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4. Mars

Mars

Could life originate on Mars?

Liquid water is a key ingredient of life as we know it. Without water, there was no such life on Mars in the past, and there would be no life surviving now. Life might have thrived on Mars billions of years ago, when abundant water flowed down its valleys and across its plains. And the possibility of recent liquid water on Mars enhances the prospects for finding microscopic life there now.

No one is suggesting that the Martians are out there relaxing in warm springs or hot tubs, but microscopic life could be thriving in wet, warm areas on Mars. They could have thrived in the muddy gullies, as recently as a million years ago or just the other day. Some of them might be hanging on at the bottoms of former lakes, or maybe they have moved deep underground, where there is enough warmth and water to survive.

Or the microbes might have existed billions of years ago, when water flooded the terrain, and only fossils remain. And maybe there has never been a living thing on Mars. Of course, it is all speculation, until we land on the planet and find out. That is exactly what NASA is doing, sending robotic spacecraft to the sites of past water flow in the search for past or present life.

The question of whether or not life exists on Mars is intimately related to hypotheses for the origin of life on Earth. It could have arisen more than 3.5 billion years ago as the result of chemical reactions in tidal pools, on mineral surfaces, or in the primeval oceans. Ultimately molecules capable of reproducing themselves within tiny membrane-bound cells formed and life began.

But could life originate on Mars? The Earth and Mars were formed out of similar material at about the same time, and at a similar distance from the Sun. There is evidence for an ancient period of flowing water on Mars and suggestions of recent subsurface water and heat. Martian life may have breathed carbon dioxide, and the Martian air may even have contained substantial amounts of oxygen that is now locked into its soil. In short, all the basic ingredients for life may have once been present on Mars, and the hypothesis of chemical evolution suggests life could have arisen there.

If life did arise on Mars, how could we find evidence for it? Martian life ought to be based on the chemistry of the cosmically abundant atoms, including carbon, which is a key substance in building complex molecules. Carbon atoms can form large molecules by combining with other atoms, including other carbon atoms. Complex molecules based upon carbon are called organic molecules. Every living thing on Earth is composed of organic molecules, providing them with the capacity to evolve, adapt, and replicate. The discovery of organic molecules on Mars could therefore provide evidence that life might exist, or perhaps once existed, on the red planet.

Real live organisms, or fossils of former ones, might even be detected on Mars, but if anything now lives there it would have to be very strong, tough and exceptionally small. After all, an ice age now prevails on Mars.

Can living creatures survive on Mars?

Chemical evidence and ancient fossils indicate that primitive life existed on Earth 3.8 billion years ago, when our planet was still a fairly inhospitable place for current terrestrial life. The Earth was then cooling off from the intense bombardment that marked the last stages of its formation, and our planet lacked oxygen in its atmosphere. Since there was no oxygen, the Earth had little or no ozone to protect life from harmful ultraviolet sunlight. Yet, life developed in this apparently harsh environment.

Similar conditions existed on Mars at about the same time, up until about 3.5 billion years ago. Both the Earth and Mars probably had protective magnetic fields and thick atmospheres back then, and water seems to have flowed across the Martian surface 3 or 4 billion years ago. Thus, life could have developed on early Mars, at about the same time that life was establishing a foothold on Earth, and fossils of ancient life might be found on Mars now.

Subsequently, however, Mars lost its magnetic field and most of its atmosphere, making the Martian surface an extremely hostile place by Earthly standards. Today Mars does not have enough oxygen, liquid water or heat for most, or possibly any, forms of terrestrial life. Its atmosphere is nearly all carbon dioxide, with very little oxygen or water vapor, and it is a hundred times thinner than our air.

If any hypothetical Martian creatures avoided being asphyxiated by carbon dioxide, or frozen to death at night, they would be faced with damaging ultraviolet sunlight during the daytime. Most organisms that are found on Earth today would be quickly killed if they were exposed to the Martian surface levels of ultraviolet rays. On Earth the lethal ultraviolet is absorbed in the ozone layer and never reaches the ground. On top of that, Mars has no magnetic field or thick atmosphere to keep potentially destructive solar particles or cosmic rays from reaching the ground. Many scientists therefore remain skeptical about the chances of locating living organisms in the dry, cold Martian world.

