The Red Planet

Wednesday, 25 September 2013 - 1:00pm
Museum of London

Subject:






Overview

Many robotic spacecraft have been sent to explore the cold, dry and dusty surface of Mars. They reveal a world not so dissimilar to Earth, shaped by meteor impacts, volcanic activity, weather and flash flooding. In addition, recent discoveries inform us about the possibilities for life on Mars – both in the past and the present.

 






Transcript of the lecture

25 September 2013
 
The Red Planet
 
Professor Carolin Crawford
 
 
Red Mars is one of the four rocky planets in orbit around the Sun. At about half the size of the Earth, it is larger than both Mercury and the Moon, although it contains only one-tenth the Earth’s mass. It is our most accessible planet, and the member of the Solar system that most resembles Earth – and for these reasons it has long captured the human imagination, and been a focus for space exploration. 
 
The light of Mars can only be resolved into a disc with the aid of a telescope; even so it wasn’t until 1659 – a full 50 years after Galileo first turned a telescope to the heavens – that any features on the surface of the planet were discovered. When the Dutch astronomer Christiaan Huygens (1629-95) first observed Mars in 1656 it appeared featureless – it was only during a later attempt (in 1659 when its orbit brought it relatively close to the Earth) that he noticed the large, dark irregular surface feature that we now call Syrtis Major. By monitoring how this blotch rotated into and out of view over a period of several weeks, Huygens found that the rotation period of Mars was very similar to that of the Earth. More detailed observations by Gian Domenico Cassini (1625-1712) a few years later established the Martian day as only about 37 minutes longer than our own. Both Cassini and Huygens also noted the bright white caps at the Martian poles. 
 
Towards the end of the 18th century, William Herschel (1738-1822) demonstrated that Mars rotated about an axis inclined by 25° from the vertical, an angle again very similar to that of Earth. This implied that Mars must have comparable seasons – except that they would each much longer, given that it take Mars 1.9 Earth-years to complete its orbit around the Sun. 
 
Mar’s orbit is also slightly more squashed in shape than the Earth’s, so there is a bigger difference in its distance away from the Sun between the near and far approaches. Accordingly, although the seasonal changes are similar in nature, they are more extreme than ours, and the climate in the Southern hemisphere of Mars is more extreme than those in the North. This is because that the Southern hemisphere is tilted towards the Sun when Mars makes its closest approach to the Sun, and away at the far point; it thus experiences both far warmer summers and far colder winters. 
 
Mars has two irregular Moons that look very different from our own. They are both small and dark in colour, and probably for both these reasons weren’t discovered until 1877 (by Asaph Hall). At less than 30 km across, and composed of very low density material, Phobos and Deimos don’t exert sufficient gravity to be pulled into a spherical shape. Their surfaces are plastered with craters of a whole range of sizes, and the close resemblance of both moons to members of the nearby asteroid belt strongly suggests that this to be their origin. Captured by the gravity of Mars when they ventured too close, they became its natural satellites, orbiting around its equator. 
 
The larger moon, Phobos, orbits at such a low altitude (passing only 6000km above the planet’s surface) that with an orbital period of only 7h 39m, it is seen to rise and set three times every Martian day – except from the polar regions, where it cannot be seen at all. Phobos’s orbit is steadily shrinking at a rate of 1.8m per century; it is expected to either crash onto the Martian surface or be shattered by gravitational tidal forces to form a temporary ring around the planet, some half-billion years in the future. On the other hand, the smaller moon Deimos orbits so far above the surface of the planet that it is barely retained by the gravity of Mars. 
 
