Science and Ethics in the Exploration of Mars

Abstract

The scientific exploration of Mars might yield results of extraordinary importance for our own planet, particularly the search for extant or fossil Martian life, which would make it possible to understand terrestrial life in a more profound way. This potential scientific treasure places on us an ethical obligation to minimize the disruption of the Martian environment until our scientific exploration has been greatly advanced. We also have ethical obligations to the human scientific explorers of Mars, ethical obligations that require a series of scientific investigations, e.g., about how the low Martian gravitation may affect those explorers’ physiology.

1 Introduction

This chapter will discuss the interplay of scientific and ethical considerations relevant to the exploration of Mars . I will be concerned both with such interplay in terms of our relationship with the red planet and in terms of the well-being of the scientists doing the exploring.

There are many excellent reasons for the scientific exploration of Mars. Learning about its geological and atmospheric history could give us important insights about the history of our own planet, which in turn can give us clues about the Earth ’s future. But the most important potential discovery concerns Martian life , whether extant or fossil. Having another form of life to compare with terrestrial life would be of extraordinary value. Until this exploration is pretty much complete, it would be rather irresponsible, indeed unethical, to change the Martian environment in any significant way, which a large colonization is likely to do.

Since the scientific exploration of Mars is likely to demand long tours of scientific duty, given the enormous costs and distances involved in placing and equipping human scientists on Mars, intensive research efforts must be undertaken as well to minimize the potential harm to humans in the long stays involved.

That research, as well as the knowledge to come from our scientific study of the planet, will make future colonization efforts more likely to succeed. At that point, many practical reasons for colonizing Mars will become apparent. One is to give humanity and the terrestrial life we take with us to Mars a chance to avoid extinction in case that a cosmic catastrophe destroys much of the Earth’s biosphere (e.g., collision with an asteroid much larger than 10 km across). A second is the many inventions that would come from trying to develop large human settlements on another world, just as the space program itself produced many inventions and stimulated the use of many others. A third is the experience itself of adapting to another world, for which our descendants in the long future will thank us (Munevar 2014).

2 The Value of the Scientific Exploration of Mars

Hypotheses about the Earth are inevitably tied to more general theories about the nature and behavior of the other bodies of the solar system. As we challenge our understanding of that system, we place ourselves in a position to learn new things, not only about other worlds, but also about our own. The bounty of space science will thus not be scattered by alien winds over alien lands. It will be handed down to the children of the Earth . This is one reason why a scientific opportunity to explore Mars imposes on us an additional ethical obligation not to squander it.

As an example of how trying to understand our planet benefits from comparative planetology, consider that the Earth’s gravitational energy keeps its atmosphere from dissipating into space, and thus it determines to a high degree the density of that atmosphere. That density in turn influences the chemistry of the environment and the climate of the planet. To see the point clearly it pays to compare our planet with others. For example, the density of Mars’ atmosphere is so low (about one hundredth that of Earth’s) that water cannot exist in liquid form: It goes directly from ice to vapor.

The Moon has influenced terrestrial climate in at least an important way: Its gravitational influence stabilizes the tilt in the Earth’s axis of rotation so that it varies only a few degrees. Mars, by contrast, may have suffered wild swings in the tilt of its axis, and this instability might have had devastating consequences for the Martian climate (Toon et al. 1980). In other words, the Moon may have played a crucial role in ensuring that life on Earth endured and prospered while Mars became a barren world.

In investigating other worlds we find:

• (1) Valuable information that serves to refine our theories of the origin and evolution of the solar system, and hence of the Earth.

• (2) Unusual phenomena that stretch our views of basic terrestrial mechanisms.

• (3) Opportunities to test our ideas about the Earth—the solar system serves as a natural laboratory.

In the standard account of the evolution of a rocky planet, the denser the planet, the more heat it will have available from radioactive elements; and the larger the planet, the more retarded the loss of heat. By this account, no planets much less massive than the Earth could still have active volcanoes or relatively young surfaces. Mars, for example, shows evidence of recent volcanism (within the last two million years); but even if Mars is not a dead planet, its surface is testimony to a prolonged coma.

It seems, however, that earlier in its history, when Mars’ internal energy was much higher, Martian volcanoes might have filled the atmosphere with perhaps 100 times as much CO₂ as today (Forget et al. 2013). This factor would have raised the density and temperature enough to permit liquid water and large amounts of water vapor, and hence much more of a greenhouse effect. As we look at Mars now, that appears to have been the case. For years, spacecraft photographs showed what seemed to be riverbeds and suggested other indications of significant amounts of liquid water in the past, perhaps even an ocean. More recent evidence, discussed below, indicates at least one large deposit of underground liquid water .

This view of Mars is strengthened by the recent discovery that Mars at one time did have at least the beginning of plate tectonics as well, and may experience “Marsquakes” about every million years (Yin 2012). It seems, then, that life could have existed on Mars, and may still. If so, why did not Martian life control the climate the way Earth’s presumably did? Mars apparently did not have enough energy to run the cycles that have made a sustainable biosphere on Earth possible, just as it did not have enough heat to support the motion of tectonic plates for billions of years. Life, if it ever existed on Mars, was thus powerless to stop the ultimate collapse of its global environment. Earth was fortunate not to be besieged by such extreme conditions.

