The idea that Earth may be invaded by hostile extra-terrestrial beings has been a popular premise for many Science fiction writers for centuries. As we gather more information about our celestial neighbourhood we are finding that the existence of life outside of Earth is becoming more likely. So the question must be asked; how realistic is the idea that organisms from another world could destroy life on Earth?
Recent space missions, such as Genesis and Stardust, took samples of extra-planetary materials and returned them to Earth for analysis; Genesis crash landed causing damage to collection instruments and contaminating some of the samples collected from space. What if the threat of “alien invasion” was not the result of sentient beings coming to seize control of our planet but a consequence of bringing extra-terrestrial microbes advertently or inadvertently back to Earth in our quest for knowledge about our Solar System?
One of the main ingredients thought to be essential to life is liquid water and recent data collected from the Mars Reconnaissance Orbiter shows formations on the surface of the red planet, thought to be caused by brine flows; a super-saturated solution of salt and water. It is thought by some Science communicators that this solution could harbour life similar to halophiles .
The formations identified on Mars are in environments which can reach 27°C in the warm season but plummet to well below zero at other times of the year. Similarly, the concentration of chlorates in these flows is at a level which would suggest that the existence of life in this place is impossible. However, biologists are finding life on Earth in conditions that seem out of this world. For example, the recently discovered bacterium Planococcus halocryophilus lives in the Canadian Arctic region in seams of salty water at sub-zero temperatures, conditions which are not dissimilar to those on Mars. Other Archaebacteria thrive in Earth environments considered inhospitable to life, including extreme temperature, pH, pressure, salt concentration as well as in toxic chemicals, such as radioactive waste. This suggests that the existence of life is becoming ever more likely in all sorts of environments, including those which are extra-planetary.
NASA is currently developing technology which will go to the Jovian moon of Europa with the explicit aim of searching for life . Eight separate spacecraft as well as telescopes have provided data that shows Europa may have a massive liquid ocean located beneath its icy surface. Europa experiences huge tidal forcing due to the gravity of Jupiter which creates friction and a source of heat energy at the core of the moon. Radiation from Jupiter which hits the icy surface of Europa could potentially create simple chemicals thought to be precursors for life. The presence of liquid water, a source of energy and organic chemicals are the three ingredients thought to have brought about life on Earth, hence Europa’s ocean being a favourite location to scout for ET.
Should we find something that resembles life in the not too distant future, it is reasonable to assume that astrobiologists would be keen to get samples back to Earth for analysis in a lab. Since we only know about life from our planet, there are some potentially huge risks associated with bringing any extra-terrestrial life-forms back from another planet or moon.
The Apollo astronauts had a mandatory quarantine period of three weeks in case they brought back microbes from space; although this was largely trivial since no one could possibly know the incubation period of an extra-terrestrial germ. If life from outside of Earrth is retrieved it may be nothing like the life on our planet, meaning we may have no way to detect it. Viruses and prions are not considered “living” and can be less than 0.05µm (about 1/20th the width of a human hair). Similarly we may have no way to sterilise equipment against a non-terrestrial replicating entity, increasing the risk of exposure. It is also unlikely that Earthlings would have evolved any immunity to a space pathogen which could lead to significant population declines or extinctions.
Scientific organisations around the world recognise this risk and have devised policies concerning scenarios about returning spacecraft and the protection of humans and the Earth. The Committee on Space Research (COSPAR) first drafted the Planetary Protection Policy in the 1950’s, around the time Sputnik missions were reaching into space for the first time. 100 countries involved in space research have signed a treaty recognising this policy which states that “safeguarding the Earth from potential contamination is the highest planetary protection priority in Mars exploration”.
These outcomes, though catastrophic are extremely unlikely. An extra-terrestrial microorganism would first have to survive the journey through space. It would then have to pass containment and sterilisation protocols, survive in an environment very unlike its own, have to replicate and disperse itself as well as have some sort of characteristics which can actually cause disease or destruction on Earth.
So in response the question posed by the title; while it is not possible to estimate the probability of an extra-terrestrial microbe being harmful or not, it is possible to assume the risk of an outbreak is small. Also, the potential knowledge gained in a range of scientific fields such as Biology, Chemistry and Astronomy as well as the economic benefits associated with research and development of space programs in search of life far outweighs the risks.