The ‘Space Race’ of the 1960’s was not only about which nation would reach the Moon first; there was also intense competition in the early exploration of Mars. The first spacecraft launched towards Mars was the USSR’s Mars 1 in 1962, but communication with the spacecraft failed early on. Three years later the USA succeeded with Mariner 4 (USA 1965) performing the first close approach to Mars, passing by at a distance of about 9,500 km, and returning 22 photos. Even closer fly-pasts were carried out subsequently by Mariners 6 and 7, which jointly took another 200 images of the planet. The American Mariner 9 (1972) then became the first mission to survey Martian geology from orbit. The Mariner series of spacecraft thereby discovered a whole range of spectacular geological features hitherto unknown from Earth-based observations: impact craters of all sizes, vast plains, giant shield-shaped volcanoes, and a deep fracture in the surface forming a canyon that wraps around a quarter of the planet’s circumference.
 
The USSR placed the first lander on the Martian surface, Mars 3 in 1971, but it failed after only 20 seconds, most likely due to its unfortunate arrival in the midst of major Mars-wide dust storm. In 1976 the US deployed the very successful Vikings 1 and 2 landers from companion Viking orbiters. Each lander had a range of science experiments on board to monitor and analyse the physical and chemical properties of the atmosphere, the weather, the crust and the surface – with the particular aim looking for any indication of organic compounds within the Martian soil; a search which proved unsuccessful. 
 
Martian exploration continues apace to the present day, with ever-more ambitious probes launched towards Mars. We live in particularly exciting era, with the latest results and photos reported regularly in the news headlines each week. The planetary scientists conduct complex chemical, physical and geological experiments on the surface of Mars remotely, using mobile science laboratories such as the most recently-deployed Curiosity. Satellites such as the Mars Reconnaissance Orbiter survey the Martian surface features down to scales of only a few metres across. All these missions combined to make Mars one of the best-studied planets of our Solar System, better known than even our own Moon. 
 
The impression gained from the images returned by all these robotic spacecraft is of a world that superficially, at least, resembles regions of our own. It appears to be the closest relative Earth has in our local neighbourhood: it is a rocky planet with a similar day length and seasonal behaviour. Its slightly smaller size and more remote location from the Sun combine to make it a much dryer, frozen, and hostile world. 
 
A simple, but fairly crucial difference between the Earth and Mars is that the latter does not possess an appreciable magnetic field. It has not always been the case: the orbiting Mars global surveyor detected weak residual magnetism within the rocks in some of the oldest terrain on the surface, suggesting that the planet was magnetised some 4 billion years ago. Slightly less aged surfaces, however, do not show any magnetised regions, indicating that this global magnetic field had disappeared at some point during the next hundred million years. The reason for its removal remains unclear. Current ideas invoke either the cumulative effects of asteroidal impacts during the period of ‘Late Heavy Bombardment’ that was occurring around this time, or possible to a single but catastrophic collision that heated the core to disrupt the magnetic field it produced. Without the shielding effect of the magnetic field, Mars is left fully exposed to the damaging effects of the energetic charged particles that continually rain down on the planet from the Solar wind. Not only does this wind expose the surface to dangerous levels of harmful radiation, but it rapidly stripped away much of the Martian atmosphere. 
 
The battering of the energetic Solar wind blasted the Martian atmosphere, helping the molecules of gas to escape into space, away from the feeble gravity. The gas that is retained despite this onslaught provides only a sparse and tenuous atmosphere, and one which generates a pressure under a hundredth of that on Earth – close enough to a vacuum. 
 
The Viking landers were able to make the first detailed measurements of the composition of the Martian atmosphere, and found it to be mostly carbon dioxide (around 95%), along with a small amount of nitrogen (around 2.7%) and argon (1.6%); tiny smatterings of oxygen, carbon monoxide and water are also present. The lack of oxygen means there is also no ozone layer to protect the Martian surface from full exposure to the Solar UV radiation, again contributing to the hostile environment. Earlier reports suggesting the presence of small amounts of methane in the atmosphere – which could have originated from either biological or geological activity – have not been confirmed by the most recent and accurate experiments carried out over the last year by the Curiosity rover. 
 
