Life From Space? (Panspermia; Lithopanspermia)

eburacum

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The only problem with that is that they may be almost indistinguishable from the bacteria or viruses that are already here on this planet - so we won't be able to work out their alien origin.
That's not really true. We should be able to date the last point of divergence quite accurately - within a few hundred million years, at least, and probably much better than that. If we find bacteria on Mars we could compare the various genomes and arrive at a date for the Last Common Ancestor, using the Molecular Clock methodology.
https://en.wikipedia.org/wiki/Molecular_clock
 

Mythopoeika

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That's not really true. We should be able to date the last point of divergence quite accurately - within a few hundred million years, at least, and probably much better than that. If we find bacteria on Mars we could compare the various genomes and arrive at a date for the Last Common Ancestor, using the Molecular Clock methodology.
https://en.wikipedia.org/wiki/Molecular_clock
Well...they may be pouring out of space right now...
 

eburacum

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Well...they may be pouring out of space right now...
This would imply that all bacteria in the Solar System (at least) are genetically indistinguishable from those we find on Earth today, otherwise we'd be able to identify new arrivals. A hypothesis that could be tested with a few sample-and -return missions to comets and asteroids, and preferably to Mars as well. The Lunar samples showed nothing, except a possible streptococcus on Surveyor 3.
https://en.wikipedia.org/wiki/Reports_of_Streptococcus_mitis_on_the_Moon
(note that nothing was found in the Lunar soil).
 

EnolaGaia

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This Scientific American blog essay by an astrobiologist suggests we shouldn't be too worried about the tardigrades deposited on the moon by an Israeli probe's failed landing attempt. He thinks there's a decent chance they aren't the first of their species to land there.

Tardigrades Were Already on the Moon
It may not be smart to add more, but nature probably beat us to it anyway

The last few days a number of headlines have pointed to a previously obscure fact about a recent attempt at placing a lander on the Moon by the non-profit Israeli SpaceIL organization. That mission, which unfortunately failed to softly deposit a lander called Beresheet on the lunar surface on April 11th 2019, seems to have been carrying a set of thousands of dehydrated tardigrades as passengers. ...

Now though, the spectre of ‘deliberate’ biocontamination of the Moon is getting some scrutiny. Tardigrades, the tiny 'water-bears', are extraordinarily resilient lifeforms. For a field like astrobiology, looking for life beyond the Earth, one of the biggest challenges in our solar system is to avoid creating false-positives by allowing terrestrial biomarkers or actual organisms into alien environments, whether the Moon, Mars or elsewhere. There is also a sensible consensus that we don’t want to mess up any alien ecosystems – especially if they might be delicate and vulnerable to invasive life.

Since the dawn of the space age there have been internationally vetted protocols and broad agreements about this kind of planetary protection. But it is a tricky business. We know that our efforts to sterilize spacecraft are imperfect, and we know that human spacefarers are an enormous potential cross-contamination problem. On the Moon there are already about 100 baggies of, well, astronaut poop, from the Apollo landings. And if the far-flung ambitions of SpaceX are ever realized we will see hundreds, perhaps thousands, of microbe-oozing humans deposited on the surface of Mars.

None of this appears helpful when seen through the lens of astrobiology’s search for other life. But at the same time, we know that nature has been busy cross-contaminating worlds for the past 4 billion years. And hardy little critters like tardigrades have likely already been deposited far beyond the Earth.

The mechanism involves asteroid impacts and so-called impact ejecta. A large literature exists on both theoretical and experimental work tracking the possibilities. The bottom line is that largish asteroid impacts (i.e. roughly 1-kilometer diameter objects and up) tend to spall (shedding of surface material) stuff from a planet and eject some of it with escape velocity or higher. Furthermore, it appears that microbial life and tough organisms like tardigrades have a decent chance of withstanding the pressure and temperature extremes during these shockingly violent launches.

Big impacts can send billions of cm-scale chunks from the surface of the Earth out across the solar system. Some of those pieces may take thousands of years to drop onto other planetary bodies, wending their way through an unseen web of orbital pathways, but they will get there. Indeed, computer modeling of impact ejecta suggest that even far flung places like Titan around Saturn should – albeit rarely – be recipients of pieces of Earth over time. Places like Mars, or the Moon, get far more detritus.

From the point of view of seeking clues to the deep history of life on Earth, this kind of lithopanspermia is fascinating. It may well be that scattered across the surface of the Moon are fossil-like samples taken sporadically throughout life’s terrestrial history. It is also possible that there are samples, even if millions of years old, that contain naturally dehydrated animals like the tardigrade. It is also, of course, possible (albeit with an unknown probability) that there is a happy ecosystem on Mars populated by descendants of terrestrial life.
SOURCE: https://blogs.scientificamerican.com/life-unbounded/tardigrades-were-already-on-the-moon/
 
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