The dilemma of Mars sample return
The battle rages on: Does the scientific value of bringing Martian samples to Earth to study outweigh the risks of introducing alien organisms to our planet?
Often modern science is completely ignorant of how the microscopic world interacts with higher forms of life on our planet. Take, for example, the public statement issued in 1995 by British Health Secretary Stephen Dorrell on the ability of the bovine spongiform encephalopathy (BSE) prion (mad cow disease) to be transmitted to humans: There is no conceivable risk of BSE being transmitted from cows to people (3). Dorrell had made the comment in an effort to ease public fears about buying and eating British beef before a complete scientific study of BSE was made.
How many people put their trust in Dorrells expertise and continued to eat tainted British beef? The painful truth was that the BSE prion is a new pathogenit is not a virus or a bacterium, but a protein that resists all forms of sterilization. A year later, Dorrell made a public apology for his statement, but insisted that is was made in good faith.
Similarly, our ignorance about toxins and pathogens from other worlds could lead to false assumptions if not investigated to the best of our ability. The best protection we can offer our planet against such a threat is a combination of ongoing in situ life science analyses using robotic spacecraft with the body of peer-reviewed scientific information that already exists. What if bizarre forms of pathogens exist on planets like Mars? Are we really prepared to bring them home?
The members of the Space Studies Board (SSB; formerly the Space Science Board) of the National Research Council (established in 1916 by the National Academy of Science [NAS]) are responsible for answering such important questions. The SSB is supposed to provide the National Aeronautics and Space Administration (NASA) with estimatesbased on peer-reviewed scientific dataof the likelihood of microbial life on Mars and other planets. It also examines the survival skills of terrestrial microbes in extreme environments and the potential pathogenicity of alien microbes to Earth life. These estimates are then used to define and shape spacecraft missions to the planets as well as the sample return programs.
Yet, until very recently, the reports issued by the SSB on planetary contamination and Mars sample return (MSR) (47) did not mention the peer-reviewed papers written by Viking Mission scientists Gilbert V. Levin and Patricia Ann Straat, who conducted nine life-detection experiments on the surface of Mars in 1976. Levin and Straat claim their experiments indicated positive signals for microbial metabolism (811). Their papers appeared in such prestigious scientific journals as Science and the Journal of Geophysical Research, but were left out of the SSB reports until June of this year (12). Why?
Of all the planets in the solar system, Mars is considered to be the most like Earth. Many millions, if not billions, of years ago, Mars had tremendous volcanic activity, which perhaps continues today on a limited scale. Mars also has gigantic polar ice caps that rival the Antarctic ice sheet and consist largely of frozen water. Mars has water vapor in its atmosphere that forms wispy white clouds and low-lying fogs. It has recently been conjectured that Mars may have liquid water in biologically significant amountssomething most scientists considered impossible only 2 years ago (13).
If it were shielded from the harsh solar UV light impinging on its surface, Mars would have all the right ingredients for a sustainable microbial ecosystem. (A mere centimeter or less of dust could easily shield such life.) It is no wonder then that returning samples from Mars to Earth for study is a high priority for scientists who want to know whether Mars has or once had an active biosphere.
But do we really have to return samples to Earth to make this determination? The answer is no. The British are building an astrobiology lander that will fly to Mars in 2003 to attempt to answer the life-on-Mars question by using in situ instruments. It is the only life science lander scheduled to go to Mars this decade and the only one since NASAs Viking Mission more than 25 years ago (14).
The feasibility of returning samples from Mars has been considered periodically for more than 40 years (1517), and its roots are entwined with the establishment of the planetary protection resolution initiated by NAS in 1958. However, since the historic 1996 NASA announcement of a Martian meteorite (ALH 84001) found in Antarctica that might contain fossilized bacteria from Mars (18), NASA has stated that its highest priority for the Mars exploration program is to return Martian samples directly to Earth. Since the 1970s, MSR studies have gone through a series of design options ranging from examination of samples in a Centers for Disease Control and Prevention (CDC)like containment facility on the Moon to an Earth- orbiting laboratory.
The current MSR design uses Earth as a catchers mitt, a phrase coined by NASA Planetary Protection Officer John Rummel in 2000 (19). Engineers at the Jet Propulsion Laboratory (JPL, Pasadena, CA) describe this method as passive Earth entry; it will use a cannonball-sized sphere filled with Martian soil and rock that will directly enter Earths atmosphere without a parachute.