The huge difference in the seasonal extremes experienced on Mars (particularly in the Southern hemispheres) also generate large variations to the global atmospheric pressure. Temperatures drop so low during the Southern winter that up to a third of the atmosphere freezes – carbon dioxide precipitates from the air, leading to a pronounced drop in atmospheric pressure; in spring when this snowfall thaws and is released back into the air, the pressure increases again. 
 
Mars is much colder than Earth, with most of the planet well below 0°C for much of the year. Not only is it slightly more distant from the sun’s energy, but the tenuous atmosphere does not provide an appreciable greenhouse effect to trap what little warmth there is, and there are no oceans to temper the extreme daily temperature variations. At best, surface temperatures might achieve the giddy heights of 20°C (in summer, on the equator, at midday…), but they can sink as low as –150°C in the polar regions. 
 
The combination of such low temperatures with a lack of atmospheric pressure precludes the presence of liquid water on Mars today; were it present in any quantity, it would rapidly disperse by either freezing into ice or evaporating away as vapour. 
 
The harsh UV radiation helps oxidise the soil of Mars; the large quantities of iron present within the regolith have long been rusted, providing the surface with its characteristic red colour. The oxidisation processes would also completely destroy any living cells close to the surface. 
 
The enormous temperature and pressure changes – both on daily and seasonal timescales – power strong winds that sweep through the atmosphere, sometimes at speeds of up to 200 km/hour. The winds shape the surface deserts into large fields of corrugated sand dunes, and lift many tonnes of the tiny particles of dry, ultra-fine dust high into the sky, where they remain suspended in the atmosphere. Their presence in the air colours the sky pink, and when the dust becomes incorporated with the wind, it can exacerbate the strength and erosive effects of the wind. 
 
Relatively small vortices are seen on local scales, where spinning columns of warmed air whip up surface material to form dust devils that scour the landscape. Such structures can stretch to heights of several km, and strip away any loose material from the surface, leaving the underlying and darker-coloured deposits of rock or soil exposed, to mark their passage by dramatic long whirling tracks. 
 
Occasionally the extreme heat of the Southern hemisphere summers generate powerful winds that lift huge quantities of dust particles into the air; the dust absorbs more of the Sun’s warmth to heat the air around it, exacerbating the wind. Very rapidly a runaway effect results in a dust storm that can grow large enough to engulf the whole planet for several weeks at a time. During such periods the storm can completely obscure the surface features on Mars. Only when the dust in the air becomes so thick that it begins to obscure the sunlight will the air cool, and the stormy winds subside. 
 
Ice caps are apparent at both of the Martian poles which wax and wane in size with the seasons. Observations taken at a range of wavelengths from various orbiting spacecraft show that the bulk of the ice caps during winter is mostly frozen carbon dioxide. However, underlying both caps is a permanent cover of water ice; this is added to in the winter by the seasonal precipitation of carbon dioxide from the atmosphere. The cap shrinks once more when the temperatures rise in Spring arrives, and the thick layer of frozen carbon dioxide sublimates to resume its role in the atmosphere. The colder winters and warmer summers in the Southern hemisphere produce the most dramatic seasonal change in the size of ice cap. 
 
The cycle of seasonal deposition and removal of carbon dioxide ice creates large layered regions at the polar caps, where clean bright ice alternates with darker bands containing dusty deposits. Like the difference in the width of tree rings, the differences between such layers can reveal information about any gradual underlying changes in the Martian climate over the past few millions of years, perhaps due to precession of both its orbit around the sun, and the tilt of its rotational axis. 
 
The geology of Mars is fascinating, and the most spectacular of any Solar System planets. 
 
We think that Mars has an internal structure similar to the Earth’s, with a thin crust and a thick mantle of iron-rich silicates wrapping around a metallic core. Some differentiation due to internal melting has occurred within the core, but not to the same extent as we find within the Earth. 
 
