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 habitats exist.
The main focus is on microbial life and on potentially invasive species. Non-biological forms of contamination have also been considered, including contamination of sensitive deposits (such as lunar polar ice deposits) of scientific interest. In the case of back contamination, multicellular life is thought unlikely but has not been ruled out. In the case of forward contamination, contamination by multicellular life (e.g. lichens) is unlikely to occur for robotic missions, but it becomes a consideration in crewed missions to Mars.
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 sample-return missions (back contamination) the aim of the mission is to return extraterrestrial samples to Earth, 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 of origin and Earth. It would also require quarantine procedures for the materials and for anyone who comes into contact with them.
- 1 Overview
- 2 Evidence for possible habitats outside Earth
- 3 Planetary protection
- 4 Crewed spacecraft
- 5 The Moon as a testbed
- 6 Non-contaminating exploration methods
- 7 Back contamination issues
- 7.1 Suggested precautions for sample-returns
- 7.2 Alternatives to sample-returns
- 8 See also
- 9 References
Most of the Solar System appears hostile to life as we know it. No extraterrestrial life has ever been discovered, but there are several locations outside Earth where microbial life could possibly exist, have existed, or thrive if introduced. If extraterrestrial life exists, it may be vulnerable to interplanetary contamination by foreign microorganisms. Some extremophiles may be able to survive space travel to another planet, and foreign life could possibly be introduced by spacecraft from Earth and transform the location from its current pristine state. This poses scientific and ethical concerns.
Locations within the Solar System where life might exist today include the oceans of liquid water beneath the icy surface of Europa, and Enceladus, and Titan (its surface has oceans of liquid ethane / methane, but it may also have liquid water below the surface and ice volcanoes).
There are multiple consequences for both forward- and back-contamination. If a planet becomes contaminated with Earth life, it might then be difficult to tell whether any lifeforms discovered originated there or came from Earth. Furthermore, the organic chemicals produced by the introduced life would confuse sensitive searches for biosignatures of living or ancient native life. The same applies to other more complex biosignatures. 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 nucleic acids too (RNA or DNA).
This is true of species detected in spacecraft cleanrooms. The majority of the species isolated are not well understood or characterized and cannot be cultured in labs, and are known only from DNA fragments obtained with swabs. On a contaminated planet, it might be difficult to distinguish the DNA of extraterrestrial life from the DNA of life brought to the planet by the exploring. Most species of microorganism on Earth are not yet well understood or DNA sequenced. This particularly applies to the unculturable archaea, and so are difficult to study. This can be either because they depend on the presence of other microorganisms, or are slow growing, or depend on other conditions not yet understood. In typical habitats, 99% of microorganisms are not culturable. Introduced Earth life could contaminate resources of value for future human missions, such as water.
Invasive species could outcompete native life or consume it, if there is life on the planet. One argument against this is that the native life would be more adapted to the conditions there. However, the experience on Earth shows that species moved from one continent to another may be able to outcompete the native life adapted to that continent. Additionally, evolutionary processes on Earth might have developed biological pathways different from extraterrestrial organisms, and so may be able to out-compete it. The same is also possible the other way around for back contamination introduced to Earth's biosphere.
Evidence for possible habitats outside Earth
There is ample evidence to suggest that Mars once offered habitable conditions for microbial life. It is therefore possible that microbial life may have existed on Mars, although no evidence has been found.
It is thought that many bacterial spores (endospores) from Earth were transported on Mars spacecraft. Some may be protected within Martian rovers and landers on the shallow surface of the planet. In that sense, Mars may have already been interplanetarily contaminated.
Certain lichens from the arctic permafrost are able to photosynthesize and grow in the absence of any liquid water, simply by 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.
Although numerous studies point to resistance to some of Mars conditions, they do so separately, and none has considered the full range of Martian surface conditions, including temperature, pressure, atmospheric composition, radiation, humidity, oxidizing regolith, and others, all at the same time and in combination. Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.
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, the organisms may obtain liquid water at times of high atmospheric humidity, with salts capture enough to be capable of supporting life.