Optimists, on the other hand, have thought of plausible ways that primitive Martian life might survive when things turned hard on Mars, taking refuge from increasingly threatening conditions (Table 1) and perhaps surviving until the present day. After all, they argue, single-celled life dominated the surface of the Earth for nearly 3 billion years, well before the rise of oxygen in its air, and microbial organisms now thrive on Earth under extreme conditions that were once thought to be lethal. The most notorious of these so-called extremophiles exist at temperatures near and above the boiling point of water; they would freeze to death at temperatures that would result in severe burns to humans. Other microbes now exist at freezing temperatures underneath and within Antarctica sea ice. They even proliferate under dark, pressure-cooker conditions at the bottom of the ocean, feeding on materials emerging from volcano vents, and some of them dwell deep within the Earth’s rocky interior, thousand of meters down.

Table 1. Adapting to hostile conditions now on Mars

Obstacle to life

Adaptation required

No liquid water now on surface.Use subsurface water. Extract water from ice or rock by heat or chemical processes.
Surface temperature usually below freezing point of water. Live underground where it is warm, develop internal anti-freeze.
Very little oxygen in atmosphere. Breathe carbon dioxide.
Lethal ultraviolet sunlight and solar and cosmic particles. Live inside rocks, or underground, hide under rocks and sand, develop protective shell.
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All of these extreme life forms require water to survive, so the search for life on Mars ought to begin at places there might have been water. Even so, to send a spacecraft to Mars in search of life was an exciting long-shot gamble.

Viking 1 and 2 search for life

One of humanity’s most daring and imaginative experiments involved landing spacecraft on the surface of Mars, and searching for evidence of life there. The 3 billion-dollar gamble, in today’s dollars, began on 20 July 1976, when the Viking 1 lander came to rest on the western slopes of Chryse Planitia, the Plain of Gold, region of Mars. It appeared to have once been inundated by a great flood and was thus a promising place for life to have arisen. Six weeks later, the Viking 2 lander settled down in the Utopia Planitia region on the opposite side of the planet, near the maximum extent of the north polar cap, again a favorable site for water and possible life.

Fig. .. 

How did the Viking landers test for life on Mars? The first, most obvious, moving creature test consisted of looking to see if any creatures were frolicking on the Martian surface. The cameras could detect anything down to a few millimeters in size if it came within 1.5 meters of the landers. Pictures were taken of all the visible landscape, from the stubby lander-legs to the horizon, for two complete Martian years, but the view was always one of a desolate, rock-strewn, wind swept terrain (Fig. 8.42). A careful inspection of all these pictures failed to reveal any motions or shapes that could suggest life, not a single wiggle or a twitch, or an insect or worm. Unless they look like rocks, there are probably no forms of life on Mars larger than a few millimeters in size.

Of course, no one really expected that the Viking eyes would see living things, and each of the landers carried a $50 million biology laboratory designed to detect tiny, invisible microbes. Computerized devices inside the landers measured samples of the Martian soil for organic molecules and for signs of growth that might signal the presence of living microorganisms.

The presence of microbes could be inferred if the Martian soil contained organic molecules with carbon in them. Such a test for organic molecules might be called a dead-body test, for soil would be expected to contain a higher proportion of organic molecules derived from dead bodies than from living ones. But not a single carbon compound was detected, even though the instruments could have spotted organic molecules at a concentration of one in a billion.

The Viking experiments were so sensitive that they would have easily detected organic molecules from the most barren and desolate environments on Earth, and even carbon compounds deposited by meteorites as observed in the soils returned from the Moon. Thus, something on Mars had to be destroying carbon compounds.

As it turned out, the soil is so filled with chemicals that it would make your flesh burn. These chemicals have completely wiped out all carbon bearing, organic molecules in the Martian soil. Compounds containing carbon, denoted by C, have been oxidized, or combined with oxygen, O, to make carbon dioxide gas, CO2, that has escaped into the atmosphere.

The other experiments on board the Viking landers searched for the vital signs of living microbes. They did this by exposing the soil to various nutrients, and sniffing the atmosphere to see if microbes ate the food and released gas. Something did emit carbon dioxide and oxygen gas, but it wasn’t alive.

