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Fabulous discoveries are often followed by exceedingly dull paperwork, such as the checking and rechecking of data, graphs, statistical analyses and conclusions. This week’s announcement that scientists had found evidence of briny water on Mars will have had many experts reaching for the small print of Nasa Policy Directive 8020.7G.
The directive is required reading for those who send spacecraft to hunt for extraterrestrial life. It codifies the etiquette for “planetary protection” — preventing earthlings and their emissaries from contaminating their celestial bodies (known as forwards contamination), and arguably more importantly, guarding against the encroachment of alien microbes into the terrestrial biosphere (backwards contamination).
While the concerns about our germs hitchhiking to other worlds date back to the 1950s, this week’s revelation — and its implications for the possibility of life elsewhere in the solar system — should prompt an urgent reappraisal of how we maintain the absolute integrity of both the Martian and terrestrial biospheres.
The evidence for water flowing on the red planet was gathered by the Mars Reconnaissance Orbiter, a Nasa spacecraft launched in 2005, with images that showed dark streaks down the walls of a crater. The streaks were found to carry the infrared signature of hydrated salts, which is regarded as a proxy for water.
The results were published on Monday in Nature Geoscience. They were prefigured, however, in images dating back to Mariner 9, which began orbiting Mars in 1971. These revealed a world seemingly sculpted by liquid: valleys and canyons, ancient river beds and branching canals. Even in the 1960s, Earth-based observations suggested the spectral signature of water vapour in the thin Martian atmosphere.
A succession of orbiters and landers added layers of evidence: polar caps comprising vast quantities of water ice; rocks and pebbles rounded and smoothed as if by water; clumps of material, dug up by a robotic arm, which subsequently vaporised, indicating subsurface water; and permafrost-like patterns beneath the scarlet dust.
What made Monday’s announcement significant was its confirmation that liquid water flows on the planet’s surface today, albeit only seasonally. Mars is smaller than Earth with a much weaker gravitational field; it had been postulated that liquid water would just float away.
Astrobiologists, who study the origins of life in the universe, are thrilled: their guiding principle is to “follow the water”, since all known life, or life forms, need water for survival.
There is a class of extremophiles — organisms that survive in extreme environments — which thrive in salty, dehydrated surroundings. Scientists have found such “halophiles” in the ultra-dry Atacama Desert in Chile, in the form of microbes living in salt crystals (they absorb moisture from the atmosphere). Halophiles often contain a protein called bacteriorhodopsin; this might narrow the search for a smoking gun for life on Mars.
The obligation of space agencies to prevent contamination of Earth and other planets is stated in the 1967 UN Outer Space Treaty, which started life as a means of preventing the Moon and planets being used for hostile purposes. Earlier this year, astrobiologists raised concerns that ultra-sensitive space instruments and their associated electronics were now made of materials too delicate to withstand heat sterilisation.
While that is usually fine for orbiters, which do not land, it poses a challenge to missions such as Nasa’s Mars 2020 mission, due to touch down on the Red Planet after 2020.
There is an urgent need to develop the technology required to make sure future landers are not the bringers of bacterial doom. As humanity embarks on a search for Martian life in the brine, we must ensure that our methods are ethically watertight.
The writer is a science commentator