Only 2 years ago, MSR was going to be launched as two missions: the first in 2003 and the second in 2005. Each mission would have a Pathfinder-like Rover vehicle to look for and collect 500 g of Martian soil and rock and place it in a return canister. Then the sample container would be launched on a small rocket to rendezvous with a French-made Earth return vehicle (ERV) waiting in Martian orbit (20). The ERV would then bring the sample container to the vicinity of Earth and release it, after which it would enter the Earths atmosphere and crash land in a predetermined area in the Utah desert sometime in 2008. The capsule and samples within would then be taken for analysis to a level-4 biohazard containment facility, a high-level biological containment facility for handling deadly pathogens such as the Ebola virus (21).
The MSR program is envisioned as an international effort involving Italy and France working in partnership with NASA (20). The mission is estimated to cost more than $8 billion and will use five or more spacecraft to return just one sample. If, however, only one of its components fails for any reason, the entire mission will be lost. The confidence for such a complex mission faded in 1999 when NASA lost four of its Mars-bound spacecraft in less than 3 months.
One of these was the Mars Climate Orbiter. While the orbiter was en route to Mars, the spacecraft flight controllers failed to convert metric units to English correctly, dooming the mission (22). The incident stunned the scientific community and caused great embarrassment to NASA. Then, only 2 months later, the agencys Mars Polar Lander and the two Deep Space 2 microprobes failed and are assumed to have crashed on the Martian surface. Together, these missions cost more than $400 million. A full-scale investigation was mounted, and an independent commission re-evaluated the Mars exploration program (2325). As a consequence of the investigation, the MSR project was postponed until 2011 (26). But discussions on setting a new launch date for MSR, perhaps as soon as 2005, are in progress.
In March 2001, Colin Pillinger, the principal investigator for the British-built Beagle 2 astrobiology lander (slated for launch to Mars in 2003 on the European Space Agencys Mars Express probe) held a meeting at the Royal Society in London billed as Samples from MarsHow Shall Britain Proceed? Pillinger not only probed British interest for returning Martian samples to Earth, but also sought ways to beat NASAs new MSR schedule in 2011, thus allowing the British to be the first to return samples to the Earth. Has a new space race beguna race to return Martian samples to Earth?
The debate over MSR is a twofold argument and is a delicate balancing act based on whether or not the scientific benefits of such a mission outweigh the risks involved. So far, all that the SSB has said about the risk of back-contamination from Mars is that it is considered to be low, but not zero; therefore, any returned Martian samples will be treated as biohazardous until proven otherwise and taken to a level-4 biohazard containment facility (6). As the history of the planetary protection program has demonstrated, determining what level of planetary protection stringency is used on spacecraft missions depends on whether scientists believe that other worlds have life or the ability to sustain it. To fully appreciate how the risks of MSR should be weighed against the scientific benefits, it is necessary to review and understand how MSR evolved out of the planetary protection program (see timeline A & B).
1958: The birth of planetary protection
In February 1958, NAS drafted a resolution that examined the problem of preventing such contamination. In the resolution, NAS pleaded with scientists planning future lunar and planetary missions to use great care and deep concern so that initial operations do not compromise and make impossible for ever analysis of celestial bodies by critical scientific experiments (27).
NAS urged the International Council of Scientific Unions (ICSU) to assist in evaluating the problem of forward contamination of celestial bodies by spacecraft. In May 1958, the ICSU set up an ad hoc committee known as CETEX (the Committee on Contamination by Extraterrestrial Exploration) to begin a systematic analysis of the issues raised by NAS, such as
By June 1958, NAS had set up its own internal space activities monitoring groupthe Space Science Boardthat later strongly influenced how the Mars sample return program would be conceived. In October 1958, CETEX presented its recommendations to the general assembly of the ICSU in Washington, DC (28):
Following the meeting, ICSU established a new organization that would take over the planetary protection functions of CETEXthe Committee on Space Research (COSPAR). As an international organization, COSPAR sought to define what the acceptable levels of contamination on an outbound spacecraft might be. The SSB appointed one of its founders, Joshua Lederberg (then professor of biology at Stanford University and winner of the Nobel Prize in Physiology and Medicine in 1958 for his research in genetics) to work with COSPAR to establish spacecraft sterilization protocols.
