Of all the planets of the Solar System, the best studied is also the one most fantasised about. Mars has captured the popular imagination since astronomers began speculating that it might be a world like Earth, soon after the invention of the telescope. By the end of the 19th century, those fantasies had reached fever pitch, with suggestions by Italian astronomer Giovanni Schiaparelli and American astronomer Percival Lowell that an advanced civilisation must have excavated a planet-wide network of irrigation channels (‘canals’) in the face of global climate change. Dark markings visible using ground-based telescopes were obviously areas of vegetation fringed by encroaching Martian deserts, and fed by water artificially channelled from the planet’s icy polar caps. It was only a matter of time before we would be communicating with the Martians themselves.
All very neat and tidy, until the Mariner 4 fly-by of July 1965 (and subsequent Mariner and Viking missions) revealed that almost everything we thought we knew about Mars was wrong. With its cratered surface, a dry and windy atmosphere only 1 per cent as dense as Earth’s that stirs up frequent dust storms, and its frigid surface temperature (–65 °C on average), the new Mars was a decidedly inhospitable place. And 50-odd years of subsequent research, carried out with a flotilla of Mars orbiters and landers, has done nothing to change this view.
All the Earth’s attributes that make it a benign environment for life to evolve are absent on Mars. There’s no global magnetic field to shield the planet from the solar wind. There’s no greenhouse blanket of air to moderate the surface temperature – and nothing to regulate its carbon content, as there is on Earth. And the absence of a massive moon renders the planet’s axial tilt unstable. Yet Mars shows tantalising signs of having been very different in the past, and that suspicion drives today’s research efforts.
THE FACT THAT MARS HARBOURS THE LARGEST VOLCANIC plateau in the Solar System – the Tharsis Rise – shows that it was once a geologically active planet. Five huge volcanoes dominate the Tharsis region, one of which (Olympus Mons) is the biggest in the Solar System. These are shield volcanoes, similar in structure to those that make up the Hawaiian Islands. Their shallow slopes come from low-viscosity magma. The summit caldera of Olympus Mons stands a whopping 27 kilometres above the surrounding landscape. That extraordinary elevation is thought to be due to the absence of plate tectonics on Mars. Like an orange, the planet has an unbroken skin – a single crustal plate that may have remained stationary over a hotspot in the underlying mantle for a very long time, allowing Olympus Mons to grow to its gargantuan size.
While the growth of the Tharsis region continued throughout the most recent geological era of Mars (known as the Amazonian period, which started about 2.9 billion years ago), it is the earlier history of the planet that tantalises planetary scientists. In the oldest, or Noachian era, dating from the planet’s formation 4.6 billion years ago and lasting some 900 million years, the planet was undoubtedly warm and wet. Ancient clays and sedimentary rock formations dating from the Noachian are widespread on Mars.
The hard evidence that liquid water was abundant comes both from orbiting spacecraft like NASA’s Mars Reconnaissance Orbiter and rovers such as Spirit, Opportunity (both now defunct) and Curiosity. The orbiters look at the big picture, with cameras, radar and analytical instruments that provide coverage of the whole planet. The rovers, on the other hand, get up close and personal with the Martian surface, acting as robotic geological laboratories equipped to investigate every aspect of its rocks and soil. Curiosity even has a ‘laser zapper’ called ChemCam, which can sense the chemical composition of rocks up to 7 metres away by analysing the light emitted when the laser vaporises small areas of their surfaces.
Geographical features associated with water erosion are supported by evidence from soil and rock analysis, which reveal minerals that only form in the presence of liquid water. Moreover, the presence of gravels containing smooth pebbles demonstrates that rivers and streams flowed for significant periods of time at Curiosity’s landing site in a geological wonderland known as Gale Crater (named after the Australian amateur astronomer Walter Frederick Gale, whose discoveries during the late 19th and early 20th centuries included comets, double stars and Martian features that, like Schiaparelli and Lowell, he believed to be canals).
Further results from Curiosity’s analysis of an ancient lake bed show that it was laid down in fresh water, rather than the acidic brine that gave rise to the clays Opportunity had analysed earlier in a different region of the planet. That brine was much saltier than the Earth’s oceans (though not as saline as the Dead Sea), and the presence of an iron sulphate mineral called jarosite suggested acidity, as jarosite only forms in such environments. The acidity is also cited as a reason why the ancient Martian seabeds are not rich in carbonates, as terrestrial ocean beds are.
