Aliens In Our Backyard

When you hear the word 'alien', what do you think? The phrase 'little green men' probably springs to mind, as well as the memory of countless Star Trek humanoids, Sigourney Weaver's Alien nemesis and the iconic E.T.. Ask an exobiologist what aliens look like, however, and their answer will be quite different. Exobiology is the study of life outside the Earth, and it brings together elements of biology, chemistry, geology and planetary science to answer the question of whether alien life is possible. We may not have found E.T. yet, but as humanity has begun to explore the solar system over the past few decades, we have found a wide range of potential habitats for life.

So where is the most promising extraterrestrial real estate? And how do exobiologists determine whether a planet or moon could support life? To answer these questions, we must first understand what life is and what it needs to exist. All lifeforms on Earth share a few common features - they metabolise (convert energy into different forms through chemical reactions), maintain homeostasis (a constant internal environment), grow, respond to stimuli and reproduce. (Side note: some entities such as viruses blur the line between life and non-life; they can only reproduce and metabolise by parasitically taking over a living cell, and many scientists do not consider them to be independent organisms in their own right. Either way, where there are viruses there must be true cellular life.)

In order for life to do the things that make it life, it needs three magic ingredients: energy, nutrients and a solvent. Every biological process requires energy, and most Earth life uses the Sun as its source. Photosynthesis by plants and microbes powers most of the surface life on the Earth, and it's easy to see why - it's cheap and plentiful. However, the first organisms on Earth were thought to use the energy bound up in certain chemicals to power them through a process known as chemosynthesis [1]. The early Earth's oceans and atmosphere contained many energy-rich chemicals that simple organisms could break down, such as carbon dioxide and hydrogen sulfide. Today, deep sea hydrothermal vents are home to entire miniature ecosystems based solely on chemosynthesis, as hot water enriched by chemicals from the under the Earth's crust is released into the ocean. So we might expect aliens to use either (or both) of these energy sources.

The second ingredient necessary for life is a source of chemical nutrients. Nutrients are essential for forming molecules that perform all biological processes, and at the most basic level they allow organisms to convert energy into more useful forms. You can sit in the sunshine all day long, but unless you have chlorophyll to convert that sunlight into sugars, it's useless to you. Coincidentally or otherwise, life on Earth uses six of the seven most common elements in the Universe as the backbone for almost every biological molecule known - carbon, hydrogen, oxygen, nitrogen, phosphorous and sulfur (often abbreviated to CHONPS; the seventh element is helium which is chemically inert and hence not very useful) [2]. A multitude of exotic models of biochemistry involving different chemical elements have been dreamt up, however it is likely that alien life will use the same core elements to produce nutrients as they are common throughout the solar system.

Finally, all life on Earth needs one additional thing to survive: water. Water is often referred to as 'the universal solvent' and a liquid solvent is needed to act as a transport medium within cells, moving chemicals around to where they need to be to react and removing waste [3]. Water is an ideal solvent for this purpose as it dissolves a wide range of organic molecules and salts that are essential to life. Hydrogen bonds give water extra inter-molecular strength, meaning that it is liquid over a large temperature range. Water is good for temperature regulation as it has a high specific heat capacity and latent heat of vaporisation - this means that it takes a lot of energy to heat water up or to turn it into steam. Another quirk of H2O's hydrogen bonding habit is that in its solid form it is less dense than as a liquid. This means that when temperatures drop, bodies of water tend not to freeze right through - instead, a 'thermal cap' of ice forms on top, helping to regulate the temperature of the liquid underneath and hence keep it liquid, which is good news for anything living in it. On Earth, everywhere we find water we find life, so the existence of liquid water in other parts of the solar system is very promising for exobiology. Water isn't the only possible solvent that life could use, however; alternatives such as ammonia and methane have been proposed. These chemicals are liquid at much lower temperatures than H2O is, so we would only expect them to be useful in extremely cold conditions [4].

We now know what the basic requirements for life are, so the next step in our search for aliens is to identify the places in our solar system where these ingredients are all found. The good news is, there are a lot of places where we find energy, nutrients AND a solvent! The not-so-good news is that it would be no easy ride for any lifeforms trying to inhabit them - extremes of temperature, suffocatingly thin atmospheres, crushingly thick atmospheres, highly acidic or alkaline environments and radiation-bombarded surfaces abound. Clearly nothing as complex and fragile as a human could survive on these worlds, so why are we bothering to look at all? The answer, again, can be found on Earth. A class of organisms known as extremophiles are exactly what they sound like - organisms that love extreme environments. They populate the parts of Earth that no one else wants to, in environments that bear a striking resemblance to those of other planets and moons. Most (but not all) extremophiles are microbes; simple, single celled organisms that have proven to be the most adaptable of all Earth's life. Additionally, we would expect microbe-type lifeforms to be the first to evolve in any ecosystem due to their simplicity, and even today they make up most of the biomass of the Earth. So any aliens we encounter in our planetary backyard are much more likely to resemble E. Coli than E.T.. Where should we look first?

