Was there once life on Mars?

Scanning electron micrograph of a structural form found inside the ALH meteorite. The object is less than 1/100th the width of a human hair in size. Image courtesy Johnson Space Center/NASA [23 Kb]

On 1996 August 7, NASA revealed meteoritic samples which may be long-fossilised microorganisms from Mars. Although tantalisingly inconclusive, this is the most direct evidence ever obtained for life outside the Earth. The evidence consists of carbonaceous chemicals, carbonate and magnetite deposits, and oval grains interpreted as minute fossilised bacteria, all lodged in cracks in a meteorite that is believed to be a martian rock more than 3.6 billion years old. The discoveries were made by a team led by Dr David McKay with Dr Everett Gibson of the NASA Johnson Space Center, with a Stanford team headed by Dr Richard Zare, as well as seven other NASA and university research partners. They published the data in Science (vol. 273, p.924) on August 16.

This is not the first time life has been 'discovered' on Mars. Leaving aside the chapter of the canali a century ago, in 1976 the Viking landers produced apparently positive results in their chemical tests for life. Those results turned out to be due to oxidising agents in the soil that had mimicked the activity of living organisms; in fact, the soil was completely devoid of organic material. But it remained possible that organisms might have lived on Mars long ago, when it was warmer and wetter. Have relics of such organisms now been found?

The meteorite in question is called ALH84001, as it was picked up in the Allan Hills region of Antarctica in 1984. It is believed to have been blasted off the red planet by an impact 16 million years ago, and to have landed as a meteorite in Antarctica 13,000 years ago. Meteorites are easy to spot on the barren ice, and they are concentrated in some parts of Antarctica where glaciers converge while the surface ice is continually ablated by the dry winds. But it was only in 1994 that ALH84001 was recognised as having probably come from Mars.

Eleven other martian meteorites are known. They are basalt-like igneous rocks, with ages interpreted as either 1300 or 180 million years. Because of their similar properties, and the low probability of rocks travelling to Earth from Mars, it is thought that they may have been ejected from Mars by a single impact 180 million years ago, only to be fragmented later in space. (In contrast, we have only a single meteorite from the Moon.) They are identified as martian for several reasons. Whereas most meteorites are about as old as the solar system (4.6 billion years), these igneous rocks are too young to have originated on anything smaller than a major planet. Also, they have trapped gases that reflect the composition of the martian atmosphere, ranging from abundant CO2 to the rarest inert gases; and unlike other meteorites, they have isotope ratios that also resemble those found on Mars. In 1989, Ian Wright, Monica Grady and Colin Pillinger (Open University) found organic (i.e. complex carbonaceous) compounds in one of these meteorites. Writing in Nature (vol.340, p.220), they invited chemists to analyse the organic compounds from these martian rocks.

However, ALH84001 is unique. It is identified as martian because its mineralogy (an igneous orthopyroxene) is intermediate between those of other martian meteorites, and because it has a similar oxygen isotope ratio. But it is much older, dating back 4.5 billion years, and contains unique chemistry that was already exciting because it included organic compounds and also carbonates that must have been deposited from flowing water. This rock was apparently fractured by a nearby impact some 4.0 to 3.6 billion years ago, and warm carbonated water seeped into the cracks, at temperatures in the range 0 to 80 deg.C, depositing orange-brown globules of carbonates some 0.1mm across. This is where the signs of biological activity are found. However, the exact conditions are uncertain. Other researchers have given an age for the carbonate globules of only 1.4 billion years, when Mars was much colder and drier than it was in its early years (although liquid water could still have existed beneath the surface in volcanic regions). Others have given a temperature of around 650 deg.C for their formation, which would be too hot for bacteria.