Research published in July 2017 shows that when irradiated with a simulated Martian UV flux, perchlorates become even more lethal to bacteria (bactericide effect). Even dormant spores lost viability within minutes. In addition, two other compounds of the Martian surface, iron oxides and hydrogen peroxide, act in synergy with irradiated perchlorates to cause a 10.8-fold increase in cell death when compared to cells exposed to UV radiation after 60 seconds of exposure. It was also found that abraded silicates (quartz and basalt) lead to the formation of toxic reactive oxygen species. The researchers concluded that "the surface of Mars is lethal to vegetative cells and renders much of the surface and near-surface regions uninhabitable." This research demonstrates that the present-day surface is more uninhabitable than previously thought, and reinforces the notion to inspect at least a few meters into the ground to ensure the levels of radiation would be relatively low.
The Cassini spacecraft directly sampled the plumes escaping from Enceladus. Measured data indicates that these geysers are made primarily of salt rich particles with an 'ocean-like' composition, which is thought to originate from a subsurface ocean of liquid saltwater, rather than from the moon's icy surface. Data from the geyser flythroughs also indicate the presence of organic chemicals in the plumes. Heat scans of Enceladus' surface also indicate warmer temperatures around the fissures where the geysers originate from, with temperatures reaching -93 °C (-135 °F), which is 115 °C (207 °F) warmer than the surrounding surface regions.
Europa has much indirect evidence for its sub-surface ocean. Models of how Europa is affected by tidal heating require a subsurface layer of liquid water in order to accurately reproduce the linear fracturing of the surface. Indeed, observations by the Galileo spacecraft of how Europa's magnetic field interacts with Jupiter's field strengthens the case for a liquid, rather than solid, layer; an electrically conductive fluid deep within Europa would explain these results. Observations from the Hubble Space Telescope in December 2012 appear to show an ice plume spouting from Europa's surface, which would immensely strengthen the case for a liquid subsurface ocean. As was the case for Enceladus, vapour geysers would allow for easy sampling of the liquid layer. Unfortunately, there appears to be little evidence that geysering is a frequent event on Europa due to the lack of water in the space near Europa.
Forward contamination is prevented by sterilizing space probes 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 Earth life could reproduce there.
NASA made these policies official with the issuing of Management Manual NMI-4-4-1, NASA Unmanned Spacecraft Decontamination Policy on September 9, 1963. Prior to NMI-4-4-1 the same sterilization requirements were required on all outgoing spacecraft regardless of their target. Difficulties in the sterilization of Ranger probes sent to the Moon are the primary reasons for NASA's change to a target-by-target basis in assessing the likelihood forward contamination.
Some destinations such as Mercury need no precautions at all. Others such as the Moon require documentation but nothing more, while destinations such as Mars require sterilization of the rovers sent there. For the details, see Planetary protection.
Back 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 the Apollo missions. The Apollo regulations have been rescinded and new regulations have yet to be developed, see Precautions suggested for sample-return
Crewed spacecraft are of particular concern for interplanetary contamination because of the impossibility to sterilize a human to the same level as a robotic spacecraft. Therefore, the chance of forward contamination is higher than for a robotic mission. Humans are typically host to a hundred trillion microorganisms in ten thousand species in the human microbiome which cannot 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 appears difficult to achieve with present-day technology. In particular, adequate containment in the event of a hard landing is a major challenge.
Human explorers may be potential carriers back to Earth of microorganisms acquired on Mars, if such microorganisms exist. Another issue is the contamination of the water supply by Earth microorganisms shed by humans in their stools, skin and breath, which could have a direct effect on the long-term human colonization of Mars.
The Moon as a testbed
The Moon has been suggested as a testbed for new technology to protect sites in the Solar System, and astronauts, from forward and back contamination. Currently, the Moon has no contamination restrictions because it is considered to be "not of interest" for prebiotic chemistry and origins of life. Analysis of the contamination left by the Apollo program astronauts could also yield useful ground truth for planetary protection models.
