Tuesday, 23 July 2013

Are We Alone? The Search for Life Beyond Earth

Tuesday July 23rd, 10am

“Sometimes I think we're alone in the universe, and sometimes I think we're not. In either case the idea is quite staggering.”  This quote, sometimes attributed to the legendary science fiction writer Arthur C. Clarke, embodies one of the central questions of modern planetary science, and indeed all of astronomy.  If there really is no one else out there looking back at us, then the universe truly is a lonely place.  This second lecture aboard the Queen Mary 2 will consider our search for life away from our home planet, which will take us on a journey to define what life is, what it needs, where we might find habitable environments, what steps have been taken to announce our presence to the rest of our galaxy and how we’re listening out for signs of intelligence elsewhere.

The idea of life on our closest neighbour, the red planet Mars, stems from Schiaparelli’s mapping of the surface in the late 19th century.  Misled by optical effects within his own eyeballs, Schiaparelli carefully created a map of Martian canals, presumably constructed by a dying race of advanced technology, seeking to transport water from the moist poles to the dry, arid equatorial regions.   The idea of a Martian race entered popular culture and has remained there ever since, even though our modern exploration of the red planet has revealed no traces of life, past or present.  But how do we know what to look for, and what are the essential ingredients for life? 

The Ingredients for Life

Scientists typically refer to a habitable zone around a star, a region that works on the Goldilocks principle of being not too hot, not too cold, but just the right temperature to support the evolution of life.  The distance from the star determines how much energy and warmth it receives – too much, and water is lost to create a Venus-like oven; too little and water freezes.  The Sun provides the primary source of energy for life on Earth.  Liquid water, we believe, is the ideal solvent for chemical reactions to occur (the hydrogen bonds linking adjacent water molecules providing a superb environment for transport of reactants).  Furthermore, the promiscuous carbon atom that can bond with other atoms in a vast number of ways seems an ideal starting point for the formation of complex ‘organic molecules’.   So we have three factors: a source of energy, liquid water as a solvent, the correct chemical soup for organic molecules.  A fourth requirement is that of stable conditions over time to allow evolution to lead to complex life, something we’ll talk more about in my third lecture.

But, of course, our definition of a habitable environment (that is, one that could support life) isn’t so simple.  Jupiter’s icy moon Europa is well outside of the traditional habitable zone, but may still tick all of these boxes in its sub-surface ocean, hidden from the Sun’s energy, because tidal flexing of this moon by gigantic Jupiter provides an alternative source of energy for life.  A similar story is true for Saturn’s moon Titan, the only other body in the solar system with standing bodies of liquid.  Although not water, the hydrocarbon seas of Titan may be another suitable solvent for life.  The past few decades of biological discovery on Earth has opened our eyes to the extreme resilience of life, with bacteria known as extremophiles surviving in the strangest of places, such as near the black smokers of the deep ocean, or algal blooms around sterile lakes, bacteria found in the frozen wastes of Antarctica or even thriving within barrels of nuclear waste.  Life will find a way.

‘Our kind of life’ requires carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus (remember SPONCH, the marshmallow cookies), but also traces of elements like iron and magnesium.  But who is to say that this is the only recipe for life?  Silicon has similar properties to carbon as a promiscuous atom, and is a long-term favorite of sci-fi authors, but as it sits lower in Group 4 of the periodic table it’s also heavier, so less flexible in forming bonds.  We have to remember that our world seems so perfect for ‘our kind of life’ because we evolved and adapted to suit it, not the other way around.  Life would similarly adapt to the conditions and available chemical on other worlds, but even so, we believe that life converges on a limited number of solutions to evolutionary problems  - the compound eye for sight, or the evolution of intelligence as a means of survival. 

Following the Waters of Mars

Our most promising destination for searches for a second genesis of life remains the planet Mars, which we now know had a warmer and wetter past, satisfying the need for the solvent and the source of energy.  But what about stability over time?  How long was Mars potentially habitable for, and did life emerge during that time?  Surprisingly, despite the range of missions to Mars over the past four decades, only the Viking landers were suitably equipped to detect life on Mars.  The two landers lasted from 1976-1978 and 1976-1980, respectively, and revealed evidence for water today on Mars – frost patches seen behind Viking 2 during the morning that evaporated during the heat of the day, and the orbiters revealed networks of channels eroded by the flow of liquid water.  But their search for biology remains controversial to this day. 

Viking had three experiments dedicated to Martian biology – the gas exchange, pyrolytic release and labelled release experiments.  The labelled release experiment used Vikings arm to deliver Martian soil to a nutrient bed laden with radioactive carbon.  If organisms were present in the soil, they’d consume the nutrients and start releasing the radioactive gas to be detected by a Geiger counter.  Lo and behold, this is exactly what happened – the Geiger counter registered elevated levels of radioactive gases some 15 hours after the soil was released.  However, this result and its implications for the presence of Martian microbes has always been controversial, as one of the chemical experiments onboard, the GCMS (Gas Chromatography and Mass Spectroscopy experiment), found no evidence for organic material in the soil.  It was this cautious result, that there were no organics on Mars, which shaped NASA’s exploration of the red planet for the next few decades, despite the fact that the GCMS has since been shown to be too insensitive to firmly draw this conclusion.  The biologists largely fell from grace, and geologists and atmospheric scientists took over the mantle of Martian exploration for almost all missions since.

