THE COMPLETE COSMOS Chapter 20: Lifequest Is there life elsewhere in the Solar System? Currently, on Jupiter's moon Europa? In the future, on Saturn's moon Titan? Outline A giant star dies in a catastrophic explosion and debris splatters across space. That debris contains heavy elements only produced in gigantic stars. As it travels outwards, the debris encounters clouds of gas and dust. It enriches them with its heavy elements. One such is the cloud from which our Solar System condenses. Once formed, Planet Earth finds itself in the "habitable zone" of the system - at just the right distance from the Sun for life to be possible. Earth has water and is rich in the chemicals - heavy elements included - essential for life. Earth is the perfect incubator. Mars lies just beyond the habitable zone. Today, it is a freeze-dried desert, but it may once have been warmer and had oceans. Some scientists think we may find evidence of past life there. On Jupiter's moon, Europa, there could be a warm ocean beneath its icy crust. Hot vents could support primitive life. Saturn's moon, Titan, is like early Earth in deep-freeze. The ingredients for life are present. Heat is all they need to come alive. Scientists are also searching for life beyond our Solar System. Giant dishes listen for extraterrestrials and robot probes head for the stars. Evidence is mounting that many other stars have planetary systems - formed from clouds of gas and dust that contain water and the building blocks of life. But, so far, none of the planets identified around other stars have the conditions necessary for life. Sub-chapters Star Death, Planet Birth Death of a giant star in a supernova explosion. Shock wave passes through clouds of gas and dust, enriching them with heavy elements from the dead star, and triggering contraction. From such clouds, new stars are born, and each is the potential center of a new star and planetary system. Formation and evolution of our Solar System, with a shining star at the center. Around the Sun are rings of gas and dust that eventually form into planets. Earth forms in the 'habitable zone' endowed with the necessary conditions for the development of life. Bountiful Earth, Barren Mars Comets bring water to Earth. Life is triggered, perhaps, by lightning. The elements from the supernova are essential for living cells. Later, green planets breathe oxygen into Earth's atmosphere. Mars, unlike Earth, is outside the habitable zone. Today, its temperature is too low to sustain any type of life, but this does not discount past life. There is evidence that Mars was once warmer and had oceans. Europa's Oceans Europa, a moon of Jupiter - with a possible warm water ocean beneath an icy crust.
The chance that hot volcanic vents may support primitive life. A possibility for organisms that feed on chemical rather than solar energy. Titan - Laboratory for Life? Farther out lies chilly Saturn with few chances for life. Saturn's largest moon, Titan, may be a laboratory for the origins of life. The basic building blocks of life are present, but for the time being Titan is like Earth in deep-freeze. Five billion years from now, when the Sun swells to become a red giant star, Titan may briefly awaken. Building Blocks of Life The search for extraterrestrial intelligence (SETI). Giant radio dishes listen for signs of intelligent life. Robot envoys are traveling towards the stars. Evidence mounts that other stars have planets. The clouds of gas and dust that pervade our galaxy contain water and organic molecules. Search for New Planets Within such clouds, the Hubble Space Telescope images protoplanetary disks that could eventually form planets. Such disks may be distorted by an orbiting planet. Some stars wobble, a tell-tale indicator of a companion planet - and another star seems to have ejected a planet, but this is later discounted. Brown dwarfs - too big for a planet, too small for a star. Of the suspected extrasolar planets so far discovered, none has the conditions necessary for life - as we know it. Background Signs of Life in a Martian Meteorite In August, 1996, scientists believed they had found evidence that primitive life may once have existed on Mars. The clue came from studies of a 1.9-kilogramme meteorite fragment, labeled ALH84001. It is one a dozen or so meteorites, known as SNC meteorites, found on Earth, but believed to have originated on Mars. Why Mars as the source of these SNC meteorites? The reasoning was that gas pockets trapped inside the meteorites had an almost identical chemical composition to measurements of the Martian atmosphere made by Viking. ALH84001 is regarded as the most ancient of the SNC meteorites. It is thought to have originated in the ancient cratered highlands of the southern hemisphere of Mars. The fragment is an igneous rock that crystallized slowly from magma about 4.5 billion years ago and was fractured during intensive bombardment 3.8 to 4.0 billion years ago. Later, perhaps around 3.6 billion years ago, it spent some time immersed in water that contained