Non-contaminating exploration methods
One of the most reliable ways to reduce the risk of forward and back contamination during visits to extraterrestrial bodies is to use only Robotic spacecraft. Humans in close orbit around the target 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 back contamination risks.
Back contamination issues
Since the Moon is now generally considered to be free from life, the most likely source of contamination would be from Mars during either a Mars sample-return mission or as a result of a crewed mission to Mars. The possibility of new human pathogens, or environmental disruption due to back contamination, is considered to be of extremely low probability but cannot yet be ruled out.
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 and geological interest. The European Space Foundation report cites many advantages of a Mars sample-return. In particular, it would permit extensive analyses 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 by Martian rovers. It would also make it possible to repeat experiments in multiple laboratories with different instruments to confirm key results.
Carl Sagan was first to publicise back contamination issues that might follow from a Mars sample-return. In Cosmic Connection (1973) he wrote:
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.
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.
NASA and ESA views are similar. The findings were that with present-day technology, Martian samples can be safely returned to Earth provided the right precautions are taken.
Suggested precautions for sample-returns
NASA has already had experience with returning samples thought to represent a low back contamination risk 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, so the requirements were not very stringent. The precautions taken then were inadequate by current standards, however. The regulations used then have been rescinded, and new regulations and approaches for a sample-return would be needed.
Chain of contact
A 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 it returns to Earth. In order to eliminate the risk of parachute failure, the capsule could fall at terminal velocity and the impact would be cushioned by the capsule's thermal protection system. The sample container would be designed to withstand the force of the impact.
To receive, analyze and curate extraterrestrial soil samples, NASA has proposed to build a biohazard containment facility, tentatively known as the Mars Sample Return Receiving Facility (MSRRF). This future facility must be rated biohazard level 4 (BSL-4). While existing BSL-4 facilities deal primarily with fairly well-known organisms, a BSL-4 facility focused on extraterrestrial samples must pre-plan the systems carefully while being mindful that there will be unforeseen issues during sample evaluation and curation that will require independent thinking and solutions.
The facility's systems must be able to contain unknown biohazards, as the sizes of any putative Martian microorganisms are unknown. In consideration of this, additional requirements were proposed. Ideally it should filter particles of 0.01 µm or larger, and release of a particle 0.05 µm or larger is unacceptable under any circumstance.
The reason for this extremely small size limit of 0.01 µm is for consideration of gene transfer agents (GTAs) which are virus-like particles that are produced by some microorganisms that package random segments of DNA capable of horizontal gene transfer. 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 microorganisms in the same way. In one experiment reported in 2010, researchers left GTAs (DNA 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. Another reason for the 0.05 µm limit is because of the discovery of ultramicrobacteria as small as 0.2 µm across.
The BSL-4 containment facility must also double as a cleanroom to preserve the science value of the samples. A challenge 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 perform analyses. During all these handling procedures, the samples would need to be protected from Earthly contamination. A cleanroom is normally kept at a higher pressure than the external environment to keep contaminants out, while a biohazard laboratory is kept at a lower pressure to keep the biohazards in. This would require to compartmentalize the specialized rooms in order to combine these in a single building. Solutions suggested include a triple walled containment facility, and one of the suggestions include extensive robotic handling of the samples.
Dissenting views on back contamination
Robert Zubrin, from the Mars Society, maintains that the risk of back contamination is negligible. He supports this using an argument based on the possibility of transfer of life from Earth to Mars on meteorites.
Legal process of approval for Mars sample-return
Margaret Race has examined in detail the legal process of approval for a MSR. 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-returns
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.
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 current or past life, through biosignatures and other in situ analyses.
Instruments under development for in situ analyses
- 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.
- Several teams, including Jonathan Rothberg, and J. Craig Venter, are separately developing solutions for sequencing alien DNA directly on the Martian surface itself.
- 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.
- 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
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.
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.
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.
- S.T. Shipley , P.T. Metzger and J.E. Lane. "Lunar Cold Trap Contamination by Landing Vehicles" (PDF). Earth and Space 2014 - Proceedings of the 14th Biennial ASCE Conference on Engineering, Science, Construction, and Operations in Challenging Environments.
