The search for life: Astrobiology

By Monica Grady

Life on other planets

Knowledge of where micro-organisms flourish, and the wide range of habitats occupied by them on Earth, helps to pinpoint where life might exist elsewhere within the solar system. Given the extreme circumstances under which life can be sustained on Earth, Mars is the most likely location. The Red Planet has been extensively studied in recent years by the Viking and Pathfinder missions, and from evidence gleaned from (currently) twenty Martian meteorites. Jupiter's satellites, Io and Europa, are other good places to search for traces of life because they have conditions similar to those where extremophiles survive on Earth. In this session we will explore the bodies thought most like to harbour life--considering them in order of distance from the Sun.

 

Why should life be elsewhere?

During the formation of the solar system, the Earth and presumably other planets suffered intense bombardment by asteroids and comets. Comets in particular carry water and rich organic compounds, considered to be the crucial ingredients for life. If they could deliver such materials to the Earth, then they could also deliver them to other planets and their satellites. Therefore there is the potential for life to have started in other habitable niches throughout the solar system.

 

Mercury, Venus and the Moon - lifeless planets

It is unlikely that life exists, or ever existed on Mercury, Venus and the Moon. Mercury is home to a particularly hostile environment--daytime temperatures soar to 450°C while night-time temperatures drop to -180°C and the planet is engulfed by fierce solar radiation. Venus is further from the Sun but its dry, dense atmosphere of carbon dioxide produces a greenhouse effect that results in a surface temperature even hotter than Mercury's. The Moon has been explored first hand by astronauts who brought material back to Earth for study--it is a dry, desolate and barren body and there is no indication that life has ever existed there.

 

Exploration of Mars

Like Earth, Mars is a rocky planet, although with a diameter approximately half that of the Earth. Gravity is only about 40 percent that of the Earth, and the atmosphere, which is much thinner, is predominantly carbon dioxide rather than nitrogen. The atmosphere affords little protection from heat loss and temperatures are very low--the average daily temperature is around -60°C. Although the core and mantle structure is similar, its crust is far more rigid than the flexible plate system operating on Earth. Essentially, Mars exhibits extreme examples of the features shown by the Earth: for example, Olympus Mons, its biggest shield volcano, is almost three times as high as Mount Everest. Piles of magma have built up because limited plate movement does not allow for new crust to be taken back down through subduction zones.

Images returned by the NASA (National Aeronautical Space Administration) Mariner 9 orbiter mission of 1971-72 reveal a layered terrain in Mars' polar regions. These could indicate sedimentary rock (i.e. laid down in water), channel and valley networks that bear striking similarities to river and stream features on Earth. These were assumed to demonstrate beyond doubt that Mars must have had significant quantities of running water coursing across its surface at some time in its history. NASA's two Viking missions of 1976 confirmed Mars as a rocky planet with the two hemispheres displaying established geological histories. The northern hemisphere consists of flat, low-lying plains, with little cratering, whilst the southern region appears more ancient, with cratered highland regions cross-cut by channels, canyons and valley networks.

The Pathfinder mission of 1997 recorded spectacular images of a rock-strewn plain, showing rounded pebbles and layered structures consistent with the presence of water at times in the past. Chemical and image data collected for rocks and soil implied that some might be sedimentary. The landscape of part-rounded pebbles and boulders could also be interpreted as evidence of catastrophic flooding--further evidence of water at some time in the planet's past.

Martian surface

The surface of Mars at the Pathfinder's landing site, showing boulders of different colours and shapes. Although the Pathfinder mission did not include experiments specifically intended to test for traces of past or present life at the Martian surface, it was part of NASA's programme of missions leading up to the return of samples from Mars to Earth. © NASA/JPL/Caltech.

The most recent high resolution images of Mars' surface come from the Mars Orbital Camera, on board the Mars Global Surveyor, and these reveal the channels and valley networks in even greater detail. Surface features have thus been re-interpreted: it is clear that, unlike the river systems on Earth, tributaries do not flow into the main waterways, indicating that either the channels were formed by ice, rather than liquid water, or that the flow was below the surface of the planet. Whatever form it took, the implication is still that water was relatively abundant at some time, perhaps as recently as 1 million years ago, and indeed may still be present in sub-surface locations today.

