The past and future habitability of planet Mars

Abstract

Results from NASA’s Curiosity rover reveal traces of bygone aqueous environments with sufficiently benign conditions for some of Earth’s hardier extremophiles to thrive. If Mars hosted life in the past, it may do so again in the future, as a project to colonise our neighbour planet is now recruiting volunteers for the one-way trip. Michael Gross reports. Results from NASA’s Curiosity rover reveal traces of bygone aqueous environments with sufficiently benign conditions for some of Earth’s hardier extremophiles to thrive. If Mars hosted life in the past, it may do so again in the future, as a project to colonise our neighbour planet is now recruiting volunteers for the one-way trip. Michael Gross reports. Life on Mars was considered almost a certainty at various points in the chequered history of our curiosity about the fourth rock from the Sun. The romantic hope for a second biosphere next door always ended in disappointment, however. In 1877, Giovanni Schiaparelli (1835–1910) described a network of channels on the surface of the red planet. The American Percival Lowell (1855–1916) turned the Italian canali into English canals, evoking the presence of artificial structures and their intelligent creators. He spent more than two decades with detailed mapping and interpretation of these canals, concluding that a dying Martian civilisation built them in a last-ditch attempt to carry water from the polar regions to the lower latitudes hit by drought. Alas, the advent of astrophotography evaporated the entire Martian canalisation system along with the civilisation that supposedly built it. A century later, NASA’s twin landers Viking I and II each conducted three dedicated life-detection experiments, at least one of which reported results similar to what the mission scientists anticipated to see in the presence of life. However, as the investigation of the soil samples failed to detect any organic molecules, the nominally positive results were eventually deemed artefacts by most experts. In 1996, putative evidence of past life on Mars was discovered in the meteorite ALH84001. The rock contained inclusions of nanoscale structures rich in organic molecules, but alternative explanations for the origins of these structures have also been suggested. After heated debates dragging on for years, the situation has remained inconclusive, and most experts remain unconvinced. Since that latest disappointment, the astrobiology approach — namely, studying the development and limits of life on Earth to work out where else in the Universe life may be found — has gained considerable influence. Rather than straining their eyes to spot canal-constructing Martians or ramping up the analytics to pick up a stray extraterrestrial microbe, scientists are now starting their investigations here on Earth (Curr. Biol. (2012) 22, R207–R210). Astrobiologists have looked at extremophiles that thrive in almost Martian conditions on our own planet and have on this basis redefined the limits of what a habitable location elsewhere in the Solar System may look like. The big challenge for the two current rover missions on Mars — Opportunity in its eleventh Earth year and Curiosity in its second — is to detect evidence of past habitability. Given the currently parched and highly oxidised state of the Martian surface, evidence of the availability of water and reduced compounds is particularly sought after. Only when a convincing habitat is found can the question of past or present life be addressed again. January 24th of this year marked the completion of ten (Earth) years of operating on Mars for the Opportunity rover, an astounding achievement in itself considering that it was built to last three months and that commercial electronic devices rarely last half as long in the much milder conditions here on Earth. Opportunity was also lucky in that, as soon as it opened its camera shutters back in 2004, it became the first Mars lander to find bedrock. Luckier still, it turned out to be sedimentary rock thought likely to have been laid down in water.Curious character: A selfie of the rover Curiosity produced shortly after landing. By combining several takes, the editing removed the camera arm from the final mosaic image. (Photo: NASA/JPL-Caltech/Malin Space Science Systems.)View Large Image Figure ViewerDownload Hi-res image Download (PPT) Maybe it wasn’t quite as lucky with the habitability of the ancient environments it encountered in Meridiani Planum. The first sediments that Opportunity analysed in detail pointed to highly acidic and hypersaline environments, the kind which may have been too harsh even for the hardiest terrestrial extremophiles. However, in a paper published on the tenth anniversary of Opportunity’s touchdown, Raymond Arvidson from Washington University in St. Louis and colleagues showed that the much older sediments that the rover has found more recently with the help of the Mars Reconnaissance Orbiter are looking more promising (Science (2014) 343, 1248097). While the formation of these rocks may or may