- Staff - Purdue University (27 February 2018). "Tesla in space could carry bacteria from Earth". phys.org. Retrieved 28 February 2018.
- COSPAR Workshop on Planetary Protection for Outer Planet Satellites and Small Solar System Bodies European Space Policy Institute (ESPI), 15–17 April 2009
- COSPAR power point type presentation, gives good overview of the detailed category decisions Archived 2013-10-19 at the Wayback Machine.
- If there Are Microbes on Mars, We Might Have Put Them There
- Archaeal diversity analysis of spacecraft assembly clean rooms, he ISME Journal (2008) 2, 115–119; doi:10.1038/ismej.2007.98
- Kaeberlein, T; Lewis, K; Epstein, SS. "Isolating "uncultivable" microorganisms in pure culture in a simulated natural environment". Science. 296: 1127–9. Bibcode:2002Sci...296.1127K. doi:10.1126/science.1070633. PMID 12004133.
- Queens University Belfast scientist helps NASA Mars project "No-one has yet proved that there is deep groundwater on Mars, but it is plausible as there is certainly surface ice and atmospheric water vapour, so we wouldn't want to contaminate it and make it unusable by the introduction of micro-organisms."
- Rachel Courtland Should Mars be treated like a wildlife preserve? New Scientist, February 2009.
- [https://www.liebertpub.com/doi/pdfplus/10.1089/ast.2012.0891 Ethical Considerations for Planetary Protection in Space Exploration: A Workshop]. (PDF.) J.D. Rummel, M.S. Race, G. Horneck, and the Princeton Workshop Participants. Astrobiology, Volume 12, Number 11, 2012. doi:10.1089/ast.2012.0891
- Planetary protection—A microbial ethics approach. Space Policy. Vol. 21, Issue 4. November 2005. pp 287-292.
- The need of an ethics of planetary sustainability. Andreas Losch. International Journal of Astrobiology. 10 January 2018. doi:10.1017/S1473550417000490
- Christopher P. McKay Planetary Ecosynthesis on Mars: Restoration Ecology and Environmental Ethics NASA Ames Research Center
- Chang, Kenneth (December 9, 2013). "On Mars, an Ancient Lake and Perhaps Life". New York Times.
- Various (December 9, 2013). "Science - Special Collection - Curiosity Rover on Mars". Science.
- Experimental evidence for the formation of liquid saline water on Mars , Erik Fischer, Germán M. Martínez, Harvey M. Elliott, Nilton O. Rennó, Geophysical research letters, 7 July 2014 DOI: 10.1002/2014GL060302 "Finally, our results indicate that liquid water could form on the surface during the spring where snow has been deposited on saline soils [Martínez et al., 2012; Möhlmann, 2011]. These results have important implications for the understanding of the habitability of Mars because liquid water is essential for life as we know it, and halophilic terrestrial bacteria can thrive in brines"
- Water and Brines on Mars: Current Evidence and Implications for MSL G. M. Martínez1 and N. O. Renno, Space Science Reviews, 2013
- Summons, Roger E.; Amend, Jan P.; Bish, David; Buick, Roger; Cody, George D.; Des Marais, David J.; Dromart, Gilles; Eigenbrode, Jennifer L.; et al. (2011). "Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group" (PDF). Astrobiology. 11 (2): 157–81. Bibcode:2011AsBio..11..157S. doi:10.1089/ast.2010.0506. PMID 21417945.
There is general consensus that extant microbial life on Mars would probably exist (if at all) in the subsurface and at low abundance.
- Didymus, JohnThomas (January 21, 2013). "Scientists find evidence Mars subsurface could hold life". Digital Journal – Science.
There can be no life on the surface of Mars because it is bathed in radiation and it's completely frozen. However, life in the subsurface would be protected from that. - Prof. Parnell.
- "Mars: 'Strongest evidence' planet may have supported life, scientists say". BBC News. January 20, 2013.