The topography of the Martian surface divides it into two very distinct regions. The southern hemisphere is dominated by ancient uplands, heavily marked by successive layers of impact craters. In stark contrast, the northern hemisphere appears much younger (the number and size of its impact craters imply it has undergone a more recent resurfacing), and is composed of flat plains, all at an average altitude that is a good 5km lower than the Southern uplands. The disparity in terrain could originate in either a very uneven upwelling of subsurface molten material that covered only the northern regions; or from an early but devastating collision of young Mars with another proto-planet that tore away the outer layers of the crust in the northern half of the planet. 
 
The thin atmosphere has also failed to protect the Marian surface from meteoritic bombardment through the ages, so that impact craters cover the planet. Most, and the most prominent, date from the Late Heavy Bombardment that occurred 4 to 3.8 billion years ago. Termed the ‘Noachian’ era in Martian geology, this is the period when the largest impacts gouged out extensive amounts of new material to both form large basins – for example the giant Hellas Planitia that is up to 2300 km in diameter in the South – and to squash up the surrounding terrain to form mountains. 
 
One strange geological feature of Mars is the giant Tharsis bulge, a swelling some 5000km across and 10km high that sits across the equator to spread over both the Northern and Southern landscapes. It was thus formed after the crustal dichotomy was created, perhaps as recently as about 2 billion years ago. The bulge is located close to the Martian volcanoes in the Northern hemisphere, and it was most likely pushed out from a magma hotspot under, but not breaking through, the mantle. 
 
Martian volcanoes are the largest to be seen anywhere in the Solar System, and the most prominent are located on top of the Tharsis bulge. The giant Olympus Mons volcano spreads over 550km across, and stretches to a height of 25km above its surroundings. The most substantial of the volcanoes date from 3.5 billion years ago, but a lack of impact craters on the flanks of some suggest that they could have remained active till even a few hundred million years ago. They are all broad shield volcanoes, which are created by a continual eruption of lava at one spot. On Earth such activity results in chains of volcanic islands such as those in Hawaii as the terrestrial tectonic slide over a fixed magma hotspot. On Mars the outer crust and mantle cooled quickly to form a thick strong layer that instead remains static, allowing a volcano to grow from a gradual accumulation of material in one place for billions of years. The reduced gravity (only about 1/3rd of that of Earth) enables the volcanoes to grow to much greater heights than are possible on Earth, and the thicker Martian mantle is also able to support the total weight of a much larger mountain. Today Mars has cooled with very little internal heat; all volcanic activity is dormant. 
 
The geological features of Mars use names that are different from the similar (albeit usually less dramatic) structures on Earth: 
a Mons is the name of a large mountain volcano 
Planitia and Planum describe a low-level plain or a high plateau, respectively
Terra is given to a large land mass
Vallis is the name for a valley; a steep-walled canyon is a Chasma; and a network of valleys or canyons form a Labyrinthus 
 
The largest canyon on Mars is a spectacular rift valley up to 8km deep in places, and which stretches to over 4,000km in length. Unlike similar canyons on Earth, it was not formed by water erosion. Instead it was created as the planet’s crust fractured and split along a fault line - probably coincident with upwelling of the Tharsis bulge to the other side of the planet. Later landslides weakened and widened the canyon walls over the millennia. 
 
All the images relayed back from the surface of Mars reveal much of it to look like a dry stony desert. But the experiments carried out by many of the orbiters, landers and rovers all point to Mars not always having been such a barren and hostile environment; indeed, it seems very likely that liquid water once flowed across the landscape. This is thought to be mostly a long time ago, some 3-4 billions years in the past, but there are also suggestions of more recent and temporary flooding events having occurred. 
 
Water vapour has long been detected in small amounts in the Martian atmosphere, and it can be seen gathered into thin clouds of water ice that cling to the peaks of the tallest volcanoes. Similarly, water ice frost has been observed – directly by Viking 2 as a thin coating of the rocks and soil at the surface, or from orbit as larger accumulations nestling under the shadowed walls of craters. 
 
