The search for life in the Universe is not an easy pursuit. There is such a vast repository of information about the many wondrous forms life has taken over the history of the Earth that it is theoretically possible (perhaps almost inevitable) that life on another world could take on any number of guises as it would be affected by and respond to its particular environment. At the very least we have some educated ideas about what to look for and what types of environment on the Earth they like to inhabit. Now we just need to figure out where to look in the cosmos. Planets, moons, asteroids and comets are all options but they are not small targets. The search needs to be narrowed down to particular environments or geological features that hold the greatest chance of supporting any life forms. The Earth, as with life, has an incredible selection of sites from which we can choose and learn from.
A Home Away From Home
Ultimately, the search for life in the Universe begins by looking for localities where all the conditions needed for life to exist can be found together, in the exact same spot. This is called the search for habitability. Habitability requires a set of physical and chemical conditions, such as the availability of water, energy and carbon, that if found would first give life the opportunity to become established; and second, sustain it and allow it to flourish over the longer term. Unsurprisingly, any location that displays such must-have parameters is termed a habitable environment and it is these areas astrobiologists are most interested in finding. One such environment is the Earth itself: a perfect example of a globally habitable world, with billions of smaller niche habitable environments present within it.
The Earth is unique and incredibly special. More than 70 per cent of its surface is covered in liquid water – some might say the most important prerequisite for life. Unlike any other world in the Solar System, it is able to support this liquid water because of its thick insulating atmosphere with a greenhouse effect that prevents wild swings of temperature, and keeps the water from globally freezing or boiling away – the greenhouse effect is not always a negative thing. The atmosphere both generates an oxygen-rich ozone layer that absorbs harmful UV radiation from the Sun, and serves as a barrier to protect the fragile life below. The Earth also has a global magnetic field, something that other planets such as Mars have long since lost. The magnetic field protects the planet’s atmospheric bodyguard from being blown away by solar winds, and additionally provides another line of defence against cell-damaging cosmic radiation. Finally, the Earth is a rocky or terrestrial planet built mostly of silica-rich rock instead of metal or gas. This allows plate tectonics to be sustained, with this process thought to be crucial for the long-term sustainability of Earth’s climate and life. In the search for habitable environments on a global scale, astrobiologists are trying to locate a world similar to the Earth – looking to rocky terrestrial planets and moons with some element of an atmosphere and magnetic field.
The search for life and habitable environments so far has focused on finding a world with water, a source of energy (to power life) and a source of organic carbon. This is, of course, because all terrestrial organisms observed so far are reliant on liquid water for their survival, although fluids other than water have been found on other planets and these may have the potential to support life (see Chapter 8). Life and therefore life-friendly environments also need energy to drive and sustain metabolic processes and encourage growth and reproduction. Ultimately, this energy comes from the Sun, even though it is 149,600,000km (almost 93 million miles) away, and is readily available to any organism or habitat located on or close to the surface of a planet or moon. If conditions do not allow for life on the surface, however, then habitats may be found lurking underground where chemical energy can be used as a replacement for solar. Chemical energy sees microbes break complex compounds into simpler ones to obtain just a small amount of energy from the chemical reaction. Finally, a habitable environment needs organic carbon-based molecules – the building blocks for life. These are not necessarily biological (as they can exist without being part of or created by an organism), but life cannot exist without them. Luckily for life, they are found in every corner of the known Universe, within meteorites that have pelted planets and moons for billions of years, and in comets travelling through our Solar System and throughout the interstellar medium. This is extremely important, as the universal nature of carbon molecules and their delivery across the cosmos greatly increases the habitability potential of millions of worlds across our Galaxy and beyond.
