Andrew May - Astrobiology: The Search for Life Elsewhere in the Universe

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.

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)