In H.G. Wells’ classic War of the Worlds (1898), Earth narrowly survives an invasion from Mars, thanks to its smallest residents - microbes. But what if the roles were reversed, and Earthlings became the unwitting exterminators? A bold hypothesis from Dirk Schulze-Makuch, an astrobiologist at the Technical University of Berlin, suggests NASA’s Viking missions in the 1970s may have inadvertently wiped-out Martian life in their eagerness to find it.

Humanity’s fascination with Mars predates modern science. Italian astronomer Giovanni Schiaparelli’s observations in 1877 of ‘canali’ (channels) on the planet’s surface sparked speculation about intelligent life. (‘canali’ in Italian was mistranslated as ‘canals’ in English)

A century later, the Viking program became a milestone in planetary exploration. In 1976, two landers - Viking 1 and Viking 2 - touched down on Mars with an ambitious goal to uncover evidence of life. Equipped with cutting-edge instruments, the spacecraft conducted experiments to detect metabolic activity in Martian soil. Central to these efforts was the assumption that, like Earth, Martian life would rely on liquid water. Soil samples were doused with water and nutrient solutions, simulating conditions hospitable to Earthly organisms.

Initially, the experiments yielded tantalizing results. One test detected chemical reactions that hinted at biological activity. However, the excitement was short-lived. Further analysis deemed the reactions non-biological, likely the result of peculiar Martian chemistry. Over decades, the Viking results were largely dismissed as a cosmic dead end - until now.

Schulze-Makuch argues that the Viking methods may have been too blunt for the delicate ecosystems they sought to probe. His hypothesis draws parallels with extremophiles in Earth’s driest places, such as Chile’s Atacama Desert. In this hyper-arid environment, microorganisms survive by extracting moisture from the air using hygroscopic salts. These microbes are marvels of adaptation, thriving on trace amounts of water in conditions harsher than most Mars-like analogues. Yet they are paradoxically vulnerable: an influx of liquid water can rupture their osmotic balance, causing them to die.

If such salt-dependent microbes existed on Mars, Schulze-Makuch posits, the Viking landers’ generous application of water might have proved fatal. What was intended as a life-giving elixir could have triggered osmotic shock, killing any Martian organisms present. This raises a troubling question: did the Viking experiments, in their Earth-centric design, erase the very evidence they sought?

NASA’s Viking missions of the 1970s were a watershed moment. Building on the success of the Mariner program, which provided the first close-up images of Mars, Viking 1 and 2 represented the most ambitious attempt yet to answer a question as old as science itself: Are we alone? Equipped with a suite of experiments, the landers sought to test Martian soil for signs of metabolism and organic compounds, markers of potential life.

The missions were a triumph of engineering. Yet, the search for life yielded ambiguous results. One experiment, the Labeled Release (LR) test, seemed to show signs of biological activity when soil samples were exposed to water and nutrients. But when other tests failed to detect organic molecules, scientists dismissed the positive results as chemical reactions rather than biological ones.

The Vikings were emblematic of 20th-century scientific optimism, but they operated with limited knowledge of Mars’s unique environment. The planet’s surface is an extreme desert, colder and drier than any place on Earth. Over billions of years, Mars transitioned from a wet and temperate climate to a barren, hyper-arid landscape. Its soil chemistry - rich in perchlorates and other reactive compounds - is alien to terrestrial biology. Any Martian life that survived would have evolved extraordinary resilience, perhaps even a biochemistry fundamentally different from Earth’s.

In some ways, Schulze-Makuch’s hypothesis calls to mind classic Hollywood 1950s science fiction like The Day the Earth Stood Still (1951) where Earth’s reckless behaviour nearly brings about its destruction when encountering extraterrestrial intelligence. Similarly, Forbidden Planet (1956) – an outer-space version of Shakespeare’s ‘The Tempest’ – warned of the unintended consequences of technological overreach, much like Viking’s invasive water-based experiments.

Schulze-Makuch’s critique extends beyond the Vikings to the broader philosophy of planetary exploration. The tendency to view extraterrestrial life through an Earthly lens, he argues, risks blinding us to otherworldly possibilities. Recent missions, such as the Perseverance rover, have incorporated these lessons, adopting less intrusive methods to study Mars. Yet the shadow of Viking lingers, a cautionary tale of how the pursuit of knowledge can inadvertently lead to destruction.

It is tempting to dismiss Schulze-Makuch’s hypothesis as speculative. The Viking results remain enigmatic, and definitive proof of past or present Martian life continues to elude us. But his work underscores a critical shift in astrobiology: the need to adapt our methods to alien worlds, rather than forcing alien worlds to conform to Earthly expectations. If life exists elsewhere - whether microbial or intelligent - what responsibility do we bear to protect it? In The War of the Worlds, the Martians succumb to Earth’s smallest creatures. On Mars, it may have been Earth’s invaders that sealed the fate of its smallest denizens.