At Pioneer Labs, we create biotechnology to make Mars more habitable and support a human presence. Space agencies around the world plan to send humans to Mars, but the existing Planetary Protection policy was written for landers and rovers, which can be sterilized. Humans cannot be sterilized, and neither can crops, life-support-system microbes, or the biomanufacturing organisms that support them. Today there is no framework for assessing the safety of sending life to Mars on purpose.
Scientists and policy makers are actively discussing how to create policy that governs the upcoming era of human missions to Mars. Today, we’re releasing our proposal for a starting point. In the simple case where we’re sending well-characterized laboratory organisms to Mars, we can simply measure how likely they would be to survive and reproduce if accidentally released onto the Martian surface. We propose PRIM, Propagation Restricted Inert on Mars, as a designation a strain earns that qualifies it as safe to send to Mars.
🚧 Who sets policy
Planetary Protection is a guiding principle for space exploration that aims to protect Earth and the celestial bodies we explore from unknown hitchhikers. It prevents forward contamination of Mars by Earth organisms and backward contamination of Earth by potential Martian ones. It dates to the Space Age and the formation of the Committee on Space Research (COSPAR) in 1958. It is grounded in Article IX of the 1967 Outer Space Treaty, which prohibits ‘harmful contamination’ of planetary bodies. The COSPAR Planetary Protection Panel continues to update policy as our scientific understanding and mission goals progress. These policies are then enacted by space agencies and public and private space exploration organizations around the world.
We propose PRIM, Propagation Restricted Inert on Mars, as a designation a strain earns that qualifies it as safe to send to Mars.
⚠️ Known vs unknown risk
So far, planetary protection has caused all Mars landers and rovers to be thoroughly sterilized, obeying low limits for total final bacterial spores at launch. This prevents forward contamination in which Earth life might negatively impact potential Martian life or the search for biosignatures.
Complete sterilization makes sense for equipment because it is protecting against unknown hitchhikers. Environmental microbes can be hardy. Spacecraft carry a mix of organisms whose identities are poorly understood, and pose unknown amounts of risk to Mars. Several times, after launching a spacecraft, we have retrospectively found hardier organisms that can survive the high temperature and bleach sterilization that Mars landers and rovers undergo. Since we can’t reliably estimate how likely unknown hitchhikers are to grow on Mars, the only option is to reduce how many we send.
In contrast, when we deliberately send a specific laboratory strain, we have the ability to fully characterize its risk to Mars. Can we turn measurements of a microbe’s growth into an estimate of contamination risk?
If you know exactly which organism you’re sending, its probability of growing on Mars stops being an assumption and becomes an experiment.
One approach under discussion is using a probabilistic risk assessment (PRA) for flight-qualifying missions to the surface of Mars. This accounts for factors such as how many organisms a mission carries, how likely they are to escape containment (as opposed to staying with the spaceship, habitat, or spacewalk suit), and whether they could grow after release. We must assume the worst case scenario for unknown organisms - that they could grow if released in any ‘special region’ on Mars, making equipment sterilization still important. But, if you know exactly which organism you’re sending, its probability of growing on Mars stops being an assumption and becomes an experiment.
📋 Our framework: PRIM-certifying microbes
Inspired by GRAS status that recognizes food-safe substances, like microbes found in yogurt, we want to PRIM certify microbes as safe for Mars. PRIM stands for Propagation Restricted, Inert on Mars. It means that when exposed to Mars’ environment - not protected by a spaceship or sealed container - the microbes will be incapable of propagation.
In the PRIM framework, we test for two things:
Survival. How many cells remain after exposure to a Martian stressor?
Propagation. Can any surviving cells reproduce under that stressor?
Both matter. Surviving biomass might confuse life detection devices. But propagation matters even more. Evolution can’t proceed without growth, so a strain that cannot divide on the Martian surface cannot evolve its way into one that can.
There are numerous stressors on Mars. We chose just two stressors, carbon starvation and low water activity exposure, and applied PRIM to take measurements of two biomanufacturing chassis organisms, an engineered Cupriavidus necator and Pseudomonas putida, both adaptively evolved for growth in Defined Mars Media.
🍔 Carbon starvation. On Mars, the essential building block of life, carbon, is hard to come by. While there is a low-pressure carbon dioxide-rich atmosphere, there is not much fixed carbon present, to our knowledge, in the soil. Our biomanufacturing strains cannot eat carbon dioxide, so without the supplemented microbe-edible-carbon we give them, they both largely die and also do not show observable growth in the carbon-starved conditions. This gives us a controllable parameter to enable or disable cell survival.
🍯 Low water activity exposure. All life as we know it needs liquid water to survive. One of the factors most detrimental to life is the low water activity of Mars. Because the atmosphere has such low pressure, transient liquid water would instantly evaporate and dry out water-based life. There are some very hardy organisms that can survive low water activity conditions, but most cannot. Our biomanufacturing strains can only grow inside a bioreactor.
We choose to quantify just two of the stressors on Mars, making the numbers we obtain a conservative estimate of Mars’ ability to kill organisms.
🔢 What the numbers say
In our PRIM paper, we combine these measurements to estimate an upper bound on the risk that either of these organisms pose for contaminating the Martian environment. Under the framework’s assumptions, both strains meet its proposed one-in-ten-thousand risk threshold in the bioreactor spill scenarios evaluated in the paper. Even a full, catastrophic leak of an industrial-scale Mars bioreactor would still clear the planetary protection threshold by a factor of several hundred. These particular biomanufacturing organisms will be very dead, very fast if they get outside on Mars!
For our worked case of known biomanufacturing organisms, we can experimentally quantify how dead the microbes would be if exposed to Mars stressors. This is not a complete solution for the upcoming era of human missions to Mars. The human microbiome is a complex undefined mixture of microbes, and it’s unclear how this framework could extend to flight-qualifying humans and their associated microbiota. In that sense, sending a well-characterized microbe to Mars is safer than sending a human, whose microbiome is of unknowable composition and carries uncharacterizable risks. Deliberately and safely sending useful, clonal organisms to Mars allows us to prepare for, discover and implement solutions that will enable humans-to-Mars missions.
Sending a well-characterized microbe to Mars is safer than sending a human.
With organisms that are harder to experimentally assess, we can implement safeguarding solutions such as secondary and tertiary physical containment or biological containment. Biomanufacturing organisms are a tractable, and useful, starting point for enacting policy change.
💬 What do people think?
We’ve been circulating versions of the PRIM framework at space exploration conferences like AbSciCon and COSPAR. Early feedback on our experimental design and its limitations was extremely helpful and has been incorporated into the paper.
There is a noticeable split in the community on how cautious forward-contamination policy should be. At one end are scientists who believe that we should keep Mars as pristine as possible until we have undoubtedly ruled out the possibility of past or present life. At the other end are people who believe Mars has never hosted life and that current policy is far too conservative. While the PRIM framework does not settle this debate, most of the scientists we talked to agree that it is heading in the right direction: a data-driven experimental measurement of a specific organism’s forward contamination risk.
👀 Keep an eye out
We’ve published PRIM and are looking to present it to the planetary protection community at large to obtain more feedback and help move policy forward. If you have thoughts we’d like to hear from you — leave a comment below or write to bioisru@pioneer-labs.org.
Today, we’re releasing our Planetary Protection PRIM framework, but keep an eye out for two more technical reports coming soon. 🚀