Another important event in 1958 was the creation of NASA, born out of the National Advisory Committee on Aeronautics. NASAs early role was to oversee U.S. Department of Defense space efforts and serve as an authority for making international recommendations for the prevention of celestial contamination.
Also in 1958, out of concern for contamination issues and nuclear weapons placed in space, the United Nations founded UNCPUOSthe Committee on the Peaceful Uses of Outer Space (29). By 1967, the U.N. Outer Space Treaty was signed by the United States, the Soviet Union, and the United Kingdom, and then ratified by the Secretary General of the United Nations and signed by every member of the U.N. General Assembly. Article IX calls for parties to the treaty to conduct exploration of the Moon and other celestial bodies so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter (30). The stage was set for one of the most interesting scientific debates in history.
Remember that the major concerns at this time were only about forward contamination of celestial bodies. Back-contamination was not foreseen because it would entail bringing materials from other planets to the Earths biosphere. The only rocket technology available in 1958 was that of outbound spacecraft.
Contamination on the Moon
Microbiologists knew that grenades and shells used in bacteriological warfare contained microbes that could survive the explosive force of impacts. Had Luna 2 contaminated the Moon? The Moon has an unimaginably hostile environment with an almost nonexistent atmosphere; it is exposed to harsh solar and cosmic radiation and has no liquid water. Surely no microbes from Earth could survive thereor could they? The answer to that question would have to wait until 1969 when Apollo 12 astronauts returned a section of the Surveyor III camera.
The various reports submitted to SSB and COSPAR indicated that complete sterilization of a spacecraft was impossible (31). Microbiologists knew that under favorable ecological conditions, just one surviving spore could multiply into billions. Still, they had to do their best to ensure that scientific instruments on future landers would not find terrestrial contaminants from earlier unsterilized orbiters. It was determined that the most rigorous method for limiting microbial spores on outbound spacecraft was to use a combination of dry heat sterilization (105108 °C for 1300 h) and treatment with ethylene oxide, a toxic gas (32). Ethylene oxide had to be used cautiously to avoid being adsorbed on spacecraft materials made from polymers such as rubber. The entire spacecraft payload would have to be assembled in a class 100,000 clean rooma room in which the concentration of >0.5-µm airborne particles does not exceed 100,000/ft3. Electronic components had to be designed to withstand heat sterilization and that would mean added costs and time.
With the success of the Luna 2 probe, the Moon became a dartboard for early probes from the Soviet Union and United States as engineers tried to perfect their celestial navigation and targeting skills. Between August 1961 and October 1962, NASA launched six lunar probes in its Ranger series designed to take pictures minutes before crashing on the Moon. All six Rangers failed, and a NASA commission determined that prolonged heat sterilization of the Ranger probes likely damaged some electronic components, causing the spacecraft to fail. The agency immediately dropped the heat sterilization protocols on all lunar probe missions and opted to use the germicidal gas ethylene oxide. It was much the same method that the Soviet Union claimed it had used with the Luna 2 probe that SSB and COSPAR had earlier protested. NASAs unsterilized Ranger 7 successfully completed its mission in 1964 and was followed by Rangers 8 and 9.
The ethics of contaminating other worlds would have to be compromised to some degree if we wanted to strive for greater knowledge. It now appeared that science would have to accept the fact that the Moon would be contaminated in small local areas where incoming spacecraft spread their debris.
The Apollo back-contamination problem
In early 1965, an SSB committee agreed that a sample receiving laboratory was needed, but it also raised the question of biological contamination and the need for quarantine. Biologists working with the U.S. Public Health Service also had raised concerns during the development of the Apollo program about astronauts bringing back pathogenic material from the Moon. In 1966, the Interagency Committee on Back Contamination (ICBC) was formed to study the problem. Their recommendations resulted in the building of the $16 million Lunar Receiving Laboratory (LRL) at the Johnson Space Center in Houston. The engineers responsible for designing the LRL used the U.S. Army biohazard containment facility at Fort Detrick, Frederick, MD, as a model. Fort Detrick is equipped to deal with the worlds deadliest germs and chemicals, and the LRL was designed with that concept in mind (34).
