"Science and science fiction have done a kind of dance over the last century, particularly with respect to Mars. The scientists make a finding. It inspires science fiction writers to write about it, and a host of young people read the science fiction and are excited, and inspired to become scientists to find out more about Mars, which they do, which then feeds again into another generation of science fiction and science"

Carl Sagan

Many space scientists, if you ask them and they are being honest, were inspired to become the next generation of Universal explorers due to influences in the media during their childhood. It may have been a wish to ‘Hitchhike’ around the galaxy, to work alongside Kirk or Spock, fly space craft fighting the Cylons in Battlestar Galactica, or they were simply inspired by one of Sir Patrick Moore’s broadcasts about the stars. Whatever the cause, science fiction and science fact are intricately linked driving forward innovation and imagination. This has never been truer than now!

The human mind is fascinated by the possibility of other life in the universe. As early as the fifth century B.C., the Greeks considered the possibility of an infinite universe housing an infinite number of civilizations. Much later, in the 16th century, the Copernican model of a heliocentric solar system opened the door to all kinds of extra-terrestrial musings. Since then speculation about alien life kept pace with scientific progress well into the twentieth century, but it wasn’t until the 1950s that anyone proposed a credible way to look for these distant, hypothetical neighbours. The space age dawned and science was anxious to finally know what existed beyond the confines of our planet. With this, astrobiology was conceived and in the last 60 years has gained momentum to become one of the most exciting multi-disciplinary fields of planetary science. It is a completely unique discipline, as it strives to study and understand a subject matter, i.e. life in space, which has not actually been discovered yet. But through detailed investigations into the ability of life on Earth to survive even the harshest environments, we are beginning to understand what life might exist on other planetary bodies, where it might be hidden and how we can find it.

Where on Earth?

Scientists looking for signs of life outside of the Earth have a very different idea of what these life forms might look like compared to popular culture. They aren’t looking for little green men but instead are looking for micro-organisms, organic molecules such as amino acids, and biosignatures indicative of past life. At present the search is focused on investigating habitable environments on other planets and moons, and not the identification of life itself. We are looking for places where conditions exist now, or have existed in the past, that are conducive for life, namely regions that exhibit evidence of liquid water, an energy source, and organic molecules. Target worlds include Venus, Mars, Europa, Enceladus, Titan and even the Moon.

The search for habitable environments on Mars is currently underway thanks to NASA’s Curiosity rover that landed on Mars in 2012. In 2018 the search for biosignatures of past or present life in its subsurface will begin with the launch of the ESA/Roscosmos ExoMars rover. Today Mars has the most clement environment in the Solar System after the Earth despite its sub-zero temperatures (on average -63°C), thin CO2-rich atmosphere and savage global dust storms. It also displays abundant evidence that a warmer, wetter environment existed in its past and that liquid water once flowed across its surface1. The Earth’s other neighbour, Venus, although nearer to the Sun than the Earth, might still have the potential to house evidence of life. Under the influence of a runaway greenhouse effect with surface temperatures of a sweltering 460°C, today Venus is a desiccated world where no life could survive. However, it seems likely that it once also had liquid water oceans just like on Mars, and that any life originating in these oceans may have migrated upwards into the atmosphere as the conditions on the surface turned hostile.

We are beginning to understand what life might exist on other planetary bodies, where it might be hidden and how we can find it

Some of the moons of the many planets in the Solar System are proving to be even more interesting than the planets they orbit, and may even harbour evidence of life. In particular, the ice-covered moons Europa, Enceladus and Titan have been observed by orbiting satellites to display evidence indicating that subsurface liquid bodies, organic molecules and even atmospheres are present. Europa is a key astrobiological target, with the search for habitable niches focusing on possible deep-sea hydrothermal vents at the base of its ocean. These vents are fissures in the moons crust from which heated water erupts. Currently we cannot study these as they are hidden from view by Europa’s icy shell, so the surface of the moon, and the sulphates seen on its fractured icy exterior, has to act as our window into its possible ocean ecosystem. This potential habitat for life will be investigated by the JUpiter ICy Moons Explorer (JUICE) mission, launching in 2022.

Saturn’s moon Enceladus has attracted a lot of interest recently thanks to dramatic images of icy jets of water vapour, simple organic molecules and volatiles such as nitrogen and methane, shown erupting from its south polar region. All these components must have come from a subsurface source region that feeds the jets implying that organic molecules used by life are present deep inside the moon. Finally one of Saturn’s other moons, Titan, with its substantial atmosphere and Earth-like lakes and seas, is a crucial target in the search for life. Surface temperatures of around 94 Kelvin have led to suggestions that the liquid bodies on the surface are composed of a mixture of methane and ethane with heavier hydrocarbons and possibly dissolved atmospheric gases. Life might be present here within a range of habitats, from the liquid hydrocarbon lakes on the surface to kilometre depths into the subsurface, creating a potential biosphere volume double that of the Earth.

Alien Life

The study of planets and moons other than our own highlights the diversity of habitats that life might have to contend with. Therefore any life that we find on these worlds will be very different to us. This life falls into the category of extremophiles, a group of some of the hardiest microorganisms that can withstand and thrive in extremes of temperature, pressure and radiation, as well as salinity, toxicity, pH and limited availability of liquid water2. The best-known examples are thermophiles, such as ‘Strain 121’ and Methanopyrus kandleri, that savour the conditions around geothermal hot springs found in volcanic environments and at the bottom of the ocean.

