SATURDAY, 29 JANUARY 2011
Like the bubbles on a river
Sparkling, bursting, borne away.
—P.B. Shelley, “Hellas”, 1821
Humans don’t want to be alone. Scientific and philosophical speculation about life in the Universe is one of our most ancient and frequently sensationalised pursuits. As early as the 5th century BC, Democritus hypothesised that there are ‘innumerable worlds’, some colliding, some declining, others flourishing, and many, he controversially claimed, harbouring life itself. Our most recent century has taken this curiousity to its respective extremes. On the one hand, mass cults, billion dollar investments in a so-far fruitless search for intelligence, and even the development of ‘new’ psychopathologies (such as the abduction phenomenon). On the other, religious denial of cosmic pluralism and the assertion of human uniqueness. All of these phenomena have, to a certain extent, used extraterrestrial life as their canvas.
Indeed, 2010 could act as a case study in extraterrestrial hyperbole. The discovery of Gleise 581 (the so-called ‘Goldilocks planet’) and arsenic-eating bacteria in Mono Lake, California, both hit international headlines for weeks, with the latter subject to a highly cryptic, hype-building marketing campaign prior to the press conference. The explosive reaction to both announcements—and even the claims of the findings themselves—later came under heavy scrutiny from science writers and field specialists, yet it barely registered. The public demand for results reigned supreme.
Not many individuals have figured more prominently in public discussions of astronomy and the search for extraterrestrial life than Lord Martin Rees. As president of the Royal Society and master of Trinity College, he writes and gives passionate public lectures on issues of human and non-human life in the universe, with venues ranging from TED to Charlie Rose. At once a proponent of space exploration and a realist about the limits of current human understanding, Rees describes the search for life as “the grandest of our environmental sciences”.
“It is one of the big questions which fascinates the public far beyond those who are interested in other kinds of science,” says Rees. “But there is another issue, which is how would we react, because it would make the galaxy much, much more interesting. On the other hand, if there isn’t any other life out there, then it has a compensation in that it allows us to feel more important, though we are very small in the cosmic context. But we could conceivably be the only place in the galaxy where there is interesting and advanced life.”
“When we understand that, it will allow us to decide how likely it was. Was it just a very rare fluke, like shuffling a pack of cards and getting a perfect set? Or was it something we would expect to happen in any environment like that which prevailed on the young Earth?”
Our ability to probe our solar system for possible life signs is accelerating rapidly with technological developments in robotics and remote control. With increasing observational and computational modelling power, we are even getting a better idea of what might lie in the wider Universe. Earth-based projects hope to detect signals from intelligent life or investigate the extremes in which life can exist. But will this satisfy public curiosity? How safely can we make assumptions about life in the universe with our limited data? Will ‘interesting’ life be found?
“I don’t know if there is much chance of success,” says Rees, “but it will be so important if they succeed, that even though I’d assess the chance as much less than one per cent, it’s still good that people are doing it.”
The universe is certainly an inhospitable place, given the freezing temperatures, colossal impacts and high-energy radiation. Hopes of finding extraterrestrial life may rest partly on ‘goldilocks planets’ where conditions are ‘just right’ for life to thrive.
Of course, the assumption that life on other planets will be similar to life on Earth may be flawed. Could we be missing out on a panoply of planets with their own specialised fauna, radically different to ours? Maybe. But we know of no alternative biochemistry as versatile as our own carbon-based one, and with no evidence to build on, our attempts to predict other life systems are purely speculative. With so many potential planets out there, it makes sense to prioritise those which look most hospitable to the kind of life we already know.
So, what are these planets like, and how common are they? Conditions on Earth are finely balanced. It receives enough solar energy to melt ice, but not too much so that water is vapourised. It is massive enough to retain an atmosphere and maintain water in a liquid form, but not so dense that gravity becomes a crushing force. It has a magnetic field which deflects radiation that can damage genetic material. The ocean and atmosphere further provide a suitable ‘reaction flask’, where a host of chemical reactions that enable and sustain life can occur. Very small changes in conditions can cause environmental chaos. The Rare Earth hypothesis, popularised by Peter Ward and Donald Brownlee, argues that habitable planets are rare precisely because these conditions for habitability are so restrictive.
Ward and Brownlee also argue that the positioning of habitable planets is also very limited. Like stars, galaxies have habitable zones around them. Too close to the galactic centre, intense radiation, supernovae and collisions would quickly irradiate and pulverise life. Too far from the centre, the availability of heavy elements required for planet formation falls. Stars move around their galaxies over hundreds of millions of years, with their planets in turn orbiting around them. If a star leaves the habitable zone at any point, life on its planets will be extinguished. Arguably, the emergence and survival of life depends on the precise path which its planet takes through the galaxy.