In 2008, the Phoenix mission landed near the north polar ice cap of Mars.  Using its robotic arm to scoop up soil, it again revealed the presence of water ice in the grooves that it excavated.  Again, there were no organics or nitrates in the soil, but a new discovery was made – perchlorate.  This ‘rocket fuel’ is the most oxidised form of the chlorine atom, and would be responsible for totally destroying organic material when heated, producing chlorine bearing gas molecules as a by-product.  So any experiment heating up Martian soil (known as pyrolysis), in the presence of perchlorate, would destroy the very organic matter that we’ve been looking for all this time!  This could have happened to the Viking GCMS experiment.  So there could still be organics on Mars, either past or present, and we’re now learning how to solve this mystery.

Another tantalising possibility is that we might already have evidence for past Martian life here on Earth.    In 1996 an enormous furore was made about the discovery of nanoscale structures in a piece of meteorite blasted from the surface of Mars thousands of years ago to land in the Allan Hills region of Antarctica (ALH84001).  The gases in the rock were in exactly the balance expected for the Martian atmosphere, and those structures looked suspiciously like tiny worms.  It even prompted the US President, Bill Clinton, to announce the discovery in a press conference.  Today, the scientific consensus is that some form of geochemical processing or contamination is responsible for these structures, and teaches us the cautionary lesson that just because it looks like life doesn’t mean it actually is life!

Planets Beyond Our Solar System

“The apparent size and age of the universe suggest that many technologically advanced extraterrestrial civilizations ought to exist.  So where is everybody?”   This is one way of stating the Fermi Paradox, and is being brought into focus now more than ever as planetary scientists discover and characterise planets around other stars.  Over 900+ planets have now been identified, with many thousands of additional candidates to be verified.  Results from NASA’s Kepler mission suggest that planets of ‘super earth’ and ‘mini-Neptune size’ (that is, some 2-5 times larger than Earth) are commonplace, but simulations suggest there could be as many as 100 billion Earth-sized planets in our galaxy alone.  So where is everyone?

This brings us to the famous Drake equation to estimate the number of civilisations out there in our galaxy.  We multiply (i) the average rate of star formation by (ii) the fraction of stars with planets; (iii) the average fraction of planets that could potentially support life; (iv) the fraction of those that actually develop life; (v) the fraction of those worlds with intelligent life; (vi) intelligent life that could develop the technology to render signs detectable from space; and finally (vii) the length of time that this civilization will be beaming their signals into space.  The first three terms in this equation are slowly being studied, the remainder are open to significant interpretation, but it gives you some idea of the level of complexity in the problem!

Saying Hello?

An old Nature paper by Morrison and Cocconi (1959) made a prediction that an advanced alien civilization would exploit the simplest possible techniques for a transmission that would travel across the stars.  They would use radio as the simplest and lowest power method of transmission, and emit a very narrow bandwidth signal so that it could not be confused with any natural sources.  They would also choose some universal frequency associated with hydrogen at 1.4 GHz, the simplest of all the elements, and in a region of the electromagnetic spectrum where the interruption from water was kept to an absolute minimum.   It is at this particular frequency that all our searching has taken place, turning enormous radio antennae to the sky and hoping to hear that narrow band signal at 1.4 GHz. 

In August 1977 it finally arrived, picked up by Ohio State University’s Big Ear receivers, at exactly the right frequency and characteristic.  Immortalised forever as the ‘wow signal’, it was gone three minutes later when the second receiver had swung into the line of sight of the source.  It was never heard again, violating one of the principle rules of scientific experiment, that a result must be reproducible.  Several alternative explanations have been proposed and discarded over the years, and this signal remains an enduring mystery.  It originated from a region of seemingly empty space in the constellation Sagittarius, but was never received again.

This lack of reproducibility plagues own attempts to announce our presence to the universe.  In 1974, a radio signal was sent from the Arecibo radio telescope bearing information about the human form, DNA, and some basic mathematical principles that should be understood by an advanced civilization.  We beamed it in the direction of the globular cluster M13, but made the same crucial mistake.  It was sent only once.  When the message arrives at the globular cluster in 21000 years time, a scientist there might also discount this message as a singular freak event, one that is destined not to be reproducible.  We might have messed up our first communication with alien civilisations.

Our own interplanetary ambassadors, the Pioneer and Voyager spacecraft, carry with them discs and plaques describing the sights and sounds of Earth, and greetings in 55 languages.  Once again, we’re announcing ourselves to the cosmos and hoping that these probes will be found, by some civilisation, in the distant future.  Voyager 1 is aiming for constellation Ophiuchus.  In 40,272 AD, it will come close to an obscure star in Ursa Minor.  Voyager 2 is heading south toward Sagittarius and Pavo, and in about 40,000 years it will come within about 1.7 light years of a star called Ross 248, a small star in the constellation of Andromeda.  Who knows what the future holds for these intrepid explorers and their message to the stars?

Looking to the future, many of our ambitions to search for life among the stars are stalled.  Exploration of the outer planets and their habitable satellites is underfunded.  SETI and the search for transmissions from extraterrestrial life is unfunded.  There are some rays of hope – missions to search for planets similar to our home world are being assembled right now, and will hopefully discover other ‘Earths’ within the coming decades.  Martian explorers are closing in on the search for organics, and evidence for life past or present.  If we indeed discover this second genesis of life elsewhere in the cosmos, it will be the most important and profound discovery of all time, causing us to contemplate our presence in the universe and our eventual expansion across the solar system.  It’s a scientific goal worthy of our closest attention.

No comments:

Post a Comment