Life in the Solar System

Mercury

This airless ball of rock and metal is well outside the habitable zone and is a most inhospitable body. Its proximity to the Sun means that daytime temperatures can climb to 430° C (hot enough to melt lead), whereas it’s slow diurnal rotation means the night side plummets to -170° C without any substantial atmosphere to keep the heat in.

Venus

Since the surface of Venus was obscured by clouds and it was nearer to the Sun than Earth, C^19th astronomers pictured it as a warm, wet world - either a planet of tropical rainforests or a water world with little or no land. Early science fiction writers used both of these options, from the jungles of Edgar Rice Burroughs’ Pirates of Venus series to C. S. Lewis’ floating islands in Perelandra.

Because Venus is named after the goddess of love, several of these works assumed that the inhabitants would be peaceful and gentle. Nearly all of these fictional Venusians are virtually indistinguishable from humans in appearance.

The discovery of the true nature of Venus as an extremely hot, arid planet with a crushing atmosphere led to a loss of interest by the majority of SF authors, although a few have speculated on the possibility of terraforming the planet for human habitation.

Physical Properties and Conditions post Magellan

Venus is in many respects the Earth's sister planet. It is almost the same size as Earth and is our nearest planet. It can be seen in the dawn or evening sky as a dazzling object, hence it is often referred to as the "morning" or "evening" star. Small telescopes reveal that it shows phases, but any surface detail is hidden beneath thick, highly reflective pale yellow clouds.

Venus takes 224 days to orbit the Sun and is unusual in that it revolves clockwise on its axis as seen from above (retrograde motion) - the only planet to do so. It takes 243 days to do this, hence it would take over a century between sunrise and sunset at any place on the surface - if you could see the Sun.

Venus remained a mystery until radar technology enabled us to get under the clouds and measure temperatures and map surface features in the 1960s. Scientists got a shock. Venus is the hottest planet in the solar system - on average 495° C. Most of the planet is rolling plain, but there are large continent sized plateaux rising several kilometres high, the most prominent of which are Aphrodite Terra (about the size of (Australia) and Ishtar Terra (about the size of Africa). There is no surface water. The atmosphere consists mainly of carbon dioxide (95%) with nitrogen (3.5%) and sulphur dioxide (0.015%) as the next most abundant chemicals. The cloud layer is some 50km thick and gives rise to a surface pressure of 90 atm. Seen in ultra-violet light, characteristic "Y" shaped patterns in the upper atmosphere indicate global winds moving at 300 km/h. The clouds of Venus consist, not of water droplets, but of concentrated sulphuric acid. This rain never actually reaches the ground because it evaporates in the hellish heat. There is a well-defined lower edge to the cloud deck rendering the air clear on the ground. Terrific blasts of lightning occur in the clouds and may be responsible for the ashen light reported by astronomers looking at the dark side of Venus.

In the 1970s, the Russians soft-landed a whole series of Venera probes on the planet. The pictures they sent back in the brief period before they were crushed, fried and corroded out of existence confirmed the radar measurements. They also provided evidence of volcanic activity.

In 1978 Pioneer 10 radar mapped the surface with unprecedented detail.

In 1990 the American Magellan probe used synthetic aperture radar to produce the most stunning map of the Venusian surface yet. It revealed craters - volcanic and impact, active volcanoes, and other vulcanism, yet there is no plate tectonics like on Earth owing to the high surface temperature (there is no temperature gradient across the crust to drive the convection currents in the mantle). One of the most vital questions in comparative planetology is why should the Earth and Venus, two planets that are nearly the same size, density and similar distances from the Sun, with similar compositions, evolve into such dissimilar worlds?