significant amounts of carbon dioxide which penetrated the fractures. The Martian climate then changed leaving the planet barren with no liquid water. Then, about 15 to 16 million years ago, another major impact blasted out rocks (including ALH8400l) from Mars. These entered orbit around the Sun. Approximately 13,000 years ago, the rock which became meteorite ALH8400l fell to Earth in Antarctica - to be found by scientists in 1984. ALH84001 is slightly friable and highly fractured It has a concentration of carbonates 50 times greater than other SNC meteorites. While under Martian water, small beads of calciummagnesium-iron-rich carbonates formed in fissures and pores of the rock. There is some debate as to when the carbonates formed. An initial study fixed their age at about 3.6 billion years, but
more recent dating indicated a much younger age of 1.3 to 1.4 billion years. The carbonates occur as 'globules', typically about 50 micrometers across, although some are up to 200 micrometers across. Concentrated within the carbonates are tiny grains of magnetite (iron oxide) and iron sulphide. Their occurrence indicates that the water was hot and chemically reducing at the time. This suggests volcanic vents as the formation source - and that is of great to those studying origins of life. Nevertheless, the existence of carbonates and grains of magnetite and iron supplied could be explained by non-biological processes. Examination of fracture surfaces in ALH84001 show the presence of organic compounds called polycyclic aromatic hydrocarbons (PAHs). There are also tiny tubular or ovoid structures on the surface of the carbonates that have been dubbed 'microfossils'. Interestingly, the mixture of PAH's found on ALH84001 is very different to that found on dust grains and in other types of meteorite, suggesting the possibility of a non-biological origin. Furthermore, the microfossils are 100 times smaller than the smallest microfossils of ancient bacteria found on Earth. Although they are similar in size and shape to fossilized so-called 'nanobacteria', apparently present in some terrestrial rocks, there is considerable skepticism that these objects are fossils. While all the individual pieces of evidence for signs of past (fossil) life in ALH84001 can be explained by nonbiological (geochemical) processes, some scientists think it significant that all these pieces of evidence were found together in one sample. They think that the idea of many separate nonbiological processes occurring together is highly unlikely. It is this which, in their view, suggests a biological origin. All the evidence requires further rigorous scientific examination to test its authenticity, and the possibility of primitive life existing or having existed on Mars cannot be ruled out. The Possibility of Life on Europa Many scientists believe that Europa, one of the four largest moons of Jupiter, may be one of the few places in our Solar System where we might find extraterrestrial life. The other are Mars and Saturn's moon, Titan. What makes Europa such an attractive place to look for some form of life is the possibility that it has both liquid water and volcanic activity. Liquid water is essential for life on Earth. Volcanic activity would provide some of the heat necessary to prevent the water on Europa freezing. It could also provide important dissolved chemicals needed by living organisms. It was NASA's Galileo spacecraft that sent back high quality images of Europa's surface. That is how we know it is covered with ice. These images also hint at a layer of liquid water - a subsurface ocean - beneath the ice. True, there is still no concrete evidence for such an ocean - but the Galileo has considerably strengthened the case. Firstly, the images provide clear evidence of near-surface melting together with movements of large blocks of icy crust in ways similar to icebergs or ice rafts on the Earth. Secondly, there are very few impact craters on the surface. This suggests that this activity took place recently, geologically speaking. The problem is that we have no precise way to measure the exact age of the surface. It is possible that we are looking at an ancient "frozen" ocean, not one that presently exists. Scientists feel the evidence favors an ocean - but that is by no means conclusive. As for the possibility of life in Europa's oceans, this depends on when there was an ocean and how long it lasted - including up to the present. When the Viking landers went to Mars in 1976, most life scientists felt that to have a chance for extraterrestrial life you had to have light (for photosynthesis), liquid water, and oxygen. Since then we have discovered places on the Earth (for example, hydrothermal vents on the ocean floor and geothermal hotsprings) where life is currently sustained in the dark, without oxygen, using the heat and chemical energy from volcanic gases dissolved in superheated water. Some of these life forms such as thermophyllic (or heatloving) bacteria are among the most ancient types of life on Earth.
