Interplanetary contamination

• 1. Introduction

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  Interplanetary contamination names a danger that most people never think about: every spacecraft humanity sends into the Solar System carries life with it. Not intentionally, but inevitably. The microbes on a spacecraft's surface, the bacterial spores embedded in a rover's joints, the hundred trillion microorganisms living inside any human astronaut - these travel wherever the spacecraft travels.

  Two directions of risk concern scientists most. Forward contamination moves outward from Earth, potentially seeding other worlds with Earth biology before we ever find out what was already living there. Back contamination moves the other way, bringing something unknown home. If Mars, or Europa, or Enceladus harbors life, and if that life boards a returning spacecraft, the consequences are impossible to predict in advance.

  The questions that animate this documentary are not abstract. How close are we to a Mars sample-return mission? What would it actually take to contain something we have never seen? And is the risk of bringing Mars back to Earth real, or is it the anxiety of an era that has seen too many disaster films? Carl Sagan thought the question deserved a serious answer. NASA and the European Space Agency think so too.

• 2. Worlds That Might Harbor Life

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  Enceladus, the small moon of Saturn, became a focal point for astrobiologists after the Cassini spacecraft flew directly through its plumes. The data showed that those geysers contain salt-rich particles with an ocean-like composition. Scientists concluded the material originates from a subsurface ocean of liquid saltwater rather than from the moon's icy surface. The plumes also contain organic chemicals. Heat scans of Enceladus revealed temperatures reaching -93 degrees Celsius around the fissures where the geysers originate. That figure is 115 degrees Celsius warmer than the surrounding surface, pointing to a heat source below.

  Europa, Jupiter's ice-covered moon, makes a compelling case through different evidence. Tidal heating models of Europa require a subsurface layer of liquid water to explain the linear fracturing visible on its surface. Observations by the Galileo spacecraft of Europa's magnetic field interaction with Jupiter's field strengthened the argument for a liquid rather than solid inner layer. In December 2012, the Hubble Space Telescope appeared to observe an ice plume spouting from Europa's surface, which would support the case for a liquid subsurface ocean dramatically.

  Mars sits in its own category. There is substantial evidence that Mars once offered habitable conditions for microbial life, though no life has been confirmed there. Scientists believe many bacterial spores from Earth have already been transported to Mars aboard spacecraft. Some may be sheltered within Martian rovers and landers on the planet's shallow surface. In that sense, Mars may have already received Earth life without anyone intending it.

• 3. The Mars Surface Problem

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  Research published in July 2017 delivered an uncomfortable finding for anyone hoping Mars might still host surface life today. When bacteria were irradiated with a simulated Martian UV flux, perchlorates on the Martian surface became significantly more lethal. Even dormant spores lost viability within minutes of exposure. Two other compounds present in the Martian soil, iron oxides and hydrogen peroxide, acted together with irradiated perchlorates to produce a 10.8-fold increase in cell death compared to cells exposed to UV radiation alone, measured at 60 seconds of exposure.

  The researchers also found that abraded silicates, specifically quartz and basalt, promote the formation of toxic reactive oxygen species. Their conclusion was direct: the surface of Mars is lethal to vegetative cells, and much of the surface and near-surface regions should be considered uninhabitable. They recommended inspecting at least a few meters underground to find regions where radiation levels would be relatively low.

  Certain lichens from Arctic permafrost have shown partial resilience to Mars-like conditions in laboratory settings. They can photosynthesize and grow without liquid water by using atmospheric humidity, and they tolerate UV radiation using melanin and other specialized chemicals. However, every study of their resistance has tested conditions separately. No study has subjected lichens to the full combination of Martian temperature, pressure, atmospheric composition, radiation, humidity, and oxidizing regolith at the same time. Laboratory simulations consistently show that whenever multiple lethal factors are combined, survival rates fall sharply.

• 4. Protecting Earth From Mars

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  Carl Sagan was the first person to bring the back contamination question to public attention. In Cosmic Connection, published in 1973, he wrote that because Mars is an environment of great potential biological interest, there may be pathogens there that, if transported to Earth, might do enormous biological damage. Seven years later, in Cosmos, he was more measured but still cautious, writing that he would want to be very sure before endorsing a returned-sample mission.

  NASA and ESA are now actively developing a Mars Sample Return Program using samples collected by the Perseverance Rover. The European Space Foundation report cited a central advantage of sample return: analyses could be carried out on Earth without the size and weight constraints that limit instruments sent on rovers. Researchers could also repeat experiments in multiple laboratories with different instruments, which is not possible when the instruments are sitting on Mars.

  NASA has proposed building a dedicated facility to receive and study the samples, tentatively called the Mars Sample Return Receiving Facility, or MSRRF. The facility must be rated biohazard level 4, the same level used for the most dangerous known pathogens on Earth. But a BSL-4 facility for extraterrestrial samples faces a challenge that ordinary BSL-4 labs do not: the organisms, if any, are completely unknown. The facility would need to filter particles as small as 0.01 micrometers or larger, because gene transfer agents, which are virus-like particles capable of horizontal gene transfer, can be extremely small. In one experiment reported in 2010, researchers left such agents overnight with marine bacteria in natural conditions and found that by the next day up to 47 percent of the bacteria had incorporated genetic material from the agents. Building the MSRRF is expected to take 7 to 10 years from design to completion, with an additional two years recommended for staff to become familiar with the facility.