This appears a fairly simple formula – locate carbon, water and energy on a terrestrial world or even within a single palm-sized rock – and you will find a habitable environment flourishing with living organisms. However, nothing is ever that simple. First, think about the relative nature of the term habitable and how it changes depending upon the type of life you are looking for. A boiling acidic hot spring in Yellowstone National Park might be the perfect home for a heat-loving bacterium yet it would be fatal for a human. In fact, no world yet discovered in the Universe would be entirely suitable or even remotely habitable for a human being without extensive artificial help. Most have such extreme environmental conditions that it is almost inconceivable that any form of terrestrial-like biology could exist there. Second, environments are not static as they fluctuate and change over time. The conditions present during the origin and first appearance of life are not necessarily those needed to maintain the life forms that are created, or even capable of supporting their long-term survival. Mars is a good example of this. Mars once had a warmer, wetter, life-friendly environment, but today is an inhospitable frozen wasteland. Life that might have arisen early in Mars’ history may not have been able to survive as its environment degraded and froze. Third, science is making some mighty assumptions. It assumes that any location that has, or has had in the past, the ability to support life will definitely have contained it. It has to be considered, however, that there are environments where there is no life in spite of there being ideal conditions for it. These are called uninhabited habitats and can range in size from a single blade of grass to an entire planet. At present, astrobiologists are searching the Universe for environments that have the potential to support life either now or in the past, but may not prove to be inhabited. We are not just searching for conditions that we as humans would thrive in, of course, since this would be a very short and ultimately futile venture. Rather, we are hunting for environments that push the limits of biological survival to its very extremes, acutely aware that life has the resilience to become established and to survive in some incredibly unusual places.
Alien Environments All Over the Place
When searching for habitable environments, it goes without saying that we have to think about the type of life that might be able to live in them. So in locating potential habitats in the Solar System, it is important to consider the physical limits of life and the constraints this places on a suitable home. The main defining factors affecting cell-based life are temperature, acidity and salinity. Excitingly, certain terrestrial life would be perfectly content in extremes of these conditions and, even more excitingly, such environments are common on extraterrestrial worlds. Wonderfully, these can also be found in hundreds of pockets across the Earth that we can visit.
Alien worlds and even alien life forms can essentially be studied right here on our doorstep. These places are called analogue environments, as their biology, geology, chemistry or physical appearance (or a combination of all four) mimic an environment that was once found or currently exists on another planetary body. At present, terrestrial analogue studies are the best way for scientists to examine the habitability potential of alien environments, and they are able to help us design and develop tools and technologies for their exploration. No analogue environment is ever a perfect replica of a location or the conditions present on another world, however. For example, there is nowhere on Earth that naturally mimics the different forces of gravity found on other planets and moons, nor usually their atmospheres. The analogues we have do, however, display a number of extraterrestrial features that can be compared with those on other planetary bodies, to try to understand them better.
Some habitable analogue environments are considered more significant for exploration than others. These are based upon direct observations of their existence on other worlds through space missions, by orbiting satellites and from Earth-based telescopes. For example, we have data and images to certify that Mars has volcanoes, is composed of volcanic rocks similar to those found in Iceland or Hawaii, is covered in impact craters and has ice caps and glaciers, which are all possible sites where life might be hiding, as it is on Earth. This means that volcanoes, basalts, impact craters and ice-covered habitats on Earth are all extremely important analogues, albeit never perfect ones.
Other analogues are based on theories and circumstantial evidence. These are identified by indirect or highly suggestive evidence of their existence, which is still awaiting data to provide confirmation of the theory. Research into the conditions and biota within Lake Vostok in Antarctica is a perfect example, as it is used by scientists to explore the habitability potential of a brine ocean that may lie under the icy crust of the Galilean moon Europa (it was one of three Jupiter satellites discovered by Galileo). There is abundant indirect evidence to support the existence of this ocean and its possible composition, but we have yet to prove that it is there. Another example is the lack of flowing rivers of liquid water across the surface of Mars. Evidence of past water action and seasonally stable liquid water is globally present in the form of features that look identical to dried-up river channels, deltas, flood plains, lakes and seas on the Earth. Such indicators would include dark recurring slope lineae (RSLs) and specific minerals that can only form in the presence of water. While the connection between a river channel and liquid water seems almost undeniable, direct evidence for permanently flowing liquid water is still hard to find.
Finally, we have the analogues that are not backed up by any scientific evidence whatsoever. Unconfirmed UFO sightings aside, there is no physical evidence that life exists anywhere other than on Earth; nonetheless, terrestrial analogues for a mythical alien life form are incredibly important and are integral to the development of scenarios for planetary habitability. The premise that because life is found on Earth in a specific niche environment, and that a similar environment present on another planet could therefore be a habitable environment for life, is perhaps a reasonable one to maintain. The existence on Earth of extremophiles, extreme environment-loving organisms described in the next chapter, is an example of this life. A combination of this analogue research with laboratory experiments and data collected from the Earth itself and other planets and moons is allowing astrobiologists to develop some reasonably educated guesses as to where to look for life.