The Mars Express spacecraft surveyed the global location of minerals that form only in the presence of water, such as a range of hydrated clays and salts. These appear to be most strongly concentrated in lower-lying geological features in the older, cratered terrain in the southern highlands, suggesting that over four billion years ago vast regions of standing water that evaporated away.
 
Spectrometers and ground-penetrating radar on board the Mars Odyssey spacecraft and the Mars Reconnaissance Orbiter both detected buried ice deposits all across the planet – even at locations close to the equator. The true extent of these ice reservoirs is not yet known, as the radar can only test to a depth of 1m – but they could easily extend down several km. Evidence for the presence of frozen water locked under the surface as permafrost is supported by the appearance of several more recent impact craters, which are surrounded by ejecta that suggest a fall into material reminiscent of damp muddy sludge that has rapidly melted, splattered and re-frozen to produce a very different structure than that produced by an impact onto a hard dry surface. 
 
The flat Martian plains within the Northern polar regions are seen – both from orbit, and from the Phoenix lander – to be broken by a regular pattern of polygons. Where similar structures are studied in terrestrial permafrost landscapes, they are known to be created by the seasonal thawing and freezing of the water in the shallow subsurface regions, causing the ground to undergo cycles of repeated shrinking and swelling. Assuming a similar origin on Mars, then the polygonic structures suggest that a large portion of the polar landscape is underpinned by ice locked into the soil. 
 
The surface of Mars displays a variety of deep winding valleys and channels which are thought to have been carved out by water flow. 
 
Some of the first detailed orbital mapping was carried out by the Mars Global surveyor, which revealed that the tops of inside walls of some craters, and the crests of some sand dunes, are lined with thousands of gully-type features; these appear to spring from the land to then run downhill. Fresh gullies have even appeared over the last few years, so these features are still forming today. Despite their appearance of being shaped by meltwater, many of the gullies are most likely caused either by small landslips of loose surface material, or the melting and release of carbon dioxide ice. The few gully features that do show a clear braiding pattern indicative of flowing liquid tend to be located on shadowed slopes facing away from the sun – a location where a quantity of of snow and ice could accumulate, thawing only very occasionally. 
 
In the Southern highlands, small tributaries meander and merge to form deeper channels in networks of streams that can stretch over 50-1000km long. On Earth similar features are drainage systems generated gradually by a steady and long-lasting flow of water; on Mars this water could be supplied most simply by a low but continual seepage from underground rivers and reservoirs. 
 
Much wider, shallower features suggest the occurrence of much more dramatic and rapid flooding events, such as seen in the Mangala or Ares Valles. These do not develop slowly from a network of tributaries, but instead spring up from places in the southern uplands to flow downhill to the north. Streamlined structures such as teardrop-shaped islands within the channels indicate a surge of water has flowed around any slight raised obstruction, such as a crater, piling up sediment in its wake to form a tail-shaped structure. These broad outflow channels are created when a large volume of water is suddenly released; flooding across the landscape quickly, it will then collect and pool when it reaches low-altitude regions to form temporary bodies of standing water. The level of subsequent cratering over such features suggest that many occurred relatively early on in the geological history of Mars. 
 
The sources of some of these large outflow channels lie in regions where the terrain is chaotically fractured and broken in a way that implies a downwards landslip. Any rapid melting of sub-surface ice deposits and reservoirs – themselves triggered either by the simple pressure of the weight of overlying sediments, or from a nearby impact event – would collapse the overlying area of land as a sudden and catastrophic torrent of water was released, providing a flood that could last for a few weeks or months. The water scoured the outflow channels which fed into low-lying ground to form ancient lakes and seas. Gradually these bodies of accumulated water would have disappeared through the combined processes of evaporation, freezing and seepage back down into the underlying ground to become re-incorporated into the underground reservoirs. All that now remains of their occurrence is the erosive effects of the water on the Martian surface, and the minerals and salts left behind as the water evaporated away. 
 
Discoveries by the spacecraft landed on Mars consistently point to a lack of liquid water anywhere on the surface now.
 
