Interplanetary contamination

Interplanetary contamination refers to biological contamination of a planetary body by a space probe or spacecraft, either deliberate or unintentional.

There are two types of interplanetary contamination:

• Forward contamination is the transfer of life and other forms of contamination from Earth to another celestial body.
• Back contamination is the introduction of extraterrestrial organisms and other forms of contamination into Earth's biosphere, it also covers infection of humans and human habitats in space and on other celestial bodies by extraterrestrial organisms, if such exist.

The main focus is on microbial life and on potentially invasive species. Non biological forms of contamination have also been considered including e.g. contamination of sensitive deposits (such as lunar polar ice deposits) of scientific interest by rocket exhausts. In the case of back contamination, multicellular life is thought unlikely but not been ruled out, and in case of forward contamination, then again, forward contamination by multicellular life (e.g. lichens) becomes a consideration if you have human missions though unlikely for robotic missions.

Current space missions are governed by the Outer Space Treaty and the COSPAR guidelines for planetary protection. Forward contamination is prevented primarily by sterilizing the spacecraft. In the case of backward contamination, however, the aim of the mission is to return biological material to Earth if such exists, and sterilization of the samples would make them of much less interest. So back contamination would be prevented mainly by containment, and breaking the chain of contact between the planet and Earth. It would also require quarantine procedures for the materials and for anyone who comes into contact with them.

Vulnerability of the Solar System to contamination by life

Most of the solar system seems to be hostile to life. However the debate about whether life exists elsewhere in the Solar System has not been resolved and there are several locations where some possibility of it remains.

If such life exists then it may be vulnerable to contamination by Earth micro-organisms. If the life does not exist but the habitats exist, then the habitats are vulnerable to contamination too, which would transform them from their current pristine state.

There is enough evidence to suggest that Mars once offered habitable conditions for microbial life.^[1][2] and some think it is also possible that life may exist there today in the subsurface and shallow subsurface (top few cm).^{[3][4][5][6][7][8][9]}

Other locations where life might exist today include the ocean of liquid water beneath the icy surface of Europa, the interior of Enceladus which is currently sending water into space, and the possible ocean of liquid water which some astronomers believe might exist beneath the surface of Titan (its surface has oceans of liquid ethane / methane, but it may also have liquid water below the surface and ice volcanoes).^[10][11]

Some extremophiles are able to survive space travel to another planet, and it is believed that there are currently many dormant micro-organisms on the surface of Mars, for instance, protected within the rovers on the planet.^[12][13] So in that sense we have already contaminated Mars.

However it is believed that the contamination so far is only by dormant micro-organisms that have not yet encountered conditions that permit them to metabolize and reproduce.^[14][15] If so then our colonization may be biologically reversible. If needed, it may be possible in the future to remove all the life we have brought to Mars and return the planet to its previous biologically pristine state. This is a suggestion made by Christopher McKay.^[16]

Several studies have shown potential for life to survive on Mars if the conditions are right. In particular, certain lichens from the arctic permafrost are able to photosynthesize and grow in the absence of any liquid water, simply using the humidity from the atmosphere. They are also highly tolerant of UV radiation, using melanin and other more specialized chemicals to protect their cells. The air on Mars approaches 100% humidity in evening and morning, and at those times frosts form on the surface. These are also conditions which might make it possible for lichens to survive there.

On 26 April 2012, scientists reported that lichen survived and showed remarkable results on the adaptation capacity of photosynthetic activity within the simulation time of 34 days under Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center (DLR).^[17][18]

Other studies have suggested the potential for life to survive using deliquescing salts. These, similarly to the lichens, use the humidity of the atmosphere. If the mixture of salts is right, then at times of high atmospheric humidity, they may support liquid water that is warm enough (warmer than about -25C) and with the salts dilute enough to be capable of supporting life, and that is also cold enough so that it doesn't immediately evaporate in the low atmospheric pressures of Mars. The salts and perchlorates required are known to exist on Mars, though whether they occur in the correct mixtures to support these marginally habitable deliquescing droplets and thin layers of salty brines is unknown.

Direct evidence for liquid water in the Solar System outside of Earth where life could reproduce

Our best confirmation of these potential habitats come from Enceladus, Europa and Mars.

