Firstly I would like to start the question with the knowledge that we have, that the most basic form of life we want and expect to find in our Solar System is the primitive kind, such as viruses or bacteria, since they have the highest chance of existing, more than intelligent life (Fermi Paradox).

Now with that in mind we also know that viruses are also half alive, and half dead, and they are really tiny, much tinier than bacteria or cells. These viruses are everywhere around us, even in the air, and also, in our capsules to where we send probes to Mars.

Since our last probes that got sent to Mars, many years have past, and many more will pass until we really check for microscopic life on Mars, meaning that these viruses might mutate and evolve, maybe even bacteria could do that (I am not a biologist, please don't quote me on my ignorance). How would we know and differentiate between life brought by us, or real life in the red Planet?

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    $\begingroup$ This is a good question in the context of bacteria and other microorganisms, but you should note that viruses cannot reproduce (or evolve) in the absence of these other organisms. Notably, they at least need an organism with a ribosome to synthesize the proteins that they are made out of, since viruses do not have ribosomes themselves and can't make their own proteins. $\endgroup$ Oct 23, 2020 at 15:23
  • $\begingroup$ Not that I really know, but genetics is a big deal in the study of the history of life on earth. They estimate the accumulation of mutations over time, make taxonomies based on genetics, and so on. If they find life on Mars I'm sure they will sequence it, compare it to life on Earth, and consider whether it came from here. $\endgroup$
    – Greg
    Oct 24, 2020 at 19:56
  • $\begingroup$ I think we will be able to tell if life came from contamination - likely known species or types. Meteorite impact ejecta carrying life from Earth to Mars in the far past, with long evolution since would be harder to distinguish. Even then there could be unique DNA strings indicating shared ancestry. $\endgroup$
    – Ken Fabian
    Dec 5, 2020 at 21:03
  • $\begingroup$ Related: Is NASA hiring a 'planetary protection officer'? $\endgroup$ Dec 5, 2020 at 21:18
  • $\begingroup$ Related, not answered, not quite dupe: How are interplanetary spacecraft protected from biological contamination while being integrated onto launchers? $\endgroup$ Dec 5, 2020 at 21:20

3 Answers 3


One tool in the astrobiology toolbox is chirality, the property that many chemical compounds exist as "left-handed" and "right-handed" mirror images.

These have the exact same properties of course, as they are the exact same compounds. But biochemical processes will typically only be able to produce one of them.

Which means that if a compound from a sample of extraterrestrial life has the opposite chirality of what the same compound in Earth-based life has, we can be pretty sure it's alien. The converse is not true, if they have the same chirality, it could be either alien or from Earth, we would not know.

By determining the chirality of several compounds in the sample, we can with a very large degree of certainty (Or maybe not? I have asked about that) tell if the life shares origin with life from Earth. If every single compound has the same chirality, it likely has the same origin. If on the other hand some compounds are mirrored the other way, it's likely alien.

chirality illustration

  • $\begingroup$ Life can evolve independently with the same chirality as us. Different chirality provides a clear answer but the same chirality leaves the question unanswered. Other means - likely DNA comparison - would show evidence of common descent. $\endgroup$
    – Ken Fabian
    Dec 5, 2020 at 20:50
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    $\begingroup$ @KenFabian "The converse is not true, if they have the same chirality, it could be either alien or from Earth, we would not know." $\endgroup$ Dec 5, 2020 at 20:51
  • $\begingroup$ Posted before I finished - keep bumping a key. That was my point, yes. $\endgroup$
    – Ken Fabian
    Dec 5, 2020 at 20:56
  • $\begingroup$ I'll have to look this up, but I've seen a statement that chirality can affect survivability, or more specifically, that terrestrial life has the chirality it does because it is slightly more tolerant of radiation than the mirror. If true, it would suggest that any carbon-based life of non-terrestrial origin would probably share the same chirality, so reverse-chirality would not be a useful indicator of non-terrestrial origin because we'd never see it. $\endgroup$
    – Anthony X
    Dec 7, 2020 at 0:47

First of all, some effort is made to prevent contamination by earth organisms for precisely this reason, but it is not perfect.

But, if scientists do detect life it will probably be possible to tell that it is not of earthly origin. Here are some possible lines of evidence:

  1. Biochemistry is complicated. We don't know that DNA is the only way genetic information could be encoded, and even with DNA, there are different ways it could encode proteins. If an organism is found with a completely different genetic molecule, we will know it isn't contamination.
  2. Even if the basic biochemistry were the same, an alien microorganism would likely be very different in other ways from an earthly one.
  3. If a teeming underground ecosystem is found thousands of kilometers from the nearest prior landing, it will be hard to believe it resulted from contamination.
  • $\begingroup$ Adding to the above comment: information can be encoded in RNA as well (en.wikipedia.org/wiki/RNA#RNA_genomes), and we've also managed to synthesise nucleic acids that don't occur in Earth life (en.wikipedia.org/wiki/Xeno_nucleic_acid). $\endgroup$
    – Pitto
    Oct 25, 2020 at 6:13
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    $\begingroup$ The most difficult challenge might be distinguishing life transferred from Earth to Mars in the last 50 years by bacterial spores on space probes from life transferred from Earth to Mrs in the last billion years by bacterial spores on meteorites, or even native life which seeded Earth 4 billion years ago by bacterial spores on meteorites. With time, though, enough evidence could sure be found in the genomes. $\endgroup$ Oct 25, 2020 at 11:46
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    $\begingroup$ Related to #3, note that there's ecosystems on Earth that have been sealed away under deep ice for millions of years, with surface "contamination" far more severe than anything humans will produce or be capable of producing on Mars in the near future. $\endgroup$ Dec 5, 2020 at 14:56

One key aspect not mentioned so far is the possibility of contamination of Mars with Earth life that is not due to a failure of planetary protection. Carl Sagan was one of the first to suggest the possibility of cross contamination of life between Mars and Earth due to asteroid impacts as mentioned in his Cosmos series and as chronicled in "Carl Sagan's Universe" (1). A very detailed analysis is presented in Rachel Worth's paper "Seeding Life on the Moons of the Outer Planets via Lithopanspermia" published in the Journal of Astrobiology (2). Besides the possibility of transporting life as far as the moons of Jupiter and Saturn, she discusses in detail the probability of transport between Earth and Mars during the Late Heavy Bombardment about 4 billion years ago. This is especially important, since evidence suggests that Mars had significant surface water at that time. Although there is speculation that the Chicxulub asteroid impact may have transported life to Mars (3), Mars had lost most of its atmosphere and surface water by then (4)(5).

Because of the possibility of transport of life between Earth and Mars in ancient times, Earth life transported to Mars could have evolved over several billion years, although it is unlikely that the chirality of the life chemicals would have changed. It also raises the question, is life on Earth due to contamination from Mars? So, if we find life on Mars based on DNA, with the same familiar base pairs, and proteins incorporating the same 22 left-handed amino acids we find in Earth life--it does not necessarily mean that it's due to contamination from Earth. Rather, it is quite possible that we are due to cross contamination from Mars!

Of course the most significant discovery would be of a clear second instance of life.

(1) https://www.amazon.com/Carl-Sagans-Universe-Yervant-Terzian/dp/0521576032

(2) https://www.liebertpub.com/doi/full/10.1089/ast.2013.1028

(3) https://www.history.com/news/dinosaur-asteroid-may-have-sent-life-into-space

(4) https://worldbuilding.stackexchange.com/questions/100932/planet-loses-its-core/101030?noredirect=1#comment592728_101030

(5) https://jatan.space/nasa-maven-mars-orbiter/


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