- Michalski, Joseph R.; Cuadros, Javier; Niles, Paul B.; Parnell, John; Deanne Rogers, A.; Wright, Shawn P. (2013). "Groundwater activity on Mars and implications for a deep biosphere". Nature Geoscience. 6 (2): 133–8. Bibcode:2013NatGe...6..133M. doi:10.1038/ngeo1706.
- RADIATIVE HABITABLE ZONES IN MARTIAN POLAR ENVIRONMENTS "Finally there are other harmful radiation sources reaching Mars: ionizing and neutron radiation caused by galactic cosmic radiation and solar particle events. Due to the lack of a magnetic field and the low shielding of the Martian atmosphere (the Martian overhead airmass is 16 g cm-2 instead of the terrestrial 1000 g cm-2) the doses of ionizing radiation at the surface of Mars reach values about 100 times higher than those on the Earth. However, since a great variety of microbes tolerate this type of radiation at similar or even greater doses than those found on Mars, ionizing radiation cannot be considered a limiting factor for microbial life on Mars and thus here we will limit our study to solar UV shielding and VIS radiation pentration."
- Debus, A. (2005). "Estimation and assessment of Mars contamination". Advances in Space Research. 35 (9): 1648–53. Bibcode:2005AdSpR..35.1648D. doi:10.1016/j.asr.2005.04.084. PMID 16175730.
- MEPAG Special Regions-Science Analysis Group; Beaty, D.; Buxbaum, K.; Meyer, M.; Barlow, N.; Boynton, W.; Clark, B.; Deming, J.; Doran, P. T.; et al. (2006). "Findings of the Mars Special Regions Science Analysis Group". Astrobiology. 6 (5): 677–732. Bibcode:2006AsBio...6..677M. doi:10.1089/ast.2006.6.677. PMID 17067257.
- Space Station Research Shows That Hardy Little Space Travelers Could Colonize Mars
- NASA Press Release, May 2014 "In another investigation, spores of Bacillus pumilus SAFR-032 and another spore-forming bacteria, Bacillus subtilis 168, were dried on pieces of spacecraft-quality aluminum and subjected for 1.5 years to the vacuum of space, cosmic and extraterrestrial solar radiation and temperature fluctuations on EuTEF. These samples also were subjected to a simulated Martian atmosphere using EuTEF. Most of the organisms exposed to solar UV radiation in space and in the Mars spectrum were killed, but when UV rays were filtered out and samples were kept in the dark, about 50 percent or more of those subjected to other space- and Mars-like conditions survived. That makes it likely that spores could survive a trip on a spacecraft to Mars if they are sheltered against solar radiation, perhaps in a tiny pocket of the spacecraft surface or underneath a layer of other spores."
- Baldwin, Emily (26 April 2012). "Lichen survives harsh Mars environment". Skymania News. Retrieved 27 April 2012.
- de Vera, J.-P.; Kohler, Ulrich (26 April 2012). "The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars" (PDF). European Geosciences Union. Archived from the original (PDF) on 8 June 2012. Retrieved 27 April 2012.
- Is Searching for Martian Life a Priority for the Mars Community? Fairén Alberto G., Parro Victor, Schulze-Makuch Dirk, and Whyte Lyle. Astrobiology. February 2018, 18(2): 101-107. doi:10.1089/ast.2017.1772
- Q. Choi,, Charles (May 17, 2010). "Mars Contamination Dust-Up". Astrobiology Magazine. Archived from the original on August 20, 2011.
Whenever multiple biocidal factors are combined, the survival rates plummet quickly,
- Wadsworth, J; Cockell, CS (2017). "Perchlorates on Mars enhance the bacteriocidal effects of UV light". Sci Rep. 7 (1): 4662. Bibcode:2017NatSR...7.4662W. doi:10.1038/s41598-017-04910-3. PMC 5500590. PMID 28684729.