Cassini has directly sampled the plumes escaping from Enceladus^[19] - the salt-rich particles have an ‘ocean-like’ composition which indicates that most, if not all, of the expelled ice comes from liquid saltwater, rather than from the icy surface of the moon, giving our best direct evidence of liquid water in the Solar System.

In the case of Europa, evidence for its sub-surface ocean is indirect but strong.^[20] Recent discovery for plumes from Europa suggests the possibility of direct sampling of its ocean in the future, as for Enceladus.^[21]

In the case of Mars, there is good indirect isotope evidence from the Phoenix lander which shows that some liquid on the surface, likely to be water, has exchanged oxygen atoms with the CO2 in the atmosphere in the recent geological past. This evidence is not sufficient yet to determine whether this is due to intermittent flooding or flows, for instance after geothermal or volcanic activity or meteorite impacts, or whether the liquid water is present every year, as in some of the recent suggestions for shallow subsurface habitats.^[22]

Dissenting views on vulnerability of the Solar System

Though all agree that these potential habitats exist or are likely to exist, there are dissenting views about whether it matters to introduce Earth microbes to them.

In a paper "The Over Protection of Mars" in nature Geosciences, in 2013, Alberto Fairén and Dirk Schulze-Makuch suggested that we no longer need to protect Mars, arguing that Earth life has already reached it on meteorites.^[23]

This was rebutted in a follow up article "Appropriate Protection of Mars", in the same journal by the current and previous planetary protection officers Catherine Conley and John Rummel.^[23][24]

Issues arising from biological contamination

• If a planet becomes contaminated by Earth life it might then be hard to tell whether any lifeforms discovered originated there or came from Earth.^[25] Also the organic chemicals produced by the introduced life would confuse sensitive searches for biosignatures of native life, either present day or in deposits from the ancient past on the planet.

For instance the astrobionibbler^[26] is a sensitive tool proposed for Mars that would be able to detect a single amino acid in a gram of sample. If the sample is already contaminated by Earth life then discovery of an amino acid in it is of considerably less interest than if the sample is uncontaminated.

The same applies to other more complex biosignatures. Searches for biological remains of ancient life or life precursors using biosignatures would be much easier on an uncontaminated planet.

The same also applies to search for DNA sequences on Mars. Life on other planets could have a common origin with Earth life, since in the early Solar System there was much exchange of material between the planets which could have transferred life as well. If so it might be based on DNA too.

Most species of micro-organism on Earth are not yet well understood or DNA sequenced. This particularly applies to the uncultivable archaea, and the majority of micro-organisms are uncultivable, and so hard to study. This can be either because they depend on the presence of other micro-organisms, or are slow growing, or depend on other conditions not yet well understood. In typical habitats, 99% of micro-organisms are uncultivable.^[27]

The same is true of species detected on spacecraft in clean rooms. The majority of the species detected are not well understood or characterized and can't be cultivated, and are known only from DNA fragments.^[28]

On a contaminated planet, it might be hard to distinguish the DNA of extraterrestrial life from the DNA of life brought to the planet by our spacecraft, because of our incomplete knowledge of the DNA sequences of Earth micro-organisms.

• Invasive species could out compete native life or consume it, if there is life on the planet.^[29]

One argument against this is that the native life would be more adapted to the conditions there. However we have experience on Earth that species moved from one continent to another may be able to outcompete the native life adapted to that continent, for instance for rabbits introduced to Australia able to outcompete the marsupials already there. The same may be true for micro-organisms introduced to other planets.^[29] Evolution on Earth might have explored biological pathways not explored by life on other planets and so be able to out compete it. (The same is also possible the other way around for back contamination).

• Introduced Earth life could contaminate resources of value for future human missions, e.g. contaminate possible sources of water.^[30]

Current methods in place to prevent biological contamination

Currently, forward contamination is prevented by sterilizing rovers sent to sensitive areas of the Solar System. Missions are classified depending on whether their destinations are of interest for the search for life, and whether there is any chance that life could reproduce there.

Some destinations such as Mercury need no precautions at all. Others such as the Moon require documentation but nothing more. And others such as Mars require sterilization of the rovers sent there. For the details, see Planetary protection.

