s Perseverance rover begins looking for fossil evidence in Jezero Crater and the European Space Agency prepares its Rosalind Franklin Mars rover for launch next year.

Both rovers are searching for signs of life in ancient Martian terrain, which, according to another new paper by Allan Treiman from the Lunar and Planetary Institute in Houston, had three distinct episodes of potential habitability. Treiman’s work supports Davila’s, in that any transfer of microbial life from one planet to another—known as panspermia—most likely occurred in the same time window, early in Martian history. Not only were environmental conditions favorable during that time, asteroid exchange between Earth and Mars was much more frequent.

Davila postulates that the biochemistry of any putative Martian life forms would depend on when the panspermia occurred. And that affects, in turn, which method of life detection is most suitable. If the exchange happened before the Last Universal Common Ancestor (LUCA) of life appeared on Earth, the likelihood of a transfer would have been high. But the chances that such primitive organisms, which might not have “invented” DNA yet, survived the trip is rather low. If they did survive, these (now) Martian life—or pre-life—forms would be expected to share a common biochemical foundation with life on Earth, but otherwise could be very different, as they would have followed separate evolutionary trajectories. And that affects how wide a net we cast in our search for “life” on Mars.

The highest chance of panspermia succeeding would be shortly after LUCA evolved, about 3.9 billion to 3.5 billion years ago. During that time the exchange of planetary mass due to asteroid bombardment was very high, and the bacteria or archaea living on Earth were robust enough to survive space travel. And any newcomers from Earth would have found a habitable environment on Mars.

After about 3.5 billion years ago, panspermia would be less likely, because the amount of exchanged material trailed off rapidly, and most of Mars became hyperarid and much less habitable. But it may not have been impossible. If the transfer occurred after LUCA, the template for how life functions would have been mostly set, and we wouldn’t expect many biochemical changes following the arrival at Mars. If we find fossil evidence of Martian life, we might even be able to pin it somewhere on Earth’s tree of life. The later the panspermia occurred, the more similar Martian organisms would be to those on our own planet.

Of course, there is still the possibility that life developed independently on Mars. In that case, biochemistry may be quite different on the two planets, which would be the most challenging (as well as the most exciting) scenario for life detection. And the scientific rewards would be greatest, since we would get a glimpse of what might be universal traits of life and how they differ under specific planetary conditions. It would also suggest that life is common in the universe.

Which direction of panspermia was more likely? Based on estimates given in Davila’s paper, a transfer of rocks containing viable microorganisms from Mars to Earth was nearly 100 times more frequent than from Earth to Mars! Here “viable” means that the transferring rock had a diameter of at least three meters, enough to protect dormant microorganisms from radiation during their interplanetary journey of up to 10 million years. This is because the weaker Martian gravity makes it easier for rocks to escape the planet, and they would naturally “fall” gravitationally inward toward the Sun (and Earth). Mars also should have become habitable before Earth did, since, being farther from the Sun, it cooled down faster, and did not experience the same calamitous impact that created Earth’s Moon.

All of this led Joseph Kirschvink and Benjamin Weiss, both from the California Institute of Technology, to conclude in a seminal paper published 20 years ago that our distant microbial ancestors most likely came from Mars, and not the other way around. I think chances are good that they were right!

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