Most tantalising of all is the suggestion that the whole of the low-lying northern hemisphere of Mars was once covered by water. In high-resolution images of the planet taken from orbit, we see features normally associated with water erosion here on Earth: river valleys, oxbows, canyons, outwash flows, and evidence of beaches and sea cliffs along what is now taken to have been an extensive coastline. Laser altimetry from orbiting spacecraft (especially NASA’s Mars Global Surveyor, operational from 1997 to 2006) has shown the northern hemisphere of Mars to be flatter, lower-lying, and less cratered than its southern counterpart, leading to the idea that it once harboured an ocean. An earlier objection that the supposed shoreline varied in height around the ocean rim (and therefore couldn’t be a shoreline) has been refuted with the suggestion that large-scale shifts in the inclination of Mars’ rotation axis occurred, due, perhaps, to eruptions of Olympus Mons.
The question of whether the 5-kilometre height dichotomy between the northern and southern hemispheres is the result of an oceanic basin – or of some other cause such as convection in the planet’s mantle or a major asteroid impact – is still controversial. While it is generally accepted that a stable body of water did cover most of the northern hemisphere, the jury is still out on how deep it was, and how long-lasting. Were there tens of millions of years of constant cover, or wet episodes interlaced with long periods when the seabed was dry? Either way, the demonstrated existence of liquid water in the ancient Noachian era is an exciting find for astrobiologists, whose studies of the prospects of life having arisen elsewhere in the Universe invariably begin with a watery environment – because all life on Earth uses water as its working fluid.
It is believed that wet conditions on Mars lasted well into the Hesperian era, which occurred between 3.7 and 2.9 billion years ago. This is the period during which we know life was beginning on Earth, the oldest undisputed fossilised terrestrial bacteria dating from three billion years ago, with more controversial evidence of micro-organisms existing half a billion years earlier.
Thus it is that the search for life beyond Earth is entering a critical phase. Soon after it arrived on Mars in 2012, Curiosity achieved its mission’s primary goal – to discover whether Mars was ever habitable. Having established that, it now remains for us to find whether that ancient habitability actually spawned living organisms. And, if it did, to discover what happened to them. We’ll pick up that story again in the next chapter, but there is one further intriguing prospect that relates to the panspermia theory outlined in chapter 9. Is it possible that Martian micro-organisms could have been the source of life on Earth, having made their interplanetary journeys on the meteorites that are known to have travelled between the two planets? Did life on Earth share a common origin with its putative Martian counterpart? This is just one of the many possibilities astrobiologists are investigating today.
SO, IF MARS DID HAVE A WET PAST, WHAT HAPPENED TO change it, and is there a lesson in climate change for we dwellers on planet Earth? The geological evidence points to the Martian sea or ocean having disappeared between two and four billion years ago – that is, somewhere in the first half of the planet’s 4.6 billion-year lifetime. And the trigger seems to have been its small size – about half the diameter of Earth. With a proportionately smaller iron core, Mars had only a limited reservoir of internal heat left over from its fiery birth and, while it is thought that the core remains at least partly liquid, it is no longer hot enough to sustain either an internal dynamo or plate tectonics. It seems likely that these processes are long gone, having shut down during the ancient Noachian period.
The motion of rock plates in the Earth’s crust plays an important role in stabilising the atmosphere, because it circulates carbon between the atmosphere and the mantle beneath. But as Mars’ molten core cooled more rapidly than Earth’s, plate tectonics shut down early in its history, removing the ‘thermostat’ that allowed carbon dioxide to keep the planet warm. Thus the planet lost most of its greenhouse blanket, gradually cooling to become the frigid world we see today. The cooling core on Mars is also the reason the planet has no appreciable magnetic field, resulting in unmitigated exposure to the solar wind. That would have enhanced the process of atmospheric water vapour being dissociated into hydrogen and oxygen – and lost to space.
That’s not to say that Mars is now devoid of water, however. Much of it is still there, locked up as ice in the polar caps, or beneath the surface soil as permafrost at lower latitudes. Ground-penetrating radar aboard orbiting spacecraft has revealed glaciers overlaid by a thin layer of soil, even at temperate latitudes. And during its six-month mission in 2008, NASA’s Phoenix lander discovered a permafrost of ice only millimetres beneath the surface soil in the Martian arctic. It also demonstrated that a limited water cycle exists between atmosphere and ground, with occasional observations of snowfall. By contrast, the overall quantity of ice on Mars is far from limited. Data from ESA’s Mars Express orbiter has revealed that if just the southern polar cap were melted, it would produce enough water to flood the entire planet to an average depth of 11 metres.
The fact that Mars was once a habitable planet but isn’t now highlights how delicate the balance of the Earth’s atmosphere is. So the lesson for we Earth-dwellers is: don’t tinker with it. Especially if it involves plate tectonics.