• 5. The Human Problem

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  Robotic spacecraft can be sterilized. Humans cannot. A person carries roughly a hundred trillion microorganisms in approximately ten thousand species as part of the human microbiome. Removing them while keeping the person alive is not possible. This is the fundamental challenge for any crewed mission to Mars, and it applies in both directions.

  A crewed spacecraft represents a much higher risk of forward contamination than a robotic one. Containment is the only viable alternative to sterilization, but achieving containment to the same standard as a robotic rover appears very difficult with present-day technology. A hard landing poses a particular challenge for any containment system.

  The Apollo program addressed some of these concerns at a smaller scale when visiting the Moon. Beginning with Apollo 13, the Lunar Module carried an anti-bacterial filter on its cabin relief valve. This was placed there to prevent contaminants from the cabin being released into the lunar environment during depressurization of the crew compartment before EVA. The Apollo 11 mission had already generated thousands of letters to NASA from Americans worried that the returning astronauts might bring lunar microbes home. The Apollo quarantine regulations were eventually rescinded and new regulations for any future sample-return missions have not yet been developed to replace them.

  If humans travel to Mars, they would also shed Earth microorganisms through their stool, skin, and breath into any water supply on the planet. This contamination of water resources could have a direct effect on any long-term human presence on Mars.

• 6. The Case Against Rushing

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  Several exobiologists have argued that a Mars sample-return is premature. Their reasoning is specific: life on Mars, if it exists today, is likely to be sparse and confined to only a few niche habitats. Past life on Mars is likely to have been degraded by cosmic radiation over geological time if exposed within the top few meters of the surface. Only certain special deposits of salts or clays would have the capability to preserve organics for billions of years.

  The concern is that a sample-return conducted before more thorough in situ study of Mars would likely return samples no more conclusive about life than the Martian meteorite samples already available on Earth. Returning inconclusive samples would not settle the question of life, but it would carry the back contamination risk all the same.

  These researchers instead advocate sending more sensitive instruments on Mars surface rovers. A rover can examine many different rock and soil types across a wide range of terrain. A sample-return mission, by contrast, can only bring back what fits in the container, which is a fraction of what a rover can survey.

  Several instrument development efforts are underway to support in situ analysis. Jonathan Rothberg and J. Craig Venter are separately developing approaches to sequence alien DNA directly on the Martian surface. Levin is working on updated versions of the Labeled Release instrument that flew on Viking, including versions capable of detecting chirality, which is useful because it can detect life even if that life does not use standard biochemistry. NASA Marshall Space Flight Center is leading development of a Miniaturized Variable Pressure Scanning Electron Microscope for future lunar and Martian missions.

• 7. Governance and the Law

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  Current space missions operate under the Outer Space Treaty and the COSPAR guidelines for planetary protection. NASA made these policies formal with the issuing of Management Manual NMI-4-4-1, the NASA Unmanned Spacecraft Decontamination Policy, on the 9th of September 1963. Before that document, the same sterilization requirements applied to all outgoing spacecraft regardless of their destination. Difficulties in sterilizing the Ranger probes sent to the Moon were the primary reason NASA shifted to a target-by-target approach.

  Margaret Race examined the legal pathway that any Mars sample-return mission would need to navigate. Under the National Environmental Policy Act, which did not exist during the Apollo era, a formal environmental impact statement would likely be required, along with public hearings. That process alone could take several years. Other agencies, including the Environmental Protection Agency and the Occupational Health and Safety Administration, might become involved in the decision.

  The laws on quarantine would also need to be clarified, since the Apollo-era regulations were rescinded. During the Apollo program, NASA delayed announcing its quarantine regulations until the day Apollo launched, bypassing the requirement for public debate. Race found that this approach would likely not be tolerated today. The presidential directive NSC-25 would probably also apply, requiring a review of alleged large-scale environmental effects, eventually requiring presidential approval of the launch. International treaties relating to environmental protection and health would add further layers of negotiation. Race concluded that the public of necessity has a significant role in shaping the policies governing any Mars sample-return.

• 8. Tardigrades on the Moon

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  In August 2019, scientists reported a real-world incident that made the abstract concerns of planetary protection suddenly tangible. A capsule containing tardigrades, tiny resilient animals capable of surviving in a cryptobiotic state, may have survived for a while on the Moon after the crash landing of Beresheet, a failed Israeli lunar lander, in April 2019. The tardigrades were in a dehydrated, essentially suspended state at the time of the crash.

  The Moon is currently classified as "not of interest" for prebiotic chemistry or the origins of life, which means it carries no contamination restrictions under existing guidelines. But the Beresheet incident illustrated the gap between formal classifications and what actually happens when missions fail. The Moon has also been proposed as a testbed for new technology to protect Solar System sites from forward and back contamination. Analysis of the contamination left behind by Apollo program astronauts could provide ground truth for planetary protection models. Whatever conclusions that analysis yields will matter when planning missions to places that are not classified as safe: Europa, Enceladus, and Mars.