Backward contamination would be prevented by containment or quarantine, however there have been no sample returns thought to have any possibility of a back contamination risk since Apollo. The Apollo regulations have been rescinded and new regulations have yet to be developed, see Precautions suggested for sample return

Human colonization challenges

Human colonization is of particular concern for interplanetary contamination.

The main challenges are

• It is impossible to sterilize human missions to the same level as a robotic spacecraft. Therefore the chance of forward contamination must be higher than for a robotic mission.^[31]

Humans are host to typically a hundred trillion micro-organisms in ten thousand species in the human microbiome, and these can't be removed while preserving the life of the human. Containment seems the only option but effective containment to the same standard as a robotic rover seems hard to achieve with present day technology. In particular adequate containment in the event of a hard landing is a major challenge.

• Human explorers would be vulnerable to back contamination on the planet, if any native life is hazardous to humans directly or indirectly.^[32]

• Contamination of materials on the planet particularly water supply by Earth micro-organisms could have a direct effect on the human colonization^[30]

Use of the Moon as a testbed for human planetary protection protocols

The Moon has been suggested as a testbed for Planetary protection for human missions in space. Currently the Moon has no contamination restrictions because it's considered to be "not of interest" for prebiotic chemistry and origins of life. It could be used to test new technology to protect sites in the Solar System, and astronauts, from forward and backward contamination. Analysis of the contamination left by the Apollo astronauts could also yield useful ground truth for planetary protection models.^[33][34]

Methods of human exploration that do not lead to contamination issues

Robotic exploration, telerobotics and telepresence give a method of exploration of a planet that do not have the same issues of forward and backward contamination. This could be of special interest during early stages when the celestial body is not yet well understood.

Michael Meltzer in his "When Biospheres Collide" - a history of NASA'S Planetary Protection Programs" wrote:^[31]

"One of the most reliable ways to reduce the risk of forward contamination during visits to extraterrestrial bodies is to make those visits only with robotic spacecraft. Sending a person to Mars would be, for some observers, more exciting. But in the view of much of the space science community, robotic missions are the way to accomplish the maximum amount of scientific inquiry since valuable fuel and shipboard power do not have to be expended in transporting and operating the equipment to keep a human crew alive and healthy. And very important to planetary protection goals, robotic craft can be thoroughly sterilized, while humans cannot. Such a difference can be critical in protecting sensitive targets, such as the special regions of Mars, from forward contamination.

"Perhaps a change in the public's perspective as to just what today's robotic missions really are would be helpful in deciding what types of missions are important to implement. In the opinion of Terence Johnson, who has played a major role in many of NASA's robotic missions, including serving as the project scientist for the Galileo mission and the planned Europa Orbiter mission, the term "robotic exploration" misses the point. NASA is actually conducting human exploration on these projects. The mission crews that sit in the control panel at JPL, "as well as everyone else who can log on to the Internet" can observe in near real-time what is going on. The spacecraft instruments, in other words, are becoming more like collective sense organs for humankind. Thus, according to Johnson, when NASA conducts it's so-called robotic missions, people all around the world are really "all standing on the bridge of Starship Enterprise". The question must thus be asked, when, if ever, is it necessary for the good of humankind to send people rather than increasingly sophisticated robots to explore other worlds"

Humans in close orbit around the planet could control equipment on the surface in real time via telepresence, so bringing many of the benefits of a surface mission, without its associated increased forward and backward contamination risks.^[35][36][37]

In site resource utilization is possible in orbital missions as for surface missions e.g. using resources mined from Deimos in the case of Mars.

Back contamination issues

Since the Moon is now generally considered to be free from life, the most likely source of contamination is from Mars either during a Mars sample return or as a result of colonization of Mars.