One month before the first Apollo landing in July 1969, Time magazine featured a cover article entitled Is the Earth Safe from Lunar Contamination?, with Carl Sagan gracing the cover (35). Sagan told Time that although scientists were 99% convinced that lunar contamination posed no problems, the 1% chance of contamination was too large to ignore.
Apollo 11 and 12
Unfortunately, to the horror of the biologists on the ICBC, the Apollo 11 capsule hatch door was opened prematurely in the Pacific Ocean, contrary to established planetary protection protocols. Someone on the ship had determined that the crane was unsafe, and frogmen were sent to open the crew hatch while the capsule was still floating in the ocean. This was a very serious breach of the planetary protection protocolwhich was not pointed out in the news.
When the hatch door was opened, any lunar dust or microbes from inside the spacecraft could have been carried out into the air and the ocean. If there were any pathogenic organisms living in lunar soil, they now had a very warm and wet breeding ground in which to grow (36).
Years later, astronaut Buzz Aldrin said in a television interview that the mobile quarantine trailer in which the Apollo 11 crew was isolated had one serious flaw: Ants appeared to be going into and out of the trailer (37). If there were any Moon bugs, they would have gotten out with the ants.
Sagan accused those responsible of playing loose with the biosphere. Most of the lunar geologists considered Sagan a thorn in their side and resented him for what they thought was his alarmist commitment to planetary protection.
I always thought the most significant thing we found on the whole [expletive deleted] Moon was that little bacteria who came back and lived and nobody ever said [expletive deleted] about it, said Apollo 12 Commander Pete Conrad in 1991 (38). What was he talking about?
In April 1967, the unmanned Surveyor III landed near the eastern shore of Oceanus Procellarum on the lunar surface. It stayed on the Moon for 21/2 yearsin a near vacuum, at temperatures of +250 °F in the sunlight and 250 °F in the dark, and completely exposed to solar and cosmic radiation (UV, -, and X-rays)until November 20, 1969, when the crew of Apollo 12 landed only 535 ft away. Part of the missions objective was to retrieve selected components of the probes camera, place them in sterile containers, and return them to Earth for analysis.
Incredibly, scientists working at the LRL isolated a colony of 50100 Streptococcus mitis bacteria that were still viable from a sample of polyurethane foam insulation taken from inside the Surveyor III camera housing (39). Here was strong evidence that NASAs decision to have COSPAR eliminate the sterilization protocols for lunar probes was completely wrong. Just as in the case of British Health Minister Dorrells erroneous comments on BSE, NASA and COSPAR based their decision on belief rather than hard scientific evidence. If terrestrial microbes could survive on the Moon for 21/2 years, what might happen on Mars?
To contaminate Mars?
Microbial survival on Mars was another matter entirely. On November 30, 1964, the Soviet Union unintentionally focused attention on this issue when it lost contact with its Mars spacecraft Zond 2 and became embroiled in another heated controversy with COSPAR and SSB. Deep-space tracking stations calculated that the Soviet spacecraft collided with the surface of Mars (40). Because Zond 2 was designed to fly by the planet and take photographs, no attempts had been made to sterilize the vehicle. Soviet engineers did not consider that the vehicle might malfunction and crash into Mars.
Carl Sagan was an early member of COSPAR and a strong supporter of planetary protection measures. Aside from being one of the minority of scientists concerned about microbial survival on the Moon, he had an ongoing dispute with JPL scientists who wanted to ease the planetary protection burdens for spacecraft going to Mars to make them cheaper and easier to design (41).
Sagan, Lederberg, and Elliott Levinthal published a famous rebuttal to this argument entitled Contamination of Mars in a 1968 issue of Science (42). They wrote, One terrestrial microorganism reproducing as slowly as once a month on Mars, without other ecological limitations, in less than a decade would result in a microbial population of the Martian soil comparable to the Earth. Although it was considered an esoteric exercise in math by most who read it, this single sentence illustrated how unforgiving microbial contamination could be, given the right conditions for reproduction.
