Australasian Science: Australia's authority on science since 1938

Life On Mars?

By Morris Jones

New NASA claims of Martian life in a meteorite discovered in Antarctica haven’t convinced astrobiologists.

We’ve looked across space through telescopes, thrown dozens of space missions at the planet and studied chunks of Mars that have fallen to Earth. Still, we don’t know for sure if there is life on Mars, but that hasn’t prevented vigorous and sometimes caustic debate for decades.

Throughout all this, one Australian scientist has held his ground as a well-informed skeptic, rebutting much of the opinions and wishful thinking that has surfaced. Prof Malcolm Walter is the Director of the Australian Centre for Astrobiology at the University of New South Wales. He has more than three decades of research experience in tracing the development of early microbial life on Earth, and has extended his reach to the search for life on other planets.

He genuinely believes that life existed on Mars in its ancient past, and probably still exists there today. But let’s get irrefutable proof before we make any claims to have discovered it! So far, according to Walter, solid evidence for life on Mars is still lacking despite the claims of some other scientists.

The first phase of seriously searching for life on Mars came in the 1970s, when two Viking landers touched down on the surface. Each lander had a small biological laboratory. Soil samples were incubated with various mixtures of water, gases and nutrients to see how they would react. The goal was to find evidence of metabolic processes such as photosynthesis or the consumption of sugars by microorganisms in the soil.

Most of the tests produced no evidence of life. One experiment, known as “labelled release”, tracked the absorption of radioactively labelled carbon for evidence of biochemistry, and produced some strange chemical reactions. Some scientists, especially the principal investigator for the experiment, are convinced that this was evidence of life. Walter disagrees.

“I don’t think that Viking discovered any evidence of life. The labelled release experiment could have been biological or non-biological. But overall, there is no evidence that convinces any skeptic that Viking detected life on Mars.” Walter’s opinion is shared by most astrobiologists, but the debate is not entirely resolved.

In 1996, interest in life on Mars flared again after a meteorite retrieved from Antarctica in 1984 was found to have originated from Mars. The meteorite, dubbed ALH84001, was a small, potato-sized rock. Studies of gas bubbles fused within the meteorite matched Viking data on the Martian atmosphere, leading to few doubts about the origin of the meteorite.

Other claims, however, were more debatable. A team of NASA scientists claimed that the meteorite contained evidence of fossilised microorganisms. If this were true, it would be the first evidence of life beyond Earth.

Evidence for the claim was based on a handful of independent factors, including the discovery of organic chemicals in the meteorite and strange mineral structures that resembled rod-like bacteria. There was also the discovery of tiny crystals of magnetite, a mineral sometimes produced by microorganisms to help them navigate. The organisms use the crystals as magnets, helping them to orientate themselves with respect to the Earth’s magnetic field.

After a blaze of publicity, debate about the alleged microfossils in the meteorite settled down and the initial wave of interest gave way to skepticism as the channels of evidence were attacked.

Walter is especially critical of some rod-like microscopic structures in electron microscope images, which some say could be fossils. “Palaeobiologists are used to seeing lots of shapes that could be biological. We have learned from experience that it’s necessary to have the mineral fabric context to make convincing interpretations. Shapes seen in scanning electron micrographs such as those published for the Martian meteorite lack that context.

“Initially, it was proposed that there were five or six lines of evidence for life in the meteorite. After extensive study by the authors themselves and others, that reduced, in my view, to one line of evidence. That was the suggestion that the magnetite crystals might be biogenic.”

So, the magnetite is still a possible indicator of life, but Professor Walter is holding his ground. “I think the evidence (of magnetite) is intriguing, but one line of evidence is not sufficient to establish that there was life on Mars.”

Discussions went fairly quiet after 2000, but the pace has changed recently. In 2009 and 2010, some of the NASA scientists behind the 1996 studies of ALH84001 returned to the fray, brandishing two new scientific papers. One even ran under the strongly assertive title: “Life on Mars: New Evidence from Martian Meteorites”. With veteran NASA scientist David McKay as its lead author, this paper not only claimed that “biomorphs”, or evidence of life, were found in ALH84001, but in two other Martian meteorites. Much of the evidence focused on strange shapes and features shown by electron microscopes.

As Walter explains: “We have two papers, one of which is the result of years of thorough geochemical and mineral research, which has added substantially to our understanding of the meteorite, and another which is a study of vague fossil-like structures in meteorites.”

The authors of the paper seemed more convinced by their latest studies than in 1996, when scientific discussions by Martian optimists were still tinged with skepticism. Despite the revival of faith, there were few waves beyond the pool of strong believers.

“The detailed mineralogical study is a substantial advance over what we used to know,” Walter says, “but I also noticed that there was almost no discussion of the biological significance of the observations. What they concluded is that the mineral and geochemical analyses were not inconsistent with the biological interpretation of some of the evidence, which I read as a very weak statement about the possibility of biological evidence within the meteorite.”

The new studies place a greater emphasis on the magnetite crystals that were previously studied in the 1996 releases. Does this evidence look any stronger to Walter? “No, and other studies of the process of formation of magnetite crystals have opened the possibility of non-biological origins for such magnetite crystals.”

So, more than a decade after a tiny rock from Mars made a very public debut, the debate over its possible fossil content still rages, but Walter feels it’s time to move on. “I personally think we’ve gone as far as we can go with meteorites. It was an interesting approach to the search for life on Mars, but it has failed.”

Walter feels that the next logical course of action is to continue the robotic exploration of Mars with spacecraft. He is eagerly anticipating the launch of new missions such as the Mars Science Laboratory, a huge rover scheduled for launch by NASA in late 2011. Other missions on the cards include ExoMars, a joint US–European rover expected for later this decade. Russia is expected to launch a sample return mission to the tiny Martian moon of Phobos in 2011, carrying a small Chinese orbiter with it. Other NASA missions to land on the surface and explore the thin Martian atmosphere from orbit are also anticipated.

“Beyond the current missions planned, the most significant mission we could fly is sample-return. This may not be launched until 2018 or later. The reason why it would be important is that we would then have samples from defined and known locations on Mars, which provides context. That contrasts with the meteorites, where we don’t know their context. Context allows more convincing interpretations of the data.”

Walter does not expect any samples returned from the surface to contain life, or pose a risk to life on Earth. “It’s a hazardous environment, and it’s unlikely that organisms will be collected from there. In the extremely remote chance that there were live organisms on the surface, they would be very different from those on Earth, and would be unlikely to survive outside a laboratory.”

Any life on Mars today is likely to be found in an underground region, according to most scientists.

Despite the controversy and lack of evidence, Walter remains convinced that search for life on Mars must go on. “I think it is probable that life was and still is on Mars. We know from all the robotic missions that, early in its history, it had an environment similar to early Earth, and we know from our work in the Pilbara region of Australia that there was life on Earth in the same period. Why not on Mars?

“If there was no life there, that’s interesting in its own right.”

Dr Morris Jones is an Australian space analyst and writer.