Julian Hiscox reports on a search for life a billion kilometres away under 30km of ice.
The origin of life poses one of the most vexing questions in modern biology. While great strides have been taken to follow the fossil record on Earth all the way back to the earliest microbes, the road stops about 3.6 to 3.8 billion years ago.
Before this time, we have no idea what happened on our planet because the very processes that permit planetary habitability, such as tectonic plates, rainfall and volcanism, have completely obliterated the geological traces. We know that 4 billion years ago life on this planet would have been impossible because the Earth had just formed and was continually subjected to meteoroid bombardment, the like of which would make the meteorite that wiped out dinosaurs look like a mild downpour.
Looking for extraterrestrial life could be more fruitful. The traces might be slight but could be strewn throughout our solar system. A handful of British scientists are helping compile a chemical catalogue that might help future space missions recognise whether they have encountered life.
We are fortunate to live in an era where DNA manipulation, the synthesis of a plethora of organic molecules and modern cosmology are on the way to being understood in some detail. It is in these subjects that an answer to the origin of life has traditionally been sought. We know that simple biological precursor molecules such as amino acids were abundant on ancient Earth, the so-called primordial soup.
The components of DNA (the A, G, C and T of our genetic alphabet) were also around. These molecules were randomly shuffled and joined together to form short chains that were able to make copies of themselves. At this point, natural selection kicked in and life took off. Biology hints that our earliest ancestors were probably similar to modern-day hyperthermophiles, bacteria than can flourish at 113C.
The search for extraterrestrial life has focused on Mars for quite some time, for the simple reason that wherever we find water we also find life. Mars Global Surveyor , orbiting the red planet, as well as its venerable predecessors, has returned marvellous images of dried-up rivers and oceans. If the climate of the planet was once similar to that of the primordial Earth, there is no reason why life could not have originated on Mars. However, no liquid water can be found on the Martian surface today.
Curiously enough, the answer to the origin of life might be circling around Jupiter, one of the most hostile places in the solar system - with the exception of the surface of the Sun. The spacecraft Galileo is exploring the Jovian system. This probe was named after Galileo Galilei, the early modern astronomer who discovered the four largest moons of Jupiter that still bear his name, the Galilean satellites. Among other priceless data, Galileo has returned a wealth of images that suggest that one of these satellites, Europa, has a liquid water ocean containing more water than is found in the oceans and polar icecaps of Earth.
Unfortunately, we are separated from this ocean by not only several billion kilometres, but also by a layer of ice at least 30km thick. This makes exploration somewhat problematic. However, scientists at the British Antarctic Survey are helping develop a strategy to one day use submersible robots to navigate this ocean. We are getting a trial run, deep beneath a 2km-thick Antarctic ice layer in Lake Vostok. While this is a major engineering undertaking in itself, the idea is to send a spacecraft to Europa capable of melting through the ice and launching a submersible once it hits the water. This robot would then be free to explore the subsurface ocean using a variety of instruments.
Choosing the right instruments for a spacecraft is a complex task - there are many questions that need investigating and only so much room on the spacecraft platform. In the case of Europa, the questions that need answering are: what is the nature of the subsurface ocean and is there any trace of past or present life? One versatile instrument that is a leading contender for a place on a Europan flight is a raman spectrometer. This can simultaneously investigate both inorganic materials (minerals) and material of a biological origin. Raman works by shining a laser on a target surface and analysing the reflected beam. Each compound has a unique signature - a bit like a barcode.
Scientists in the United Kingdom, such as David Wynn-Williams of the BAS and Howell Edwards at Bradford University, are building a database of such barcodes that might one day be used to unravel the mystery of the Europan ocean.
Like the geological and fossil record, we have no clear understanding of the composition of the atmosphere on early Earth - which is important to understanding the origin of life. What we need is a world where the organic building blocks of life are being synthesised in our own era. These processes may occur on Titan, a satellite of Saturn. This place holds a number of distinctions - it is the largest satellite in the solar system and the only one with an appreciable atmosphere. However, being so far from the Sun, the temperature is about - 180C - although such low temperatures are an advantage, as compounds made in the atmosphere are preserved for a long time.
Delivery by comets is thought to be one of the major mechanisms for seeding the primordial Earth both with the building blocks of life and also with water.
A joint Nasa-European Space Agency mission called Cassini is on its way to Saturn. In 2004 it will drop an entry probe called Huygens into Titan. By identifying the compounds that are synthesised and delivered on Titan, Huygens probe may go a long way to helping to provide answers, or lead to better-defined questions, for solving the origin of life on Earth.
Julian Hiscox is a lecturer in the School of Animal and Microbial Sciences, Reading University.