not have involved water, the authors report evidence suggesting that these rocks later interacted with water in several different ways, leading to the deposition of different minerals, including iron-rich smectite, clay, and hydrated materials rich in silica. This water appears to have been less acidic than the fluids whose traces Opportunity observed earlier at other sites, and hence would be better suited to support life. In July 2012, the much more capable Mars Science Laboratory mission, renamed Curiosity, arrived in Gale crater with an impressive suite of analytical instruments designed to find traces of ancient habitats. Taking a detour on the way to its primary science target, the foothills of Mount Sharp, Curiosity investigated sediment structures in a trough known as Yellowknife Bay, where several promising geological features meet. As CalTech geologist John Grotzinger and colleagues describe in a recent paper, analysis of the clay sediments at this study site suggests that both flowing and standing water has been present for a duration between hundreds and tens of thousands of years (Science (2014) 343, 1242777). Together with an additional five papers from the Curiosity team (introduction and abstracts: Science (2014) 343, 386–389), the report by Grotzinger and colleagues presents evidence of an environment that may once have been habitable for life forms comparable to terrestrial chemolithoautotrophs. Considering the sterilising effect of radiation at the unprotected surface of Mars, life underground feeding on minerals would be the only viable option. By drilling into a sediment formation called the Sheepbed Mudstone, and analysing the chemical composition of the materials, the teams gathered evidence on the conditions under which this sediment formed less than 3.7 billion years ago. Specifically, the teams used Curiosity’s ChemCam, APXS and CheMin instruments to demonstrate the presence of the elements hydrogen, oxygen, sulphur, carbon, nitrogen and phosphorus. All with the exception of phosphorus were readily converted to volatile compounds, from which the authors conclude that these elements would have been available to resident life forms. Phosphorus may only have been available in small amounts or with difficulty, e.g. as a trace or minor element in minerals like olivine. Elements alone aren’t sufficient for life, however. For life to gain energy from chemical sources, there must be compounds at different degrees of oxidation, which is a particular concern in view of the current highly oxidised conditions at the surface of the planet, which appears red due to the abundance of oxidised (ferric) iron. For the first time, Curiosity teams have detected reduced materials that, on the otherwise largely oxidised surface of Mars, could serve as energy sources. The SAM (sample analysis) instrument found sufficient amounts of reduced sulphur to envisage sulphur redox chemistry as an energy source for life. The CheMin instrument discovered abundant amounts of magnetite, a mineral that contains both ferric and ferrous iron. Apart from providing encouraging hints regarding the availability of redox chemistry in earlier environments, the conditions under which magnetite would form on Earth also point to a fairly habitable environment. Given the basic requirements of chemical elements and chemical energy, the researchers also addressed a range of stress factors that might reduce the habitability of such an environment. Salinity is an important factor, as it may reduce water activity, i.e. the availability of water for the hydration of biomolecules and metabolites. While there are archaea in environments such as the Dead Sea, the reduced water activity can become limiting for life in high-salinity environments. Some sediments explored by Opportunity, for instance, suggest the presence of brines that were too saline to support life. In contrast, the Sheepbed mudstone formation seems to have formed under low-salt conditions, and liquids that transformed it subsequently appear to have had low salinities as well. Fluids that later percolated through the sediments may have been somewhat more salty, but the presence of the poorly soluble calcium sulphate suggests there was still sufficient water activity for life. Investigations of the likely pH range of these fluids showed that they were more compatible with life, with near-neutral or weakly acidic pH. Curiosity found no minerals that might suggest a strongly acidic fluid, such as iron sulphates. The authors conclude that “the simplest interpretation of the sequence of diagenetic [rock-transforming] events would involve progressive desiccation of mildly saline, pH-neutral waters — a very Earth-like scenario.” Given that Mars doesn’t have plate tectonics to rework its surface, rock formations tend to be much older than those found on Earth. The precise age of the Sheepbed mudstone is unclear, but the likely date of formation of the Gale crater around 3.7 billion years ago sets an upper limit that already makes the rocks quite young in comparison to much of the Martian landscape. Thus, the water