There are no immediate plans for a Mars sample return, but it remains a high priority for NASA and the ESA, because of its great potential biological interest. The European Space Foundation report cites many advantages of a Mars sample return. In particular, it would permit extensive analysis with any of the equipment available on Earth, without the size and weight constraints for instruments sent to Mars on rovers. These analyses could also be carried out without the communication delays for experiments carried out on Martian rovers. It would also make it possible to repeat experiments in multiple laboratories with different instruments to confirm key results.^[38]

Carl Sagan was first to raise and publicise back contamination issues that might follow from a Mars sample return. In Cosmic Connection (1973) he writes:

Precisely because Mars is an environment of great potential biological interest, it is possible that on Mars there are pathogens, organisms which, if transported to the terrestrial environment, might do enormous biological damage.^[39]

He makes the analogy of the plot twist in The War of the Worlds, by H.G. Wells, where Earth pathogens made the Martian invaders of Earth sick and then they die. Perhaps, he suggests, the same could happen to us on Earth if we return samples containing micro-organisms from Mars. On the one hand, he points out, this possibility seems unlikely because of the lack of contact between the two planets and because pathogens adapt to their host. But on the other hand, the lack of contact also means that we would have never evolved any defences against any pathogens.

Also, pathogens when they adapt to a host normally evolve to be less rather than more lethal. Also, some pathogens such as Legionnaire's disease attack humans using essentially the same mechanism they use to infect other microbes (in this case amoeba) so a disease of microbes on Mars could become a pathogen of animals on Earth.

Ledeberg wrote ^[40]

“Whether a microorganism from Mars exists and could attack us is more conjectural. If so, it might be a zoonosis to beat all others. On the one hand, how could microbes from Mars be pathogenic for hosts on Earth when so many subtle adaptations are needed for any new organisms to come into a host and cause disease? On the other hand, microorganisms make little besides proteins and carbohydrates, and the human or other mammalian immune systems typically respond to peptides or carbohydrates produced by invading pathogens. Thus, although the hypothetical parasite from Mars is not adapted to live in a host from Earth, our immune systems are not equipped to cope with totally alien parasites: a conceptual impasse”

This possibility has been confirmed in all the later studies, as the worst-case scenario. Other possibilities have also been raised such as micro-organisms that have harmful effects on crops, or that disrupt natural cycles, and pathogens that infect other micro-organisms.^{[41][42][43][44][45]}

As a result, the possibility of new human pathogens, or environmental disruption due to back contamination is considered to be of extremely low probability but can't yet be ruled out completely.

Later in Cosmos (1980) Carl Sagan wrote:

Perhaps Martian samples can be safely returned to Earth. But I would want to be very sure before considering a returned-sample mission.^[46]

The PPO and NASA and ESA view is similar to this. The findings were that with present day technology, Martian samples can be safely returned to Earth provided the right precautions are taken.

The risks of environmental disruption resulting from the inadvertent contamination of Earth with putative martian microbes are still considered to be low. But since the risk cannot be demonstrated to be zero, due care and caution must be exercised in handling any martian materials returned to Earth.

^[47]

Precautions suggested for sample return

We have already had experience of sample return thought to be a back contamination risk, during the Apollo era, when samples were returned for the first time, by Apollo 11. At the time, it was thought that there was a low probability of life on the Moon. The precautions taken then were inadequate by modern standards, however. Also the regulations used then have been rescinded, and new regulations would be needed.^[48] A different approach would be needed for a modern sample return.

Chain of contact

The sample return mission would be designed to break the chain of contact between Mars and the exterior of the sample container, for instance, by sealing the returned container inside another larger container in the vacuum of space before return to Earth.^[41][49] In order to eliminate the risk of parachute failure, the current plan is to return the capsule to the Earth without the use of parachutes: the capsule will fall at terminal velocity and the impact will be cushioned by the capsule's thermal protection system. The sample container will be designed to withstand the force of the impact.^[49]

Receiving facility

To receive the returned samples, NASA has proposed to build a biohazard containment facility - known as the Mars Sample Return Receiving facility (MSRRF).^[50]

The proposed sample return facility must be a biohazard level 4 laboratory. However, the facility must also contain unknown biohazards, as the sizes of any putative Martian micro-organisms are currently unknown. In consideration of this, the ESF proposed additional requirements. Ideally it should contain particles of 0.01 µm, or larger, and release of a particle 0.05 µm or larger is unacceptable under any circumstances.^[51]

The reason for this extremely small size limit of 0.01 µm is because of the consideration of Gene Transfer Agents (GTAs). These randomly incorporate segments of the host genome and can transfer them to other evolutionarily distant hosts, and do that without killing the new host. In this way many archaea and bacteria can swap DNA with each other. This raises the possibility that Martian life, if it has a common origin with Earth life in the distant past, could swap DNA with Earth micro-organisms in the same way.