NASA’s Spirit and Opportunity rovers were deployed to equatorial regions on opposite sides of Mars in Jan 2004. Both landing sites were chosen (from prior orbital mapping) to already show indications of past water activity, and the rovers’ mission was to examine a wide range of rock and soil in each location, in order to characterise the water history of Mars. 
 
Spirit landed in the ancient 166-km wide impact crater Gusev, which was already known to contain layered deposits; orbiter images also revealed a water network cutting across the crater rim suggesting that the crater was once flooded to form a lake. Despite today appearing much like a flat and barren desert, Spirit discovered plenty of minerals within the soil that would have required signification hydration to form, thus confirming the presence of standing water at the location. Spirit embarked upon a journey to the nearby Columbia Hills, where it imaged further evidence for flowing water action from erosion features in the rocks. 
 
Opportunity was landed on a flat plain where orbital mapping had detected the presence of haematite; this mineral is formed by a slow evaporation process, requiring the surface to have remained wet for an extended period of time. The rover confirmed its presence directly by imaging small spherical ‘blueberries’ composed entirely of haematite, and scattered extensively across a wide ‘pavement’ of dry rock slabs. 
 
These two very effective rovers thus returned clear evidence that quite different sides of Mars were covered with water before about 3.7 billion years ago; however, their analysis showed that this water was quite inhospitable compared to most standing water on Earth, being acidic, oxidising and very salty. 
 
Direct evidence for the presence of water ice just below the outer layers of soil was discovered by the Phoenix spacecraft, which landed in the Northern polar regions if Mars in 2008. Unlike the rovers, Phoenix was only able to examine its immediate environment, albeit in great detail. A robotic arm was able to dig down into the surface, and return samples to the body of the lander for microscopic imaging and detailed analysis. Small trenches excavated to depths of 5-15 cm into the soil revealed buried chunks of white water ice which then gradually sublimated away (over a couple of days) once exposed to sunlight and thin dry air of Mars. Crucially, the chemical analysis carried out by Phoenix discovered the presence of perchlorates in the Martian soil samples; perchlorate is a mineral salt which is very soluble in water – to find it in any quantity immediately reveals that it must have been a long time since any liquid water washed across the surface. Phoenix also recorded snowflakes falling from high clouds, but in a snowfall that vapourized long before reaching the ground. 
 
This complete lack of liquid water is also confirmed much closer to the equator, by the most recent lander, Curiosity, which has already spent over a year on Mars since arriving in August 2012. During this time it has already driven a distance of over 2.9km and ¬– like all tourists – has snapped some tens of thousands of images. Its mission is to establish whether or not Mars ever hosted what we would regard as habitable environments in its history. The spacecraft was landed in Gale crater, chosen as the target for exploration because orbiter images already showed the central peak known as Mount Sharpt to have sedimentary layers at its base, suggesting the past presence of water. 
 
Curiosity started its investigations close to its original landing site in a region since named as ‘Yellowknife Bay’, which contained large eroded rock formations. Its close examination of the outcrops revealed many of them to be composed of a jumble of small, rounded pebbles cemented together by finer sandy material. Known as ‘conglomerate’, such formation is often found associated with riverbeds on Earth. The Martian pebbles are a similar size to those found in typical terrestrial river gravel, and they are too large and heavy to have been transported by the wind; their smooth, rounded edges suggest a period of water erosion in streams that are not particularly deep or fast. The variation in the pebble colour also indicates that they have been eroded from a range of different rock types, and transported across distance of up to 10-15km. The location of the outcrops is fits well with the satellite images showing a nearby network of water channels that spread away from a cut through of the northern rim of the crater; water appears to have flowed along these channels to deposit their load of rounded, eroded gravel.
 
