The question How certain are we that we have not accidentally sent life to other planets/moons? talks about all the precautions taken to avoid spreading life to other planets/moons. But why even try it? Wouldn't we want to actively spread bacteria around our Solar System to see if something can stick and grow? If some bacteria could thrive on Mars for example, it would be a great experiment that would also tell us which mutations will occur over time in that environment.

I do understand that finding native bacteria on Mars would be somewhat valuable but surely it would also be valuable to manually seed life across the universe?

Update: I’m not advocating for this to be done. Just asking why this is considered anathema.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – called2voyage
    Jul 29, 2021 at 14:06
  • $\begingroup$ @PeteBecker by default I imagine any alien explorers would outright destroy our civilization, similar to how Homo Sapiens destroyed all other Homo members... $\endgroup$ Jul 30, 2021 at 18:29
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    $\begingroup$ "I do understand that finding native bacteria on Mars would be somewhat valuable but surely it would also be valuable to manually seed life across the universe?" These really aren't equal, opposite choices? Also why would it be useful? Maybe, we're about to go there ourselves in the next 1000 years, great, resources are waiting. Or, Sun is going to die, turned out we couldn't go, next best thing. These don't seem like problems on remotely the same timescale of "We definitively find life on Mars or not" $\endgroup$ Jul 30, 2021 at 21:19

7 Answers 7


The answer to your formal question is: we cannot be certain we haven't already contaminated another world with our life, although we try not to. See NASA's notes on planetary protection protocols (1) for more information about that.

However, I think it is more important to address the other question you propose. Paraphrasing, you ask: "Why wouldn't we want to actively spread bacteria around our Solar System to see if it might thrive and grow?". The answer is because if we actively contaminate other worlds, such as Mars, Europa, Enceladus, or Titan, we drastically reduce our chance of discovering a second instance of life, independent of that on Earth. At the molecular level, all life we know--from the yeast under my fingernails to blue whales--is the same. All incorporate the same DNA and RNA with the same base pairs and proteins built from the same 22 left-handed amino acids. Certainly one of the greatest scientific questions is "Is life in the universe ubiquitous and exists in every nook and cranny where it possibly could, or is it extremely rare, with the closest even primitive life maybe hundreds of light years away?". At present, we have absolutely no direct evidence one way or the other. How can we possibly not want to know the answer to that?

The value of discovering an independent instance of life in the Solar System cannot be overestimated. To be an independent instance, it would be different from Earth life in some way. Understanding that difference would be extremely valuable toward our understanding of the nature of life in general. Also, the discovery would give us some indication that life is more likely to be abundant beyond the Solar System.

(1) https://sma.nasa.gov/sma-disciplines/planetary-protection

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    $\begingroup$ The logic becomes even more clear if you look at categorization of missions and planetary bodies, for example on pages 5-6 and 31 respectively of this document. The more likely a body (or region, on Mars) is to have once supported life, the more protection it is afforded. $\endgroup$
    – Cadence
    Jul 28, 2021 at 18:58
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    $\begingroup$ It would also be amazing to find life out there that's NOT different from that on Earth -- but only if we can be sure it's not some goo that we tracked in on our shoes. $\endgroup$
    – A C
    Jul 29, 2021 at 4:34
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    $\begingroup$ Oh wow. I did my PhD on selenocysteine, the 21st amino acid. I am so used to people forgetting it, that I got completely confused and thought it was the 22nd and you had just forgotten pyrrolysine. You hadn't forgotten anything! Not only did you include selenocysteine, you even counted pyrrolysine so my suggested edit is completely off. Please reject it with my apologies. $\endgroup$
    – terdon
    Jul 29, 2021 at 10:32
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    $\begingroup$ @Terdon, I did reject the edit as you suggested. No worries. The fact that someone with you level of knowledge now concurs with my statement is reassuring. $\endgroup$
    – Vince 49
    Jul 29, 2021 at 12:10
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    $\begingroup$ "hundreds of light years away" Or billions of light years away. We don't know how life started on Earth (abiogenesis). And without that information, and only this one data point, we cannot estimate the probability of any kind of life existing elsewhere. For all we know, life is unique to Earth. The discovery of any other life would allow us to start estimating such probabilities, as well as giving us clues about how abiogenesis happened on Earth. $\endgroup$
    – PM 2Ring
    Jul 30, 2021 at 6:06

Once we introduce microorganisms into an environment, it makes it more difficult to study any microorganisms that might already be there, and it's basically impossible to undo. We won't be able to tell if any products of biological processes came from our contamination or from what was there before us. We won't be able to tell whether a particular microorganism was already there, or if it mutated from something we introduced.

