Raring to go: Explorers practise weightless walking during a simulated Mars mission on a Tyrolean glacier in Kaunertal, Austria, in 2015. Picture: REUTERS
Raring to go: Explorers practise weightless walking during a simulated Mars mission on a Tyrolean glacier in Kaunertal, Austria, in 2015. Picture: REUTERS

The closest place in the universe where extraterrestrial life might exist is Mars, and human beings are poised to try to colonise this planetary neighbour within the next decade.

Before that happens, there has to be recognition that a very real possibility exists that the first human steps on the Martian surface will lead to a collision between terrestrial life and biota native to Mars.

If the red planet is sterile, a human presence there would create no moral or ethical dilemmas. But if life does exist on Mars, human explorers could easily set off the extinction of Martian life. As an astronomer who explores these questions in my book Life on Mars: What to Know Before We Go, I contend that earthlings need to understand this scenario and debate the possible outcomes of colonising the neighbouring planet. Maybe missions that would carry humans to Mars need a timeout.

Life, scientists suggest, has some basic requirements. It could exist anywhere in the universe that has liquid water, a source of heat and energy, and copious amounts of a few essential elements such as carbon, hydrogen, oxygen, nitrogen and potassium.

Mars qualifies, as do at least two other places in our solar system. Europa, one of Jupiter’s large moons, and Enceladus, one of Saturn’s large moons, appear to possess these prerequisites for hosting native biology.


Below their thick layers of surface ice, Europa and Enceladus have global oceans in which 4.5-billion years of churning of the primordial soup may have enabled life to develop and take root. Nasa spacecraft have imaged spectacular geysers ejecting plumes of water out into space from these subsurface oceans.

To find out if these moons have life, planetary scientists are developing the Europa Clipper mission for a 2020s launch. They also hope to plan missions to Enceladus.

Since the start of the space age, scientists have taken the threat of biological contamination of other worlds seriously. In 1959 Nasa held meetings to debate the necessity of sterilising spacecraft that might be sent to other worlds. Since then, all planetary exploration missions have adhered to sterilisation standards that balance scientific goals with limitations of not damaging sensitive equipment, which could potentially lead to mission failure. Nasa protocols exist for the protection of all solar system bodies, including Mars.

Since avoiding the biological contamination of Europa and Enceladus is an extremely well-understood, high-priority requirement of all missions to the Jovian and Saturnian environments, their moons remain uncontaminated.

Nasa’s Galileo mission explored Jupiter and its moons from 1995 until 2003. The possibility existed that the spacecraft, once out of rocket propellant and subject to the whims of gravitational tugs from Jupiter and its many moons, could crash into and contaminate Europa.


Seeding life: An artist’s concept of a comet approaching Mars, as orbiters prepare to make observations. Picture: REUTERS
Seeding life: An artist’s concept of a comet approaching Mars, as orbiters prepare to make observations. Picture: REUTERS

To completely eliminate any such risk, in 2003 Nasa used the last bit of fuel on the spacecraft to send it plunging into Jupiter’s atmosphere. At a speed of 30 miles per second, Galileo vapourised within seconds.

Nasa repeated this protect-the-moon scenario 14 years later. The Cassini mission orbited and studied Saturn and its moons from 2004 until 2017. In September 2017, when fuel had run low, Cassini’s operators deliberately plunged the spacecraft into Saturn’s atmosphere, where it disintegrated.

Mars is the target of seven active missions, including two rovers, Opportunity and Curiosity. On November 26 Nasa’s InSight mission is scheduled to land on Mars, where it will make measurements of the planet’s interior structure. With launches planned for 2020, the European Space Agency’s ExoMars rover and Nasa’s Mars 2020 rover are designed to search for evidence of life on Mars.

Robotic rovers pose little risk of contamination to Mars, since all spacecraft designed to land there are subject to strict sterilisation procedures before launch. These rovers are likely to have an extremely low number of microbial stowaways.

Any terrestrial biota that manage to hitch rides on the outside of those rovers would have a very hard time surviving the half-year journey from Earth to Mars. The vacuum of space combined with exposure to harsh X-rays, ultraviolet light and cosmic rays would almost certainly sterilise the exteriors of any spacecraft sent to Mars.


Any bacteria that sneaked inside one of the rovers might arrive at Mars alive. But if any escaped, the thin Martian atmosphere would offer virtually no protection from high-energy, sterilising radiation from space. Those bacteria would be likely to be killed immediately. Because of this harsh environment, life on Mars, if it currently exists, almost certainly must be hiding beneath the planet’s surface. No rovers have explored caves or dug deep holes in search of possible Martian microbes.

Given that the exploration of Mars has so far been limited to unmanned vehicles, the planet is likely to remain free from terrestrial contamination.

But when Earth sends astronauts to Mars, they will travel with life support, energy supply systems, habitats, 3D printers, food and tools. None of these materials can be sterilised in the same way that spacecraft can. Human colonists will produce waste, try to grow food and use machines to extract water from the ground and atmosphere. Simply by living on Mars, human colonists will contaminate the planet.

It is critical that every attempt be made to obtain evidence of any past or present life on Mars well in advance of future missions to Mars that include humans. What we discover could influence decisions on whether to send colonists there.

Even if we ignore or don’t care about the risks a human presence would pose to Martian life, the issue of bringing Martian life back to Earth has serious societal, legal and international implications that deserve discussion before it’s too late.

What risks might Martian life pose to Earth’s environment or human health? Does any one country or group have the right to put all of life on Earth at risk?

Nasa, the United Arab Emirates’ Mars 2117 project, SpaceX, Mars One and Blue Origin already have plans to transport colonists to build cities on Mars.

• Weintraub is a professor of astronomy at Vanderbilt University, in Nashville, Tennessee. This article first appeared on www.theconversation.com.

The Conversation