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Evans Rhodri - Astrobiology: The Search for Life Elsewhere in the Universe

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Evans Rhodri Astrobiology: The Search for Life Elsewhere in the Universe
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Extraterrestrial life is a common theme inscience fiction, but is it a serious prospect in the real world? Astrobiologyis the emerging field of science that seeks to answer this question.

The possibility of life elsewhere in the cosmosis one of the most profound subjects that human beings can ponder. AstrophysicistAndrew May gives an expert overview of our current state of knowledge, lookingat how life started on Earth, the tell-tale signatures it produces, and howsuch signatures might be detected elsewhere in the Solar System or on the many exoplanetsnow being discovered by the Kepler and TESS missions.
Along the way the book addresses key questions such as the riddle of Fermisparadox (Where is everybody?) and the crucial role of DNA and water theyreessential to life as we know it, but is the same true of alien life? And the reallybig question: when we eventually find extraterrestrials, will they be friendlyor hostile?

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CONTENTS

In July 2018, the UK tabloid newspaper the Daily Express carried a story dramatically headlined Aliens on Europa: NASA Hunts for Life Just 1 cm under Surface of Jupiters Moon. To reinforce the message, it was accompanied by an eye-catching composite image. On one side of the graphic was a photograph of Europa: an enigmatic-looking world, nothing like our own Moon, with a smooth surface of solid ice, criss-crossed by dark cracks. On the other side of the image, an artists impression of a typical alien: grey-skinned but otherwise distinctly humanoid in appearance, with a high-domed, hairless forehead, large eyes and delicate features.

Confusingly, however, the articles strapline read as follows:

Scientists hoping to find alien life on Jupiters moon Europa may not have far to search after a study revealed microbes could be surviving just one centimetre beneath the surface.

So whats going on? Is NASA going to Europa to hunt for big-brained, humanoid aliens, or for tiny little microbes? Delving further into the fine print, it turns out the answer is neither. The Express article was prompted not by a space mission thats about to take off, but by a clever piece of scientific deduction. It is widely accepted that, if life exists on Europa and yes, were most likely talking about microscopic organisms here its to be found in the ocean of liquid water believed to exist several kilometres below the icy surface. The new development the Express picked up on is the suggestion that chemical traces of life for example proteins or complex DNA-like molecules might be found near to the surface of the ice, making them much easier for a space probe to detect. This idea originated in a scientific study that had just been published in the journal Nature Astronomy, which came to the following conclusion:

These results indicate that future missions to Europas surface do not need to excavate material to great depths to investigate the composition of endogenic material and search for potential biosignatures.

This is real science, and in principle its no bad thing that it found its way into a widely read tabloid like the Daily Express. But the way the newspaper chose to report it and the way the popular media treats stories of this kind in general is likely to leave readers more confused than enlightened. Are they saying that NASA believes there are humanoid aliens on Europa? Why go all the way to Europa when the same newspaper also frequently reports anecdotal sightings of humanoid aliens here on Earth? Is NASA on the point of sending a space probe to look for life on Europa, or is that just an idea for the future? Why do scientists keep going on about extraterrestrial microorganisms, when everyone knows that aliens are pretty much like us except for their big eyes and high foreheads?

All these things and many more will be clarified in the course of this book. Astrobiology is a wide-ranging subject, dealing with the possibility of life beyond Earth from every conceivable angle. To start with, however, lets kick off with a much simpler question.

Is There Life on Earth?

From our perspective on the surface of the planet, its obvious theres life on Earth. From out in space, too, its not that difficult to detect. The night side of the planet is lit up by city lights, there are thousands of small artificial satellites in orbit, the radio spectrum is buzzing with structured signals that have no natural explanation, and the atmosphere is laced with industrial pollutants.

But all those things have existed for just a century or so: a tiny fraction of the Earths lifetime, which is about 4.5 billion years. Nevertheless, life at a less obvious level has existed for a significant fraction of that time, perhaps as much as 4 billion years. Until just under a billion years ago, all of that life (and the vast majority of it even today) took the form of tiny single-celled organisms the microbes that scientists are so fond of talking about. The following table shows how, over the course of time, increasingly complex forms gradually evolved and were added to the mix of life on Earth.

Milestones in the evolution of life on Earth (all dates are approximate)

Time before present (in millions of years)Evolution of life on Earth
4,500Formation of the Earth
4,000First single-celled life forms
1,700Microscopic multicellular organisms
600Small marine animals; seaweed
450Fish; land plants; insects
350First land vertebrates (amphibians); trees
180Jurassic dinosaurs; flowering plants
50First lemur-like primates
3.5First tool-making hominids
0.3Homo sapiens

This means the question of life on Earth is a matter of definition. To a scientist, life includes any kind of living thing even if it can only be seen through a high-power microscope. By that definition, Earth has been home to life for almost 90 per cent of its history. On the other hand, people brought up on a diet of sci-fi movies and tabloid stories about UFO encounters are more likely to equate life with a technologically savvy civilisation in which case that 90 per cent figure drops all the way down to 0.000002 per cent.

If were going to look for life on other Earth-like planets, what are the relative chances of finding it by those two definitions? We can make a rough estimate by picking random snapshots of the Earth at different points in its 4.5 billion-year history. On that basis, the chance of finding life by the ufologists or sci-fi fans definition is so tiny as to be virtually zero. By the scientists definition, on the other hand, the chances are pretty good.

So lets look a bit more closely at that scientists definition of life. The nature of life turns out to be surprisingly difficult to pin down, and precise definitions tend to vary between specialists working in different branches of science. As far as astrobiology is concerned, a good starting point is the working definition devised by NASA in the 1990s:

Life is a self-sustaining chemical system capable of Darwinian evolution.

Thats refreshingly concise, but it packs a lot into a small number of words. The first part, self-sustaining chemical system is clear enough. But the latter part, capable of Darwinian evolution, hides a lot of detail. It doesnt just mean that our self-sustaining chemical system has to be able to evolve, or change its form over time. First, theres an implicit assumption that the change occurs over successive generations, each of which is born, grows and dies. Then theres that word Darwinian after Charles Darwin, the Victorian naturalist who did far more than suggest that living species evolve. He argued that they do this for a reason to adapt to the changing circumstances of their environment and that they do so by means of natural selection, or survival of the fittest.

The beauty of this definition is that it encompasses everything from the single-celled organisms that emerged on Earth 4 billion years ago and may possibly be hiding under Europas ice sheets via semi-civilised primates like ourselves, all the way up to super-advanced lifeforms we can hardly even imagine. Astrobiology the subject of this book deals with the possibility of life beyond Earth wherever it falls in that spectrum. As the astro prefix implies, its essentially a sub-branch of astronomy, using the same sort of telescopes, space probes and theoretical techniques that astronomers apply to any other facet of outer space.

Earlier in this chapter (page 2) we saw a quote from a scientific paper featuring a lot of multisyllabic words. One of them, biosignatures, will turn out to be one of the most important words in this book. A moment ago, we saw how all the obvious ways an outside observer might detect life on Earth artificial lights, satellites, radio signals, etc. relate to our own civilisation. But there are other, subtler, ways of detecting more primitive lifeforms and these are collectively known as biosignatures. Most importantly, living organisms produce, as waste products, tell-tale chemicals that would be very difficult to account for in terms of non-living processes. These chemical signatures are potentially detectable to astrobiologists through telescopes or spacecraft-based sensors.

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