If science fiction is to be believed, Mars is the place in the solar system where we can expect aliens to originate from, and exobiologists do indeed think it might harbour life. Early Mars was probably much like the Earth - warm and wet, covered in water oceans [5]. Mars is much smaller than the Earth however, meaning that it didn't have enough gravity to hang on to its atmosphere. The interior of the planet cooled and solidified quickly, both ending tectonic activity which would replenish its dwindling atmosphere and stopping the internal dynamo responsible for generating a magnetic field. So Mars today is a cold desert with a carbon-dioxide atmosphere only 5% as thick as the Earth's and no magnetic protection from dangerous solar radiation. Scientists believe that Mars has a large subsurface water ice reservoir, parts of which occasionally break through the surface temporarily due to seasonal temperature fluctuations. Today, only lifeforms adapted to such an extreme environment with only occasional access to liquid water could survive. Fortunately, several organisms on Earth could cope with Martian conditions, and perhaps the most notable is Deinococcus radiodurans. The 'world's toughest bacterium', D. radiodurans is resistant to cold, vacuum, dessication and extreme radiation, meaning it could survive quite happily on or just below the surface of the red planet.

Further away from Earth, Jupiter's moon Europa had garnered much interest as a potential home for life. Europa's surface is covered in water ice, and certain surface features such as dark streaks and 'chaos terrain' are thought to be caused by water from a subsurface ocean seeping through. The interior of the moon should be frozen solid, just like Mars, but Europa steals energy from Jupiter as it orbits through a process known as tidal flexing. The liquid ocean is pulled around by the subtly changing gravitational field it experiences, heating it up [6]. The icy crust could be anything from a few hundred metres to a hundred kilometres thick - either way, it is too thick to allow any sunlight through. It is thought that a chemosynthetic biosphere could exist in this subsurface ocean, fed through hydrothermal vents much like some life on Earth. Two of Jupiter's other large moons, Ganymede and Callisto, are also thought to have subsurface oceans, although the amount of tidal heating they receive from Jupiter is much lower so they are less robust exobiological targets.

Perhaps the most exotic place we might find life is on Saturn's moon Titan. With a nitrogen/methane atmosphere thicker than the Earth's, ice volcanism and chilling surface temperatures of around -180oC, Titan is a fascinating chemical playground. Titan's is the only surface outside the Earth that lakes have been found, filled with methane and ethane instead of water [7]. Perhaps Titan is host to an ecosystem based on methane instead of water? If much-slowed chemical reactions due to the extreme cold aren't a barrier, such an ecosystem could be fuelled by an abundance of organic chemicals present in Titan's atmosphere.

?Enceladus is another Saturnian piece of real estate that could harbour life. Resembling Europa, Enceladus is an icy moon that is also thought to contain a subsurface ocean. Excitingly, the Cassini spacecraft has seen plumes of material being ejected from the moon's surface. Cassini flew through the plume and found water, carbon dioxide, salts and an abundance of organic, energy-rich compounds such as methane, ethane, ammonia and propane [8]. All these are extremely promising signs of the habitability of such an ocean.

?So when can we expect to find our aliens, if they are out there? Will we ever detect them directly, or will we have to rely on secondary 'biomarkers' that indicate life processes are occurring? The answers depends on the development of international space programmes over the coming decades. In the next decade, there are two big missions to Mars - NASA's MAVEN orbiter which was launched in late 2013 and the European Space Agency's ExoMars rover in 2018. We may have to wait a bit longer to explore the outer solar system; the proposed Europa Jupiter System Mission (EJSM) and Titan Saturn System Mission (TSSM), which would have launched in the early 2020s, have been postponed due to funding difficulties. If these missions survive the inherent economical and technical difficulties involved, we would still need a huge dose of luck to make such a discovery - after all, if aliens were easy to find, we'd have found by now.

On Earth, life abounds. Extremophiles have taught us that even in the most hospitable environments, as long as the three magic ingredients are there - energy, nutrients and water - living organisms not only survive but often thrive. Since there are many locations so close to home where these ingredients come together, it is entirely possible that life clings on in these habitats. The caveat is that life on Earth has it easy - we inhabit a goldilocks planet - and that allows Earth to host an extensive biosphere with complex organisms such as ourselves. Alien organisms are likely to be simple, extremophilic microbes living in a very scaled-down and largely hidden ecosystem. Whatever is out there, future exploration of our solar system is likely to be very exciting - stay tuned!

[1] Chela-Flores, J. (2000): "Terrestrial microbes as candidates for survival on Mars and Europa", in: Seckbach, Joseph (ed.) Journey to Diverse Microbial Worlds: Adaptation to Exotic Environments, Springer.

[2] http://www.phschool.com/science/biology_place/biocoach/biokit/chnops.html

[3] http://www.astrobio.net/interview/453/water-the-molecule-of-life

[4] http://www.ncbi.nlm.nih.gov/pubmed/15556414

[5] Hartmann, William K.; Neukum, Gerhard (2001). "Cratering Chronology and the Evolution of Mars". Space Science Reviews 96 (1/4): 165-194.

[6] Greenberg, Richard; Europa: The Ocean Moon: Search for an Alien Biosphere, Springer Praxis Books, 2005.

[7] Stofan, E. R. et al (2007). "The lakes of Titan". Nature 445 (1): 61-64.

[8] Waite, J. H. et al (2006). "Cassini Ion and Neutral Mass Spectrometer: Enceladus Plume Composition and Structure". Science 311 (5766): 1419-22.