Did Mars and Earth swap microbes?

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https://cosmosmagazine.com/biology/did-m...p-microbes

Paul Davies (excerpt): . . . From time to time, Earth and Mars take a hit from a comet or asteroid with enough force to blast rocks around the solar system. Some terrestrial rocks will fall on Mars and vice versa [...] If Earth and Mars can trade rocks, surely they can trade life too? Shielded within a rock, a hardy microbe could easily withstand the harsh environment of outer space and so arrive at the other end still viable.

When I suggested this in the early 1990s I received nothing but derision. It was objected that microbes wouldn’t survive being kicked off a planet, or the fiery plunge through the atmosphere. However, it has been shown that the Mars meteorites generally do not show signs of shock heating, only the outer layer of a meteorite becomes incandescent, and it all happens so fast that the interior doesn’t get hot. Today, these objections have largely melted away. (Extremophiles)

[...] Evidence suggests that until about 3.5 billion years ago Mars was warm and wet and far more earthlike than it is today. As we know there was life on Earth at that time, it seems inevitable that the transfer of viable organisms from Earth to Mars would have occurred, thus seeding the red planet with Earth life. Of course, the same mechanism works in reverse; indeed, it is easier to knock rocks off Mars because of its lower gravity and thinner atmosphere.

All of which raises the intriguing question of whether life on Earth may have started on Mars and come here in impact ejecta, implying that we are all the descendants of Martians. Mars does have a few favourable aspects as an incubator of life; certainly early Mars was no less congenial than early Earth for biology to get started. But whichever way around it was, it seems that if we ever find traces of life on Mars, chances are it will just be good old terrestrial life.

[...] The spread of life between near-neighbour planets could be common throughout the universe. But what about longer journeys?

MORE: https://cosmosmagazine.com/biology/did-m...p-microbes
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