When samples of the Martian soil were exposed to liquid food laced with radioactive carbon, large amounts of radioactive carbon dioxide poured out from the soil. This certainly suggested that animal-like microbes were digesting the food and exhaling carbon dioxide gas. But when additional nutrients were added to the soil, there was no additional increase in radioactive gas. Living creatures would have continued to ingest the food.

When the Martian soil was exposed to water, a burst of oxygen flowed from it. At first, the surprised scientists thought that plant-like microbes were emitting the oxygen, but they soon realized that the release was too fast and brief. Microbes would grow and produce more oxygen as time went on, thereby releasing oxygen at a steady rate. Moreover, the oxygen was released when the experiment was performed in the dark, and this behavior would not be expected from plant-like photosynthesis that depends on sunlight.

After further experiments, scientists concluded that the biological tests failed to detect any unambiguous evidence for life on Mars. Instead of being produced by organisms of any kind, all of the results were attributed to non-biological, chemical interactions. Highly oxidized minerals in the Martian soil were reacting with the nutrients, breaking them up and liberating some oxygen gas and even more carbon dioxide.

Since Mars has no ozone layer, the planet’s surface is exposed to the full intensity of the Sun’s ultraviolet radiation, and these rays have turned the surface into an antiseptic form. The lethal soil has apparently destroyed any cells, living, dormant or dead, wiping them out with chemical reactions, somewhat like pouring hydrogen peroxide into a cut. The only safe haven for any living organism would thus be under the chemically reactive ground.

The highly oxidizing material has even turned Mars red. Its distinctive hue looks like rust, and for a good reason. Analysis with Viking lander instruments has shown that the reddish soil has a high content of iron, denoted by Fe, which has been oxidized to produce a form of rust known as ferric oxide, or FeO. It accounts for the planet’s pronounced red color.

The suspension of fine reddish dust can color the Martian sky pink. Because the atmosphere is so thin, its light-scattering properties, which determine color, are dominated by dust particles lofted into the atmosphere by periodic dust storms. In contrast, abundant molecules in the Earth’s thick air preferentially scatter blue sunlight down to the surface, making our sky blue.

As it turned out, the pioneering Vikings paved the way for the next spacecraft to land on the reddish-brown surface under a cold pink sky.

Mars Pathfinder and Sojourner Rover

The primary objective of the next spacecraft to land on Mars, the Mars Pathfinder lander and its diminutive roving vehicle, Sojourner, was to demonstrate a low-cost means of landing a small payload and mobile vehicle on Mars. Unlike previous spacecraft, that went into stately orbits around Mars and descended gently to the surface, Mars Pathfinder was shot directly at the planet, using a small parachute on arrival and cushioning the impact with air bags.

Landing on 4 July 1997, in celebration of Independence Day and America’s 221st birthday, the mission became a celebrated example of NASA’s new mantra of “faster, better, cheaper”. Mars Pathfinder and The Sojourner Rover were developed in four years, at a total cost of $250 million for launch, lander and rover. That is comparable to the budget of a major motion picture, and a mere fraction of the $1 billion cost of the twin Viking missions in the 1970s – which amounts to more than $3 billion in 1997 dollars. Still, Mars Pathfinder was not designed to do anywhere near the amount of science as the Vikings; science was not even a factor allowed to drive the Mars Pathfinder design.

Fig. .. 

Although the Pathfinder mission was not designed to look for direct signs of life, it did search for very indirect evidence of a formerly warm, wet Mars that might have supported life. The landing site in Chryse Planitia, the Plain of Gold, was chosen because it lies at the mouth of a large outflow channel, called Ares Vallis, apparently carved by catastrophic floods in the distant past (Fig. 8.43). It is thought that running water flowed down the Ares Vallis and flooded the plain at the landing site between 3.6 and 4.5 billion years ago.

Fig. .. 

One of the major unexpected results of Mars Pathfinder was the inability to detect chemical diversity in the rocks. The data are consistent with all measured rocks being chemically the same and covered by different amounts of the same dust. This was a surprise, because the landing site was expected to contain a wide variety of ancient rocks washed down by the flooding waters (Fig. 8.44).

The distant, streamlined hills, known as Twin Peaks, appear to have been smoothed by water (Fig. 8.44), and evidence of layered sedimentation shows up in both the nearby rocks and the Twin Peaks. Even the presence of sand, as opposed to smaller dust particles, suggests the widespread action of flowing water. The sand detected from Pathfinder was light in color, just like beach sand on Earth; and sand on Earth is formed by running water. In addition, magnets on Pathfinder found that the airborne dust is very magnetic, which can be explained if liquid water helped embed magnetic minerals in the dust.