Of course, Rare Earth has its critics. Though only one supposed ‘Goldilocks planet’ has been found, the tens of billions of exoplanets in our galaxy gives a high chance that other Earth-like planets do exist. Critics also maintain that there is no reason why conditions on Earth could not be replicated elsewhere. In the same line of argument, should the evolution of life not be just as commonplace?
The Kepler mission, launched by NASA in 2009, investigates these questions by examining stars with potential planet systems outside our solar system. This uses enormously sensitive equipment to detect the shadows of planets moving in front of their star. Whether any of them support life will not be known for some time, but these surveys do at least give us some idea of whether Earth is especially unusual, or if we might be just one planet of many where life could exist.
Even if there are other habitable planets, what are the chances that life will have evolved? If it has, will it be intelligent? We do not know if the event that produced the first organism was an isolated incident or an inevitability on any hospitable planet given sufficient time. One line of argument is that the evolution of intelligent life must be quite unexceptional. After all, it happened here, and intelligence has proved so useful on Earth that it must be an evolutionary inevitability—intelligence might evolve wherever life exists. Others consider the moments in Earth’s history where our entire ecosystem was threatened with extinction—by meteor impacts or sudden climate change, for example—and claim it’s a miracle that intelligent life survived at all.
The Search for Extra Terrestrial Intelligence (SETI) project hopes to detect radio signals from any intelligent life. As radio signals from space can penetrate Earth’s atmosphere, these signals can be collected using radio telescopes on the ground using two strategies. The first is to carry out a sky survey, sweeping a large telescope over vast areas looking for strong signals. The second strategy is to target the search, by pointing a telescope at a particular star for a long period of time, increasing sensitivity to weaker signals.
The Allen Telescope Array (ATA), currently in development, will boost these efforts considerably. Its unique design allows continuous use, which will speed up the search and allow up to a million nearby stars to be studied. For the first time, SETI will be able to survey a significant proportion of the Universe. Should it so happen that any life is emitting radio frequency signals, the ATA will considerably increase the probability of detecting them. Despite these efforts, no signals have yet been detected, lending credence to one description of SETI as “searching for a gold nugget buried in a field”.
“Now, if that is the case,” says Rees, “you might think that it would indicate life to be a sort of trivial feature in the Universe. But that would be wrong, because there’s another thing that we will learn from astronomy, which perhaps people outside of astronomy are not so aware of, and that is that the future is longer than the past. Life could become important even if we only have the initial spark of it.”
Our search for life in the Universe naturally starts within our own solar system. The proximity is certainly an advantage: we can send probes and rovers relatively easily, which enables us to directly examine the physical characteristics of planets, moons and meteoroids.
One possible place we might find signs of life is Mars. This planet is apparently inhospitable: dry, dusty, with a thin and slowly dwindling atmosphere. Its atmospheric pressure is so low that water cannot exist in a liquid state. However, Mars used to be more Earth-like, with a thicker atmosphere allowing liquid water to exist. Microbial life may therefore have existed—or may even still exist—on the planet. To explore this, the state of ancient Mars is being studied using meteorites found on its surface. In early 2010, findings from ‘Spirit’, one of NASA’s rovers, further hinted at a warm, wet climate on Mars some four billion years ago, and since then the discovery of certain carbonate rocks provides further evidence for the possibility of living organisms. Future missions to Mars are also planned to explore dry riverbeds, ice and rock types that only form when water is present, which should provide new insights.
Europa, one of the moons of Jupiter, is seen as one of the most promising habitats for extraterrestrial life in our solar system, despite its dissimilarity to Earth. The Galileo space probe orbited Jupiter from 1995 to 2003 and showed that Europa’s surface is made entirely of ice, approximately 150 kilometres thick. Despite being too small to retain a dense atmosphere, traces of oxygen and small amounts of ozone have been detected, most likely held in the planteary ice, but could also potentially contribute to the formation of an extremely fragile atmosphere. Also, there is the enticing possibility of a vast underlying salty ocean where organisms might flourish.
While future study of Europa is planned, comparison with organisms on Earth can help to explore possibilities for life in Europa’s potential oceans. In Antarctic sea ice, where temperatures can fall to -18 degrees centigrade, cold-tolerant microbial communities do exist. If similar organisms exist on Europa, they might lie dormant in subsurface ice for long periods, becoming active during brief episodes of local heating generated by tidal energy. This may also lead to the formation of hydrothermal vents similar to those found on Earth. They also might offer a feasible habitat for chemosynthetic organisms to survive, similar to those around deep sea hydrothermal vents on Earth. Investigations at Lake Vostok, lying beneath the ice of the Antarctic Plateau, may give an insight into what life could be present on Europa. Vostok is believed to contain microbial life which has been isolated for 18 million years, and survived just the kind of harsh conditions likely to be found on Jupiter’s moon.