Comparative Planetology and the Possibility of Terraforming

4.5 billion years ago both Earth and Venus had atmospheres high in carbon dioxide and water vapour. Both of these gases have a greenhouse effect, trapping heat that would otherwise radiate back to space. Carbon dioxide has a much more dramatic greenhouse effect than water vapour. Both the early Earth and the early Venus would have been very hot. Earth, however, being significantly further from the Sun than Venus, received less solar energy. This meant that even with the greenhouse effect of Earth’s carbon dioxide, the surface temperature was low enough for there to be liquid water in the form of surface oceans as well as water vapour in the atmosphere. Because carbon dioxide is soluble in water, much of the gas was removed from the atmosphere and dissolved in the oceans, eventually to be deposited as carbonate rocks (e.g. limestone). This therefore lessened the greenhouse effect and Earth’s temperature dropped. As the temperature dropped, the amount of water vapour that can be held in the atmosphere would also decrease, thus dropping the temperature further. When life evolved the whole process was accelerated, as micro-organisms and plants removed carbon dioxide by photosynthesis and converted it into organic material that was buried in sediments (later forming oil, natural gas & coal).
Therefore, although the Sun has increased its luminosity through time causing more heating, simultaneously on Earth the amount of carbon dioxide in the atmosphere has decreased, leading to global cooling. The overall result is a stable temperature for billions of years.
On Venus, however, the amount of solar radiation received is higher then on Earth, so the initial temperature would also have been much higher. It seems likely that Venus had much less liquid water, or possibly none at all, and therefore lacked Earth’s mechanism for removing carbon dioxide from the atmosphere. This would mean that as the Sun warmed up then so would Venus. Any liquid water present would then evaporate, causing the greenhouse effect to become stronger. Eventually Venus lost even its water vapour, as the molecules were broken down into hydrogen and oxygen by sunlight in the upper atmosphere, and the lighter hydrogen escaped into space. So roughly speaking, to convert Venus into an equivalent of Earth suitable for human habitation, we would have to emulate several billion years of geological and biological development on Earth.
Reducing the amount of solar energy reaching Venus would have to be done by raising the planet’s albedo (reflectivity), e.g. by seeding the clouds with something reflective, or by putting mirrors in orbit to reflect the sunlight back into space.
Reducing the carbon dioxide content of the atmosphere could be done industrially by chilling atmospheric gases until the CO₂ solidified and then releasing the remaining gases. The solid CO₂ would then have to be stored somewhere where it was unable to heat up and reach the atmosphere again. In Kim Stanley Robinson’s Blue Mars, he suggests burying it under an impermeable seal and letting oceans form on top of it. The seal would also have to be earthquake and volcano proof!
A safer, if more drawn out method would be to import water to Venus as it cooled and let the water wash the CO₂ out of the atmosphere as it did on the early Earth. When the temperatures are low enough to sustain micro-organisms (120^oC), strains which photosynthesize could be introduced. These would remove more CO₂ and start to add oxygen to the atmosphere. However, the first oxygen produced will be soaked up by the rocks and oceans, so even culturing micro-organisms on an industrial scale might take billions of years for there to be a breathable atmosphere.

Mars

The "Red Planet" has fascinated mankind for centuries; in fact it is the only world whose surface features can be discerned through even a modest telescope. As a subject of amateur observation it is still a challenge due to the subtlety of these features and the small size of its disk. It is these two last facts that have conspired to create the mythology of Mars.

Giovanni Cassini first glimpsed the caps of Mars in 1666. A seasonal wave of darkening on the globe was observed in 1837 by William Beer, so too the expansion and retreat of the ice caps.
In 1877 Giovanni Schiaparelli really accelerated speculation when he reported linear features on the surface. He called these canali - meaning channels or grooves. Across the Atlantic the American diplomat-turned-astronomer Percival Lowell picked up on these reported features, and replicated the observations himself. His mistranslation of canali as canals implied artificial construction and created a mindset that persisted for the next 40 years. In 1895 he published a book called Mars in which he described a cool, arid dying world kept barely fertile by the network of irrigation canals designed to transport water from the ice caps to the deserts. The wave of darkening was explained as vegetation that bloomed every Martian spring. The sheer size of this canal network implied a technology far in advance of our own. This enthusiasm also led him to build the Flagstaff Observatory specifically for the purpose of studying the planet in more detail.

Science fiction writers around the world responded to this evocation, none more so than Edgar Rice Burroughs, who in his 11 volume Barsoom series described a Lowellian Mars inhabited by egg-laying princesses!
H.G.Wells succumbed to the lure, but in a chillingly different vein. The War of the Worlds (1898) imposed aggressive territoriality onto the dying world scenario, and also pointed out the catastrophic impact of microbial infection - a lesson very much at the fore of today’s planetary exploration strategies (Incidentally Earth gets its revenge in the sequel Edison’s Conquest of Mars, written by G.P.Serviss). No one capitalised more on the belief in intelligent Martians than Orson Welles in his legendary radio broadcast of 1938 where he described the invasion of Earth to a panic-stricken audience.

