Many hypotheses abound regarding the conditions under which life began on Earth, and many origin scientists have compared these scenarios with conditions on other worlds with the hope of discovering extraterrestrial life. As space science and technology surge ever forward, the possibilities for life developing elsewhere within the Solar System – and beyond – continue to broaden.

The Cassini-Huygens mission, developed by NASA and the European Space Agency, has been exploring the saturnian system since entering Saturn’s orbit in June 2004, studying its magnetosphere, complex ring system and numerous moons. Saturn’s sixth largest satellite, Enceladus, has recently become the subject of intense interest to scientists studying the origins of life, as it may have the essential criteria for chemical evolution to take place.

Located within Saturn’s diffuse blue E ring, Enceladus is intensely bright, reflecting almost all solar radiation that strikes it. The high geometric albedo is caused by its reflective icy surface, which has a temperature of about -200 degrees Centigrade [1]. It has a complex frozen topography of ancient cratered terrains, mountain ridges, plus recently resurfaced ice flows and fractured plains [2]. The moon’s south polar region shows signs of active geology including cryovolcanism, faulting and other crustal deformities: a landscape resulting from internal activity such as tidal force, indicating a liquid interior. Parallel surface fractures at the south pole are stained with blue/green organic material and show signs of an underground heat source. Instruments on board Cassini detected an enormous plume emanating from this region, emitting water vapour, ice and dust particles [3]. The water being ejected from these warm vents supplies oxygen not only or Enceladus’ substantial atmosphere, but also the entire saturnian system [4]. Analysis of the moon’s spectra suggest the presence of ammonia, carbon dioxide and simple organics [5].

By comparison, the conditions on the young Earth of 4 000 Ma ago were similar to the conditions on present day Enceladus. From the planet’s origin until about 3 800 Ma ago, the Earth had a very violent setting [6]. There is no physical evidence on Earth of bombardment from space due to the planet’s early geological activity. However, evidence from geologically less active bodies within the inner solar system suggest that Earth was subjected to an intense early bombardment of projectiles left over from the formation of the solar system. There is, likewise, no evidence today of when the first oceans formed. Ancient rocks in Greenland reveal a sedimentary structure likely to have been deposited partially in water, inferring the Earth’s oceans were formed by 4 000 Ma ago [7]. The composition of the Earth’s early oceans is likely to have been saline, containing high levels of the reduced ion, iron(II) – a soluble compound derived from rock minerals – which would have been abundant due to the lack of oxidising conditions at the time, which would otherwise convert iron(II) into its alternative ionic form, the insoluble compound, iron(III) [8]. The oceanic chemical environment is also likely to have contained methane and carbon dioxide, originating from marine volcanoes and hydrothermal vents.

4 000 Ma ago, Earth’s early atmosphere probably contained substantial amounts of carbon dioxide and nitrogen. These gases are abundant in the atmospheres of neighbouring Venus and Mars (unaffected by life-giving oxygen), and are emitted during volcanic eruptions, common to the young Earth [9]. Atmospheric oxygen did not accumulate until about 2 000 Ma ago [10]. Consequently, there was no protective ozone layer – conditions, therefore, on land, would have been unsuitable for the evolution of life due to harmful UV radiation from the Sun.

The high levels of atmospheric carbon dioxide may have caused a strong greenhouse effect, this, coupled with potentially less cloud cover (reducing planetary albedo) may have resulted in a high global mean surface temperature (GMST). Another scenario for Earth’s climate of 4 000 Ma ago is one which considers a low solar constant due to a weaker Sun (emitting 20 – 30% less energy compared to today), resulting in an increased planetary albedo and a GMST below 0 degrees Centigrade, with a net result of the freezing of the oceans [11] – comparable conditions to those on present-day Enceladus.

A view accepted by the majority of origin scientists is that life on Earth came about by the process of chemical evolution. This idea has spawned many theories concerning the origin of life on Earth. Until the 1950s, many believed that chemical evolution took place in the atmosphere, which was assumed to have contained high levels of ammonia, hydrogen and methane – a highly reducing environment – where the synthesis of complex organic molecules could take place, resulting in protocells: precursors of biological molecules. This concept was abandoned when atmospheric scientists discovered that UV radiation from the Sun would have destroyed the methane and ammonia.

Another idea was put forward in 1984 by C R Woese, who assumed that chemical reactions occurred on pyrite surfaces which were then thrown up into the atmosphere during bombardment of projectiles [12]. This idea also had flaws due to the assumption that further reactions took place in the atmosphere, now known to have contained mainly carbon dioxide and nitrogen – chemically inert and, therefore, unsuitable for life to evolve.

Other scientists, such as Robert Hazem, have suggested that life may have evolved around maritime hydrothermal vents, using rock minerals as catalysts, reactants and shelters for chemical reactions to take place. Hazem assumes that layered clay minerals provided the surfaces for molecules to assemble and react [13]. The mineral surfaces may have aided the process of polymerisation (subject to the process of hydrolysis), creating long-chain polymers on rocks – a scenario consistent with Earth’s earliest fossils [14]. In 1995, Jeffrey Bada suggested that 3.8 billion years ago, the Earth’s surface was about 97% water, frozen to a depth of about 300 m [15]. He and his colleagues also believed that life started in the oceans around hydrothermal vents.

In conclusion, the reducing environment necessary to kick-start chemical evolution may exist today (or have existed in the past) on Saturn’s moon, Enceladus. Assuming it has a liquid water ocean beneath its icy skin and that, like Earth, is has a hot core producing marine volcanism and hydrothermal vents, the associated minerals and gases (including methane) could, theoretically, provide the necessary chemistry for the formation of biological molecules – the essential ingredients for the chemical evolution of life.

© Melanie Davies 2006