Thus, there is abundant evidence that liquid water once flowed across Mars, and that the climate must have once been warmer and wetter than at present. Perhaps the planet had a thicker atmosphere in its early history, and conditions were then conducive to the survival of life.

The immediate vicinity of the Pathfinder landing site, however, appears to have been dry and unchanged for eons. The region seems to have remained almost unaltered since catastrophic floods sent rocks tumbling across the plain more than two billion years ago. It has apparently been untouched by water ever since the ancient deluge. Only the winds remained to erode and shape the surface.

Fig. .. 

The small rover, called Sojourner, added an important scientific component to the Pathfinder mission, along with elements of drama and excitement that captivated the public. The tiny vehicle weighed just 10.6 kilograms, about the same as a house pet, and had the overall size and rectangular shape of a small microwave oven (Fig. 8.45). Equipped with six-wheel drive, with each wheel driven separately, Sojourner explored about 250 square meters of the Martian surface, measuring the chemical makeup of the rocks and soil. In contrast, each of the two Viking landers were shackled to one location, unable to roam across the surrounding terrain, somewhat like getting sick in bed on vacation.

Sojourner moved slowly across the terrain, at less than a hundredth, or 0.01, of a meter per second, like a baby learning to walk. It could “think” with behavior programmed into its computerized “brain”, albeit with an intelligence quotient less than that of an insect. Five laser beams, sent out in different directions, were used to sense obstacles and determine how far away they were, enabling the rover to find and analyze rocks, and to either steer around or climb over them. A gyroscope, spinning like a top inside Sojourner, kept the course steady.

Sojourner’s behavior captivated the imagination of the public, which followed the mission with great interest via the World Wide Web; at its peak the web site recorded 47 million hits in just one day. In addition, the diminutive rover operated 12 times its design lifetime of seven days in the unforgiving cold, earning the title “the little engine that could”. The tried-and-true technology embodied in the pioneering Sojourner will be incorporated in larger roving vehicles that NASA plans to send to Mars, probably to scout the planet’s surface for signs of water.

The rover examined an array of Martian rocks of different shapes, sizes and texture (Fig. 8.46), provided with colorful names like Barnacle Bill, Yogi, Scooby Doo, Casper, Wedge, Shark and Half Dome. It also analyzed the Martian soil in the vicinity, showing that it is very similar to the soil at the Viking 1 and 2 landing sites.

Scientists expected to find a type of volcanic rock known as basalt, which forms by partial melting of the mantle. This kind of igneous rock is typical of lava found on the Earth’s ocean floor, as well as on the maria of the Moon. Many of the rocks analyzed by Sojourner, however, contained much more silicon, or quartz, than pure basalt. On Earth, such volcanic rocks are called andesites – named after the Andes Mountains that are made out of a mixture of quartz and basalt.

The Martian rocks tell of a heated, tumultuous internal history. To form quartz, the crustal material must have been heated, cooled and reheated many times by volcanic reprocessing, much like quartz rocks on Earth but unlike rocks returned from the Moon. The implication is that Mars has been convulsed by internal heat through much of its 4.6-billion-year history, marked by repeated internal melting, cooling and remelting that produced an abundance of telltale quartz.

Moreover, Mars, like the Earth is an internally layered globe, with a crust, a mantle and an iron core. The evidence that the red planet is not merely a solid ball of rock comes from changes in radio communication with Pathfinder as Mars rotated about its axis. By combining these signals with similar measurements from the Viking landers, scientists have determined the density at various depths in Mars, inferring the presence of a dense, metallic core and the thickness of its mantle. Early in the planet’s history, the molten rock on Mars became differentiated, with heavy elements like iron sinking to the bottom and light ones rising to the top.

So Mars Pathfinder and Sojourner Rover have shown that Mars was Earth-like in its infancy. Both planets sustained enough internal heat to produce surface quartz and distinct internal layers. This, in turn, suggests that ongoing volcanic activity could have produced a thick atmosphere during the planet’s youth, becoming warm and wet enough to sustain life.