- Shielding biomolecules from effects of radiation by Mars analogue minerals and soils. G. Ertem, M. C. Ertem, C. P. McKay and R. M. Hazen. International Journal of Astrobiology. Volume 16, Issue 3, July 2017, pp. 280-285 DOI: https://doi.org/10.1017/S1473550416000331
- "Silicates Eroded under Simulated Martian Conditions Effectively Kill Bacteria—A Challenge for Life on Mars". Ebbe N. Bak1, Michael G. Larsen1, Ralf Moeller, Silas B. Nissen, Lasse R. Jensen, Per Nørnberg, Svend J. K. Jensen and Kai Finster1. Front. Microbiol., 12 September 2017. https://doi.org/10.3389/fmicb.2017.01709
- Why Life on Mars May Be Impossible. Jeffrey Kluger. Time - Science; Jul 6, 2017.
- Mars Soil May Be Toxic to Microbes. Mike Wall. Space.com. July 6, 2017
- Mars soil is likely toxic to cells — does this mean humans won't be able to grow vegetables there?. David Coady. The World Today. 7 Jul 2017
- Cassini samples the icy spray of Enceladus' water plumes; June 22, 2011
- Cassini Tastes Organic Material at Saturn's Geyser Moon; March 12, 2008
- What Makes Us Think There is an Ocean Beneath Europa's Icy Crust?, Paragraphs 4 - 7
- Water plumes spark a race to Jupiter moon Europa, Lisa Grossman, New Scientist 31 December 2013
- Hubble Space Telescope Sees Evidence of Water Vapor Venting off Jupiter Moon, Paragraph 4; December 12, 2013
- Signs of Europa Plumes Remain Elusive in Search of Cassini Data; December 17, 2014
- Meltzer, Michael (May 31, 2012). When Biospheres Collide: A History of NASA's Planetary Protection Programs. pp. 46–51. ISBN 978-0-16-085327-2.
- When Biospheres Collide - a history of NASA'S Planetary Protection Programs, Michael Meltzer, May 31, 2012. See Chapter 7, Return to Mars. Quote: "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 its 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."
- Safe on Mars page 37 "Martian biological contamination may occur if astronauts breathe contaminated dust or if they contact material that is introduced into their habitat. If an astronaut becomes contaminated or infected, it is conceivable that he or she could transmit Martian biological entities or even disease to fellow astronauts, or introduce such entities into the biosphere upon returning to Earth. A contaminated vehicle or item of equipment returned to Earth could also be a source of contamination."
- Mars and the Moon (C. A. Conley & J. D. Rummel Acta Astronautica 63 1025–1030 (2008))
- Biological contamination studies of lunar landing sites: implications for futureplanetary protection and life detectionon the Moon and Mars, D.P. Glavin, J.P. Dworkin, M. Lupisella, G. Kminek and J.D. Rummel,International Journal of Astrobiology (2004) doi:10.1017/S1473550404001958
- Almost Being There: Why the Future of Space Exploration Is Not What You Think
- First Exploration Telereobotics Symposium Archived 2015-07-05 at the Wayback Machine.
- [HERRO: A Science-Oriented Strategy for Crewed Missions Beyond LEO HERRO: A Science-Oriented Strategy for Crewed Missions Beyond LEO]
- European Science Foundation - Mars Sample Return backward contamination - strategic advice Archived 2016-06-02 at the Wayback Machine. July, 2012, ISBN 978-2-918428-67-1 - see 2. From remote exploration to returning samples. (for more details of the document see abstract )
- Carl Sagan,The Cosmic Connection - an Extraterrestrial Perspective (1973) ISBN 0521783038
- Carl Sagan Cosmos Random House Publishing Group, 6 Jul 2011
- Assessment of Planetary Protection Requirements for Mars Sample Return Missions (Report). National Research Council. 2009.| Quote: "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."
- M. S. Race Planetary Protection, Legal Ambiguity, and the Decision Making Process for Mars Sample Return Archived 2010-06-19 at the Wayback Machine. Adv. Space Res. vol 18 no 1/2 pp (1/2)345-(1/2)350 1996
- European Science Foundation - Mars Sample Return backward contamination - Strategic advice and requirements Archived 2016-06-02 at the Wayback Machine. July, 2012, ISBN 978-2-918428-67-1. (for more details of the document see abstract )
- "Archived copy" (PDF). Archived from the original (PDF) on 2013-02-16. Retrieved 2013-08-12. Mars Sample Return Discussions, as presented on February 23, 2010.