Extreme Living
The more we explore other worlds, the more we see that their environments are extreme in comparison to those on the Earth. As such, when looking for places on our planet where we can study life forms that might be able to thrive elsewhere, we turn to those localities that depart from our norms, that house conditions where we as humans would struggle to survive without help. These include hot springs and geysers, deep-sea hydrothermal vents, hypersaline environments such as salt flats, deserts both hot and cold, glacial ice, evaporites (sediments precipitated when water evaporates), and even the atmosphere itself. Instead of describing each type of environment, it is more meaningful to show how certain places on the Earth function as analogue sites for some of the most astrobiologically attractive worlds in the Solar System.
A Home Fit For a Martian
In the search for alien life, one instantly thinks about the possibility of life on Mars. This single Solar System body is of more public and scientific interest than any other because it is the most Earth-like planet we have seen and have fairly good access to, not that you would think so. I believe we will one day find life there, or at the very least evidence of it having once existed. Owing to their similarities, the Earth boasts hundreds of analogue environments that mimic closely not just the Mars of today but also the various environmental conditions it has experienced over its 4.5-billion-year existence: as it changed from a warm and wet oasis to a cold and arid desert. One of the main steps in assessing the habitability of Mars, therefore, is to study a similar range of Earth-based environments, understand what makes them habitable and what life forms can exist in them, and unearth their level of dependency on water – this is what I do! Needless to say, establishing the presence of life on Mars is a huge challenge, because as the availability of liquid water on the surface of Mars has fallen, so too has the planet’s habitability potential and sadly the chances of it hosting life today.
The early years of Mars could be considered an Eden for life, as liquid water was abundantly available on its surface, temperatures would have been higher and its thicker atmosphere would have protected the planet. Rio Tinto, located in southwest Spain, is a fascinating analogue for habitable environments and the life forms that may have existed on Mars during this period of its history. Rio Tinto is a natural acid-rock drainage system, flowing with blood-red, iron-rich waters that teem with life. The pH of the river is an extremely acidic 2.3 (the same acidity as if it were flowing with lemon juice), directly created by its microbial community. Organisms here are chemoautotrophs that derive energy from chemical reactions between inorganic compounds such as iron (a process called chemolithotrophy), with iron-oxidising, acid-tolerant filamentous bacteria such as Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans dominating it. Rusty iron-rich terraces of rock have been forming along the banks of the river for more than 2 million years, and these have trapped bacteria and other microscopic organisms inside them. This site is therefore a wonderful natural laboratory where living organisms can be observed and then matched to their fossil ancestors in the surrounding rocks. This naturally gives rise to the questions ‘Might these or similar types of organism once have lived in ancient rivers on Mars?’ and ‘Could we find evidence of organisms like these preserved in rocks on Mars?’
Time has taken its toll, however, and the Mars of today has lost most of its surface liquid water, has begun to rust and has become a dusty frozen desert. The tendency is to imagine a desert to be a golden, sandy, dry landscape baked by the intense heat of the Sun, but this is not the case on Mars. For starters, it is extremely cold. The most Mars-like environment on the Earth today is actually found at the South Pole, in the vast white Antarctic desert and the spectacular Antarctic Dry Valleys. These are the coldest and driest regions on the Earth, and mimic the harsh arid conditions now prevalent on Mars. Temperatures in the Valleys plummet to –40°C (–40°F), which, combined with the highest UV-B radiation levels on the planet and no source of liquid water, create extreme environmental stresses for life. On first inspection they appear completely barren – what could possibly live in these conditions? Yet within glacial ice and sub-surface rocks, life flourishes. Buried under the surface are warmer wet niches that create microscopic habitats for algae, fungi, nematode worms and tardigrades (the water bears whom we will meet in the next chapter). These endolithic (within rock) realms give hope that similar habitable environments could persist on Mars today and may have provided a refuge for life when conditions on its surface started to deteriorate.
Observations and investigations on Mars have driven the need for greater analogue research which has in turn allowed for the discovery of more habitable environments, not just on Mars but also throughout the Universe. This has meant the discovery of hundreds of extreme sites on Earth that have drawn attention to new, potentially habitable patches of planets that had been previously overlooked and are even providing an insight into the origins of life itself.