Many scientists now speculate that Earthly life may actually have arisen under such conditions. So, of course, we are now more interested in places like Mars and Europa where there may have been liquid water and volcanic activity as a place to look for primitive life. If there is an ocean on Europa, there is no easy way to estimate the chances of life could existing there. But that is exactly the question scientists are trying to answer by continued exploration. In other words, we must go and look. It is no easy task. Europa's distance from the Earth and Sun and its thick layer of ice, will make exploration difficult. For the time being, the Galileo spacecraft will continue to observe Europa. Beyond that, NASA is considering a possible Europa orbiter mission, perhaps with landers. Such a mission might answer the ocean question by combining a number of techniques. One is to use radar to penetrate the ice and measure its thickness. Another is to make very precise gravity and altimetry measurements to observe the tides raised by Jupiter on Europa. These will be much larger - as great as 40 meters or so - if there is a liquid layer. Titan - An Example of Early Earth in Deep-Freeze Titan is the largest of Saturn's moons. With a diameter of 5,150 kilometers, it is the second biggest moon in the Solar System. Titan is the only planetary moon known to have a dense, opaque atmosphere. Many scientists believe that the chemistry in Titan's atmosphere may be quite similar to that existing on the Earth several billion years ago before life began releasing oxygen into the air. For this reason, Titan may provide clues to the primeval Earth. Titan's fascination, meant that the Voyager 1 spacecraft was targeted to make a close fly-by in November 1980. Unfortunately, Voyager found that Titan's surface was completely hidden by a thick layer of photochemical haze at about 200 km altitude. Several distinct, detached haze layers were visible above the opaque haze layer. The haze layers merged with the main layer over the north pole of Titan, forming what scientists first thought was a dark hood. Under the better viewing conditions of Voyager 2, the hood was found to be a dark ring around the pole. The southern hemisphere was slightly brighter than the northern, possibly the result of seasonal effects. When the Voyagers flew by, the seasons on Titan were early spring in the northern hemisphere and early autumn in the south. The Voyagers confirmed that most gas in Titan's atmosphere was nitrogen, as on Earth. The second most abundant was methane. There was also ethane and several other hydrocarbons. The thickness - the extent - of Titan's atmosphere was about ten times that of Earth. Titan's atmospheric pressure near the surface was 1.6 bars, some 60 percent greater than Earth's at sea level. The atmospheric temperature near the surface was about -178 degrees Celsius, only 4 degrees above the triple-point temperature of methane. This is the temperature at which methane can exist simultaneously as a solid, liquid and gas at the same temperature and pressure. Methane, however appeared to be below its saturation pressure near Titan's surface. This means that rivers and lakes of methane probably don't exist, in spite of the tantalizing analogy to water on Earth. All the same, scientists believe that lakes of ethane do exist and that methane is probably dissolved in the ethane. Through a series of photo-chemical reactions, stimulated by ultraviolet light from the Sun, the methane in Titan's atmosphere is converted to ethane, acetylene, ethylene, and (when combined with nitrogen) hydrogen cyanide. Such carbon-based compounds represent the first stages in the formation of the building blocks necessary for the formation of life. But Titan's very low temperature almost certainly inhibits more complex organic chemistry. Nevertheless, in some five billion years time, when the Sun swells into a
blazing red giant, Titan may receive sufficient heat for more complex reactions to take place. This, however, will be a brief spell. The lifetime of the red giant Sun will probably be too short for the sustained evolution of life. The Cassini spacecraft will provide more information about Titan. Launched on October 15, 1997, Cassini is due to arrive at Saturn in June, 2004. Later that year, Casini will release the European-built Huyghens probe for a parachute descent through Titan's atmosphere, followed by a touchdown (or splashdown!) on the surface. The Cassini orbiter will then have more than 30 encounters with Titan during its orbital tour of Saturn. The orbiter will map the moon's surface with a synthetic aperture radar similar to that used by the Magellan spacecraft when mapping Venus. Links for Further Information Lunar and Planetary Institute site, featuring a balanced view of the debate concerning the evidence for life in Martian meteorites, including explanations of recent scientific papers, with insightful and objective commentaries. http://cass.jsc.nasa.gov/lpi/meteorites/mars_meteorite.html The possibility of life on Europa and investigations of its surface, with lists of related web sites, books and journal articles. http://www.msoe.edu/~tritt/sf/europa.life.html The Galileo spacecraft page with many images of Europa and the other moons of Jupiter. http://www.jpl.nasa.gov/galileo Images of the surface of Europa acquired by NASA spacecraft. http://www.msoe.edu/~tritt/sf/europa.images.html A page with clues to the possibility of life on Europa, and supporting evidence which may gathered from the Antarctic. http://www-b.jpl.nasa.gov/galileo/news11.html A page of facts about Titan, with evidence for the possibility of life. http://www.msoe.edu/~tritt/sf/titan.html A page on the nature of possible life on other planets - including Europa - within our Solar System. http://www.msoe.edu/~tritt/sf/life.html From Space Science News, a page about looking for life in extreme environments. http://science.msfc.nasa.gov/current/events/ast21aug98_1a.htm Jean Schneider's comprehensive Extrasolar Planets Encyclopaedia, with latest news, overview of detection methods, searches, bibliography and reports, list of related web sites, and the Extrasolar Planets Catalog, which includes brown dwarfs and all extrasolar planet candidates to be confirmed. http://www.obspm.fr/planets The Extrasolar Planets Catalog (extracted from the Jean Schneider's site). http://wwwusr.obspm.fr/departement/darc/planets/catalog.html
Questions and Activities for the Curious 1. When massive stars die in supernova explosions, why is this important for the formation of planets and the evolution of life? 2. Explain what is meant by the term "habitable zone". Give examples of one planet which lies in the habitable zone around our Sun and one that is outside. 3. Describe the conditions on the young Earth. Show how they were very different from the Earth we know today. 4. Explain why some scientists believe we might find primitive life on Jupiter's extraordinary moon Europa. 5. Research the scientific program known as SETI (Searches for Extraterrestrial Intelligence). Give arguments for and against this type of research activity. 6. What message would you send to an alien civilization and why? 7. If astronomers did detect conclusive signs of life on another planet, what effects do you think this would have on our society? 8. Describe the search for extrasolar planets - the hunt for planets around other stars. Give two examples of such planets. Say whether you think the conditions are right for life as we know it to have evolved there.