This story starts with an auspicious day in the annals of spaceflight – Wednesday, 19 November 1969, during the unforgettable era of NASA’s lunar landings. A few minutes before 7 am GMT on that day, Apollo 12 became the second human-occupied spacecraft to touch down safely on the surface of the Moon, after the historic landing of Apollo 11 the previous July. Of course, I’m quoting Greenwich Mean Time here, rather than Houston time, because that was the time zone in which I was glued to a blurry TV screen in wintry Scotland, watching every last detail of the astronauts’ extra-vehicular activity – or moon-walk, to you and me.
Despite my attentiveness, I managed to miss one of the most memorable events of the mission. The pinpoint landing had brought Apollo 12’s astronauts Pete Conrad and Alan Bean to within walking distance of a robotic spacecraft that had been sitting on the Moon’s surface for two and a half years. Surveyor 3 was part of NASA’s intensive preparation for the Apollo flights back in April 1967, and mission scientists wanted to investigate the effect of long-term exposure to the harsh lunar environment. What would the near-complete vacuum, monthly temperature range of –150 °C to +120 °C, and relentless bombardment by subatomic particles do to the fragile components of a spacecraft?
Conrad and Bean duly removed various bits and pieces from Surveyor, including its TV camera, and packed them up for the return to Earth. Actually, that sounds a bit more meticulous than it really was, because the camera was stuffed into a nylon duffle bag rather than one of the special airtight boxes normally reserved for lunar samples. Nevertheless, it made a safe splashdown in the South Pacific Ocean on 24 November 1969 in Apollo 12’s Command Module, along with Conrad, Bean and pilot Dick Gordon.
For the camera, though, that was just the start of the story. Scientists examining it after its return were surprised to find spores of a common bacterium, Streptococcus mitis, residing in its insulating foam. These little critters are found in the mouths and throats of humans. When the spores were cultured, they proved to be perfectly viable, leading to the remarkable conclusion that microbes deposited on the camera before lift-off – perhaps by someone sneezing on it – had survived on the Moon for more than two years. This conclusion, published in the academic literature in 1971, was regarded by Pete Conrad as ‘the most significant thing that we ever found on the whole…Moon’.
More recently, however, the claim has been disputed, since there is evidence that the contamination may have occurred after the camera left the Moon. A ‘breach of sterile procedure’ in the lab has been cited, as well as the possibility that the camera was contaminated while it was in its duffle bag in the Apollo 12 Command Module – in close proximity to the three returning astronauts. It does seem hard to imagine that none of them sneezed during the three-day trip back to Earth. On the other hand, there are aspects of the culturing results that suggest the bacteria were present on the Moon. For example, they took some time to spring into life, and they were only found within the insulating foam rather than on its surface – both of which would be unlikely to happen with later contamination. The bottom line is that we will probably never know the truth, but the episode did highlight the possibility that microbes could survive the rigours of space.
SINCE THE APOLLO 12 EPISODE, THERE HAVE BEEN OTHER examples of space-hardy microbes coming back to Earth with a decided zest for life. Perhaps the most memorable is the 553 days endured on the outside of the International Space Station by a batch of terribly British microbes from Beer. (Not the beer you drink, but a fishing village in Devon that got its unusual name from the Old English word for a grove of trees – bearu.) A few lumps of rock from the cliffs of Beer – complete with their microbial inhabitants – were mounted onto the outside of the space station in 2008. Eighteen months later, they were returned to Earth and examined. Why?
Scientists from the Open University in the United Kingdom, who were responsible for the experiment, wanted to test the effects of the space environment on a completely random sample of microbes, rather than preselecting the ones that we already know are pretty hardy. Those are usually known as extremophiles, because of their lust for extremes, and include organisms that can survive below freezing point, as well as ones that don’t mind being in boiling water. The Beer microbes weren’t extremophiles; they were just ordinary workaday microbes, constituting several different communities of micro-organisms. What the scientists wanted to know was which of these are sufficiently hardy in space to be useful to future space explorers in naturally recycling waste products in life-support systems. And now they know – because a sizeable fraction of the population survived to tell the tale back on Earth. It could be a very useful discovery, and certainly bolsters our understanding of the way single-celled organisms react to the harshness of space.
But – amazingly – there is at least one animal species that can survive the vacuum and radiation environment of space. Meet the tardigrade, an eight-legged animal that is also known as a water-bear – although with a maximum length of a millimetre, this is not a bear you’ll ever have to flee screaming from. Some of the thousand or so known species of tardigrade are able to survive incredible extremes of temperature, pressure and radiation: they are simply the toughest animals on the planet. They survive by shutting down their metabolism and curling up into a dehydrated ball. On Earth, specimens have endured for decades in this state, before being successfully rehydrated in water. The space tardigrades, lofted on 14 September 2007 in a European experiment called Biopan-6, survived the rigours of open space for ten days. Or, at least, some of them did. Survival rates were not particularly high, but several of the returning spacefarers did go on to produce perfectly normal offspring. As the prestigious scientific journal Nature cheerfully reported, for these creatures, space suits are optional.