Possible life in a rock from Mars

Rocks that arrive from space and survive their fiery descent to the ground are given the name meteorites. Thousands of them have been recovered from the ice sheets of Antarctica, and most are chipped fragments of asteroids, a ring of rubble located between the orbits of Mars and Jupiter.

Only about a dozen meteorites have been identified as coming from Mars. They have been collectively named the SNC meteorites after the initials of the locations where they were first observed to fall from the sky – near Shergotty, India in 1865, Nakhala, Egypt in 1911, and Chassigny, France in 1815.

How do we know that the SNC meteorites came from Mars? These igneous rocks have characteristics similar to terrestrial lava, but the ratios of certain elements, the oxygen isotopes, are distinct from all rocks on Earth. This means that they came from a different planet, a moon, or the asteroid belt. Most of these meteorites solidified from molten material less than 1.5 billion years ago, so they had to come from a place that was volcanically active long after the origin of the solar system 4.6 billion years ago. This rules our Mercury and the Moon, as well as the asteroids, which are not large enough to be volcanically active anyway. The minerals in some of the meteorites have been chemically altered by water, so they originated in a place with water, ruling out the Moon and Venus. That leaves Mars as the only place with the required attributes that is close enough for a rock to be propelled to Earth. The clinching argument for an origin on Mars comes from analysis of pockets of gas trapped in the SNC meteorites. This gas has a unique composition that exactly matches that of the same gases in the Martian atmosphere, as measured by the Viking landers.

The origin of each meteorite from Mars can be traced back to the jolt of a much larger impacting object. Most of the debris of this violent collision would fall back to the Martian surface to form the rim of a crater, but some of it would be blasted off at a high enough speed to escape the weak tug of Martian gravity. That material would move in its own orbits around the Sun. These orbits would be gradually skewed by the gravitational pull of the distant planets, and very occasionally redirected on a collision course with Earth. Over an interval of 10 to 100 million years, a small fraction of the ejected debris would eventually strike the Earth.

Interest in the possibility of Martian life was heightened when scientists found possible signs of ancient, primitive bacteria-like structures inside just one of these rocks from Mars. This meteorite crash landed on the blue ice near the South Pole toward the end of the last ice age, resting there in frigid isolation for millennia, most likely compressed in the snow and later exposed. Then in 1984 a geologist spotted it in the Allan Hills region of Antarctica, bagged it, and sent it to the United States for analysis. Since it was the first meteorite to be processed from the 1984 expedition, the unusual rock has been designated ALH (for Allan Hills) 84001.

The resume of ALH 84001 has been established from laboratory analysis of different sets of radioactive chemical elements or isotopes. This Martian meteorite differs from the other ones in being very ancient, having solidified from molten material about 4.5 billion years ago. The rock then stayed on the surface of Mars more or less undisturbed for eons. Then 16 million years ago, the blast of an impacting object excavated the rock from the surface and flung it into space. There it wandered in an increasingly eccentric orbit, eventually moving close enough to be captured in Earth’s gravity and falling on the Antarctica ice sheet 13 thousand years ago. That much every one agrees on. ALH 84001 originated on Mars soon after the planet formed, and was found in Antarctica 16 million years after being blasted away from the red planet and 13 thousand years after arriving at Earth.

Fig. ..  Fig. .. 

The controversy and excitement centers on suggestions that microbial life took refuge in cracks within ALH 84001 a very long time ago. The microscopic and chemical evidence includes globules of calcium carbonate; the first organic molecules thought to be of Martian origin; several mineral features characteristic of biological activity; and what looks like fossils of extremely small bacteria-like organisms that lived on Mars billions of years ago (Figs. 8.47, 8.48). Some scientists argue that these observations collectively provide strong circumstantial evidence for past, primitive life on Mars; others reason that the evidence is not conclusive and that there are non-biological explanations for all of it.

Regardless of the outcome of the ongoing controversy, the evidence only concerns very ancient structures. The carbonates that cover the walls of cracks in the meteorite are probably about 3.6 billion years old. Thus, even if there was something once living in ALH 84001, we are now looking at fossils of long-dead corpses and have no reason to think life might still be around.