It will always be possible to introduce our bacteria later. Careful study of anything pre-existing can only be done before we do that.

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    $\begingroup$ I mostly agree, although it should be possible to tell if an organism originated from Earth based on the sequencing of it's genome. Any truly Martian organism should be very different if it was from a separate abiogenisis (it might well not share the same genetic code). Even if it shared a common ancestor with a microbe on Earth the divergence in genetic material should be detectable. $\endgroup$
    – Slarty
    Jul 28, 2021 at 7:11
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    $\begingroup$ @Slarty what about the side effects of micro-organisms, such as their waste products? Was that CO2 there naturally or did our bacteria put it there? $\endgroup$
    – user253751
    Jul 28, 2021 at 10:03
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    $\begingroup$ @Slarty should..should..should Maybe. We've never observed Martian life, or any extraterrestrial life, so we really have no basis to assert what should or should not be the case when studying them. If we find any. $\endgroup$
    – Seth R
    Jul 28, 2021 at 14:42
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    $\begingroup$ @Slarty Re Any truly Martian organism should be very different if it was from a separate abiogenisis. That is a big if. The panspermia hypothesis (and it is just a hypothesis) says life is universal and has a deep common origin. If we find life on Mars that is similar genetically to life on Earth, how are we to know that that Mars life was on Mars for billions of years versus recently introduced by Mars landers/rovers? $\endgroup$ Jul 28, 2021 at 14:50
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    $\begingroup$ "It will always be possible" -- life on Earth, and especially human life, won't be around "always". If we wipe ourselves out, there won't be more chances for landers to seed Europa with life until a new intelligent civilization (if ever) arises. $\endgroup$
    – Yakk
    Jul 28, 2021 at 14:51

Update: There is a thorough and well-sourced summary too long and detailed to reproduce here published in Nature Feb. 13, 2020: Lessons in space regulations from the lunar tardigrades of the Beresheet hard landing

Why are we trying to prevent (Earth) life from spreading via landers rather than actively encouraging it?

I've added "Earth" to the quoted title parenthetically. Even though it's not written explicitly, we are not currently trying particularly hard to prevent non-Earth life from spreading via landers.

...but surely it would also be valuable to manually seed life across the universe?

If life is there already, then "to seed" would not be the most accurate verb.

tl;dr: per NASA:

(there is) a legal and ethical consideration to protect potentially viable organisms and ecosystems beyond Earth.

The two current answers here are based on protection of the site from contamination to prevent loss of future science. However NASA also discusses the ethical concern of impacting any preexisting life that may already be present.

Since science puts so much time, effort and money into searches for life, it indicates that science acknowledge that there might actually, conceivably already be life in places where it looks for it.

In comments below Is there any demonstrated or even proposed technology that can sterilize a spacecraft with 100% certainty and yet leave it electronically functional? there is a link to Europa Lander Study 2016 Report; Europa Lander Mission; JPL D-97667, Task order NNN16D011T; Europa Lander Mission Pre-Phase A. In the beginning of Section 7, Planetary Protection it says:

The mission design of Europa Lander (Chapter 10), as a spacecraft that will land and also potentially impact the europan surface prior to landing, should be compliant with the requirements of PP mission classification Category IV under current COSPAR and NASA planetary protection policy, i.e., less than 1×10−4 probability of contaminating the europan ocean by a viable Earth microorganism. The ultimate landing, a relatively low-velocity impact of Europa Lander onto Europa’s surface without the benefit of atmospheric heating, drives the derived requirement for bioburden reduction processing of the spacecraft hardware.

The importance of preventing forward contamination is two-fold, first as a legal and ethical consideration to protect potentially viable organisms and ecosystems beyond Earth, and second as a scientific imperative to ensure that false positives are minimized (NAS, 1958; COSPAR, 1964; Rummel and Billings, 2004; UNOOSA, 1966; and see Hand et al., 2009 for a more detailed discussion). The specific language to which the United States, and therefore NASA, committed to in 1967 is that of the Outer Space Treaty, Article IX: “States Parties to the Treaty shall pursue studies of outer space, including the moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination ... and, where necessary, shall adopt appropriate measures for this pur- pose.”