For water to have been present on Mars in the past, the atmosphere must have been much thicker and surface temperatures much warmer than they are today. A thicker atmosphere ensures greater protection from solar radiation, and with wetter conditions, all the elements suitable for the emergence of life, in theory, would have been in place. However, experiments conducted on materials as the result of the Viking lander expeditions concluded that there was no detectable trace of organic matter and thus no likelihood of the presence of life.

Despite these conclusions, the discovery of micro-organisms on Earth that can survive in conditions previously thought hostile to life have continued to drive the search for extraterrestrial life on Mars. Future expeditions, such as the European Space Agency's Mars Express, due to arrive in December 2003, and the Japanese Space Agency's orbiter Nozomi, due to arrive a month later, are hoping to reveal, through sophisticated testing techniques, traces of life in Mars' surface and sub-surface sediments.

 

Meteorites from Mars

Information obtained from meteorites has been vital to the interpretation of data returned by the Viking and Pathfinder missions. There are (currently) twenty Martian meteorites, distinguished from 'regular' meteorites on the basis of their younger ages down to 165 million years. Some of the meteorites contain gases and pockets of black glass: the glass formed by shock melting of mineral grains during the impact event that broke the meteorites from their parent. The composition of the gas trapped within the glass is identical to that of the atmosphere of Mars, so scientists concluded that the rocks undoubtedly originated there.

The Pathfinder data showed that rocks on the surface were much more silica- and calcium-rich than the meteorite material, implying that they had been through several cycles of heating and melting, rather than having simply crystallised from a single magma. Thus there is evidence of a diversity of rock-types on Mars, but many remain unidentified due to lack of mineralogical data. Studies of minerals within Martian meteorites has, however, revealed how much Mars' atmosphere has changed since the planet formed, and how much water has flowed across the planet's surface.

 

Life on Mars?

Shapes found embedded in carbonate patches in a meteorite in Antarctica in 1994 (Allan Hills 84001) led NASA scientists to claim that they had discovered evidence for a primitive 'fossilised Martian biota'. This, however, remains highly controversial and is the subject of much debate. Carbonates almost certainly did form on Mars as the result of the slow evaporation of warm, briny water locked in enclosed basins. This is an environment in which, as has been seen on Earth, sulphur-loving micro-organisms may be sustained. Despite the surface water having now dried up, it is possible that liquid still resides in the subsurface soil layers, housing such bacteria. Until sufficient Martian material is available for detailed study, such theories will remain purely speculative.

 

Exploration of Jupiter's satellites

Jupiter is a gas giant planet beyond the Asteroid Belt and is the largest planet in the solar system. The movement of cloud belts encircling the planet and the huge bolts of lightning that have been recorded by spacecraft flying nearby, indicate its turbulent nature and energetic environment. Jupiter's environment is extremely inhospitable and it is unlikely that anything could survive there. Some of Jupiter's satellites are a better prospect--there are 16 and they all vary in composition. Two of the largest, Io and Europa, are similar in the size to the Moon and are judged to possess a range of properties and environments that maintain the potential to harbour life.

 

Io

Io is composed of metal and silicates; it has a dense core of iron and iron sulphide surrounded by molten silicate overlain by a thin silicate crust. Thermal activity (volcanoes, geysers and vents)--the result of its heating by the tidal forces of Jupiter's gravitational attraction--ensure a steady silicate lava flow that is constantly refreshed. Surface temperatures range between -150°C and 300°C depending on locality and proximity to thermal vents. Despite no evidence of water, it is considered a potentially habitable body for sulphur-loving species, based on its thermal properties alone.

 

Europa

Europa is seen as the most exciting body in the solar system on which to search for traces of life. Information and images from NASA's Galileo mission in 1995 indicated that it is made of metal and silicates, but with a significant water-ice content. Sunlight reflecting from the ice makes Europa's surface one of the brightest in the solar system. Scientists have found evidence of a sub-surface salty ocean beneath the thinner water-ice shell. Europa features many of the ingredients necessary for life, including water, and heat energy (generated by the gravitational tug from Jupiter). There has been much speculation that Europa's ocean might be heated from the bottom upwards by hydrothermal vents similar to those found in Earth's oceans. If this is the case, these vents might host a rich variety of organisms.

Further exploration of Europa is being planned. Scientists hope to be able to verify the presence of sub-surface water and are testing their experiments at Lake Vostok in Antarctica. The viable micro-organisms that have been found in ice cores provide valuable information on primitive terrestrial micro-organisms that might have evolved along a different path from their surface-located relatives.