Floating above Venus
On the opposite side of Earth is an unlikely target in the search for habitable environments in the Solar System: its sister planet, Venus. It is common knowledge that Venus is pretty inhospitable for life, now known to resemble our clichéd imaginings of hell rather than a lush tropical paradise. It has the highest surface temperature (an unimaginable 460°C/860°F) of any planet in the Solar System (including that of Mercury, even though this planet is nearer to the Sun), therefore liquid water is an impossibility on its surface. It also has a toxic, unbreathable, carbon-dioxide-rich atmosphere and clouds dripping with sulphuric acid. The Venusian surface is not a habitable environment for life as we know it. High above the sweltering ground, however, is a potential habitable zone where temperatures lie between 0°C (32°F) and 120°C (248°F), and water vapour is available. The lower and middle cloud decks within this high-altitude region may therefore support an aerial biosphere. These clouds offer long-lasting droplets of water, although they are highly acidic and rich with dissolved hydrogen sulphide. Energy sources are available through chemical reactions such as sulphate reduction, or indeed photosynthesis, as happens in plants on Earth. Acid-loving organisms found in hot acidic waters on Earth, such as the bacterium Acidianus infernus, which grows at 88°C (190°F) and in acidic conditions down to pH0.5, are key analogue organisms for Venus. In addition, there is a precedent set for floating life, from Earth. Evidence shows that bacteria may be actively metabolising, and even reproducing, in clouds way above the Earth. So why not on Venus?
So, although most similar to Earth in composition and distance from the Sun, the terrestrial planets of the Solar System do not present much in the way of enticing habitable environments on their surfaces today. As we’ve seen, our strongest hopes lie with Mars that it may contain preserved biosignatures of past life forms within surface rocks and minerals, or active communities enduring deep in the sub-surface, while Venus may offer habitable conditions high in its atmosphere, with life floating within the clouds. As such, the traditional formulation of the habitable or Goldilocks zone around a star is perhaps too restrictive as we start to understand just how far life can go to survive. A planet or moon may not need to be around 1AU (the distance from the Earth to the Sun) from its host star to house environments suitable for life. Suddenly, habitable environments existing throughout the Solar System and in other solar systems across the Universe are becoming a possibility.
The Jovian Moons
Surprisingly, some of the likeliest candidates for life-hosting habitable environments in the Solar System are to be found on moons. These are becoming more important in the search for life than the planets they orbit. The most promising are found among more than 210 frozen natural satellites orbiting the gas giant worlds of Jupiter and Saturn. Recent space missions have revealed many of these moons to be geologically active bodies, with volcanoes spewing ice as well as molten lava, geysers the size of whole countries on Earth, impact craters in their thousands, and vast channels and valley networks. Excitingly, these moons are displaying a wealth of potentially habitable environments. The problem for astrobiologists, however, is that although these geological features could be housing alien life, these moons are so incredibly extreme compared to the Earth that suitable terrestrial analogues of their environments and potential life forms are fewer in number. In addition, our knowledge of the conditions actually present on these icy moons is mainly based on inferences rather than on deductions supported by definitive data. There is a great deal more educated guesswork and imagination needed for detecting life on these worlds than those closer to home … but that is half the fun.
The Oceans of Europa
One of the most important moons in the search for habitable environments in the Solar System is Europa. This moon is actually built like the Earth and the other terrestrial planets, mainly of silicate rock, but instead of a liquid it is covered in a smooth sheet of water ice that is many kilometres thick. This icy shell is believed to hide a secret ocean of briny water beneath. You might think that this ocean is fairly small, but in fact Europa is only slightly smaller than the Moon, and the volume of its ocean is estimated to be 3 × 1018 cubic metres: this is twice the volume of all Earth’s oceans put together. Europa’s frozen temperatures (–220°C/–364°F at the poles), resulting from its huge distance from the Sun, provide an extreme environmental challenge for life, although a number of ice-dominated habitats on the Earth could supply analogues for liveable environments here. Most significantly, there is an important analogue site for the ocean itself. Lake Tirez in Spain contains very salty, sulphate-rich waters that may be similar to Europa’s salty interior. In addition, salt-loving organisms growing in these Spanish waters provide an insight into how a habitable environment could exist on or beneath Europa’s surface.
