[baja_blast]

One thing that has annoyed me is in the 1970s the Viking landers did experiments to check for the presence of life on Mars known as the Labeled Release experiments https://en.wikipedia.org/wiki/Viking_lander_biological_exper... which dropped a nutrient solution with radioactive Carbon-14 to detect if there was any off gassing to detect if anything metabolized the soil. And both experiments showed positive results but it has been dismissed since chemical reactions could not be ruled out. But here is the thing, the experiments then did a sterilization control where they heated up the soil to 320 F for 3 hours and attempted the experiment again and no gasses were detected which is something you'd expect to see if the gasses were produced by microbes and not chemical processes.

Now is this a positive detection of life? No because other possible factors can not be ruled out. But what puzzles me is why we have never followed up with any further experiments to try and detect life? After the Viking missions we never conducted any further experiments that could rule out any other possible chemical reactions to get closer to confirming the presence of microbial life.

So I would say with the Labeled Release experiments coupled with the seasonal Methane detections strongly imply that there is still microbial extremophiles on Mars.

[deelowe]

Read the wikipedia link and it seems there are credible explanations for what is going on:

> "With unsterilized Terrestrial samples, though, the addition of more nutrients after the initial incubation would then produce still more radioactive gas as the dormant bacteria sprang into action to consume the new dose of food. This was not true of the Martian soil; on Mars, the second and third nutrient injections did not produce any further release of labeled gas."

> "Albet Yen of the Jet Propulsion Laboratory has shown that, under extremely cold and dry conditions and in a carbon dioxide atmosphere, ultraviolet light (remember: Mars lacks an ozone layer, so the surface is bathed in ultraviolet) can cause carbon dioxide to react with soils to produce various oxidizers, including highly reactive superoxides (salts containing O2−). When mixed with small organic molecules, superoxidizers readily oxidize them to carbon dioxide, which may account for the LR result. Superoxide chemistry can also account for the puzzling results seen when more nutrients were added to the soil in the LR experiment; because life multiplies, the amount of gas should have increased when a second or third batch of nutrients was added, but if the effect was due to a chemical being consumed in the first reaction, no new gas would be expected. Lastly, many superoxides are relatively unstable and are destroyed at elevated temperatures, also accounting for the "sterilization" seen in the LR experiment."

[baja_blast]

> This was not true of the Martian soil; on Mars, the second and third nutrient injections did not produce any further release of labeled gas

But that's probably because martian microbes are less tolerant of high temperatures when compared to Earth microbes. But, yes I am aware there are other non life factors that could have resulted in a positive detection. But my point is why would we never follow up with further experiments to test for possible chemical reactions?

[deelowe]

Because we have enough evidence already to draw conclusions? Space exploration is expensive.

[tivert]

> But what puzzles me is why we have never followed up with any further experiments to try and detect life? After the Viking missions we never conducted any further experiments that could rule out any other possible chemical reactions to get closer to confirming the presence of microbial life.

According to Wikipedia, the radiation levels are too high:

https://en.wikipedia.org/wiki/Life_on_Mars#Cosmic_radiation

> Even the hardiest cells known could not possibly survive the cosmic radiation near the surface of Mars since Mars lost its protective magnetosphere and atmosphere.[63][64] After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that over time, any life within the first several meters of the planet's surface would be killed by lethal doses of cosmic radiation.[63][65][66] The team calculated that the cumulative damage to DNA and RNA by cosmic radiation would limit retrieving viable dormant cells on Mars to depths greater than 7.5 meters below the planet's surface.[65] Even the most radiation-tolerant terrestrial bacteria would survive in dormant spore state only 18,000 years at the surface; at 2 meters—the greatest depth at which the ExoMars rover will be capable of reaching—survival time would be 90,000 to half a million years, depending on the type of rock.[67]

[baja_blast]

People have said the same thing many times yet we keep discovering extremophiles thriving the some of the most hostile environments. And in 2020 they conducted an experiment on the ISS that exposed Earth bacteria to direct cosmic radiation for 3 years and it turns out they survived https://www.smithsonianmag.com/science-nature/scientists-dis.... And this was just Earth bacteria that did not evolve under these conditions, any remaining microbes on Mars would have developed adaptions to survive in such conditions.

[Qem]

So we just need to find a recently excavated impact crater, a few thousand years old, and send a probe there, to inspect freshly exposed layers.

[sgt101]

>But what puzzles me is why we have never followed up with any further experiments to try and detect life?

simples.

If life is definitively detected on Mars there will never be another mission to determine if life is present on Mars. So what clever scientists have figured out is that they need to do all the science that they want before checking for life.

[dotnet00]

I feel like if life was definitively detected on Mars, it'd kick off a serious new space race for sending crew there. The prestige of either potentially studying alien life, or of having the ability to set foot on the fundamental origin of life on our own planet would be even more historically significant than Apollo.

[Qem]

> I feel like if life was definitively detected on Mars, it'd kick off a serious new space race for sending crew there.

Nope. More probable for it to get quarantined, for fear of cross contamination. IIRC NASA purposefully doesn't land craft on the locations with conditions closer to habitability in Mars today, fearing stowaway microbes from our probes run amok and eradicate any extant martian biota, just like rabbits and other critters brought by Europeans wrecked ecosystems in Australia.

[dotnet00]

IIRC a big part of Perseverance's mission is to stash away pristine, provably untouched samples of Mars, because NASA sees a human landing on Mars in the nearish (ie 20-30 years) as an inevitability.

I don't think they'd quarantine the planet due to it, even moreso because even if the US decides to quarantine it, other nations might not and even then, there'd be value to sending a crew to say, a station in Mars orbit or on Phobos/Deimos, for more effective control over survey vehicles and to reduce the feedback loop between retrieving samples from the surface and studying them.