The life-like structures in ALH 84001 are also very, very small. The globules are smaller than the period at the end of this sentence, and the putative microfossils within them are less than one-hundredth the width of a human hair. So, we are talking about structures that can only be seen with the most powerful electron microscopes, and there certainly isn’t any evidence that any higher life form ever existed on Mars.

Still, even the possibility that rudimentary life once existed on another planet is so important that we should carefully examine the pros and cons of the controversy, and we first discuss the supporting evidence for alien life on Mars. All of that evidence is found within the meteorite’s round carbonate globules. They were apparently formed billions of years ago when water-bearing fluid percolated through the crustal rocks of Mars. The carbonate globules that are found in ALH 84001 are similar in size and texture to carbonate features within fossils of ancient bacteria on Earth. The organic molecules that are located in and on the meteorite’s carbonate globules provide the first evidence that such molecules ever existed on Mars. Certain magnetic minerals are also found within the globules, and they resemble those created by terrestrial bacteria. Highly magnified images reveal structures resembling bacterial fossils found on Earth. These tube-like features seem to have formed in colonies within the confines of the globules.

But there are alternative, non-biological explanations for all of the supporting evidence. Geological and geochemical processes can explain the carbonate globules and the complex molecules, magnetic minerals and apparent fossils within them. Carbonates can be deposited in changing chemical environments in the absence of life. Organic molecules are formed by non-biological processes as well as by biological ones. Hundreds of organic molecules are distributed throughout interstellar space and in meteorites known to have come from the asteroid belt, created by lifeless processes in these sterile locations.

Detailed study of the structure of the magnetic crystals found in ALH 84001 indicates that they were probably produced by non-living, inorganic processes rather than by living bacteria. The fossils may also be too small to contain all the genetic essentials of a living cell, for a good-sized protein cannot fit inside them. And the so-called fossils do not seem to contain cells, or the cell walls and membranes that could protect a cell from its environment while also extracting energy from it.

An informed, objective jury would probably examine the different interpretations of the evidence, and conclude that no one has proven, beyond a shadow of doubt, that ancient microscopic Martian life existed in ALH 84001. Indeed, the majority of scientists now think there is nothing in the meteorite that conclusively indicates whether life once existed on Mars or exists there now. They aren’t saying that there couldn’t be life on Mars, or doubting the arguments for it, but just that there is enough uncertainty to cast doubt on the claim that microscopic life once lived in ALH 84001. This does not refute the general arguments for possible life on Mars, either past or present, and many scientists subscribe to the belief that life will be discovered in some location other than Earth.

The continuing hunt for extraterrestrial life

It seems that modern civilization has always anticipated finding life on Mars, perhaps because of its Earth-like seasons, clouds, past flowing water, ice caps and similar daily rhythm. And Mars remains the most likely, nearby place in the solar system to find extraterrestrial life. We have visited the Moon and concluded that there is no life there. The intense heat on the surface of Venus boiled away any water long ago; it would fry and vaporize all living things that we know of. Mercury has essentially no protective atmosphere, and its temperature extremes from day to night would alternately boil and freeze anything on its surface, except perhaps at the polar regions.

Thus, as dry and cold as it might be, Mars remains the most plausible nearby home for life in the solar system outside Earth itself, and future voyages to search for life on Mars are inevitable. Robotic spacecraft, and eventually humans, will visit the most likely places to contain life, returning rocks and soil to be scrutinized in the terrestrial laboratory.

Terrestrial microbes will surely accompany astronauts to Mars, perhaps contaminating the planet and complicating the search for life. Conversely, rocks and soils brought back to Earth from Mars by a future space mission could be full of deadly microbes that might cause a global catastrophe on Earth. Visiting astronauts could conceivably get infected with an alien Martian plague, and be forced into quarantine if they make it back home. Precautions should also be taken against an unexpected crash landing of the returning spacecraft, which could release dangerous alien organisms. Since the surface of Mars is now sterilized, these are unlikely possibilities, but precautions are being taken just in case the improbable occurs.

As a matter of fact, it is possible that Earth and Mars have regularly exchanged microbes over the years, without any modern spacecraft or astronauts being involved. Life forms may have arisen on Mars first and then migrated to Earth, hitching a ride on a meteorite; or it might have been the other way around, with life originating on Earth and traveling to Mars. Cosmic impacts with Mars have sent hundreds of tons of nomadic Martian rocks to Earth over the centuries, and much more during the past eons. The rain of impacting debris was hardest in the early days of both planets, increasing the likelihood of biological exchange between them. And even now, two tons of Martian rocks are thought to rain down on Earth each year, and about the same amount of terrestrial rocks annually smashes into Mars.