The Earth also has analogues for the capability of life to survive buried under a shield of ice. Liquid water lakes hidden up to 3.2km (2 miles) beneath the ice sheets of Antarctica, such as Lakes Vostok, Ellsworth, Bonney and Vida, are thought to be similar to Europa’s salty sub-surface ocean. Core samples taken from the ice surrounding Lake Vostok in 2012 revealed DNA from an estimated 3,507 organisms. Similar under-ice realms on Europa are believed to have the best potential to host microbial ecosystems in the entire Solar System, bar the Earth. A habitable sea-floor environment may also occur on Europa. There are extensive communities in the dark, cold, high-pressure environment of the Earth’s ocean floor, particularly around deep-sea hydrothermal vent fields such as Lost City on the Mid-Atlantic Ridge and the Mariana Trench in the Pacific Ocean. It is important to investigate these analogues, even if at present any actual search for a deep-ocean biosphere on Europa is impossible. We must instead settle for hunting for indicators of their activity and the presence of this ocean on the surface ice.
The Fountains of Enceladus
One of Saturn’s many moons, Enceladus, has attracted great interest thanks to dramatic images of powerful icy jets currently erupting from more than 100 cryovolcanoes near its southern polar region. These geysers explode over 645km (400 miles) into space (about the distance from London to Paris!). Measurements taken of these gigantic plumes by the Cassini Orbiter have found water gas, simple organic carbon-based molecules and volatiles such as nitrogen and methane to be present. These life-essential compounds must have come from source regions far inside the moon, from the area that feeds the jets. The assumption, therefore, is that organic molecules used by and needed for life are present deep within Enceladus. The young volcanic landscapes of Iceland provide a good analogue for these plumes. Iceland is covered in geysers and hot springs, cracks in the Earth’s surface where near-boiling water erupts in spectacular fountains, blanketing the ground with mineral- and nutrient-rich waters. Surrounding the hot springs of Iceland are heat- and acid-loving bacteria that form mats of microorganisms, creeping across the surface and thriving in the hot acidic waters. Such terrestrial features are miniscule versions of the gargantuan jets seen erupting from Enceladus but they can inform us about the processes involved in their formation and their ability to create and support a habitable environment.
The Lakes of Titan
One place that greatly resembles the Earth in appearance is another of Saturn’s moons, Titan. It is the only moon in the Solar System known to possess a thick atmosphere, and a substantial one at that, and there is evidence that it has Earth-like lakes and seas, rivers with running fluids, sand dunes and weather systems. One lake, Kraken Mare, is three times larger than Lake Michigan-Huron. Titan’s lakes are crucial targets in the search for habitable environments and life on this moon. However, surface temperatures of around –179°C (–290°F) suggests that the liquid bodies on the surface could not be composed of water, but are more likely to be a mixture of methane and ethane – hydrocarbons. Life might be present here within a range of habitats, from the liquid hydrocarbon lakes on the surface to great depths into the sub-surface, creating a potential biosphere volume double that of the Earth. Owing to the very different chemistries of the liquids on Titan, we can only speculate about what life might be like there, and as such there are very few analogues for this world. The best known one is Pitch Lake, on the island of Trinidad – a natural liquid hydrocarbon or asphalt lake just like those found on Titan, albeit a far smaller version. A unique microbial community is found here, one that includes archaea and bacteria that are actually anaerobic – they are able to live without any oxygen at all. The natural asphalt-soil seeps of the Rancho La Brea Tar Pits in California and the Alaskan Oil Field petroleum reservoirs are also potential habitable analogue sites for Titan.
The Signatures of Life
Once we find these habitable environments on Mars, Europa or Titan, and can send either robotic landers to hunt them down or even, one day, humans to investigate them, what will we look for? It is highly unlikely that organisms will be caught scurrying across the surface of Mars or swimming in the lakes of Titan (although never say never). We will instead be searching for the evidence that life leaves behind. These signs of life or biosignatures will be recognisable as they will be composed to a variable extent of carbon.
A biosignature is any substance, be it an element, isotope or molecule, that provides scientific proof of past or present life. The usefulness of a particular biosignature is determined not only by the probability of life creating it, but also by the improbability of non-biological processes producing it. Life processes may produce a range of biosignatures such as nucleic acids (the building blocks of DNA), lipids (fats), proteins and various morphological or visual features that are detectable in rocks and sediments (think dinosaur bones, trilobites, ammonites and any other kind of fossil). In addition, life interacts with its surroundings; for example, it can cause changes through chemical reactions with rocks and fluids, altering their chemistry or creating new materials. These processes will leave features in the geological record that indicate that life was once present.