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    $\begingroup$ That article in Nature is labeled opinion. There are some, apparently including the OP, who have the exact opposite opinion: That it is morally and ethically irresponsible not to spread earth life throughout the universe. (I am not in that camp.) $\endgroup$ Jul 28, 2021 at 13:00
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    $\begingroup$ Towards whom would this ethical consideration be? Towards future humans? Since the only life that could possibly exist in the Solar System is bacteria like, I imagine there’s no ethical obligation towards those hypothetical creatures as they lack consciousness. $\endgroup$ Jul 28, 2021 at 18:28
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    $\begingroup$ @JonathanReez there's an ethical obligation to being good stewards: cultivate what exists, don't change it without good reason and most assurededly not without understanding both what is there and what the consequences of the change are. This is both an obligation to the life we care for and a practical preservation for ourselves. The last thing we need is an interplanetary equivilent of kudzu, killer bees, hornet wasps, or any of a dozen other dangerous environmental contaminations we've seen on earth. $\endgroup$
    – RonLugge
    Jul 28, 2021 at 23:11
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    $\begingroup$ @DavidHammen my flag was denied, so I've gone on to ask the following in meta: How extensively should we interrogate/pursue post authors about suspected ulterior motives or suspected thoughts or beliefs? Please feel free to comment or post an answer there. $\endgroup$
    – uhoh
    Jul 29, 2021 at 1:02
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    $\begingroup$ @JonathanReez, it's the little E. Coli that taught us gene splicing! And life doesn't mean animals only. The ethical considerations are towards you, me and the entire US. Even a single dead or alive bacteria on Mars can teach us a lot about the story of life, ours and theirs, how it started, when it may end. And then, who knows, we may discover a form of life that is not carbon-based, somewhere not too far ... $\endgroup$
    – Ng Ph
    Jul 29, 2021 at 21:10
  1. Go ask an Aussie about rabbits. They've been trying to get quit of them for, I think, well over 100 years - but they breed, well, like rabbits, and getting them OFF the damned continent is thoroughly uphill work.

  2. Get a Terran microorganism in an alien environment, you've just contaminated that environment, and your studies are already skewed. And you'll never ever get rid of that Terran microorganism, it's there to stay. Full Stop.

  3. You've just introduced a mutating factor into the the alien environment you're trying to study, you will not be able to study its true form anymore. It is, as I said, contaminated. Oops!

And that, dear friends, is why we go to pains to make sure that interplanetary satellites are so clean. Particularly if we think that they may be going to an inhabited planet - remember how War of the Worlds ended? Our "encouraging" seeding of Terran life on other planets could end up wiping out another species or three - what if one of those could have offered the cure for cancer?

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    $\begingroup$ Or perhaps the local lifeforms find out that Earth ones are quite tasty, and want more... $\endgroup$
    – Jon Custer
    Jul 28, 2021 at 16:17
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    $\begingroup$ Didn't War of the Worlds end with the alien lifeforms (i.e. the invading Martians) dying out because they couldn't tolerate Earth's microbes? $\endgroup$ Jul 29, 2021 at 5:52
  • $\begingroup$ I think a more realistic scenario than rabbits in Australia (given the difference in environments between Earth and Mars) would be to move a great white shark to the Serengeti and move a lion into the Atlantic ocean and see how well each copes with the environment of the other. $\endgroup$
    – Slarty
    Jul 29, 2021 at 12:18
  • $\begingroup$ @Jon Custer, or Earth lifeforms decide to teach the Martians a few tricks to defend themselves when Mars is invaded by human colonizers. $\endgroup$
    – Ng Ph
    Jul 29, 2021 at 22:20
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    $\begingroup$ Rabbits in Oz are the second-most costly invasive species in the country. The development of biological control systems for rabbits in Oz during the 1950s, such as myxomatosis was used by the US biological weapons program - particularly ways to quickly make vast quantities of pathological agents. $\endgroup$
    – Fred
    Jul 30, 2021 at 5:14

I think the answer to this question is concise and can apply broadly to any similar situation.

If both experiments are valuable, they should be performed in a manner that allows them both to be done.

Seeding a foreign world with terrestrial life would, at the very least, make it more difficult to study any native life.

It could be possible to simulate extra-terrestrial environments on Earth for your proposed experiment on Earth organisms - But, studying extra-terrestrial life can only happen one way.

So, ethics aside, the logical thing to do is ensure that no native life is present before seeding a world.

This will allow the maximum amount of research possible to be done.


Several answers have already identified strong ethical and scientific arguments for valuing and preserving possible extraterrial life.

But even supposing for the sake of argument that we were able to satisfy ourselves that there was no life on Mars whatsoever, and everybody was agreed that we ought to go ahead with terraforming it... surely we would want to plan what life we introduced and when.

Nobody wants to spend billions engineering a bacterium designed to terraform Mars, only to discover that a previous mission has already colonised it with another microbe that treats our fancy designer bacterium as a tasty snack.