By the 1930s however, science had revealed a new model of Mars that contradicted the Lowellian view. The thin atmosphere, intense cold and virtual lack of water forced fiction writers to reappraise the romantic myths. Stanley Weinbaum (A Martian Odyssey - 1934) and Arthur.C.Clarke (The Sands of Mars - 1951) both portrayed the grim reality of life on a desert planet, while Ray Bradbury (The Martian Chronicles - 1950) seeks romance in a Martian past, which comes to haunt the present day explorers of this arid world.

The space age began with a desire to reach Mars. Werner von Braun, the famous rocket engineer proposed a grandiose mission in 1952 (Das MarsProjekt) and NASA had ambitious plans post-Apollo for a manned Mars landing. Sadly these hopes were dashed for a variety of reasons, not least of which were the grainy results from Mariner 4 and subsequent space probes that showed an absolutely desolate world - more like a red version of our Moon than any desert on Earth.

However, the mythology lives on in two images returned by the Viking orbiters in 1976 - one shows a series of "pyramids", the other a sculpted "face" on Mars - both wind carved features have been adopted as evidence for a race of Martians that predated our own civilisations, and possibly influenced them. High resolution images returned by the Mars Global Surveyor have affirmed the rational hypothesis that these features are random rock formations.

Physical properties

Mars is half the diameter of the Earth and takes 687 days to go round the Sun in one of the most elliptical orbits in the solar system (radius varies from 128 to 156 million miles). It rotates once every 24 hours 37 min. - a Martian "day", or sol. The surface gravity is approximately 1/3 that of the Earth, and its mass is only 1/10 of Earth’s. It has a smaller density than the Earth and is not thought to possess an iron core, rather a semi-molten iron sulphide/iron oxide core.
Until the arrival of Mars Global Surveyor in 1997, there was no measurable magnetic field, but the existence of a very weak field has now been confirmed for the first time, which has important consequences for the ability to sustain life.
The Martian atmosphere is 95% carbon dioxide, 2.7% nitrogen with the rest made up of argon, carbon monoxide, oxygen and water vapour. The air is very thin - the surface pressure is less than 1/100th that of the Earth. Atmospheric dust makes the sky pink. The winds vary from gentle gusts of 20 km/h up to supersonic migrations of polar gases from north to south, which are responsible for major dust storms.
The great distance from the Sun and lack of retaining atmosphere means that on average, Mars is very cold. Summer temperatures can climb above freezing, but winter temperatures are down to -130° C. This causes the carbon dioxide to freeze, especially at the poles where it forms a temporary cap.

At its closest approach even small instruments can discern detail on its disk. The polar ice caps are seen to grow in winter and retreat in its summer. There are darker markings on its surface that shift in outline and sometimes disappear as the planet is enveloped in global dust storms. These are the result of soils of different albedos. Mariner 9 was the first probe to send back high-resolution pictures of the Martian surface in 1971. It was followed by the twin Viking spacecraft in 1976. The pictures they took revealed a heavily cratered world, like the Moon. Many of these craters are eroded, implying weather.
The biggest volcanoes in the solar system are to be found on Mars. The most awesome is Olympus Mons which rises 24 km above the surrounding desert and has a base 500 km wide. Some of these volcanoes may still be active, but it is uncertain. Mars doesn't appear big enough to have fully fledged plate tectonics. A huge canyon big enough to cross the United States - the Valles Marineris seems to be a rift fracture similar to the Red Sea.
Geologically, Mars divides into a recently resurfaced northern hemisphere which comprises smooth lowland lava floodplains and is the site of most of the volcanoes, and a much older heavily cratered southern hemisphere which is the site of large impact basins.
The red colour is due to rich abundances of iron oxides/sulphides in the soil - it really is a rusty planet!

Water on Mars

The current temperatures and pressures on Mars mean that liquid water is unstable on the surface - it would either boil away or freeze solid, as some has done at the North Pole. In fact any water present in the soil should be frozen to a depth of several kilometres.