flows that shaped these deposits happened at a time when Earth may already have hosted living cells (pinning down that date hasn’t been much easier than finding life on Mars). Considering the very similar starting conditions that both planets had, this supports the argument that life could have originated and spread on either or both of them, until the loss of the atmosphere made Mars less habitable, while stratospheric ozone made Earth more so. If life did exist in the aqueous environments that Curiosity describes so tantalisingly, the rover isn’t equipped to find its fossilised remains, but future missions building on this information may be able to do so. More challenging still is the question of whether any survivors are still hiding deep underground. Although there is now abundant evidence of frozen water and soil humidity, researchers are still struggling to pin down conclusive proof of current water flows on Mars. One promising avenue is the observation of recurring slope lineae (RSL) on the edges of certain craters, a seasonal phenomenon that appears to follow warmer conditions, and thus could indicate a thawing and flowing of water just below the surface. In a study due to appear in the journal Icarus, Lujendra Ojha and colleagues have detected 13 sites with RSL features after analysing images taken by the current orbiter missions from 200 candidate sites. However, definitive proof that these features are linked to water is still lacking. Looking at the development of our relationship with Mars from the canali through to the rovers, it is remarkable how science has had to scale down its targets in order to move from romantic imagination to more pragmatic questions. The search for grand civilisations was followed by attempts to detect carbon metabolism, and now we are getting excited about evidence of past habitability — and there is even a study of how this evidence can survive the hard radiation on the surface of Mars. Rather than giving up after negative results concerning canal builders and carbon consumers, researchers have reframed the quest in the terms of modern astrobiology, which uses life on Earth as a key example for life in the Universe. Taking first steps first, the limits of the life we know must inform the search for habitats, and once they are found, the search for their inhabitants, dead or alive, may follow. As part of this new era in space exploration, Mars has seen an unprecedented amount of research activity and success in the last two decades. Since the arrival of Mars Pathfinder in July 1997, there has always been at least one probe reporting from the red planet, and at the moment there are five – two rovers and three orbiters – with further missions in preparation. A side effect of the progress made is that the idea of establishing a human colony on Mars, once the exclusive domain of science fiction, has become the goal of a non-profit company, Mars One (www.mars-one.com). While the cost of sending human explorers to Mars and bringing them back safely is still beyond the scale of what any government or organisation would be prepared to pay for space research, the company reckons that one-way trips could be financed by selling ‘reality show’ style TV rights to this unique event. The price tag for sending a crew of four to live on Mars has been estimated to be six billion US dollars, while the cost of a return mission might run into hundreds of billions. The company is already selecting participants from the more than 2,000 applicants. The plan is to launch a demonstration mission and a communications satellite in 2018, a rover in 2020, six cargo missions in 2022, and the first team of settlers in 2024, with further teams following at two-year intervals. Thus, by 2033, all going well, there might be up to 20 human settlers living on Mars. Apart from the obvious ethical complications, critics have cast doubt on the viability of the business model of the enterprise. Wired magazine, not usually averse to futuristic thinking, estimated that the Mars One plan “will most likely struggle to get off the ground” and awarded only two points on a one-to-ten scale of plausibility. Still, the plans have succeeded in inspiring would-be space travellers and the media alike. Considering the primitive hardware that enabled Neil Armstrong to walk on the moon, there is no reason why today’s much more advanced technology shouldn’t enable people to live on Mars if they want to and if they can find someone to pay for it. They might even find ways of making the red planet slightly more habitable, for instance by thawing out some of the carbon dioxide in the pole caps. As we are using up resources faster than Earth can replenish them and survival of our civilisation is by no means assured (Curr. Biol. (2013) 23, R1017–R1020), expanding onto a second planet may become a logical step for humanity in the near future, as well as an insurance policy against planetary disasters. Maybe, just maybe, there will be canals on Mars one day, and the planet that appears to have been habitable a few billion years ago will become so once again.