What would Martian life enable us to learn about our own kind of life? The answer is clear. All life on this planet is based on the same carbon chemistry and apparently, all have the same genetic code. Of the many possible amino acids, only 20 are used to build proteins. DNA , the reproductive code for terrestrial life, makes use of only four bases. Moreover, organic molecules can be left handed or right handed, but terrestrial life prefers left-handed amino acids and right-handed sugars. Are these circumstances mere accidents of organic evolution, or are there fundamental reasons why life has taken these particular turns on this planet? Even one other kind of life would permit us to make great strides in examining these matters. For that, other life may use a wider range of amino acids and bases, or it may prefer right-handed amino acids or left-handed sugars. One result of such a finding may be that, say, a particular chemical balance in the Earth’s oceans caused the preference for left-handed amino acids. Or the alien life may be similar to life on Earth , which would reveal to us some sort of organic inevitability. The new perspective would be very fruitful in trying to understand our own biology at all levels.

It would be especially useful to observe stages of organic evolution and to study life as it begins in a new world, or at least to find fossil records of such beginnings. Even if perchance organic evolution produced in two similar planets similar primitive cells with essentially the same genetic code, the subsequent evolution would have much to teach us, for life in those two planets would undergo different histories of adaptation. Imagine, for example, that the now famous Alvarez asteroid had not crashed on the Earth. No one knows how dinosaurs would have continued to evolve, but it is possible that their grip on the surface of the planet would have been further strengthened. Mammals might have been thus forever condemned to crawl and scratch in the night like so many other vermin.

Even similar planets are likely to exhibit different tectonic histories. Plate tectonics brings continents together or breaks them apart; it throws chains of mountains up over the landscape; and it creates volcanoes where the plates rub against each other. In doing so, it brings some habitats to an end and others into existence. It destroys. It influences. It changes life in many ways. Slight differences at the beginning of the history of a planet would alter the make-up of the crossroads that life has to face, first at the level of organic chemistry, and then at the level of cells—presuming that cells are common to living things. A eukaryotic cell (a cell with a nucleus) may well be the result of symbiosis between different varieties of prokaryotic cells (without a nucleus) (Bylinsky 1981). For example, the mitochondria in eukaryotic cells may be the remnants of prokaryotic cells that discovered how to use oxygen for energy and were swallowed but not digested by larger bacteria. Since eukaryotic cells are the building blocks of all complex organisms on Earth , we can imagine that different symbiotic relationships between primitive cells might have led to forms of life vastly different from those of our acquaintance. On planets so endowed, the subsequent interaction of life with the rest of the environment would have a multiplier effect, for they would change their environment in novel ways, and those new environments would lead life to adaptations that on Earth could meet only with misfortune.

Acquaintance with such alternative biotas would inevitably lead to profound transformations in biology, since biology would grow, and scientific knowledge seldom grows without changes. In this, scientific knowledge resembles animals. Mammals, for example, did not just get bigger after the extinction of the dinosaurs. As their size increased, the structure of their skeletons had to change to accommodate their larger weight. In a planet with gravity similar to ours, a dog the size of an elephant would probably look much like an elephant. In an analogous manner, a science of biology that were suddenly much larger in subject matter would have to grow connections and supporting structures for which there was little need in the days of a single biota.

On Mars , the unprotected surface is not the most promising place to look for life. There are sites in Antarctica devoid of life on the surface, but if we care to dig we may find it in porous rocks below. Some raise the possibility that Mars might offer extreme forms of life. There are numerous examples of life surviving under extreme conditions: in the core of nuclear reactors, in underground streams with temperatures of hundreds of degrees Fahrenheit, under incredible pressures at the bottom of the ocean. Organisms have been found even deep in the Earth’s crust. No extreme habitat, though, is as challenging as the Don Juan Pond in Antarctica , where the salinity is so great that a random sample is likely to fail the Viking test for the presence of carbon compounds (NASA’s two Viking landers used tests that, in combination, failed to find life in Mars). But living organisms exist in the Don Juan Pond ! (Siegel 1970, 1968; Siegel and Spettel 1977; Siegel and Siegel 1980). And they can also survive in space (Jönsson et al. 2008).

Of course, the Viking experiments were designed to detect average life on the surface, or inches from the surface, whereas it is clear that if any life exists on Mars it should be in extreme conditions, or at least in protected habitats probably not very close to the surface.

If Mars fails us, life might have nevertheless made a stand in the organic clouds of Jupiter or perhaps in some underground caves in active Io . Another moon of Jupiter, Europa , is covered by smooth ice, which indicates a good amount of internal heat, and probably an ocean of water under the ice. Another notable prospect is Titan, the large moon of Saturn with a dense atmosphere and at least traces of organic compounds. Unfortunately, Titan is too cold for life as we know it—cold enough (−288 F°) that the argument about the ability of life to adapt to extreme habitats begins to wear thin.