- Mars Sample Return Receiving Facility. (PDF).
- Planning for the Analytic Environment to Conduct Life Detection Experiments on Samples Returned from Mars: Observations and Issues (2012) D. S. Bass, D. W. Beaty, C. C. Allen, A. C. Allwood, L. E. Borg, K. E. Buxbaum1, J. A. Hurowitz and M. D. Schulte. Lunar and Planetary Institute. 2012. Accessed: 19 August 2018.
- European Science Foundation - Mars Sample Return backward contamination - Strategic advice and requirements Archived 2016-06-02 at the Wayback Machine.
- Amy Maxmen Virus-like particles speed bacterial evolution published online 30 September 2010
- Lauren D. McDaniel, Elizabeth Young, Jennifer Delaney, Fabian Ruhnau, Kim B. Ritchie, John H. Paul High Frequency of Horizontal Gene Transfer in the Oceans Science 1 October 2010: Vol. 330 no. 6000 p. 50 doi:10.1126/science.1192243
- Mars Sample Return Receiving Facility - A Draft Test Protocol for Detecting Possible Biohazards in Martian Samples Returned to Earth (PDF) (Report). 2002.
A Sample Return Facility will require combining technologies used for constructing maximum containment laboratories (e.g. Biosafety Level 4 labs) with cleanroom technologies which will be needed to protect the Mars samples from Earth contamination.
- A Draft Test Protocol for Detecting Possible Biohazards in Martian Samples Returned to Earth Archived 2006-02-22 at the Wayback Machine.
- CLEANROOM ROBOTICS – APPROPRIATE TECHNOLOGY FOR A SAMPLE RECEIVING FACILITY 2005 update on the Draft Test Protocol .
- 2010 Mars Sample Return Orbiter decadal survey: Quote: The NASA Planetary Protection Officer commissioned the development of a draft test protocol that would represent one "necessary and sufficient" approach to evaluate the safety of the samples while safeguarding the purity of the samples from terrestrial contamination. A Draft Test Protocol for Detecting Possible Biohazards in Martian Samples Returned to Earth was published in October 2002 . In 2003, three architectural design teams independently examined the scope, approach, cost, and technology required for the SRF, using the Draft Test Protocol for requirements. The approaches varied from allrobotic handling of samples to more traditional glove box implementations. The studies indicated that the principles and techniques required are generally mature. Biosafety laboratories, the NASA Lunar Sample Facility, pharmaceutical laboratories, and electronic fabrication cleanrooms perform most of the required individual functions. However, there are some areas needing early development, such as ensuring sample preservation and bio-safety together, representing new challenges that were addressed by techniques like dual-walled containers (and gloves) with positive pressure clean inert gas in between the walls. This, as well as some further development in ultra-clean sample manipulation, safe and pure transport of samples, and sample sterilization techniques, are planned in the technology program.
- "7: "Sample-Receiving Facility and Program Oversight"". Assessment of Planetary Protection Requirements for Mars Sample Return Missions (Report). National Research Council. 2009. p. 59.
It has been estimated that the planning, design, site selection, environmental reviews, approvals, construction, commissioning, and pre-testing of a proposed safe room facility (SRF) will occur 7 to 10 years before actual operations begin. In addition, 5 to 6 years will likely be required for refinement and maturation of SRF-associated technologies for safely containing and handling samples to avoid contamination and to further develop and refine biohazard-test protocols. Many of the capabilities and technologies will either be entirely new or will be required to meet the unusual challenges of integration into an overall (end-to-end) Mars sample return program.