In one striking experiment first reported in 2010, researchers left GTAs (conferring antibiotic resistance) and marine bacteria overnight in natural conditions and found that by the next day up to 47% of the bacteria had incorporated the genetic material from the GTAs.^[52][53]

The reason for the 0.05 µm limit is because of the discovery of ultramicrobacteria as small as 0.2 µm across, and bearing in mind that these smallest size limits may be reduced further in the future..

It also must double as a clean room to preserve the science value of the samples. The problem here is that, while it is relatively easy to simply contain the samples once returned to Earth, researchers would also want to remove parts of the sample and use them in experiments. During all these handling procedures, the samples would need to be contained to prevent contamination of Earth. However at the same time the samples would need to be kept free from contamination by Earth micro-organisms and biological material; even a single amino acid of Earth origin could confuse the analysis. This introduces conflicting requirements. They surely can be reconciled but to date, no facility has had to do this, so new building requirements would need to be imposed.

A clean room is normally kept at a higher pressure than the external environment to keep contaminants out, and a biohazard laboratory is kept at a lower pressure to keep the biohazards in. The challenge is to combine these in a single building. Solutions suggested include a triple walled containment facility, and one of the suggestions includes extensive use of robot handlers for the samples.^{[54][55][56][57]}

Timescale for building the facility

The short timescale of two years used for the quarantine facilities for Apollo 11 is now considered to be too short and probably the main reason why the facilities failed to contain the samples adequately even by the standards of the time, with two failures of containment, one at splashdown and the other in Houston. The facility would be expected to take some years from design to completion.^[58][59] Preliminary studies have warned that it may take as many as 7 to 10 years to get it operational and an additional two years is recommended for the staff to become accustomed to the facilities.^[41][60]

Dissenting views on back contamination issues

The International Committee Against Mars Sample Return^[61] maintains that it is not possible to return samples to Earth safely at this stage. Their main reason for saying this is the novelty of the containment procedures required combined with the possibility of human error and mission design mistakes, either during the return flight or after return of the samples. They urge more in situ studies on Mars first, and preliminary biohazard testing in space before the samples are returned to Earth.

At the other extreme, Robert Zubrin (Mars surface colonization advocate and director of the Mars Society) maintains that the risk of back contamination has no scientific validity. He supports this using an argument based on the possibility of transfer of life from Earth to Mars on meteorites.^[62][63]

For more about these issues see Planetary Protection - Controversies

Legal process of approval for Mars sample return

Margaret Race has examined in detail the legal process of approval for a MSR.^[48] She found that under the National Environmental Policy Act (NEPA) (which did not exist in the Apollo era) a formal environment impact statement is likely to be required, and public hearings during which all the issues would be aired openly. This process is likely to take up to several years to complete.

During this process, she found, the full range of worst accident scenarios, impact, and project alternatives would be played out in the public arena. Other agencies such as the Environment Protection Agency, Occupational Health and Safety Administration, etc., may also get involved in the decision making process.

The laws on quarantine will also need to be clarified as the regulations for the Apollo program were rescinded. In the Apollo era, NASA delayed announcement of its quarantine regulations until the day Apollo was launched, so bypassing the requirement for public debate - something that would be unlikely to be tolerated today.

It is also probable that the presidential directive NSC-25 will apply which requires a review of large scale alleged effects on the environment and is carried out subsequent to the other domestic reviews and through a long process, leads eventually to presidential approval of the launch.

Then apart from those domestic legal hurdles, there are numerous international regulations and treaties to be negotiated in the case of a Mars Sample Return, especially those relating to environmental protection and health. She concluded that the public of necessity has a significant role to play in the development of the policies governing Mars Sample Return.

Alternatives to sample return

Several exobiologists have suggested that a Mars sample return is not necessary at this stage, and that it is better to focus more on in situ studies on the surface first. Although it is not their main motivation, this approach of course also eliminates back contamination risks.