Viking Lander 1 set down on the surface of Mars on July 20, 1976, and Viking Lander 2 on September 3, 1976. The biological results are discussed in detail elsewhere in the scientific literature (44), but all three Viking biology experiments from both landers returned positive indications of activity in the soil. However, based on the lack of evidence for organic molecules provided by the Landers GC-MS organic analysis instrument (45), many of the scientists looking at the Viking data thought the positive reactions of the biological instruments were caused by chemicals that seemed to be mimicking life.
Recent evidence suggests that the Viking GC-MS would not have detected certain carboxylate salts that could have been present as metastable oxidation products of high-molecular-weight organic species (46). Also, the possibility remained that very low levels of organic matter, below the GC-MS instrument detection limit, could have been present. Such low levels of organic matter would not have been inconsistent with the presence of very low levels of microbes. Other problems existed too: The high sulfur content of Martian soil (50100% higher on Mars than on Earth) may have poisoned the Viking GC-MS hydrogen separator coil (made of palladium), thereby preventing pyrolyzed samples from entering the gas chromatograph portion of the instrument for analysis.
Levin, one of the three Viking principal investigators in biology, along with his co-experimenter Straat, maintained that the Viking labeled-release (LR) experiment found living micro organisms on Mars (47). They tested an Antarctic soil sample with their LR instrument before it went to Mars and found organisms thriving in it. The same soil was given to the Viking GC-MS team for preflight testing. They claimed the soil was sterile and contained no organic matter. This was reason by itself not to trust the results of the Viking GC-MS.
The origin of the MSR program
With the incredible success of NASAs twin Viking landers, the prospect of returning samples from Mars became more of a reality. NASA grants were made available to scientists and engineers who wanted to study the problems associated with a Mars sample return mission. Not surprisingly, the issue of planetary protection was considered a high priority for any of these studies.
One of the most comprehensive NASA reports on MSR was published in 1981 (49); its focus was maximum planetary protection. A highlight of the 134-page report was the suggestion that NASA build a CDC-like Earth-orbiting facility for the analysis of planetary sample returns under conditions of maximum protection against contamination, with minimal damage to the samples. The idea was to have an MSR spacecraft with 1 kg of samples aboard that could be analyzed in low Earth orbit by a qualified biohazard team using automated procedures, tissue cultures, and microassays.
The orbiting quarantine facility was envisioned to consist of five separate modules of different sizes, each with a different purpose: examination facility, power supply, habitation, supplies, and waste storage. Donald L. DeVincenzi, editor and codirector of the Antaeus Project (the study to design the facility ) describes it thus: Three barriers are envisioned to protect the biosphere from any putative extraterrestrial organisms: sealed biological containment cabinets within the laboratory module, the laboratory module itself, and the conditions of space surrounding the facility (49).
Another study published in 1988 for the Lunar and Planetary Institutes Workshop on Mars Sample Return called for a laboratory setup on the lunar surface (50). The authors cited the advantage of the Moon over an Earth-orbiting laboratory as having gravity, being a protective distance from the Earth, and taking advantage of the vacuum of space. Again, the emphasis of this report was to protect Earth from contamination.
By 1990 NASA had asked SSB once again to evaluate and change the sterilization requirements for outbound Mars spacecraft so that landers without life-detection instruments could be sent economically to Mars. SSB cooperated and published new guidelines in one of their 1992 booklets distributed by the National Research Council (3). Then in 1994, a resolution addressing these recommendations was adopted by COSPAR and incorporated into NASAs planetary protection policy. Mars Pathfinder would be the first of NASAs new landers not to be sterilized.
Klaus Biemann, emeritus professor of chemistry at the Massachusetts Institute of Technology and principal investigator for the Viking organic analysis instrument, said in 1997 that a complete Viking-class sterilization was necessary for all Mars landers because it had been calculated that contamination of Mars by terrestrial organisms living inside spacecraft could occur in as little as 25 to 100 years (44, p 207).
NASA began to change its view of MSR as well. With the adoption of its faster, better, cheaper slogan and philosophy in 1992 (51), the agency now sought economical ways for bringing samples back to Earth. NASA Administrator Daniel Goldin saw MSR as being a crowning achievement of the Mars exploration program and put it at the top of his priority list. He waited for any engineering reports that might satisfy the new NASA philosophy.
Passive Earth-entry or Earth-orbiting examination?