In contrast, Titan, a moon of Saturn, is thought to be one of the most Earth-like worlds found to date because it possesses a hydrocarbon-based hydrological system and a thick, dense atmosphere. Saturn and its 30 satellites have been explored by the Cassini-Hyugens mission, a cooperative effort between the United States and Europe using robotic spacecraft. This mission revealed Titan’s rich atmospheric chemistry, which may be a lead into pre-biotic chemical pathways capable of kick-starting life. There may also be seasonal effects, suggested by variations in relative concentrations of gases. On the other hand, Titan was found to have extremely turbulent weather: high wind speeds, methane and ethane-based storm clouds, as well as temperatures plummeting to -180 degrees centigrade.
The search for another independent origin of life highlights our ignorance about the origin of life on our own planet. Since we have yet to synthesise an artificial cell system starting from basic chemical components, there is not one definite explanation, but rather many conflicting theories.
One theory suggests that life originated elsewhere in the Universe, travelled through space and reached the Earth. This theory is called panspermia, meaning ‘seeds everywhere’, and arose from the discovery of possible organic molecules inside meteorites. Life is thought to have originated 500 million years after the Earth formed, only a quarter of its current age. Supporters of the panspermia hypothesis speculate that life could not have originated at such a rapid pace in geological terms. However, panspermia does not solve the mystery of the origin of life on Earth, it only shifts it to another place in the Universe.
It may be difficult to imagine living micro-organisms travelling through space, withstanding radiation and landing safely on Earth. On the other hand, many micro-organisms on Earth are very good at surviving incredibly harsh environments. They have adapted to thrive at extreme temperatures, in corrosive environments, and possibly even under high pressure and high salt concentrations. Bacteria have been known to thrive in radioactive waste and water boiling out of deep-sea volcanoes. Some propose that life began in these places, where carbon oxide-rich water came into contact with hydrogen-rich fluids rising from below the sea floor.
An American initiative, the Deep Carbon Observatory, was launched in August 2009 and attempts to find the deepest forms of life on Earth by sampling rocks and microbes from the crust to the core of Earth. They are also building devices to simulate interior conditions of Earth in the lab. The observatory aims to investigate if the biochemistry deep in the structure of Earth played a role in the origin of life.
We are, as the saying goes, ‘all made of star dust’. But how did organic molecules arise from inanimate matter? In the 1950s, Stanley Miller and Harold Urey produced amino acids from a simple set of chemicals likely to be present in the early Earth’s atmosphere. It seems probable that basic building blocks can be formed thanks to the properties of carbon. However, for more complex biomolecules, the question is still open.
There are two models that try to address this question. Some scientists hypothesise that the formation of basic molecules was catalysed on the surface of iron sulphide minerals, leading to a very primitive form of metabolism. Others suggest a very basic life form based on the self-replicating capability of RNA. This can both store genetic information like DNA, and catalyse chemical reactions like proteins. However, experiments are still under way to investigate the sequence of chemical events that led to the synthesis of the first molecule of RNA.
“Physics and chemistry are much easier subjects to answer than biology,” says Rees. “We understand how from simple life, monocellular life developed, then multicellular life, etc., but no one understands how the very first reproducing organisms formed. And that’s obviously a question that’s key in biology, irrespective of any interest in what happens beyond the Earth. And I would say—I’m not an expert in that subject—but I think many biologists would hope to have clearer ideas in the coming decades about how life got started on the Earth.”
What drives the search for extraterrestrial life? Curiosity about our surroundings is an integral part of our psychology, and key to human progress. Nevertheless, we have invested more time, energy and speculation into investigating other planets than we have the inaccessible areas of our own planet. We discovered communities living on deep-sea hydrothermal vents only after we had already been to the moon. We can model the state of the Universe to within one second of its beginning, 18 billion years ago, yet have not fathomed how life began a mere six billion years ago. Though we know only a fraction of the species on Earth, we scour space for the tiniest signs of life.
One could argue that, in the midst of a rapid biological catastrophe, we should focus on protecting what we have left before gallivanting around the solar system in search for extraterrestrial microbes. But there are ways in which this search could change our views of Earth and the life on it. We might find that life can only start on Edenic, Earth-like ‘Goldilocks planets,’ where everything is ‘just right’. Then, the imperative to preserve the Earth would be redoubled. If we find nothing else out there, the implications would be even greater. Along with feelings of isolation, this would leave us the recalcitrant custodians of something much more fragile and special than we have understood. The only home we’ve ever known.
Yvonne Collins is a PhD student in the Mitochondrial Biology Unit
Wing Ying Chow is a PhD student in the Department of Chemistry
Letizia Diamante is a PhD student in the Department of Biochemistry
Natalie Lawrence is a MSc student in the Department of the History and Philosophy of Science
Amelia Penny is a 2nd year undergraduate in the Department of Earth Sciences From creation to decay,