Professor Malcolm WalterBiography
Director, Australian Centre for Astrobiology, UNSW
Malcolm Walter is Professor of Astrobiology at the University of NSW and Director of the Australian Centre for Astrobiology based there. He has worked for 45 years on the geological evidence of early life on Earth, and more recently on the search for life on Mars. He has also worked as an oil exploration consultant and a consultant to museums. In 2004 he was elected Fellow of the Australian Academy of Science. He has been awarded a Eureka Prize for his interdisciplinary research.
The oldest convincing evidence of life is in 3.5 Ga rocks in the Pilbara region of WA and the Barberton Mountainland of South Africa. Diverse microbial communities lived in environments ranging from volcanic calderas to open marine settings. There is tenuous evidence for life in 3.8-3.9 Ga rocks of Greenland, but the record is obscured by the pervasive alteration of all known rock successions of this age and older. Thus there are no known well preserved rocks older than 3.5 Ga - so, no convincing fossil record. The best we can do at present is to infer the earliest stages of life from studies of extant organisms and from theoretical and experimental approaches to the origin of life.
The oldest known macroscopic organisms preserved in the rock record are dated at 2.1 Ga. Their affinities are obscure, but they may have been algae. There is chemical evidence for eukaryotes (nucleated sexually reproducing organisms) back to 2.7-2.8 Ga, and tentative morphological evidence at 3.0 Ga or even earlier.
Oxygen isotope studies hint at the possibility that the oceans of these times were hot, perhaps 60-800C or so, despite the low luminosity of the Sun. An enhanced greenhouse resulting from high CO2 concentrations in the atmosphere is the likely cause. The bulk of the evidence indicates an atmosphere and hydrosphere with little or no free oxygen and oceans with little sulfate. Oxygenation of the surface of the Earth resulted from oxygenic photosynthesis by cyanobacteria.
It is almost inevitable that there once was and probably still is life on Mars. There is a traffic of meteorites between Earth and Mars, and given the fact that microbes live at depths of up to several thousand metres in the Earth’s crust (the source of some meteorites) microbes are likely to have been transported from one planet to the other. However, far more significant is the possibility of a second origin of life on Mars.
Since the earliest days of missions to Mars in the 1960’s it has been known that liquids flowed on the surface of the planet early in its history. Dry river beds abound in terrains older than 3 Ga. The Viking missions of the 1970’s photographed water frost and more recently spectroscopic observations from orbiters indentified both water and carbon dioxide ice, especially at the poles. In 2008 the Phoenix lander achieved direct analyses of water ice. The evidence is that early in its history Mars was warm and wet, and thus habitable, and that habitable niches have persisted to this day.
To date no life has been detected on Mars. False leads have included Martian meteorite ALH84001 and spectroscopic observations of methane in the atmosphere, now discredited.
Missions planned for the next 20 years, including landing large laboratories and eventually the first two-way mission, to return samples, can now be targeted at the most promising sites. It is reasonable to expect that before the end of this century we will know whether Mars once was, and maybe still is, inhabited.