The Red Planet has fascinated man for centuries, ever since the first telescopic observations that revealed pole caps, seasonal changes and signs of atmosphere. A new study is likely to re-open the debate about the possible presence of fossils in Martian rocks, even though it is unlikely that the claims made will be accepted.
For many decades the nature of the pole caps was debated, with many different theories being proposed over the years. William Herschel, writing towards the end of the 18th century was the first to suggest that they might actually be composed of much thicker ice although, for many years, starting with Cassini, the most popular theory was and would continue to be that the pole caps could be no more than very thin layers of frost because they advanced and retreated too quickly each year to be composed of thick ice. Another theory that was popular for a time was that the pole caps might be formed mainly of nitrogen tetroxide, while at different times the evidence has swung most heavily in favour of solid carbon dioxide pole caps with little, or no water ice. Although Herschel’s views on the planets contained many errors, or at least things that were wildly inaccurate, his view that Mars had a very thin atmosphere, thick pole caps and transient clouds was finally to be vindicated in the 1970s.
Of course, now we know that most of the theories were, at least in part correct: the pole caps contain ice hundreds of metres thick, demonstrating that, whatever has happened to the planet in the meantime, Mars retains a lot of water, even if only in frozen form, while a seasonal frost of carbon dioxide ice coats the surface in winter. We only have to look at this amazing image of a crater in Vastitas Borealis, the immense basin around the Martian North Pole that many experts believe is an ancient ocean bed, to know that both types of ice abound on Mars. We see both a thick water ice in the centre of the crater and blue-white carbon dioxide frost around the crater rim.
There is plentiful evidence of large deposits of ice both on and below the surface of the planet – one of the features of both Mariner 9 and Viking images was the quantity of splosh craters detected on the planet, in which an impact apparently occurred in permafrost and caused a massive, temporary outflow of liquid mud from the crater. There are plenty of examples such as Yuty crater (left), imaged by Viking 1 that hint at large amounts of sub-surface ices.
With evidence that Mars still harbours water in large quantities, either in its pole caps, or in the sub-surface, the debate on life on Mars that seemed largely to have been killed-off by the Viking landers, has come back to life steadily. Even if there is no life on Mars now, maybe life did develop in the past and we can find evidence of it on the surface.
Planetologists have largely accepted that Mars had a warm, wet past, but still argue bitterly about how long this blue marble phase of Mars lasted. While some evidence points to Mars having had extensive oceans and thus a much denser atmosphere, for a significant fraction of its history, other experts point to evidence that suggests that any warm, wet past of Mars with a relatively thick atmosphere lasted a few hundred million years at most. The argument is fundamental because, at one extreme, Mars would never had had time for life to evolve – although the evidence seems to suggest that life appeared on Earth very quickly after the surface solidified – while, on the other, the chances of simple life, even multicellular organisms, developing on Mars are greatly enhanced.
However, bear in mind that complex life took four thousand million years to appear on Earth. The appearance of such classic marine animals on Earth as Trilobites did not occur until the Pre-Cambrian explosion, around 540 million years ago… more than 4000 million years after the formation of the Earth. Even the first multi-celled Eukaryotes, with a cell nucleus have only been around for the last third of the Earth’s existence.
Although Earth’s oceans were teeming with life almost from their earliest days, for two thirds of the Earth’s history you would have needed a microscope to see evidence of it. So, unless life on Mars evolved far more rapidly than life on Earth, you would only expect to find evidence of fossils with a microscope. That is precisely what a team announced in 1996, finding both unexpectedly high concentrations of organic compounds in the Martian meteorite ALH84001 and microscopic worm-like structures that they suggested could be fossils. While the consensus now is that the “fossils” of ALH84001 are not biological, there are still a few hold-outs in the community that argue that they are true fossils.
What astrobiologists would like though, is to find clear evidence of fossils in areas where it is known that water has flowed in the past. These might well though be buried deep under the surface, even if they exist. On Earth, plate tectonics lifts up areas that were previously ocean floor and exposes them, a phenomenon that appears not to have happened on Mars. However, the jackpot for any Mars rover would be to find an exposed fossil bed somewhere on the planet, although any hypothetical pre-historical Martian lifeforms may look nothing like their terrestrial equivalents: in fact, one wonders if, with our anthropogenic and Earth-centred view, we would even recognise genuinely alien life – as Mr Spock might say, it is not logical that life, but not as we know it, should have two arms, two legs a head and look like an actor in a rubber suit! Alien may mean too alien to recognise (and certainly not English-speaking!)
A paper just submitted from the Lunar and Planetary Institute suggests that small, dark structures observed in a rock dubbed Haroldswick on Vera Rubin ridge, within Gale crater may be the first evidence of large fossil life on Mars. It is suggested in the paper that this area used to be, perhaps 3500 million years ago, a large, fresh-water, rather than brackish lake. While the Curiosity Rover team suggests that the dark features in the insert image are Gypsum crystals, deposited from salty water, growing into the rock, the rival suggestion is made in the paper that these could be burrow holes made by ancient Martian worms.
It is fair to say that, after the ALH84001 affair, this claim will be the hardest of hard sells. It is unlikely to gain much credence. After two Sols at the site, Curiosity’s controllers moved the rover on to a new site, eleven metres away, ending investigation of Haroldswick. Evidently, the Curiosity team saw nothing extraordinary in these dark structures and, had there been any suspicion in their minds that they might not be ordinary, crystalline deposits, you can bet your bottom dollar that they would have stayed at that rock for as long as was necessary. Haroldswick may not be evidence of ancient, fossil life, but the debate about the possibility of finding fossil evidence of life on Mars is not going to go away any time soon. The different claims that have been made show though just how hard it will be to settle the issue definitely until we are able to analyse Mars rocks brought back to Earth from multiple different sites on the Red Planet.
Extraordinary claims require extraordinary proof, which these images are not, but the hope that one day we will obtain it should be reason enough to keep exploring our sister world. We may prove that life did develop on Mars, or that the planet has always been sterile: either way the philosophical implications are enormous.
 As always, in such cases, there are alternative interpretations including the idea that the impact occurred on a pocket of sub-surface carbon dioxide ice, causing the equivalent of a pyroclastic cloud.