The evident space-hardiness of common earthly microbes raises all kinds of interesting possibilities. For example, did microbial life come to Earth from elsewhere in the Solar System? Panspermia (‘seeds everywhere’) is an old idea reinstated several decades ago by the iconoclastic British astronomer Fred Hoyle, in collaboration with Chandra Wickramasinghe, now of the University of Buckingham. These eminent scientists developed a theory of cometary panspermia in which rudimentary life-forms are common throughout the Universe, and travel through space from one planet to another. The idea remains wildly controversial, but we do now know that carbon-containing molecules important for life processes were present in the cloud of gas and dust from which the Solar System formed 4.6 billion years ago. They are preserved today in comets, and suggest that perhaps the building blocks of life, at least, came from space. As unlikely as it might be, the cometary panspermia idea cannot yet be ruled out.
THE TWO DIFFERENT SITUATIONS OF EARTHLY MICROBES being transported to another celestial body (as might have occurred with Surveyor 3) and the transfer of extraterrestrial organisms (if they exist) back to Earth have special significance in the field of Solar System exploration. They are referred to, respectively, as ‘forward contamination’ and ‘back contamination’ – rather unimaginative terms that nevertheless highlight the need to take care whenever space missions are being planned.
Why are they so important? Forward contamination could, conceivably, introduce earthly microbes into an extraterrestrial environment where they could flourish to the detriment of any hypothetical indigenous organisms. Maybe the results of that contamination wouldn’t be apparent for a few million years, but clearly it would still be a Bad Thing. And the consequences of back contamination are just as unpredictable. Okay, maybe a few malicious Martian microbes accidentally brought home on a future sample-return mission wouldn’t wipe out all life on Earth, but we simply don’t know.
The bottom line is that the space world takes these possibilities extremely seriously – and has for years. The issue of potential planetary (or lunar) contamination was first raised as long ago as 1956 at an Astronautical Federation Congress. Not only did this predate the pioneering orbital flight of Sputnik 1 in 1957, but it occurred long before there was any real evidence that microbes could survive in space. Then, in 1967, the United Nations Outer Space Treaty was ratified, providing the foundations of space law. With great foresight, it incorporated a set of so-called planetary protection rules that are still in use today.
Article IX of the Treaty provides the legal basis. It states that: ‘Parties to the Treaty shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose.’ Bold words, but increasingly significant ones in an era in which we know contamination is a real possibility.
The planetary protection rules are now managed by the multinational Committee on Space Research (COSPAR), a large group of scientists, which meets every two years. COSPAR defines different categories of missions, ranging from Category I (any mission to locations not of direct interest for chemical evolution or the origin of life, such as the Sun, or Mercury) to Category V, which is concerned with sample-return missions that could bring extraterrestrial biological materials to Earth.
The other categories defined under the planetary protection rules sit fairly logically between these limits. Category III is for fly-by and orbiter missions to ‘locations of significant interest for the chemical evolution and/or the origin of life’, with a significant chance that contamination could compromise future investigations. Such locations include Mars, of course, and some places in the outer Solar System such as Jupiter’s moon Europa, and Saturn’s moon Enceladus. Finally, and most importantly, Category IV is for spacecraft that will actually land in such locations.
BECAUSE MARS IS OF SUCH GREAT INTEREST IN THE SEARCH for life beyond Earth, there are special rules that apply to the red planet. In fact, COSPAR defines Mars Special Regions as those within which terrestrial organisms could readily propagate, or those that are thought to be more likely to host Martian life-forms.
In particular, any region of Mars in which liquid water could occasionally occur (and there are a few, despite the planet’s subzero average temperature) are classified as Special Regions. They include a 20-kilometre-wide lake recently discovered under the ice of the southern polar cap. These places are subject to the most stringent planetary protection rules, the so-called Category IVc. This states that sterilisation must be achieved to a maximum of 30 spores per spacecraft. To a non-biologist like me, that sounds like an ultra-low level of contamination, and, indeed, is known as the ‘Viking post-sterilisation biological burden’, because NASA’s two Viking landers of 1976 were sterilised to this level. It was achieved by baking each entire spacecraft at a temperature of nearly 112 °C, and then enclosing it in a pressurised cocoon known as a ‘bioshield’ to prevent any biological contamination until the spacecraft had left Earth’s atmosphere. The bottom line, though, is that sterilisation incurred a cost of approximately US$100 million out of a total mission cost of US$1 billion.
This level of expense on lander missions to Mars has led some astrobiologists to argue that the Category IVc rules should be relaxed. One prominent scientist (Ryan Anderson, who works on NASA’s Curiosity rover) has remarked that it’s paradoxical that the most habitable parts of Mars are the toughest places to send new spacecraft to. In any case, it’s possible that Martian organisms might already have found their way to Earth on meteorites that are known to have travelled between the two planets – a variation of Hoyle and Wickramasinghe’s panspermia hypothesis. Some scientists have even suggested that terrestrial life actually originated on Mars perhaps four billion years ago. Others who support relaxing the rules claim that there will be no problem in determining the origin of any Martian microbes we may find, once a robotic DNA sequencer has been sent to the planet in some future mission.
