In 2005 the Cassini spacecraft is due to release the Huygens probe, which, if all goes to plan, will descend through the atmosphere of Titan to its surface, taking measurements along the way1. The results of these measurements, along with any taken on the surface if Huygens survives impact, will be eagerly awaited by the scientific community back on Earth, for it is hoped that conditions on Titan may give clues to the emergence of life on Earth and may even point towards the development of life on Titan.
What causes Titan, the largest satellite of Saturn, to be the subject of such intense scrutiny is its resemblance to the early Earth. It is known that the Earth was formed around 4. 5 billion years ago and it is speculated that the earliest detectable life forms existed at least four billion years ago2, so it can be reasonably assumed that the very early events in the history of the planet hold the key to the origin of life. The essential conditions for life, at least as we know it on Earth, are water, supplies of the necessary chemicals from which living cells are constructed and energy.
This energy may be light or other forms of radiation, or chemical energy. It is also necessary that these conditions exist on a planetary-sized body, in order that gravity is strong enough to prevent the loss of water to space. Surface temperature, too, is important, since it is necessary for water to exist in the liquid state. The atmosphere of the early Earth probably consisted mainly of CO2 and N2, together with small quantities of other volcanic gases such as sulfur dioxide and methane. There was no oxygen, but water vapour would have been present.
In 1944, the astronomer Gerard P Kuiper discovered the existence of an atmosphere on Titan via the recording of its spectrum. It has since been discovered that the atmosphere is comprised of 85-95% nitrogen, with most of the remainder being methane plus a few simple organic molecules. This nitrogen-rich atmosphere, together with similar pressure and density, are what promote comparisons between present-day Titan and the early Earth. However, a major difference is in the temperature. Titan is a moon of Saturn and is therefore resident in the “outer” solar system.
Consequently it has a surface temperature of just 94 K and gases which are minor, but very important in Earth’s atmosphere, carbon dioxide and water vapour, are frozen solid on Titan3. This is a crucial point because all known life requires liquid water as a solvent. However, this does not discount the existence of a different form of biochemistry, as yet unknown, on other worlds. Carl Sagan, for instance, has speculated that on extremely cold worlds where water is frozen, liquid ammonia may act as the necessary solvent.
In any event, since the Universe is cooling, there would probably have been a time when a younger Titan was home to liquid water. Life is believed to have developed through a process of chemical evolution. , whereby simple building blocks such as amino acids were able to develop, either in the atmosphere or on the ocean surface, through the energy provided by UV radiation from the Sun and from the electrical energy of lightning. Polymers are believed to have formed, largely based on hydrogen, carbon and oxygen.
The problem, however, is that hydrolysis limits the length of self-condensing polymers, preventing the formation of RNA and DNA. (It has been suggested that clay minerals may have provided the necessary environment to overcome this problem and in 1996 research was published showing that mineral surfaces can encourage longer chains to form5. ) However this problem was overcome, it is likely that very long polymers were formed with a membrane structure, allowing the development of nucleic acids and, eventually, living cells.
There is also speculation that this process didn’t occur on the Earth at all and that life had an extraterrestrial origin, being carried to Earth from space in comets, meteoroids and dust particles. The consensus, however, is for chemical evolution and the study of conditions on Titan may provide vital clues to the development of this process on Earth. Jeffrey L. Bada proposes yet another scenario – that life formed in the ocean beneath the ice layer of a frozen Earth.
Warmth from the Earth’s interior would have ensured that water remained liquid beneath an ice layer around 300 metres thick and life could have developed via a reaction known as Strecker synthesis. This involves the reaction of gaseous hydrogen and methane. The products react with ammonia in water to produce amino acids. The required elements can all be found around hydro-thermal vents. Until recently it was thought that extreme environments like this would be unable to sustain life. However, in 1977 deep-ocean explorers discovered diverse ecosystems thriving in just such conditions. Bada further speculates that the earth may then have been thawed by cosmic collisions. Ice has a higher albedo than water and if a large enough hole was made the decreased albedo could have caused a runaway melt. 7 It is known that Titan is comprised mostly of ice, but early in its existence, when it resided in a hotter Universe, it is possible that it had liquid water on its surface. If that is the case, it could be possible that life developed on Titan before it became frozen and, if it did, it may still persist deep in Titan’s interior.
It would appear that Titan – the only body in the Solar System other than Earth to have a ‘thick’ nitrogen atmosphere – is one of the prime candidates for exploration and investigation into the development of the early Earth. Although it differs from Earth in many ways, there are also many crucial similarities and in the Cassini spacecraft and Huygens probe it is hoped that some questions regarding the development of life may shortly begin to be answered.