Currently, life on Earth has been shown to survive and reproduce at an impressive 121°C but these limits are continually being tested. In contrast, the lower record limit for life is at -20 °C, around the temperature at which water inside cells freezes. The majority of chemistry in terrestrial life functions well around pH 7 or neutral, but life has been found in the most acidic (pH 0) and most alkaline (pH 12.5) of environments. Most impressively, life can also adapt to more than one extreme condition. It can be subjected to a combination of very acidic waters, limited to no oxygen availability, and extremely high pressures in a single environment and survive3.

Microorganisms living in the giant geysers of the Yellowstone Park (USA) can tolerate extreme environmental conditions, similar to those present on planets and moons in outer space. D Garding

The Future for Humanity

Investigating the habitability of other worlds does not only benefit our understanding of the versatility of life, but also enables us to take a step towards to answering one of humanities oldest questions, are we alone? Not only that, we also want to understand how humans might be able to live and work on them one day as our home. We are not talking about the moons of Jupiter and Saturn just yet, and definitely not Venus, but Mars and our own Moon are prime targets for future human outposts. In the long term i.e. another 3 billion years, our Sun will start to expand and enter its red giant phase as its gets closer to its death. It will engulf Venus, and even if it doesn’t swell enough to reach the Earth it will still boil off the oceans and heat the surface to temperatures that even the hardiest extremophile couldn’t survive. This, however, is a long way off. Until this time we have a number of other reasons why we might want to leave the Earth. Life is fragile and any number of natural or man-made catastrophes could occur such as another ice age, global warming, asteroid impact, nuclear war or complete depletion of our natural resources. This paints a very bleak picture of our future but the chances are that we will simply choose to leave Earth because we want to explore.

Our first port of call so to speak might be the Moon. It is an ideal staging post where we can accumulate materials, equipment and manpower outside of the confines of Earth’s gravitational well. From the moon we can send missions onwards to Mars or into deep space, set up astronomical stations to view the cosmos without the interference of an atmosphere or Earth’s radio chatter, and even support a bustling space tourism industry. We already have the means of getting to the Moon and our technology has proven to be advanced enough to sustain human and plant life in space: we just need to begin building.

To build a habitat on the Moon is no easy feat; it requires a number of considerations including understanding how building materials will respond to the vacuum on the Moon, the extreme temperature variations between day (120°C) and night (down to -153°C), impacts by micrometeorites (up to 10Km/s), outward forces from pressurised habitats, radiation damage, and the 1/6th gravity of that of the Earth. These habitats will be a lifeline for future colonists by providing oxygen for them to breathe, water to drink, protection from the harsh radiation of the sun, providing light and power during the 14 day nights, and keeping them comfortable in all temperatures.

One design put forward so far is a stereotypical inflatable dome. These are light weight and would be relatively easy to erect on the surface, however, they would need protection. Local materials such as the lunar regolith could be used to cover the inflatable habitats providing an addition layer of defence against radiation and micrometeorite strikes. Habitats could also be erected within ancient lava tubes. These natural cavern systems provide a structure within which habitats could be built and easily sealed, the rock provides protection from the harsh surface environment and impacts, and they are commonly interconnected allowing for the habitat to grow. A great review of how building a lunar base could become a reality has been written by Benaroya et al. (2002) titled "Engineering, Design and Construction of Lunar Bases".

A manned mission to Mars would take about 440 days to complete with astronauts visiting the surface of the planet for a period of two months. NASA/Pat Rawlings, SAIC

Building an outpost on Mars will require a lot more work, even though we have much better conditions than on the Moon. This is because of the greater distance between the Earth and Mars (at its closest point Mars is a mammoth 55,000,000km away), the difficulty of transporting materials, and the effects on human physiology and psychology. The total journey time from Earth to Mars could take between 150-300 days depending on the distance between the planets at the time of launch and the rockets, or fuel, being used. The benefits of moving to Mars are that it has a similar length of day, axial tilt and seasons to Earth. It also has an atmosphere, water ice, and habitable environments. There are a number of geological landforms such as impact craters and lava tubes which could house habitats and there is a never-ending list of scientific investigations that can be carried out.

The benefits of moving to Mars are that it has a similar length of day, axial tilt and seasons to Earth

The environment on Mars is the main challenge to be overcome as the 95% CO2 atmosphere is toxic to humans and promotes low atmospheric pressures (6 mbar), it only has 38% gravity of that on the Earth, it’s always cold (-85 to -5°C), and there are no liquid bodies of water. As with habitats on the Moon, oxygen will need to be produced for humans to breathe and suits will need to be worn whenever the inhabitants leave the outposts. Due to the time taken to travel between Mars and the Earth (not to mention the cost), any habitat on Mars will need to be self-sustainable, growing its own food, extracting its own water from the frozen ground and producing its own oxygen. Many studies are currently being conducted into the logistics of how this might be done.

We are living in a time where science fiction and scientific research are converging and, thanks to the internet and social media, the entire world can be involved. We are living in a time when humanity is capable of sending people and robots to other planetary bodies, and we are about to witness the Voyager-1 probe leave the confines of our Solar System and enter interstellar space. Who knows what lies in store for humanity in the future?