Curiosity has now started on a journey of over 8km towards its primary destination at the foothills of the mountain located at the centre of Gale crater. The lower slopes of Mount Sharp shows eroded features around rocky cliffs that have presumably been shaped by water, as well as clear sedimentary layers at its base. The rover will not take a direct route; not only will it have to be steered to avoid a wide bank of sand dunes that are in the way, but there will be stops and diversions – both planned and serendipitous – to examine rocks and geological features spotted en route. 
 
All the evidence from both orbiting and landed spacecraft point to Mars having a much warmer and wetter climate in its ancient history (before and between 3.5-3.8 billion years ago). As on Earth, this water is most plausibly delivered by comets and asteroids during the early heavy bombardment; it may have remained liquid for a while, but it will have gradually disappeared, much of it perhaps frozen to form the deep sub-surface ice deposits. 
 
Something about Mars changed, turning it from a warm wet planet to one with a cold dry climate. A key factor may well be how the Martian atmosphere developed with time. The early volcanic activity will have produced gases that contributed to building a thick atmosphere, at the same time that internal heat was warming the planet. The temperature and air pressure at the surface could well have provided conditions warm and dense enough to facilitate a water cycle of rainfall, rivers and lakes for a while. But Mars could not retain this thicker, warming atmosphere – not only did the planet’s own internal heat cool much more rapidly than here on Earth (given the larger surface area to volume ratio of a smaller planet), but the sudden disappearance of the global magnetic field left the atmosphere vulnerable to being stripped away by the Solar wind, abetted by the comparatively weak Martian gravity. 
 
Much of the human fascination with Mars over the last century or so has been stimulated by the suggestion that Mars could host life, or could have done so in the past. Although current findings confirm that it could indeed have once harboured a hospitable environment, the idea of an association of the Red Planet with alien life stems back much further, to features on the surface first noted in the latter part of the 19th century. 
 
The Italian astronomer Schiaparelli first reported the presence of narrow and dark linear features criss-crossing the surface of Mars from observations he undertook during a close encounter of Earth and Mars in 1877. He assumed they were natural structures and termed them ‘canali’, meaning ‘channels’ in Italian, but misinterpreted into English as ‘canals’, subliminally suggestive of an artificial origin. Many astronomers observed the canali, making maps of their distribution, and noting how changed in appearance with the Martian seasons; they seemed more prominent during the summer, leading to an interpretation by some astronomers that the dark patches were due to seasonal plant growth. Extrapolations of this idea even led to the hypothesis that the dark vegetation lined the sides of artificial canals that transported meltwater from the polar regions to the desert. 
 
Comparison to sharper telescopic photographs – even by the early part of the 20th century – soon knocked the idea of intelligent Martian famers on the head. The networks of long and straight features were revealed as an optical illusion where the eye tends to join small disconnected features into lines. The apparent seasonal variations in the appearance of the lines could be attributed to their obscuration by the long-lived global dust storms which also deposited new layers of dust across the terrain. 
 
To the best of our knowledge, Mars is currently lifeless. Accepting that it may have been warmer and wetter in the distant past, we can rule out all but the hardiest of microbial life surviving to the present day. Known on Earth as extremophiles – these microbes could possibly have developed in the very acidic and salty water conditions. But given the harsh UV radiation bombarding the surface, even such resilient lifeforms are highly unlikely to have survived to the present day unless they have remained buried deep underground. Here the temperatures are warmer, there might caches of liquid water, and the environment is shielded from radiation and damaging UV sunlight. After all, extremophiles have been discovered in not dissimilar conditions, living up to 1km below the Earth’s surface. 
 
Conditions favourable for life might well have arisen much earlier on Mars than on Earth. First of all, given its smaller size, Mars will have cooled from its molten state at least a billion years before Earth solidified. Additionally, very recent research on the mineralogy of the Martian soil shows that it would have been rich in elements such as boron and molybdenum which are thought to play a crucial role in the assembly of the molecular components required for living organisms – RNA, DNA and proteins. Indeed, it is not altogether clear how such processes could have been so easily initiated on Earth, where the relevant minerals would either have been dissolved in oceans or are thought to not have been available in either sufficient quantity or the correct chemical form. 
 