McKay et al. argue that the carbonate globules themselves are similar to some bacterially induced precipitates, although inorganic formation is possible. Also, the brown rims of the globules contain particles of magnetite and iron sulphides, not normally found together except when they are synthesised by certain anaerobic bacteria. Also close to the carbonate globules are abundant organic compounds called polycyclic aromatic hydrocarbons (PAHs), as detected by a dual laser mass spectrometer at Stanford. (The simplest PAHs are naphthalene, with two fused rings, and anthracene and phenanthrene with three rings.) On Earth these compounds can be produced from decay of bacteria, and they occur in coal and petroleum deposits, in diesel exhaust fumes, and in ancient sedimentary rocks – all ultimately of biological origin. However PAHs can be made non-biologically, and they have also been detected in interstellar clouds and in some non-Martian meteorites.

Most provocative are the scanning electron micrographs that may be pictures of fossil martian bacteria. These are abundant oval or rod-like shapes clustered in and around the carbonate globules. They are 20–80 nanometres across and typically about 100 nanometres long. However, these are smaller than any definitely-known terrestrial bacteria by a factor of ten, and possibly too small to sustain bacterial metabolism; moreover, although some pictures show quite distinct specimens, the earlier ones published in the Science paper seem to show the ovoids as emerging from an even finer-grained and irregular matrix, which may mean that they are only mineral structures.

The researchers are properly cautious about this evidence. 'There is not any one finding that leads us to believe that this is evidence of past life on Mars. Rather, it is a combination of many things that we have found,' McKay said. 'The relationship of all of these things in terms of location... is the most compelling evidence.' Gibson added: 'We don't claim that we have conclusively proven it. We are putting this evidence out to the scientific community for other investigators to verify, enhance, attack – disprove if they can – as part of the scientific process.'

The items in question are certainly intrinsic to the ALH84001 rock; they exist deep in cracks that were formed before the rock arrived at Earth, and had never been exposed until fractured in the lab; and samples of the meteorite were sterile, showing that the rigorous procedures used to prevent terrestrial contamination were successful. If the rock were terrestrial, life would be the only likely explanation for these items' coexistence. But two major issues remain: did this rock come from Mars? and are the items really relics of life? Either proposition alone would not be very contentious, but when combined to imply 'life on Mars' they produce an extraordinary claim that requires extraordinary evidence.

Did ALH84001 come from Mars? It is certainly a meteorite, but in view of its unique features, one would certainly like confirmation from other samples before being certain that it is not from a unique type of asteroid. And are its special features due to life? The evidence offered is similar to that used to establish the date of origin of life on Earth, where there are unmistakable fossil bacteria 3.5 billion years old. But other 'fossil bacteria' on Earth are more doubtful, and the 'martian' ones, in view of their tiny and heterogeneous sizes, seem to belong to this category. Moreover the magnetite particles are typical not of ancient terrestrial bacteria, but relatively advanced ones, so they are not an expected feature of ancient life; and cracks in an igneous rock are a strange place to find life. The authors' call for further research is certainly valid. One avenue, of course, would be to visit Mars and search for indubitable rocks of the same type – or, even better, sedimentary rocks which would be a less esoteric home for fossils. More immediately, there could be much more research on the terrestrial bacteria that have recently been discovered living inside rocks deep in the Earth, where no life was expected; much remains to be discovered about their variety, their metabolism, and how they could be fossilised.

Finally, if these are indeed martian microfossils, we find ourselves in a curious philosophical position. Because these traces of life on Mars were identified by their similarities to those on Earth, we marvel that the earliest life on the two planets was so similar. But if it were otherwise, the martian relics would not have been identified as biological! So it may have been inevitable that the first such evidence would lead to the following argument. If Mars and Earth were both populated by such similar organisms, almost as soon as the planets became habitable, then perhaps these organisms had a common origin. They might have originated in comets – though there is no evidence for such an origin. Or they might have originated on Earth, which is after all the biggest and best of the biospheres – but it is much more difficult to blast rocks from Earth to Mars than vice versa. So possibly life originated on Mars, and came to Earth in a meteorite like ALH84001. This would give new meaning to Ray Bradbury's conclusion: we are the Martians now.

John H. Rogers, Director, Jupiter Section
Richard McKim, Director, Mars Section
1996 October 10

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