Images from the Mariner and Viking orbiters hint at a much wetter past, and provide abundant evidence for the action of liquid water and ice in the planet’s history. The Martian uplands are peppered with dendritic channels that resemble terrestrial river systems. The debate has been ongoing about their cause, but now the generally accepted view is that most of these were initiated by the removal of water from beneath the surface and subsequent ground collapse, not the free flow over the surface. Steep sided channels are thought to be the product of glaciation, rather than liquid water. Michael Carr of the US Geological survey suggests that these processes finished early on in the Martian history at equatorial latitudes - elsewhere they may have persisted, especially if geothermal heating was present.

More spectacular are the outflow channels, especially concentrated around the Chryse Basin and along the edge of the highland regions. These are massive (100km wide, 2000km long) channels that contain teardrop shaped islands and are heavily scoured - indicating a high volume, rapid flow of liquid. The most likely explanation for their formation is that they are the result of catastrophic floods bursting forth from underground reservoirs or aquifers that were disrupted by volcanic activity/meteorite impacts. In some cases the flow rate reached 1 km³ per second! The inferred ages of these channels shows a wide variation, they have occurred throughout Mars’ history. This explanation does not require any change in the conditions on Mars in the past, but does support the notion that vast amounts of water are stored deep underground, especially in the highlands. The Mars Pathfinder landed at the mouth of one of these channels - Ares Vallis, and its results could throw fresh light on the possible formation of such features.

Elsewhere on Mars there is evidence of subsurface water. Many craters have a "splashed" appearance implying impact melting of the permafrost. The great Valles Marineris shows layered sedimentation that could have arisen from an ancient lake. In fact, several geologists have suggested that episodic lakes and oceans could have occurred in the northern hemisphere, producing a relatively warm, wet climate for several million years at a time before boiling off or freezing. The triggering for this would come from large scale melting of ground ice by the gigantic volcanoes in Tharsis and Elysium, together with a massive release of carbon dioxide. This periodic inundation could have occurred right through Mars’ history, each era finishing with extensive glaciation. (Parker, Baker - JPL, Kargel, Strom - University of Arizona). It must be emphasised that this is still a highly controversial area, and further evidence from Mars Global Surveyor, Mars Odyssey, Beagle and the fleet of probes destined for Mars over the next 5 years may resolve these questions.

The Viking Landers

Vikings 1 and 2 were designed to study Mars in great detail. They were launched in 1975 and arrived at Mars in 1976. Each spacecraft consisted of two parts: an orbiter and a lander. The Viking 1 and 2 orbiters studied Mars from orbit for four and two years respectively, returning thousands of images of the planet. The two Viking landers descended through the thin atmosphere and landed on the surface of Mars. They eventually became the longest-surviving active laboratories on the surface of another world, far surpassing their original six-month design lifetime. Lander 2 provided information on the Martian environment for about four years and Lander 1 for more than six years.
Numerous experiments designed to photograph, probe, and sample the Martian surface were packed into the 600-kilogram Viking lander. Each lander was powered by two radioisotope thermoelectric generators, which convert heat to electricity from the radioactive decay of plutonium 238. The landers required 70 watts of power, less than that needed by most light bulbs.
Each Viking lander was equipped with two identical cameras that scanned a vertical segment of the Martian scene using a moveable mirror, and photodetectors recorded the amount of light reflected into the camera. Completing a vertical scan and then rotating the camera slightly for the next scan made a complete picture, or "image", of the surface.
During the first three months of the Viking Primary Mission, more than 1,500 individual pictures of the surface were relayed to Earth by the landers. Following this initial flurry of activity, selected images were taken to note changes due to seasons and dust storm activity.
In addition to searching the landscape for large life forms with the cameras, three types of experiments were carried out to determine whether the Martian soil contains any form of microscopic life. The landers performed each of these experiments repeatedly on different samples of soil using the Viking Lander Soil Sampler - a robotic scoop.
The first of the biology experiments, the Gas Exchange Experiment, was designed to test whether minute organisms lying dormant in the soil would come to life after addition of water or organic compounds. Both oxygen and carbon dioxide were given off after addition of water, but their release could have been caused by decomposition of the soil.
The second experiment, the Carbon Assimilation Experiment, assumed that organisms would thrive in the carbon dioxide-rich atmosphere of Mars, and would incorporate or assimilate carbon from the atmosphere in their life processes. Although some carbon compounds were produced during this experiment, they could also have been caused by chemical reactions in the soil.
The third experiment, the Labelled Release Experiment, tested whether life processes were present by monitoring the release of radioactive gas introduced to the sample in the form of nutrients. A rapid release of carbon dioxide occurred after the first addition of nutrients, consistent with biological activity. However, the amount of carbon dioxide soon diminished, suggesting that Martian organisms were not responsible. This last experiment has caused the most controversy - especially since its designer finds it difficult to pass off the results as easily as conventional wisdom dictates.
Although the experiments underwent rigorous testing on Earth under a variety of conditions, the sheer chemical reactivity of the Martian soil seems to have taken scientists by surprise. It is now accepted that the soil contains no organic compounds, hence no carbon based life, but is capable of oxidising and reducing chemical reactions that mimic life processes. The lack of an ozone layer in the atmosphere allows powerful UV rays to strike the surface and create an abundance of highly activated molecules - in fact the soil is thought to be so reactive that it would effectively sterilise any microbial life within it. This doesn’t of course discount the possibility of life further down beneath the surface.