The question requires indeed a very serious scrutiny. This is because, the goal expressed by the OP has already been put into practice in 2019. In that year, a private organisation sent a payload of tardigrades to the Moon, the Beresheet mission.

The OP explains that the motivation, similar to that of the experiment embarked on the Israeli’s mission, is (apparently) to study Life. Quote from the question: “it would be a great experiment that would also tell us which mutations will occur over time in that environment[Mars]”. So, they (the OP, the Arch Foundation, ...) wanted to do it in the name of Science. It would be indeed “great” (in the sense that we may be able to learn new things), PROVIDED it is conducted scientifically. And this is exactly where the OP of this question didn’t show due considerations.

First, “in that environment” assumes the prerequisite that we have scientifically characterized the environment, and further we have, and can maintain strict control over it (nothing else get in after the start of the experiment).

Second, to study the mutations triggered by a certain environment condition requires that we have a pretty good knowledge of the various mechanisms of mutations under various conditions (that we can or cannot parametrize). I doubt scientists can claim that we have reached that level of knowledge.

Third, “will occur over time” means that we have that time to observe these phenomena, or at least we know the time frame. This can be problematic when it can take thousands years, as the case for some undersea microbes to replicate.

In short, this type of ideas/proposals should be pushed back categorically: people can do terrible things in the name of Science.

To the author of this question, to be clear, I am not insinuating dark motives from you. I am making a plea that you reconsider your position.

EDIT: I would like to address the recent clarifying Edit

Why this is considered anathema?

Submitting life forms from Earth to conditions found on other celestial bodies has inherent scientific values (nothing anathema about it).

What is very disputable is the approach you suggest: instead of taking precautions to not spreading Earth’s life by cleaning the landers, do the opposite. What you suggest is akin to using an entire celestial body (or an unbounded area of it) as a laboratory for some random experiments, open-ended in their expected outcomes, and decided by some individuals. This is unscientific at best, and dangerous for sure, since I bet people would use this as a pretext to justify their sloppiness (and/or to save money).

As to the second part of your question, where you ask whether we should put on equal footing, on one hand, the preservation of some “native” forms of life, on the other hand, the spreading of Earth life, it can be viewed from different angles. From the point of view of "ethic", I personally find it anathema (same debate as destroying jungles to grow industrial crops, same debate as destroying archeological sites to build modern housings). In general, I don’t find it so urgent to spend billions in colonizing other planets. I prefer that we spend the same on preserving ours, such as cleaning the oceans of the plastic wastes.

But to avoid the “opinion-based” criticism so favored on SE, we can look at it from the scientific perspective. Perhaps you are thinking that, if there is a native form of life, we can let Earth’s forms of life compete with it. We take the winner, what do we lose? I think, in case the Earth life out-competes a native form, we lose forever the knowledge of how much alien forms of life can teach us, about our origins, about our destiny, about the definition of “life” itself. It is a big lost to Science. In the case the native form is the winner, we can cross-fingers it doesn’t evolve into a super Earth-killing pathogen.

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  • $\begingroup$ I understand your sentiment and probably lots of folks concur. But "people can do terrible things" in general. Is there fact-based evidence that anybody is "do(ing) terrible things in the name of Science" now, in the present tense? It's good to keep Stack Exchange answers well founded in fact. They should strive not to be opinion pieces or editorials. SE answers should not be "pushes back against ideas" nor "pleas" and they should indeed be answers to the question and not a "not an answer". $\endgroup$
    – uhoh
    Jul 28, 2021 at 11:58
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    $\begingroup$ @uhoh, agreed that "Israeli's experiment" is an incorrect formulation. Edited. $\endgroup$
    – Ng Ph
    Jul 28, 2021 at 15:43
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    $\begingroup$ 1) If the tardigrades were "sealed in epoxy" (per previous comment) then they were clearly not put there to "seed life". 2) Tardigrades are not bacteria. Therefore "the goal expressed by the OP has already been put into practice in 2019" seems to be a false statement. 3) The question is simply a question. It is not a position. It makes no sense to make "a plea that (they) reconsider (their) position. $\endgroup$
    – uhoh
    Jul 28, 2021 at 22:53
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    $\begingroup$ In addition to not addressing the same situation the OP asks about, I think you have misunderstood some other stuff as well. Beresheet was a time capsule--it was intended to contain a collection from Earth (primarily knowledge stored on a disc) that could be recovered later. It was engineering, not a scientific experiment. The tardigrades included were in suspended animation because of dehydration, so there was no expectation that they would reproduce. The effect of space travel on tardigrades in general has already been studied. $\endgroup$ Jul 29, 2021 at 17:32

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