So, life might not have emerged spontaneously on Earth. It could have come from Mars, and in that case we might all be Martians. Or life might have originated on Earth and was then delivered to Mars. Maybe life arose on the two planets independently and spontaneously or perhaps impacting comets or asteroids pollinated both planets. And just maybe Earth is the only place to harbor life in entire solar system. It’s all a lot of fun to think about, but every one of these possibilities is mainly speculation with little hard scientific evidence.

To get back to everyday reality, how can we best widen the search for life, especially for extant and dormant life on Mars? To begin, we should understand what characterizes living things, and determine how to find these characteristics on an alien planet. Then we should decide what locations on Mars are likely to have sustained life, and go there and look for it.

What is life anyway? Perhaps the most important key to life is energy, which flows through life and powers it. All living things have to extract energy from their environment to fuel themselves. The most powerful source of this energy is sunlight striking the surface of a planet, but heat energy from the planet’s hot interior will also suffice. Tides provide an additional, but usually less powerful, source for energizing life.

On Earth, the major source of available energy is sunlight. The geothermal energy provided by the planet’s hot inside accounts for just 0.0016, or 1.6 x 10-4, the amount of available power as sunlight. Tidal energy accounts for a tenth of the available power provided by geothermal energy.

Chemical processes convert the available energy into fuel that is consumed by biological organisms, which return waste products to the environment. Photosynthesis by plants, for example, uses the energy of sunlight to extract carbon, C, from carbon dioxide, CO2, in the atmosphere, releasing oxygen, O2, into the air. With the help of water, the carbon is incorporated into organic molecules within the plants. Chemists say that the carbon dioxide, CO2, has been reduced, or broken apart into its simpler constituents, C and O2.

Despite its overwhelming energy input to Earth, the Sun is not the only source of life-giving energy on our planet. Geothermal energy powers chemical processes that produce inorganic compounds without carbon, such as hydrogen, hydrogen sulfide, ferrous iron, and iron sulfide. Organisms also use these compounds for fuel, effectively eating rocks, and sometimes living in complete darkness.

We have found giant clams and fields of tubeworms living in the blackness of the deep sea, near underwater volcanoes on the ocean floor. The Earth’s inner heat releases chemicals in the volcanic vents, providing food for tiny microbes, which are eaten by larger organisms. But they are ultimately dependent on photosynthetic oxygen, which enters the deep ocean from the surface water. Other microbes survive thousands of meters inside the Earth’s crust, sometimes living in solid formations of granite. Like the deep underwater microbes, these subterranean organisms have substituted volcanic fires for solar ones as an energy source, living off the Earth’s inner heat and inorganic chemicals.

Living things also have to be protected from their own waste products, generated while consuming either organic or inorganic substances. Cells, whose membranes wall off the cell interior from its own chemical byproducts, can do this. And cells involve structure, shape and form. In order to survive as time goes on, life must also replicate itself and eventually adapt to a changing environment.

It’s likely that hypothetical Martian microbes could similarly thrive on geothermal or geochemical energy, and tests should be designed to detect the chemistry involved in sustaining organisms with this type of energy. Since they require sunlight, other plant-like microbes would have to be at or near the sterile surface of Mars, where they cannot now live; and the Viking experiments seem to have ruled out this type of mechanism for obtaining energy.

And where should we look for the putative Martian organisms? The Viking tests literally just scratched the surface, the top layer of soil. Since the surface has been exposed for eons to freezing cold, lethal solar ultraviolet, killing cosmic particles and a lengthy parched drought, you might have expected that no life would be found on it. If simple life forms once existed on the surface of Mars, they might have survived by burrowing inside rocks or deeper underground for warmth and sustaining moisture.

The first place to look for signs of Martian life will be places where liquid water once existed, and where subsurface liquid water might still reside. The layered beds of ancient lakes, the sites of past catastrophic floods, and the possible locations of recent water flow or volcanic activity are all logical places to land a spacecraft. Robotic vehicles or astronauts could retrieve rocks from these locations, and drill core samples from deeper down. Living microbes might still be hiding inside the rocks or cores, or we might find fossilized remnants there.