Biosignatures are commonly used in geochemistry, geobiology and geomicrobiology to determine whether living organisms are or were once present within samples. Now they are applied to astrobiological exploration, founded upon the premise that biosignatures encountered in space will either mimic those found on Earth or be undeniably recognisable as originating from extraterrestrial life. An example of such biosignatures might be complex organic molecules, or structures whose formation is virtually impossible without the help of life. Some categories of space biosignatures include: cellular and extracellular morphologies (fossils, in other words); bio-organic molecular structures (e.g. lipids, proteins); chirality (a molecule’s left- or right-handedness affected by interactions with living organisms); the presence of biogenic minerals (such as opal, only formed by life processes); atmospheric gases (e.g. methane and ozone that are largely produced via biological processes); and remotely detectable features on planetary surfaces, such as photosynthetic pigments.
Many of these signatures will not be detectable without the help of rocks, minerals or ice to preserve and protect them over millennia. Encasement within these media allows for the preservation of the remains of living organisms to be studied after their death, and their identification after geologically significant periods of time (billions of years). In general, this occurs through the process of fossilisation. The most common method is mineralisation, whereby hard parts of an organism are replaced by minerals such as calcite, silica, pyrite and phosphate, as well as a number of clays that were dissolved in water present in the sediment in which the organism died, or fell into shortly after death. An environment must satisfy a number of criteria as a suitable site for fossilisation, and some of the most valuable habitable environments we have found on other planets would be excellent sites for this process. As fossilisation and replacement of the original organism progresses, cell contents, cytoplasmic details and wall structures can be destroyed, rendering the identification of the original organisms difficult. In addition, after millions or even billions of years encased inside rocks, these fossils can be broken down, cracked, rearranged, completely destroyed by tectonics, weathering and erosion, or buried so deeply that we might never find them. Ultimately, this makes it incredibly hard to find and recognise a fossil of a once-living organism. Biosignatures therefore can be extremely valuable as they are more easily preserved within rocks, they can survive for much longer periods of time, and they can tough out a number of physical processes that would easily destroy a fossil. They are the key to the search for evidence of past life on Mars, Europa or any solid planetary body, and as such identification of analogue biosignatures in the ancient rock record on Earth is crucial; it provides an opportunity to learn which geochemical signatures are unequivocally produced by life, and how they are preserved over geologic time.
It is important to avoid a false positive result in the search for life. It would be a disaster to cry wolf over such a globally important discovery. Fossil-like objects may resemble once-living life forms, but they must be proven to be biogenic before claims of life are made. To find definitive evidence of living organisms on another world and to prove an object’s biogenicity, we therefore need to observe the co-occurrence of biological morphology, i.e. fossils and carbon chemistry. Biological information needs to be extracted from any candidate life forms to prove that they were once, or are currently, alive. DNA, the basic building block of all life on Earth, as well as proteins and fatty acids – without which cells could not exist – are key pieces of irrefutable evidence of life. The use of analogue environments on the Earth, such as Rio Tinto, is therefore important in this search. Here, filamentous fossils that appear to be bacterial in origin are studied using a range of analytical techniques to identify proteins and fatty acids preserved within them. Bearing such markers, the fossils can be confidently assigned to life and are trustworthy evidence of previous habitable conditions in the area. Studies on fossil localities in Earth’s oldest rocks, such as the Greenstone Belts of South Africa and Australia, have shown that some fossils and their associated biosignatures can be preserved and identified as being greater than 2.5 billion years old. Sites such as these provide ideal testing grounds for discovering the best techniques for the identification of markers of life on Earth and on other worlds.
Exploring the Extremes
Finding life anywhere on another world is just one use of analogue sites. Another is to figure out how humanity itself might one day leave the Earth and survive on another planet or moon. As such, a good analogue site is also a location where the exploration conditions of future astronauts can be simulated. Future explorers of the Moon or Mars will need to handle various conditions, such as reduced gravity, radiation, extreme temperatures and working in pressurised spacesuits. Preparing astronauts calls for training on sites that exhibit some or all of these conditions. The operations that can be simulated extend from living in isolation, cooking and gardening, doing fieldwork in a spacesuit and extravehicular activities (EVA) in reduced gravity to the construction of future habitats for humanity.
In order to help develop the key knowledge required to prepare for human exploration of Mars, the Mars Society initiated the Mars Analog Research Station (MARS) project. A global programme of Mars exploration operations research, this project includes two Mars base-like habitats located in deserts in the Canadian Arctic (the Haughton Mars Project, HMP) and Utah (The Mars Desert Research Station, MDRS). In these Mars-like desert environments, extensive long-duration field exploration operations are conducted in a similar style and under many similar constraints as would occur on the Red Planet. MDRS is a laboratory for learning how to live and work on another planet and is a prototype of a habitat that could house humans on Mars and serve as their main base of exploration. The station serves as a home-from-home to teams of six or seven crew members, including geologists, astrobiologists, engineers, mechanics, medics, human-factors researchers, artists and others, who live for weeks to months at a time in relative isolation, as they would on the surface of Mars.
