Half a century ago, the joke was that you should never shake hands with aliens because they might be made of antimatter. Touching them in such a case would lead to mutual annihilation. Today, thanks to advances in gamma-ray and cosmic-ray astronomy, we know that antimatter is a negligible component of the cosmic mass budget. These circumstances are well understood in the context of the dense early phase of the Universe, when all traces of antimatter annihilated with matter, leaving behind a tiny (six parts in ten billion) excess of matter that is responsible for what we observe in the Universe today.
But there is another critical reason to avoid contact with extraterrestrial life: it may be a mirror image of terrestrial life.
In 1848, the chemist Louis Pasteur outdoor that some molecules necessary for life exist in mirror image forms. These two forms are referred to as “left-handed” and “right-handed”, where handedness or chirality refers to the direction in which polarized airy rotates as it propagates through a solution of molecules. Biology on Earth has broken symmetry and chosen homochirality, representing only one of these chiral forms. DNA, RNA and their components are dextrorotatory, while amino acids and proteins are left-handed. Symmetry breaking could have occurred before or after the appearance of life. This uncertainty may be resolved when we discover life beyond Earth.
If extraterrestrial life turns out to be equally likely to be left-handed or right-handed, then we would know that symmetry was accidentally broken on Earth in a way that allowed one hand to dominate Earth's ecology. This would also suggest that the transfer of life between planets, the so-called panspermiais irrelevant because it would naturally lead to one dominant hand in systems sharing a common heritage.
New Article on a political forum co-authored by 38 leading experts in the journal Science warns of existential risks of mirror life, based on 299-page technical report titled: “Mirror Bacteria: Feasibility and Risk.” In mirror bacteria, DNA, RNA, amino acids and proteins are replaced by their mirror images. Future advances in synthetic biology may lead to the production of mirror bacteria. Their interactions with natural organisms would be different from the interactions of natural bacteria with natural organisms.
The immune defense relies on interactions between chiral molecules that may fail to detect or kill mirror bacteria. As a result, mirror bacteria can spread throughout the ecology of natural terrestrial life, unchecked by natural biological controls. Mirror bacteria would prove to be unsafe pathogens for a wide range of natural life forms, including humans. The report details the resulting health risks to humans, other animals and plants, as well as the potential ecological consequences.
On this basis, an article in the journal Science states: “Unless compelling evidence emerges that mirror life does not pose extraordinary risks, we believe that mirror bacteria and other mirror organisms should not be created, even those equipped with engineered biodefense measures.” We therefore recommend that research aimed at creating mirror bacteria should not be allowed, and donors have made clear that they will not support such work.”
But what if nature created a mirror image of life on another planet? Should we worry about delivering this mirror life to our planet inside a rock or the Mars Sample Return mission? It would be the interplanetary equivalent of Amazon's delivery service, bundled with content we never ordered.
What happens naturally on other planets does not necessarily meet the recommendations of the 38 authors of the Science article. One thing is certain: if any rocks with traces of mirror life were ever delivered to Earth in the last 4.2 billion years since their emergence, The last universal common ancestor (LUCA), terrestrial life managed to survive the potential damage. Otherwise, mirror cyanobacteria, which feed on non-chiral nutrients and sunlight, could dominate Earth's ecosystem by producing mirror versions of common sugars. Mirror animals would feed on mirror food produced by mirror plants. It is possible that no package of mirror life reached Earth because the universal mechanism favors the same homochirality for life everywhere.
For now, a modern technical report and science policy paper suggest we should exercise caution when handling material that potentially carries traces of extraterrestrial life. Keep this critical caution in mind when returning samples from Mars as part of NASA and ESA's ambitious multi-mission campaign.
While pursuing Elon Musk's inspiring vision of occupying and monetizing Mars, it's also critical to be mindful of health risks humans are a multi-planetary species. The possibility that Martian soil may contain relics of mirror life increases known health risks from energetic cosmic rays and extreme day-night temperature fluctuations on the Martian surface.
Until we know that homochirality is universal, we should exercise caution in our first contact with extraterrestrial life.
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