We can be certain that life was around on Planet Earth around 3.5 billion years ago, if not before, because unmetamorphosed sedimentary rocks of that age from Western Australia in which stromatolites occur contain a black to brownish, structureless material known as kerogen. The material is a hodgepodge of organic compounds that form during the breakdown of proteins and carbohydrates in living matter. It is the source material for petroleum compounds when kerogen-rich rocks are heated during burial. The vast bulk of organic compounds preserved on Earth are in the form of ancient kerogen, whose mass exceeds that of the living biosphere by about 10 thousand times. A good sign that it does represent ancient life lies in sedimentary kerogen’s depletion in ‘heavy’ 13C compared with 12C (negative values of δ13C), because in metabolising carbon dioxide living cells preferentially use the lighter of these two isotopes. Conceivably, 13C can be removed from inorganic carbon by metamorphic processes, so low values of δ13C in metasediments from West Greenland might be organically derived or, equally, they might not.
At the time of writing, geoscientists specialising in Martian matters had become excited by some results from the geochemical system aboard the surviving functional NASA rover. Curiosity has slowly been making its way up Mount Sharp at the centre of Gale Crater near to Mars’s equator. Analysis of high-resolution images taken from orbit suggest that the rocks forming the mountain are sediments. the lowest and oldest strata are suspected to have been deposited in a crater lake when conditions were warmer and wetter on Mars, about 3 billion years ago. Curiosity was equipped with a drill to penetrate and sample sediment unaffected by ultraviolet radiation that long ago would have destroyed any hydrocarbons exposed at the surface. In late 2016, before the rover had reached the lake sediments, the drill’s controller broke down. Since then, Curiosity had moved on to younger, less promising sediments. More than a year later mission engineers fixed the problem and the rover backtracked to try again. Heating the resulting samples to almost 900°C yields any volatile components as a gas to a mass spectrometer, results from which give clues to the molecules released.
The Sample Analysis at Mars (SAM) team report a range of thiophenic, aromatic and aliphatic molecules of compounds of carbon, hydrogen and sulfur (Eigenbrode, J.L and 21 others 2018. Organic matter preserved in 3-billion-year-old mudtsones at Gale crater, Mars. Science, v. 360, p. 1096-1101; doi:10.1126/science.aas9185). The blend of pyrolysis products closely resembles those which form from heated terrestrial kerogens and coals, but also from pyrolysis of carbonaceous chondrite meteorites. So, the presence of Martian kerogen is not proven. But the results are so promisingly rich in hydrocarbons that another weapon in SAM’s armoury will be deployed, dissolving organic compounds directly from the drill cuttings. This may provide more convincing evidence of collagen. Yet only when samples are returned to labs on Earth will there be a chance to say one way or the other that there was once life on Mars. The results reported in Science’s 8 June issue will surely add weight to the clamour for the Mars 2020 sample-return mission to be funded. Whether or not there is life on Mars demands a great deal more investment still…
A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook