The Secret Life of Mars

Simon Grose finds consensus and skepticism about evidence of life from a rock on Mars and a meteorite on Earth.

High resolution images of the Martian rock formation dubbed El Capitan show the tracks of NASA’s Opportunity rover and the two round marks it made to take samples of the rock. Close examination of the pictures and analysis of the rock samples have provided the strongest evidence yet that saline water once sat or flowed in this area of Mars. For most expert observers the evidence is conclusive.

“Liquid water once flowed through these rocks. It changed their texture, and it changed their chemistry,” said Dr Steve Squyres of Cornell University, principal investigator for the science instruments on the Opportunity and Spirit rovers.

Australian geologist and Mars theorist Dr Nick Hoffman of the University of Melbourne agrees: “The Opportunity lander has found definite signs of the action of liquid water at the surface of Mars. . . perhaps 3.5-4 billion years ago”.

El Capitan is pocked with 10 mm x 2 mm indentations called vugs – a texture found on Earth where crystals of salt minerals form within rocks as they sit in briny water. If the crystals are later eroded or dissolve in less-salty water, vugs remain like pockmarks to mark their passing.

Further evidence comes from tiny round particles embedded in the rock that could have been formed if minerals dropped out of solution inside porous, water-soaked rock and clumped together. A molten birth through volcanic activity or a meteor impact is also possible for such spherules, but their relatively even distribution across the layered rock makes this less likely.

Yet more evidence comes from evidence of cross-bedding, a pattern formed when some layers in a rock lie at an angle to the main layers. This can be caused by wind or flowing water, but NASA said that the small cross-bedding and concave patterns identified may be vestiges of underwater ridges.

Hoffman says the chemistry of El Capitan provides even stronger evidence for a watery past on Mars. The rover’s spectrometers detected high levels of sulfur in the rock – possibly in salts of magnesium and iron – along with jarosite, a hydrated iron sulfate, and elements that can form chloride or bromide salts.

“On Earth, rocks with as much salt as this Mars rock either have formed in water or, after formation, have been highly altered by long exposures to water,” NASA’s announcement said. “Jarosite may point to the rock’s wet history, having been in an acidic lake or an acidic hot springs environment.”

But this does not fit the topography of the site, according to Hoffman, who points out that the Meridiani Plain where Opportunity landed is not an enclosed basin so the findings are not proof that the surface of Mars once featured lakes or oceans.

“We can’t have had a lake there,” Hoffman said, pointing out that similar “white rock” deposits in another site are above the rim of the surrounding crater.

Hoffman’s favoured theory to explain the evidence of water action on El Capitan is that the salts precipitated out of groundwater in a mechanism that occurs on Earth in low-lying desert flats known as sabkhas. In sabkhas, the water table reaches the surface and evaporates, leaving dissolved minerals to accumulate in layers. He says nodules similar to the spherules found in El Capitan are common in sabkhas, as are the kind of crystals that leave a vug behind when they disappear.

“I envisage groundwater seeping to the surface across the whole Meridiani region and forming a smooth layered deposit that drapes everything – craters, hills, valleys etc. – infilling the lows and smoothing the terrain,” Huffman said.

“All this took place long ago and that groundwater is no longer active.”

This kind of process could explain the formation of the rock and its current state, although NASA speculates that the rocks were formed from sedimentary deposits at the bottom of large water bodies.

Hoffman argues that if there is still liquid water on Mars it will be several kilometres underground – beyond the reach of frigid surface temperatures – and that only here will be a chance to find living things on Mars. While this kind of investigation is beyond the conceivable capacity of Martian exploration for many decades ahead, testing for signs of former surface life could be easier.

He believes that sulfate minerals identified by the rovers could be compared with isotopic anomalies found in sulfates produced by Earthly organisms to provide evidence for or against the presence of living things on the ancient Martian surface. The same isotopic anomalies in Martian sulfates would imply that DNA-based organisms caused them.

“This will require a new Mars mission with a mass spectrometer, similar to those carried by NASA’s original Viking landers and to that lost with the European Space Agency’s Beagle II,” Hoffman said.

One theory that Hoffman admits is looking shakier in the light of the new evidence is his own: that the Martian surface once featured large bodies of liquid carbon dioxide. He says that jarosite identified by Opportunity would not have survived a soaking in CO2. “Since the Meridiani deposits are semi-regional, and we see similar “white rocks” in many other areas of Mars, it significantly downgrades my theories of a global CO2-rich Mars,” he admitted.

Meanwhile, another Australian scientist with theories about life on Mars has been dismissed by the experts but is sticking to his guns. In the heat of the Australian summer, as NASA’s rovers began poking and scratching the chilly Martian dust, a press release lobbed into journalists’ inboxes around the country headed: “Australian Scientists Confirm Past Life on Mars” (AS, March 2004, p.4).

Surely this was a Eureka moment, changing our concept of life and its provenance forever. So why didn’t the announcement set the scientific world in a spin? After all, the research had been published in the Journal of Microscopy after being reviewed by four independent peers, and the authors were reputable. At the time Tony Taylor was a PhD student supervised by Prof John Barry of the University of Queensland’s Physics Department. He is now a postdoctoral research fellow at the Australian Nuclear Science and Technology Organisation.

But Professor Malcolm Walter, Director of the Australian Centre for Astrobiology at Macquarie University, was not impressed. “I’ve had a quick look at their published article and it is nearly all about their observations in the modern environment rather than a comparison with Mars,” Walter said. “When it comes to comparing with the Mars meteorite I don’t believe a word of it.”

This story can remind us how cleverly adapted bacteria are, how clever humans can be, and how astrobiology is trying to take itself seriously. The key characters are not bacteria but magnetosomes, which are found in some bacteria and contain magnetite, a form of iron oxide that is magnetic and known as lodestone when found as a mineral deposit. In bacteria, magnetosomes align their hosts to the Earth’s magnetic field, a survival mechanism in estuarine mud where such guidance is needed to know up from down.

When NASA scientists discovered tiny structures in a Martian meteorite found in Antarctica, one of the signs that led them to assert that these were bacterial fossils was the signature of magnetosomes.

Taylor compared the NASA results with scans of bacteria he found in contemporary Queensland mud. He did this using transmission electron microscopy, a method that does not deliver salient pictures but gives biophysicists enough information to impute the shape and thus the identity of the matter being observed.

Round particles in El Capitan indicate Mars was wet. Image: NASA/ JPL.

Taylor observed that the alignment of carbonate and magnetite structures in the living bacteria and in the meteorite samples were exactly the same. Taylor said in an interview that this “uncanny” relationship was the basis for his published conclusion that, it provides “solid evidence for past life on Mars”. Taylor said his findings puts it “beyond reasonable doubt” that DNA once populated the red planet.

“This is a matter of all of the correct chemical elements being spaced and placed in three-dimensional structures that perfectly match the structures found in bacteria here on Earth,” he claimed. “So it’s more than just looking like life, it looks like and is composed of exactly the same materials.”

Yet Taylor admits that this conclusion copped “a mixed reception” in astrobiological circles. His original article was submitted to the journal in 2001, but in the more than 2 years between then and its publication he says that NASA and Australian experts, including celebrity physicist Prof Paul Davies, had refused to debate his findings.

Davies is professor of Natural Philosophy at the Australian Centre for Astrobiology, whose head Malcolm Walter says that the magnetosome evidence is not strong enough to revive the notion that the structures in the Martian meteorite are fossilised bacteria. He says the original NASA work put forward six lines of evidence that the structures were fossils, but that five of these have been argued away by other researchers. The evidence for magnetosomes was the only line of evidence to survive this peer review.

“I follow the dictum of the late Ca\rl Sagan who said that extraordinary claims require extraordinary proof,” Walter said. He believes that Taylor and Barry “plunged into a minefield”.

“They did some pretty nice work on modern bacteria and then got carried away with their own enthusiasm. These people are very skilled in the field of studying modern bacteria but are novices when it comes to thinking about life on another planet.”

Taylor remains unfazed by such criticism. He says that Walter is not qualified to talk about magnetosomes and that “politics” is behind much of the criticism he has received. The core issue is whether the tiny structures that NASA found in the Martian rock are signs of ancient life. Walter represents the now mainstream astrobiological view that this theory has been disproved, while Taylor claims that the magnetosome signature he claims to have found in that rock should reverse that view.

He and Nick Hoffman are just two of many theorists with ideas about Mars. As more data is eked out of the Martian surface by the current mission and its followers, one sure thing is that more theories will come forward. And it is now almost certain that liquid water will play a part in them.

Copyright Control Publications Pty Ltd Apr 2004