If everything went according to plan, Mitcheltrees passive Earth-entry MSR capsule would enter the Earths atmosphere in a ballistic fashion, having no reaction control rockets to maneuver and no parachute. It would require only atmospheric drag to slow it down, provided that it came in on the correct trajectory. At journeys end, the capsule would be expected to hit the ground in the Utah desert at 80 mph.
However, in 1999 Mitcheltree experienced his first major failure. He designed the atmospheric entry shells of the two small deep space microprobes attached to the aeroshell (an atmospheric entry shell) of the Mars Polar Lander spacecraft. The two microprobes are attached to the outside of the aeroshell and are released into the Martian atmosphere before the aero shell is jettisoned. When the Polar Lander was lost, there was no indication of the fate of the small probes either. Both probes were designed to survive the shock of impact with the Martian surface. Where are they? To this day, no one knows.
The 1999 investigation into the loss of the four NASA Mars probes revealed that the agencys faster, better, cheaper philosophy was failing (23). The independent investigation team headed by Thomas Young concluded that NASA needed to re-evaluate its entire Mars exploration program, and the MSR program was placed on hold until 2011.
In 1980, Henry S. F. Cooper wrote an excellent book on the Viking Landers (53), in which Sagan voiced his views on MSR capsule concepts that would directly enter Earths atmosphere. Sagan challenged JPL engineers, stating that if they were confident enough in a design for a safe Mars sample return container, a prototype should be tested by placing living anthrax germs inside. Sagan said the capsule should be launched into space and returned on the same trajectory that a real MSR mission would use. Then, if the capsule survived the atmospheric entry and impact on the Earths surface, it could be examined inside a sample-receiving laboratory. Sagan considered this a test run before Martian samples were returned. His point was that if we could not handle the MSR procedure using terrestrial anthrax, we might not be able to bring back Martian soil safely either.
Not surprisingly, none of the JPL engineers took Sagans challenge seriously and said that they were appalled by his idea. However discomforting the challenge was, it illustrated how seriously the issue of back-contamination should be taken.
Several other notable scientists have said that they do not like the idea of a direct Earth-entry approach for MSR. Carl Woese, a Nobel Prizenominated biophysicist at the University of Illinois, UrbanaChampaign, who discovered the third domain of life called Archaea, says,
Chandra Wickramasinghe, professor of astronomy and mathematics at Cardiff University, Wales, is a leading proponent of the panspermia hypothesis, which holds that life on Earth was seeded from space, and has studied comets and interplanetary dust since the early 1970s. He said,
Former Viking biology team member Levin says, I fear that, even if a safe MSR container could be made and brought to Earth, there is a good probability that some of the sample would escape from the secure lab where the container would be opened (56). Levin also questions the scientific benefit of MSR and says, How could we get a living sample to survive the 9- or 10-month trip without knowing what any Martian microorganisms present in the sample need in the way of substrates, water, temperature, atmosphere, environmental cycling, etc.? Would we ever know whether it started out alive or dead? He recommends a 10-phase approach to MSR based on his experience working with the Lunar and Planetary Institute and COSPAR (see box, Safe methods for MSR) (56).
David A. Paige, associate professor of planetary science at the University of California, Los Angeles, was a co-investigator for the Mars Polar Lander Mars Volatiles and Climate Surveyor experiment. Paige submitted a scientific abstract to the Lunar and Planetary Institute Conference held in July 2000, in which he questioned the current wisdom of a faster, better, cheaper approach to MSR. In his abstract, he wrote (57),
Looking back over the history of the planetary protection program, it is clear that, although the participants started out with the purest of intentions, many decisions based on erroneous assumptions have led to almost the exact opposite of what the program tried to accomplish. Established to protect the Moon, Earth, and planets from biological contamination, the planetary protection program should not be manipulated to achieve the goal of producing more economically designed spacecraft. Because the risk of back-contamination from celestial sample return missions is an issue that effects everyone living on our planet, the decision process on how to proceed with such missions should not belong to one nation alone.
Note: All of the URLs were accessed in June 2001.
Barry E. DiGregorio is a research associate of the Cardiff Centre for Astrobiology in Wales and is the founder and executive director for the International Committee Against Mars Sample Return (email@example.com; www.icamsr.org). His book, Mars: The Living Planet, with co-authors G. V. Levin and P. A. Straat, was released in 1997, and reexamined the Viking biology evidence.