- Mars Sample Return: Issues and Recommendations (Planetary Protection Office Summary) Task Group on Issues in Sample Return. National Academies Press, Washington, DC (1997)
- Robert Zubrin "Contamination From Mars: No Threat", The Planetary Report July/Aug. 2000, P.4–5
- transcription of a tele-conference interview with ROBERT ZUBRIN conducted on March 30, 2001 by the class members of STS497 I, "Space Colonization"; Instructor: Dr. Chris Churchill
- Jeffrey L. Bada, Andrew D. Aubrey, Frank J. Grunthaner, Michael Hecht, Richard Quinn, Richard Mathies, Aaron Zent, John H. Chalmers Seeking signs of life on mars: in situ investigations as prerequisites to sample return missions Independent Contribution to the Mars Decadal Survey Panel
- Mars Exploration Strategies: Forget About Sample Return D. A. Paige, Dept. of Earth and Space Sciences, UCLA, Los Angeles, CA 90095
- Future Mars Missions: Can Humans Trump Robots?
- Gaskin, J.A.; Jerman, G.; Gregory, D.; Sampson, A.R., Miniature Variable Pressure Scanning Electron Microscope for in-situ imaging & chemical analysis Aerospace Conference, 2012 IEEE , vol., no., pp.1,10, 3–10 March 2012 doi: 10.1109/AERO.2012.6187064
- Mars Sample Return Mission? Naaah… Just Beam Back Martian DNA
- Biomedicine News Genome Hunters Go After Martian DNA
- Researchers Design a DNA Sequencing Microchip for Detecting Life on Mars Science Tech Daily, July 9, 2013
- Radiation Resistance of Sequencing Chips for in situ Life Detection Christopher E. Carr, Holli Rowedder, Clarissa S. Lui, Ilya Zlatkovsky, Chris W. Papalias, Jarie Bolander, Jason W. Myers, James Bustillo, Jonathan M. Rothberg, Maria T. Zuber, and Gary Ruvkun. Astrobiology. June 2013, 13(6) 560-569. doi:10.1089/ast.2012.0923
- A. D. Anbar1 and G. V. Levin A CHIRAL LABELED RELEASE INSTRUMENT FOR IN SITU DETECTION OF EXTANT LIFE., Concepts and Approaches for Mars Exploration (2012)
- Andrew D. Aubrey, John H. Chalmers, Jeffrey L. Bada, Frank J. Grunthaner, Xenia Amashukeli, Peter Willis, Alison M. Skelley, Richard A. Mathies, Richard C. Quinn, Aaron P. Zent, Pascale Ehrenfreund, Ron Amundson, Daniel P. Glavin, Oliver Botta, Laurence Barron, Diana L. Blaney, Benton C. Clark, Max Coleman, Beda A. Hofmann, Jean-Luc Josset, Petra Rettberg, Sally Ride, François Robert, Mark A. Sephton, and Albert Yen. The Urey Instrument: An Advanced In Situ Organic and Oxidant Detector for Mars Exploration Astrobiology. Volume 8, Number 3, 2008
- J.L. Bada, P. Ehrenfreund F. Grunthaner, D. Blaney, M. Coleman, A. Farrington, A. Yen, R. Mathies, R. Amudson, R. Quinn, A. Zen, S. Ride, L. Barron, O. Botta, B. Clark, D. Glavin, B. Hofmann, J.L. Josse, P. Rettberg, F. Robert, M. Sephton. Urey: Mars Organic and Oxidant Detector Space Sci Rev (2008) 135: 269–279
- LOW-LATENCY TELEROBOTICS FROM MARS ORBIT: THE CASE FOR SYNERGY BETWEEN SCIENCE AND HUMAN EXPLORATION, Concepts and Approaches for Mars Exploration (2012)
- Space Exploration Enabled by Telepresence: Combining Science and Human Exploration Archived 2013-02-17 at the Wayback Machine. Based on Findings from: "Exploration Telerobotics Symposium". May 2-3, 2012 NASA Goddard Space Flight Center
- Space Exploration Via Telepresence: The Case for Synergy Between Science and Human Exploration, Findings and Observations from: "Exploration Telerobotics Symposium". May 2-3, 2012. NASA Goddard Space Flight Center Archived 2013-02-17 at the Wayback Machine.