Detection of extraterrestrial life more difficult than previously thought

Was there life on Mars - and perhaps simple forms of life have survived on the red planet to this day? Answering these questions is an important goal of many current and planned spacecraft. However, as experiments by an international research team are now showing, the instruments developed for this purpose are probably not able to detect past and present traces of life on Mars. Only the examination of soil samples in an earthly laboratory can therefore answer these questions, the scientists write in the journal "Nature Communications".

Researchers led by Armando Azua-Bustos from the Center for Astrobiology in Madrid examined soil samples from the Red Stone region in Chile's Atacama Desert using the best methods possible in a terrestrial laboratory, as well as instruments available from current or planned ones space probes are used. A river delta in the extremely arid Atacama Desert that dried up more than 100 million years ago, Red Stone is thought to be the region on Earth most closely resembling conditions on Mars today.

Using modern techniques such as DNA sequencing, Azua-Bustos and his colleagues easily detected active microbial life in soil samples from the Atacama Desert. In some cases it was even possible to cultivate microbes from the samples. The majority of the microbes detected are of previously unknown forms. The researchers also found a large number of biosignatures – i.e. organic molecules that are typically components of microbes – from life forms that existed in the past.

The team then analyzed the same soil samples with instruments such as those on board the Mars rover “Curiosity” or those intended for the European ExoMars rover. The analogue of the "Curiosity" instrument had ten times the sensitivity of the original device, but was still only able to detect a few biosignatures at the limit of its detection limit.

According to the researchers, the original instrument would have failed here. MOLA, the Mars Organic Molecular Analyzes instrument planned for ExoMars, did not fare much better: It only detected a few organic substances in the samples.

It is therefore to be expected that neither the current nor the currently planned missions will find a definitive answer to the question of life on Mars, according to the scientists. "The limited or lack of detection by the rover instruments of a number of biosignatures of the living and extinct microbes in the samples from the Red Stone region underscores the critical importance of a Mars sample return mission," emphasize Azua-Bustos and his colleagues. Only then could the samples be thoroughly examined for signs of life in laboratories on Earth.

In fact, the US rover Perseverance has been collecting rock samples for a return mission to Earth since 2021. However, it is currently completely open when and by what means such a mission could actually take place.

In an accompanying commentary in Nature Communications, planetary scientist Carol Stoker of NASA's Ames Research Center also points out that such missions can only bring a small amount of rock to Earth. "It remains to be seen whether clear signatures of life can be found in these limited samples," says the researcher. "So we have to be careful about taking the lack of clear evidence as proof of the absence of life."

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