Martian Moons

Mars has two tiny moons, Phobos (fear) and Deimos (panic). These are irregular chunks of rock, 30km and 16km across respectively. They are scarred and pitted with numerous craters and resemble carbonaceous asteroids in composition. Jonathan Swift described their discovery by the Laputan astronomers in Gulliver’s Travels that bear an uncanny resemblance to the actual discovery made by Asaph Hall 150 years later!
In the 1940s astronomers found that that the orbits of these bodies (Phobos in particular) showed an unexplained secular acceleration towards the planet. In the 1960s several astronomers were seeking to explain this anomalous behaviour in terms of a very low density moon - one hypothesis suggested that Phobos was actually a hollow artificial satellite (for a more detailed analysis see Intelligent Life in the Universe by C. Sagan and I Schklovskii).
Some theories suggest that they were indeed captured asteroids, ensnared early on when Mars had a more extensive atmosphere to slow them down.

Martian Meteorites

A number of meteorites have been identified as originating on Mars - this is done by looking at the composition of the rock itself, and also by analysis of the gases trapped in tiny bubbles in it. The mixture of gases in the Martian meteorites matches that of the Martian atmosphere as measured in the Viking experiments.
The meteorite that was announced to have fossil microbes in it is designated ALH84001. ALH refers to the Allan Hills in Antarctica, where it was found. The "fossils" are tube-like structures found within carbonate globules in the rock. They resemble bacteria, but - at 10 to 100 nanometres in size - are about a thousand times smaller. This alone has caused much debate as to whether they are really the remnants of life - can anything as complicated as all the biological systems in a micro-organism fit into such a small space?
The tube-like structures contain tiny crystals of iron sulphides and magnetite.
Terrestrial bacteria can form crystals like these, but they are most commonly precipitated inorganically. The carbonate globules contain a lot or organic compounds, but again these can also be formed without help from biology. The fossil cells also show no evidence of any cavities inside them - living cells require lots of fluid for biological processes to take place.
The rock of the meteorite itself was formed 4.5 billion years ago, whilst the carbonate globules within it date somewhere between 1.4 to 3.56 billion years old. It was ejected into space about 16 million years ago, and fell to Earth about 13,000 years ago.
If the carbonate globules are 3.56 billion years old, then they would have formed when Mars resembled the early Earth: it would have been much warmer than today and may have had oceans - both conditions which would have encourage life to develop. This therefore adds weight to the interpretation of the microbes as biological in origin. However, if the carbonate globules are as young as 1.4 billion years old, then by that time Mars would have lost most of its atmosphere and frozen into much the state we see it in now - not a likely scenario for life to develop.
The temperature at which the carbonate globules formed is also a matter for debate. Two different techniques have been used to estimate the temperature and they have given contrasting figures of more than 500^oC and less than 100^oC. The former is far too high for the carbonates to have been deposited by biological activity - organic materials fall to pieces at much lower temperatures than this. 500^oC would indicate that the carbonates had formed during an asteroid impact. The "less than 100^oC" is an acceptable temperature for life around hydrothermal vents (deep-sea hot springs).
The carbon in the carbonate globules is enriched in the lighter of the two common carbon isotopes - ¹²C. On Earth, the major process that enriches carbonates in this way is photosynthesis. The enrichment occurs because less energy is wasted when organisms use the light ¹²C instead of the heavy ¹³C to produce organic material. This is the best evidence for the fossils being genuine Martians. A more recent analysis of ALH84001 in the US have indicated that the polyaromatic hydrocarbons (PAH’s) and some other features are more likely the result of terrestrial contamination during the 13 000 years on Earth – a dash to the recent optimism!