Such findings have revived the speculation that life could have been delivered from Mars to Earth by hitch-hiking a ride on a meteorite. Meteorites do land on Earth from Mars – about a dozen or so have been identified as of Martian origin, both from the balance of isotopes within the rock, and the composition of tiny pockets of gas contained within which exactly matches that measured in the Martian atmosphere by landers. These meteorites are pieces of the Martian surface that were blasted away in to space from a large impact – fragments of debris only need to reach the comparatively low speed of 5 km/s to be able to escape the weaker gravity of Mars. The escaped material would most likely orbit Mars for a while, until the combined gravitational pull of Jupiter or nearby asteroids might nudge it into a different orbit that would eventually intersect with that of the Earth some millions of years later. 
 
One such Martian meteorite, ALH 84001, did cause great public excitement in 1996 when scanning electron microscopy revealed miniscule structures embedded in the cracks of the rock, which had shapes resembling fossils of primitive bacteria-like life. But at only 20-100 nanometres in diameter, the structures are much smaller than any known cellular life, and in the years since the original excitement of discovery, the scientific consensus is that they are a form of mineral structure rather than a micro-fossil. Nonetheless, we cannot rule out the possibility that life did not originate on the Earth. 
 
The robotic exploration of the Martian surface is planned to continue well beyond the present day, although it will be many years before a spacecraft will explicitly search for direct evidence for life well below the surface of Mars. Future missions will extend the visual, chemical and mineralogical analysis of the soil down to microscopic scales; the most important step beyond that will be the first mission to successfully collect and return soil and rock samples to Earth for analysis in the lab. Such a mission is required to test the capability of running a return mission to Mars before we can consider the launch of a crewed flight; and before astronauts are dispatched to Mars, extensive robotic missions will be needed to investigate possible landing sites and the accessibility of local resources such as water, as well as to deliver cargo supplies, life support systems and living units. 
 
President Obama has proposed that NASA launch the first Martian astronauts by the 2030s, but there other interested parties with plans to land the first people to the Red Planet even sooner. 
 
In particular, earlier piloted missions to Mars are very possible if one drops the assumption that any mission to Mars would have to be a return mission. This reduces the technological challenges that would have to be met. Instead – in a situation similar to that of the first settlers to Australia or the Americas, the first people to Mars could be regarded as the first immigrants, departing on a one-way ticket. A Dutch not-for-profit foundation called Mars One is now actively pursuing this option with the aim of establishing a permanent human settlement on Mars in 2023; they have already received over 200,000 applications from candidates wanting to be amongst the first four-person crew to launch in 2022. The idea is that the colony will expand with regular deliveries of new crew in subsequent years, again on a one-way ticket. Mars One look to the public interest that such a mission will generate to finance the huge cost (of at least four billion dollars), aiming to recoup the financial outlay by selling the broadcasting rights to potentially be the biggest media event in the world. 
 
Funding such a mission and finding the right crew is only the start; no-one can pretend that life on Mars is going to be easy. Even the 7-9 month journey presents a whole host of risk and danger for the travellers. Medical study of astronauts enduring long stays in space already show that they lose both bone and muscle mass in conditions of low gravity – this will not be regained in the reduced gravitational field of Mars, and the inability to adjust to Earth’s strong gravity would alone most likely preclude a return. Radiation detectors currently on Curiosity demonstrate that any surface exposure would soon reach hazardous levels – and this is without considering the radiation that would be encountered on the journey through open space between planets. Add to this the cold, the poisonous and near-vacuum conditions of the atmosphere, the lack of protection from damaging UV sunlight and the corrosive and penetrating dry-dust that could infiltrate every filter, clog and short electronic equipment… and Mars no longer looks such a pleasant place to stay. If I see astronauts launched to Mars in my lifetime, I shall be cheering them all the way … but also very glad that I am not among them!
 
 
© Carolin Crawford 2013