Some of these exobiologists advocate more in situ studies followed by a sample return in the near future. Others go as far as to advocate in situ study instead of a sample return at the present state of understanding of Mars.^[64][65][66]

Their reasoning is that life on Mars is likely to be hard to find. Any present day life is likely to be sparse and occur in only a few niche habitats. Past life is likely to be degraded by cosmic radiation over geological time periods if exposed in the top few meters of the Mars surface. Also, only certain special deposits of salts or clays on Mars would have the capability to preserve organics for billions of years. So, they argue, there is a high risk that a Mars sample return at our current stage of understanding would return samples that are no more conclusive about the origins of life on Mars or present day life than the Martian meteorite samples we already have.

Another consideration is the difficulty of keeping the sample completely free from Earth life contamination during the return journey and during handling procedures on Earth. This might make it hard to show conclusively that any biosignatures detected does not result from contamination of the samples.

Instead they advocate sending more sensitive instruments on Mars surface rovers. These could examine many different rocks and soil types, and search for biosignatures on the surface and so examine a wide range of materials which could not all be returned to Earth with current technology at reasonable cost.

A sample return to Earth would then be considered at a later stage, once we have a reasonably thorough understanding of conditions on Mars, and possibly have already detected life there, either modern or past life, though biosignatures and other in situ analysis.

Instruments under development for in situ analysis

• NASA Marshall Space Flight Center is leading a research effort to develop a Miniaturized Variable Pressure Scanning Electron Microscope (MVP-SEM) for future lunar and martian missions.^[67]

• Several teams, including Jonathan Rothberg, and J. Craig Venter, are separately developing solutions for sequencing alien DNA directly on the Martian surface itself.^{[68][69][70][71]}

• Levin is working on updated versions of the Labeled release instrument flown on Viking. For instance versions that rely on detecting chirality. This is of special interest because it can enable detection of life even if it is not based on standard life chemistry.^[72]

• The Urey Mars Organic and Oxidant Detector instrument for detection of biosignatures has been descoped, but was due to be flown on ExoMars in 2018. It is designed with much higher levels of sensitivity for biosignatures than any previous instruments^[64][73][74]

• Astrobionibbler - this is a smaller instrument proposed to replace Urey on ExoMars. It is capable of detecting a single amino acid in a gram of soil.^[26]

Study and analyses from orbit

During the “Exploration Telerobotics Symposium" in 2012 experts on telerobotics from industry, NASA and academics met to discuss telerobotics, and its applications to space exploration. Amongst other issues, particular attention was given to Mars missions and a Mars sample return.

They came to the conclusion that telerobotic approaches could permit direct study of the samples on the Mars surface via telepresence from Mars orbit, permitting rapid exploration and use of human cognition to take advantage of chance discoveries and feedback from the results obtained so far.^[75]

They found that telepresence exploration of Mars has many advantages. The astronauts have near real-time control of the robots, and can respond immediately to discoveries. It also prevents contamination both ways and has mobility benefits as well.^[76]

Return of the sample to orbit has the advantage that it permits analysis of the sample without delay, to detect volatiles that may be lost during a voyage home. This was the conclusion of a meeting of researchers at the NASA Goddard Space Flight Center in 2012. ^[77]

"One possible scenario for surface exploration of Mars via LLT could be the deployment of twin telerobotic rovers on the surface with high-definition visual tools to allow low-latency communication and rapidly adaptable operation from an on-orbit crew for field astrobiology. Such “tele-rovers” could be equipped with instruments for detailed in situ reconnaissance and capabilities for recovering and sending selected samples to the human-tended on-orbit spacecraft for preliminary screening by means of lab analysis by resident astronauts. In the case of samples of biological significance, very rapid encapsulation and recovery of the sample materials at the spacecraft in orbit are required and this is enabled by this approach. Most of the required technology already exists for terrestrial telerobotics exploration of Earth, although the TRL would have to be advanced and validated for operations on Mars"^[75]

Similar methods could be used to directly explore other biologically sensitive locations such as Europa, Titan, or Encladus, once a human presence in the vicinity becomes possible.

See also

References