Future Exploration

The priorities for any future missions to Mars are:

• Look for subsurface ices
• Look for more suitable landing sites
• More searches for life
• Return samples to Earth
• Survey large areas using a Mars rover
• Manned exploration of Mars
• Exploration of Phobos and Deimos

Whilst many of these clearly lie decades in the future, the fleet of missions planned over the next 7 years will attempt to answer many of these requirements. The successful landing and deployment of Mars Pathfinder/Sojourner on July 4th 1997 was a vital first step in testing the technology for surface exploration. The Mars Global Surveyor went into orbit in September 1998 and has surveyed the surface in unprecedented detail, as well as acting as a communications satellite for the armada of subsequent probes. The 2001 Mars Odyssey orbiter added high resolution thermal imaging capability. Mars Express is a joint venture between Europe and NASA and carries with it the Beagle 2 lander designed by Prof. Colin Pillinger of the Open University. It has specific experiments on board to look for preserved life on the Isidis Planitia. Also bound for Mars are two robotic explorers as part of the 2003 Mars Exploration Rover Mission. These are very similar to the Sojourner rover. NASA's Mars Reconnaissance Orbiter, scheduled for launch in 2005, will be equipped with cameras to zoom in for extreme close-up photography of the martian surface, carry a sounder to find subsurface water and look for safe and scientifically worthy landing sites for future exploration. The long term goal is to return samples to Earth and this will happen before 2014 according to NASA's roadmap.

Moons

Jupiter has 16 named moons, the largest four of which were discovered by Galileo Galilei in 1610.

Until the Voyager flybys, very little was known about these fascinating bodies. More recently the Galileo orbiter has provided a wealth of information on Ganymede, Callisto, Europa and Io. The latter two are the most likely environments for organic processes.

Saturn has the biggest family of moons in the solar system, 18 named, many smaller moonlets. Voyagers I and II both brought back information on these bodies, but planetary scientists are eagerly awaiting the rendezvous of the Cassini probe with this gas giant in 2004. One of the primary mission objectives is to deploy a probe into the atmosphere of Titan, Saturn’s largest moon.

Europa

Europa is the smallest of the four Galilean moons, but it is still the 6^th largest satellite in the solar system. With a diameter of 3,138 km, Europa is slightly smaller than our own Moon.

Europa is the smoothest object in the solar system. The satellite has a mostly flat surface, with nothing exceeding 1 km in height. The surface of Europa is also very bright, about 5 times brighter than our Moon. There are two types of terrains on Europa's icy crust. One type of terrain is mottled, brown or grey in colour and consisting of mainly small hills. The other type of terrain consists of large smooth plains criss-crossed with a large number of cracks, some curved and some straight. Some of these cracks extend for thousands of kilometres. The cracked surface appears remarkably similar to that of the Arctic Ocean on Earth. The crust may be no thicker than 150 km.

There are very few craters observed on Europa, particularly large ones. The lack of craters indicates a young age for the surface, perhaps as young as 30 million years old. The inner core of Europa is suspected to be iron-sulphur, similar to that of Io. Since Europa has a lower density than Io (3.01 gm/cm³), the size of the inner core is expected to be smaller than Io's. A tenuous atmosphere of oxygen has been detected on Europa.

There is a possibility that a liquid ocean exists under the icy crust of Europa. The ocean may be present due to warming from a tidal tug-of-war with Jupiter and the other Galilean satellites. Similar tidal heating drives the volcanoes on Io. Recent Galileo images have provided evidence that Europa had a liquid ocean or "warm ice" underneath the crust, but it is not clear if this ocean exists to the present day. Water geysers may exist on Europa, though none has yet to be observed. The presence of a weak magnetic field implies a conductor in motion, with salt water being the favourite candidate.

Of the four Galilean moons, Europa was the most poorly observed by Voyager. Galileo has recently completed an extended mission - the Galileo Europa Mission, which focused on an intensive study of Europa.