NASA’s Hawai’i Space Exploration Analog and Simulation (Hi-SEAS) mission is another analogue site used to prepare for human space flight to Mars. Located on the slopes of the Mauna Loa volcano on the island of Hawaii, this isolated dome is surrounded by a Mars-like terrain and houses a crew of terranauts who are researching what is required to keep a space-flight team happy and healthy during an extended or even permanent mission to Mars. Research into food preparation and growth, and crew dynamics, behaviour, roles and performance is carried out by the team, who also must live their daily lives, do chores, conduct EVAs in spacesuits and contend with 40-minute delayed communications, as would happen on Mars. A global mission support team of more than 40 volunteers, including myself, provides 24/7 technical assistance and a friendly ear. The first mission in 2013 lasted for 4 months and in 2015 the first year-long mission began.
The NASA Extreme Environment Mission Operations project (NEEMO) is an analogue mission that sends groups of astronauts, engineers and scientists to live in Aquarius, the world’s only undersea research station. Operated by Florida International University, Aquarius is located 5.6km (3.5 miles) off Key Largo in the Florida Keys National Marine Sanctuary. It is deployed next to deep coral reefs 19m (62ft) below the surface. Underwater analogue sites allow for the training of aquanauts in neutral buoyancy conditions while operating in a natural but extremely hostile alien terrain. The aquanauts experience some of the same challenges beneath the waves that they would on a distant asteroid, planet or moon. During NEEMO missions, they simulate living on a spacecraft and test spacewalk techniques for future space missions. The underwater condition has the additional benefit of allowing NASA to weight the aquanauts to simulate different gravity environments and a technique known as saturation diving allows the aquanauts to live and work underwater for days or weeks at a time. Potential targets for such training are missions to the International Space Station (ISS), the Moon, Mars and asteroids, to test sampling, drilling and field explorations in one-sixth or one-third of Earth’s gravity and to test anchoring systems in microgravity. A slightly different type of underwater analogue site is based at the Pavilion Lake Research Project (PLRP) in British Columbia, Canada. Since 2004, two-week missions have been conducted every summer to train astronauts how to search for evidence of life in an extreme environment with reduced-gravity conditions – however, astronauts only do EVAs underwater; they do not live there.
There are three permanent, all-year research stations on the Antarctic Plateau: Concordia Station (French–Italian), Vostok Station (Russian) and the Amundsen–Scott South Pole Station (US) at the Geographic South Pole, the southernmost place on the Earth. One of the coldest places on our planet, and the world’s largest desert, temperatures here hardly rise above –25°C (–13°F) in the summer and the lowest natural temperature ever measured was recorded at Vostok Station: a frost-shattering –89.2°C (–128.56°F). These stations conduct a great deal of planetary and astronomical research but, perhaps most interestingly, they all allow the study of stressors associated with long-duration space missions, including extreme isolation and confinement. During the winter, crews are without the possibility of evacuation or deliveries for 9 months and live for prolonged periods, up to 6 whole months, in total darkness. Concordia station has been proposed as one of the highest-fidelity, real-life Earth-based analogues for long-duration deep-space missions.
The possibility of finding life somewhere other than the Earth seems to increase the more we understand our own planet, the conditions in which life has been found to survive and thrive, and the more we see data from the orbiters and landers that we are sending to other worlds. Although the planets and moons of our Solar System may prove to be habitable, however, it still does not mean that there is life on them. Earth remains the only example we have of an inhabited planet where life originated and evolved from a single-celled organism to the plethora of species we see today. Through working in areas across the Earth that exhibit similar traits to places on other planets and moons in the Solar System, we know that we need to target habitable environments around the numerous impact craters, ancient volcanoes and sub-surface environments of Mars, within the salty liquid oceans beneath the ice of Europa, in the source regions of the gigantic water jets erupting from Enceladus and within the Earth-like hydrocarbon lakes and seas of Titan. Each of these environments is considered to be extreme, and will exert immense stresses and pressure on any organism, including humans, trying to exist within them. Even though the conditions for life are tough … life is tougher.