Possible Life on Mars and on Moons of Outer Planets

During the second half of the 20th century, space research advanced in many ways, from the launch of the first artificial satellite to orbit the Earth in 1957. The Russian satellite Sputnik-l was followed by a programme of more ambitious spacecraft missions, and with the first manned orbital flight in 1961.

Further developments by Russia, and increasingly by the USA, produced manned space stations and the Apollo series of missions to the Moon. With wider international participation in more recent years, it became practical for unmanned and more complex spacecraft to be sent out on deeper missions into space, to investigate what conditions may be like on other planets and moons in our solar system.

From the time when telescopes were invented at the beginning of the 17th century, astronomers looking at Mars with its polar caps and some detail of surface markings, have wondered if life of some kind could exist there. Of the two nearest planets, Venus and Mars, the planet Venus was found to have very hostile conditions, with a very high temperature and atmospheric pressure, caused by its closer orbital position around the Sun. The extreme conditions on Venus were confirmed as a result of the first landing on the planet, by a Russian spacecraft in 1970.

In contrast, on the other side of Earth's own orbit, Mars is the most Earth-like planet in several ways. Although Mars is much smaller and further away from the Sun, it is considered to be within the region of the Sun's 'habitable zone', where life of some kind could have evolved. Mars has no lasting water on its surface, because of the very low atmospheric pressure at present, and surprisingly it is calculated that Mars has about the same total land area as that of our own planet. The length of a day on Mars is about the same, and the axis of rotation is also similar at present, producing seasonal variations during its rather longer orbit around the Sun.

The presence of water is regarded as being essential for the existence of life anywhere, and Mars has a plentiful supply of water ice and carbon dioxide held in its polar caps. The northern ice cap of Mars is by far the most substantial of the two, and in conditions of melting would be sufficient to cover the surface with water to a depth of many feet. A study of the Martian surface by telescope and orbiting spacecraft has shown evidence of water flows on the surface. This can only happen if there is adequate atmospheric pressure, so conditions must have been different during an earlier period of Mars history.

The astronomer Carl Sagan expressed the strong" belief that life on Mars must once have existed on Mars, and the search for evidence of past life on Mars, perhaps continuing as living organisms of some kind, has been started, with spacecraft missions and the landing of robotic vehicles on the planet.

In 1976 two VIKING mission spacecraft landed on Mars, with the aim of testing soil samples for the presence of life. In these experiments nutrients were added to the soil and heated, with the expectation that any microbes or organisms there would react, with the release of carbon dioxide, or methane. Some of the results were very surprising and positive, but the reasons for such a positive response were uncertain.

It was thought that a chemical reaction of some kind could be an alternative explanation instead of the presence of life. Some of the experimentation had failed to identify any organic molecules in the soil. However, Dr Gilbert Levin, one of the principal specialists of the mission, had conducted a key experiment in the search for life and was sure that the presence of living organisms had been confirmed.

Because of the uncertainty, the official NASA announcement as a result of the mission was that there was no evidence of life on Mars, and that experiments were continuing in attempts to find an alternative explanation for the results which had been obtained.

In 1980, Dr Robert Jastrow, founder and director of the Goddard Institute for Space Studies, announced that following investigation of data from the Viking mission soil tests, an unmistakable signal for life had been identified.

No alternative explanation for the Viking results seem to have been found, and quite recently it has been acknowledged more openly that the results were inconclusive. It has also been pointed out that the complexity of the results were to be expected in comparison with inert samples containing no life of any kind, and that this complexity was exactly what the scientists had found.

In recent years there has also been the detection of substantial areas of methane on Mars. Although this could have a biological origin, it is thought that this could also be caused by volcanic activity, but there is no evidence of such activity on the surface at present. A marked decrease in the amount of methane has also been found following its discovery, so it is thought unlikely that it is being replenished on a continuing basis.

Apart from Mars, there are several other candidates for the possible existence of life in our solar system. Proximity to the Sun and its warming effect is an obvious factor for the potential development of life, but there can be other causes of heating. The huge gravitational attraction of the giant planet Jupiter is known to have produced the extensive volcanic activity observed on its close moon Io. The extent of the activity was a major surprise when the Voyager mission passed Jupiter and near Io in 1979, before continuing a tour of the outer planets. Jupiter's moon Io was confirmed as being the most volcanically active moon in the solar system.

Jupiter's more distant moon Europa was also found to be showing the effects of gravitational attraction. Europa's greater distance from Jupiter has preserved an icy surface, but this was observed to be cracked extensively, and with evidence of movement like large ice floes.

It has been concluded from the observations that there must be an ocean below the ice, and with the water being warmed by the tidal effects of Jupiter’s gravity. In this case the conditions in a warm ocean could be very suitable for the evolution of life, but any space mission to land on Europa and to drill through its thick icy crust, with the purpose of investigating the ocean below would be a very major project.

The more recent CASSINI spacecraft mission to Saturn and its moons has also produced surprising results, with a flyby of its moon Enceladus identifying many fountains of watery jets and ice particles coming from what must be a sub-surface sea. The spacecraft has passed through the vapour from this several times, and has found from analysis the same level of salinity as that found in our own planet's oceans. The leader of the Cassini Imaging Science team has suggested that with a subsurface sea, organics and an evident energy source, there could be the same type of life as that found in similar environments on Earth.

The other major and perhaps the best candidate for life on one of the moons is Saturn's largest moon Titan. There have been significant surprises, because this moon was not expected to have a very substantial atmosphere. As it turned out, when Titan's atmosphere was sampled and analysed, it was found to be several times denser than our own atmosphere.

The Cassini mission also carried the European Space Agency's HUYGENS space probe, which separated from the main spacecraft and landed successfully on the surface of Titan. During its descent many images of the surface were obtained, with these showing lakes and rivers found to contain methane. As a result, it is thought that Titan could have some of the same conditions which existed on Earth during its early history, and that the evolution of life there is a distinct possibility.

On our own planet, we are quite familiar with many of the life forms which share the world with us, both animate and plant life, existing in its different lands and oceans -from the warmer equatorial areas to the coldest parts of the continents and Polar Regions. With recent and continuing discoveries, it now seems that life of some kind is able to exist everywhere on the planet, and in the most extreme conditions, in deep caves away from any daylight, close to the heat of volcanic activity and even near volcanic vents on the ocean floor where small creatures have been found in quite poisonous surroundings, and in conditions of great pressure caused by the depth of water. Such living creatures have been given the name extremophiles because of these conditions in which they have become able to live.

For the most recent development in the search for life, we should look back again at the planet Mars. On August 6th, 2012, the most complex landing of the heaviest and most advanced robot vehicle was achieved successfully, and with great accuracy at the site of Gale crater, a location where observational evidence had already suggested the likely presence of water in the past. The vehicle, named Curiosity, soon gathered soil samples from its immediate surroundings, before beginning further exploration of the area. Almost 40 years after the original Viking mission and soil tests, there is a preliminary report that the latest finding has already identified organic compounds in the Martian soil. If confirmed, the presence of organics may finally indicate that some kind of life elsewhere, if only in its basic forms, is an actual reality, and that life has evolved on more than one planet in our solar system.

Geoff Falla