If an ocean exists on Europa, then it may be possible that life exists in these oceans. What would the energy source for this life be? Because of Europa’s distance from the sun and the layer of ice covering the surface, sunlight would not be a viable energy source. Photosynthesis is therefore not possible.
Chemical energy - as used by bacteria round Earth’s hydrothermal vents - may be a possibility. If we assume that Europa has a sulphur-iron core and mantle similar to that of Io, then we would expect to find volcanic activity beneath Europa’s icy crust. Indeed, some of the surface features on the ice may be indicative of this degree of heating from below. Hydrogen sulphide and iron are both utilised as an energy source by anaerobic bacteria on Earth.
Whether organisms larger than a bacterium could develop on such an energy source is a matter of speculation. It is notable on Earth that multicellular creatures did not evolve until oxygen was present, and large multicellular organisms did not develop until the volume of oxygen in the atmosphere had got above 1% (today’s atmosphere contains 20%). The only free oxygen on Europa exists above the ice - it is formed by splitting of the ice molecules into hydrogen and oxygen by sunlight. The hydrogen is quickly lost to space, but the heavier oxygen is trapped for a while longer.
The sources of hydrogen sulphide and iron may be very localised, so Europa’s hydrothermal vents would be little oases in a barren desert sea. Any organisms - large or small - would have to cross this barren wasteland to reach the next source of food. Bacteria or plankton may simply shut down to a spore form and drift with the currents. Larger animals would have to store up food reserves at one vent to survive the long journey to the next. In such a situation, being able to detect the vents would be a very valuable skill, so a highly developed sense of smell to home in on vent chemicals would be expected to develop. The sulphide itself is one likely "smell" to detect, but oceanographers on Earth detect vents by looking for volcanic helium in seawater. If any animal is capable of evolving a detection system for such an inert element, then Europa is the place to find it.

Io

This is the most bizarre world in the solar system. It is about the size of our Moon but resembles a pizza in appearance. The yellow surface is peppered with active sulphur volcanoes that deposit molten sulphur in parabolic plumes. The source of all this internal energy is the tidal flexing of the moon as it is tugged by the combination of the other Galilean satellites and the massive gravity of Jupiter itself. The Galileo probe has returned some spectacular images of this tortured moon. The abundance of sulphur and thermal energy could well support life processes in the same way that undersea fumaroles do here on Earth, but they will have to contend with the high levels of radiation that swathe mighty Jupiter.

Callisto

This moon is larger than Io and Europa, but further from Jupiter. Visually it seems to be quite different, having a dark soily surface peppered by bright white impact craters. The reason for this contrast is that this moon is made primarily of soot and ice, the impacts that generated the craters excavated this ice and deposited it on the surrounding darker soil.

Galileo has detected a weak magnetic field around this moon which must have as its origin a conducting fluid. NASA scientists have postulated the existence of a salty ocean beneath the crust – another possible site for life.

Titan

Titan is a fascinating place; the second largest moon in the solar system (again, bigger than Mercury) because it is the only moon to have its own thick atmosphere. This atmosphere is mostly nitrogen (85%) and argon (14%) with a small amount of methane (1%) present. It is this that gives rise to its orange colour as it undergoes a photochemical reaction with sunlight - similar to terrestrial smogs. Under the clouds the conditions are just right (94K, 1.5 atm) for ethane to exist at its triple point, so scientists visualise deep oceans of liquid methane and ethane with ethane "bergs" floating in them under a steady rain of methane or ethane snow. These conditions closely mimic primordial compositions on Earth, despite being much colder, and complex organic molecules might have had a chance to develop.

The temperatures there are far too low for any sort of biological activity, but a large comet or meteor will however, hit Titan every so often. Impacts like these will release a lot of heat, and the surface round the impact site may take hundreds to thousands of years to re-freeze. If this timescale is large enough for life to get started, then Titan may have had many origin events. If a microbe which was tough enough to survive the millennia of freezing between each impact evolved, then it is possible (but unlikely) that Titan may have a boom and bust ecology just waiting for the next good times to roll.

The Huygens Probe will parachute into the atmosphere of Titan on Jan 14^th 2005 and will land on the surface some 2 hours later. This will give us much more of a privileged position to analyse any potential life processes.