Interplanetary contamination

Did You Know?

• In August 2019, scientists reported that a capsule containing tardigrades in a cryptobiotic state may have survived for a while on the Moon after the April 2019 crash landing of Beresheet, a failed Israeli lunar lander.
• The Apollo missions incited public concern about microbes on the Moon, and NASA received thousands of letters from Americans worried about back contamination.
• One of the most reliable ways to reduce contamination risk is to use only robotic spacecraft, with humans in close orbit controlling surface equipment via telepresence.

Overview

The solar system is mostly inhospitable to life as we understand it, and we have never found any life beyond Earth. Yet if such life does exist, it could be at risk from microorganisms brought by our spacecraft. Some hardy extremophiles might endure the journey through space and land on another world, which raises both scientific and ethical questions. Places that might harbor life today include the hidden oceans under the icy crusts of Europa, Enceladus, and Titan—the last has lakes of liquid ethane and methane on its surface but may also hold liquid water deeper down, along with cryovolcanoes. If Earth life contaminates another planet, it becomes hard to tell whether any life we find is native or came with us. The organic compounds from introduced organisms would muddy the search for biosignatures, whether from living or ancient native life. Because the early solar system saw a lot of material swapping between planets, any life out there could share a common origin with Earth’s and might even use nucleic acids like RNA or DNA. Most microbial species we isolate are poorly understood and can’t be grown in a lab; we know them only from DNA fragments swabbed from surfaces. On a contaminated world, telling apart the DNA of alien life from that of stowaway Earth life would be nearly impossible. And introduced microbes could spoil resources like water that future human missions would rely on.

Evidence for possible habitats outside Earth

Enceladus and Europa show the best evidence for current habitats, primarily due to the possibility of hosting liquid water and organic compounds. Mars once offered habitable conditions for microbial life, though no evidence of such life has been found. Many bacterial spores (endospores) from Earth are thought to have been transported on Mars spacecraft, some possibly protected within rovers and landers on the shallow surface, meaning Mars may already be contaminated. Certain lichens from arctic permafrost can photosynthesize and grow using only atmospheric humidity, and are highly tolerant of UV radiation. However, no study has considered the full range of Martian surface conditions simultaneously; laboratory simulations show that when multiple lethal factors are combined, survival rates plummet quickly. Other studies suggest potential for life using deliquescing salts that capture atmospheric humidity to obtain liquid water.

Crewed spacecraft

Crewed spacecraft pose a higher risk of forward contamination than robotic missions because humans cannot be sterilized to the same level. The human microbiome contains about a hundred trillion microorganisms in ten thousand species that cannot be removed while preserving life. Containment appears the only option, but effective containment to robotic standards is difficult with present-day technology, especially in the event of a hard landing. Human explorers could also carry microorganisms acquired on Mars back to Earth. Contamination of water supplies by Earth microorganisms shed in human stools, skin, and breath could directly affect long-term colonization of Mars. Historical measures include the anti-bacterial filter placed in the Apollo Lunar Module cabin relief valve from Apollo 13 onward to prevent contaminants from being released during depressurization before EVA.

Back contamination issues

The Moon is now generally considered free from life, so the most likely source of back contamination would be Mars, during a sample-return mission or a crewed mission. The possibility of new human pathogens or environmental disruption is considered extremely low probability but cannot be ruled out. NASA and ESA are actively developing a Mars Sample Return Program to return samples collected by the Perseverance Rover. The European Space Foundation report cites advantages such as extensive analyses on Earth without size and weight constraints, no communication delays, and the ability to repeat experiments in multiple laboratories. Carl Sagan publicised back contamination issues in Cosmic Connection (1973) and later in Cosmos (1980). NASA and ESA concluded that with present-day technology, Martian samples can be safely returned provided the right precautions are taken. NASA's experience with Apollo 11 samples, considered low risk at the time, involved inadequate precautions by current standards; new regulations would be needed. A sample-return mission would break the chain of contact by sealing the returned container inside another container in space before return. The capsule could fall at terminal velocity, cushioned by its thermal protection system, and the sample container would be designed to withstand impact.