To begin the 22nd-century quest for life on Mars, NASA safely inserted its Mars Odyssey spacecraft into Martian orbit in October 2001. It is mapping the chemical and mineralogical makeup of Mars, looking for signs of liquid water near the surface and shallow buried ice, while also evaluating potential radiation risks to future human explorers. Then in 2003, the European Space Agency is expected to launch its Mars Express orbiter carrying the British Beagle 2 lander, while NASA lands its twin Mars Exploration landers with their rovers. And in 2005, the next time Mars is at its nearest distance from Earth, NASA expects to send its Mars Reconnaissance orbiting spacecraft. Its instruments will greatly enhance the search for evidence of water, take images of objects about the size of a beach ball, and search for future landing sites on the Martian surface. One of these locations will be chosen for NASA’s first sample return mission, hopefully launched in during the second decade of the century.

The discovery of life on Mars, even primitive life in the very distant past, would have profound implications. It would give us companionship in a vast and lonely Universe, and it would also be a little humbling. The discovery would raise the likelihood that life might be found elsewhere in the Universe as well, perhaps on one of the countless planets that surely exist in our Galaxy (Focus 3). Of course, the enduring idea of life on Mars could prove as illusory as the Martian canals, but even in that event many humans will still retain a passionate conviction that there must be life somewhere else in the Universe. So the quest will continue, and whatever happens the human spirit will remain as beautiful and glorious as ever.

Focus 3. Widening the search for life

Looking beyond Mars, there are other locations that contain two ingredients for terrestrial life - water and energy. Europa, a large moon of Jupiter seems to have a global sea of liquid water just beneath its thin, cracked icy crust. Though far from our home star, and therefore bathed in diminished sunlight, alien life might thrive in Europa’s dimly illuminated seas, warmed by inner tidal heat and powered by chemicals. Underwater volcanoes, analogous to those discovered in the deep ocean floors of Earth, could exist at the bottom of Europa’s oceans, similarly teeming with microbial life. Ultimately, we would like to obtain samples from the fractured ice, but we don’t know how to accomplish a roundtrip to Europa, including a stopover, with any current technology.

Saturn’s giant moon Titan, larger than the planet Mercury, possesses a thick, Earth-like nitrogen atmosphere with abundant methane and other organic molecules. A rich chemistry powered by sunlight in the upper atmosphere produces hazes that hide the surface below. The Cassini-Huygens spacecraft, scheduled to arrive at Saturn in June 2004, will send a probe down to examine Titan’s surface, to see if it offers a sanctuary to life-forms coming in out of the cold. We may then gain new perspectives on what happened before or during the origin of life on Earth. Unfortunately, it is even harder to get to Titan than Europa, and a round-trip with a landing is not feasible using today’s technology.

And even if life within our solar system is only found on Earth, the discovery of planets around far-away stars suggests that life might be common in the cosmos at large. These stars and their planets were formed in interstellar clouds that contain vast amounts of water, one of the key molecules of life, as well as all kinds of organic, carbon-bearing molecules, from formaldehyde to benzene.

As yet, not one of these planets has been seen through a telescope. Most have been detected indirectly by the gravitational pull they exert on their star, causing it to apparently “wobble” as the planet orbits the star. Moreover, the orbiting planet has to be at least as large and massive as Jupiter to produce a detectable effect. Smaller planets, comparable in mass and size to the Earth, cannot now be detected by this method.

Telescopes must be lofted into space, outside the Earth’s obscuring atmosphere, and linked together to discover Earth-sized planets around stars other than the Sun. NASA and the European Space Agency are actively, and independently, planning such a mission, that may eventually discover planets more akin to Earth and Mars, scrutinizing them for the chemical signatures of life. Planetary atmospheres that contain mixtures of gases that are out of equilibrium, and should not be present together, would be one example of a clear signature of life. Large amounts of molecular oxygen in a planet’s atmosphere would, for example, indicated the presence of plant life.

Many scientists think that somewhere out there, among the 100 billion stars in our Galaxy, there ought to be at least one habitable planet similar to Earth, swarming with organisms and perhaps with something more advanced than us. Others discount the possibility, but now we are back in the realm of speculation that may only be clarified by future scientific investigations.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University