Most scientists, however, support the status quo, arguing that any contamination of Mars makes the task of finding putative Martian life more difficult. Significantly, they also note that if such life is contaminated by terrestrial organisms, it’s a one-way process – there’s no possible return to the pristine state.
Mars, of course, is also a target for human exploration, which will bring its own contamination issues. NASA currently plans to have astronauts walking on the surface of the red planet in the mid-2030s, a date that is more likely to be delayed than brought forward, due to the enormous technical challenges of such a mission.
I recently had the opportunity of asking two NASA luminaries – an astronaut and an astrobiologist – how the Category IV rules will be applied when crewed missions go to Mars, since complete sterilisation is clearly impossible. Despite the fact that these individuals worked in completely different areas of NASA, and were on opposite sides of the globe when I spoke with them, their answers were remarkably similar. There was an underlying assumption that robotic missions between now and the crewed landing will fail to find any signs of life. If that turns out to be the case, then perhaps the Category IV rules could be loosened a little, to allow microbe-riddled humans to visit. But the bigger surprise came when I asked what might happen if living organisms are found there. Both lowered their voices and adopted a similarly conspiratorial air to tell me that ‘Well, the planetary protection rules will probably be quietly dumped.’ It remains to be seen just how this will play out down the track.
MARS IS NOT THE ONLY PLACE WHERE STRINGENT ANTI-contamination procedures are applied. Other hot spots in the search for life in the Solar System are some of the moons of the giant planets. We know that Jupiter’s moons Europa, Callisto and Ganymede have a rocky core overlain by a global ocean of liquid water, which is itself overlain by a thick crust of ice. Saturn’s moons Titan, Enceladus and Dione are thought to have a similar structure.
Intriguingly, both Europa and Enceladus have spectacular geysers of ice crystals erupting from their south polar regions, offering free samples of the subsurface ocean to any properly equipped spacecraft that can fly through them. While the NASA/ ESA/ASI Cassini spacecraft was exploring Enceladus, we found chemical evidence of hydrothermal vents on the floor of the Saturnian moon’s sub-ice ocean. This is highly suggestive, since similar active vents on the infant Earth’s ocean floor are thought to be one of the places where life originated on our own planet.
Arguably, Saturn’s moon Titan has even more to offer. As well as a liquid water ocean underlying its hard-as-rock ice surface, it has frigid seas and lakes of liquid hydrocarbons on top. They are effectively seas of liquid natural gas, and they are in equilibrium with Titan’s thick atmosphere, replenished from time to time by heavy showers of oily rain. Moreover, the seas and lakes could harbour life-forms based not on water (as all life on Earth is), but on the constituent chemicals of natural gas – methane and ethane. We’ll visit this extraordinary world again in chapter 13.
Any living organisms in the sub-ice oceans of Europa and Enceladus, or in the hydrocarbon seas of Titan, are likely to have originated quite independently of life on Earth. The distances involved are huge, and, in Titan’s case, any hydrocarbon-based life would be totally different from earthly life. Thus there is much at stake in risking contamination. For this reason, NASA’s Galileo probe, which studied Jupiter in the early 2000s, was made to burn up in the atmosphere of the giant planet in 2003 to avoid any possible contamination of its moons. Likewise, the highly successful Cassini probe was intentionally destroyed by having it enter Saturn’s atmosphere on 15 September 2017.
The future exploration of the Solar System’s ice moons without contaminating their surfaces presents particular problems to planetary scientists. Perhaps that’s why most currently proposed missions stick to exploration from orbit, rather than risking a landing. Hence JUICE (Jupiter Icy Moons Explorer) is ESA’s spacecraft to Europa, Callisto and Ganymede, scheduled to begin an eight-year journey in 2022. Its sojourn there will allow mission scientists to take a close-up look at those worlds with the possibility of sub-ice life-forms firmly in mind.
NASA has plans, too. ELF (Enceladus Life Finder) will, if eventually approved by NASA, make multiple flights through the ice fountains of the Saturnian moon looking for suggestive molecular signatures. Whereas ELF’s predecessor, Cassini, could detect inorganic molecules like hydrogen and silicates, ELF will be looking for biological precursors such as nucleic acids, amino acids and lipids. And some life-related molecules can be detected through the submillimetre radio waves they emit, which has led to another NASA proposal called SELFI – the Submillimetre Enceladus Life Fundamentals Instrument.
As with all similar missions, JUICE, ELF and SELFI would require Category III sterilisation, prior to their long journeys to the gas giants. And their travels would almost certainly end with suicidal plunges into the respective parent planets, to avoid contaminating the moons.