At any rate, the “extremophile ” evidence can show only that once life begins it can adapt to very hostile conditions. But it does not show that life could begin in such conditions. These are two very different things. Let me illustrate this point by means of an analogy. During pregnancy, many substances can be lethal to the developing embryo, e.g., alcohol, tobacco, and hallucinogenic drugs. The chances for the new life are greatly hampered under those conditions. Once the baby is born, the situation begins to change. Eventually, it may grow into an adult who smokes, drinks and abuses drugs, none of which are conducive to a healthy life, but none of which need be immediately lethal either, as they could be to the embryo. Furthermore, life might not be able to make a start on a planet that would otherwise be exactly like today’s Earth , but it surely has no trouble flourishing in it now.

As we saw earlier, in a thin atmosphere, such as the present Martian atmosphere , water goes from solid to vapor without first becoming liquid. Therefore, the surface features that resemble river deltas and suggest past running water in turn constitute evidence that the atmosphere was much denser once upon a time.

There are alternative hypotheses on the dendritic channels, though. According to one of them, for instance, occasional but pronounced tilts in Mars’ axis of rotation would expose one of the poles to the full action of the sun. If that were the case, the melting polar cap would provide enough pressure to permit water to run off and presumably form those surface features—all without the benefit of a dense atmosphere. Photographs by the Mars Reconnaissance Orbiter , with ten times better resolution than any taken before, indicate that lava and wind-driven dust have run through those presumed river channels and gullies far more recently than water, even though catastrophic floods might have carved them once upon a time (Science 2007).

Nevertheless, the preponderance of new evidence indicates that the Martian atmosphere was far denser once upon a time and that liquid water ran on the Martian surface . The most striking findings are those of Opportunity, a Mars Exploration Rover . Opportunity found, for example, salt deposits in a region called Meridiani Planum . According to Mike Carr , the man who wrote the book on water on Mars, it is clear that a large body of water existed in that region. It is also clear that the “water had to pass through the ground to pick up the dissolved ions that ultimately were precipitated out as salts” (Carr 2004). This means that the water in that region (a lake or a sea) could not have been mere runoff from melted polar ice. At any rate, the positive side of the case for water was greatly strengthened by the discovery of a 20 km wide lake of water about 2 km under the ice of the South Pole . The discovery was made using radar measurements from the Mars Express spacecraft (Orosei et al. 2018).

If we assume that planets similar to the Earth have similar beginnings—in this case, similar distributions of organic materials, atmospheric gases, and sources of energy—and if we keep in mind that our own earliest fossils are about 3.5 billion years old, it seems plausible to suppose that life made a start in Mars. If that is so, the possibility exists that in some regions of Mars, we may find fossils of organisms that thrived in days when the atmosphere was denser and warmer.

Although such Martian fossils would perhaps not be as exciting as living organisms, they still would be invaluable in that they would permit us to compare our form of life with an alien one. We may also be able to draw some interesting lessons from a failed interaction between life and a planetary environment.

As luck would have it, though, liquid water is likely to exist in underground deposits. The permafrost is presumed to exist to a depth of hundreds of meters, which suggests that some liquid water may be found in proximity to magma deposits and other sources of thermal energy. Martian life forms , if any exist, need not be quite as extreme after all.

3 The Value of Astrobiology with or Without Specimens

The lack of extraterrestrial specimens is an objection to the pursuit of astrobiology only if we accept a narrow definition of the field. Astrobiology goes beyond the search for extraterrestrial life: It is largely the application of space science and technology to understand how life may originate and evolve anywhere in the cosmos. As a practical matter, astrobiology often devotes itself to investigating how life originated and evolved on this planet. Astrobiology tries to determine, for example, what the Earth was like 3.5–4.5 billion years ago—what was the ultraviolet flux? What were the volcanic and other tectonic activity? How much molecular oxygen was in the atmosphere? And how much ozone? How much carbon, hydrogen, and nitrogen were “recycled” through the Earth ‘s crust and how much were brought to the Earth by asteroids and comets? To decide these issues, we must go away from the Earth to study the older surfaces of the Moon and Mars , the presumably still primordial atmosphere of Titan, and the largely untouched chemistry of comets.¹ Astrobiology is thus inseparable from comparative planetology.

This connection is all the more evident when we remember that to understand the nature of an environment well we need to understand its origin and evolution. In the global environment of the Earth, life has played a crucial part, by changing the chemical composition of the atmosphere, its density, and its temperature. How life originated is thus a question of great importance if we are to understand how our global environment came to be as it is. At the same time, we cannot begin to settle that question without making some critical determinations about how the planet was formed, how its atmosphere was created, how much energy it received from the sun in its early evolution, and in general all those questions that form integral part of comparative planetology.

In trying to answer the question of the origin of life, however, there are great difficulties of substance and of method. For example, some investigators require explanations that make life somehow inevitable or at least very likely. Given the early conditions in the planet (e.g., a reducing atmosphere and later a primordial soup of organic materials), and processes that should be expected (e.g., radiation, lightening), organic evolution towards life should be highly probable. Another school of thought would have a series of extraordinary coincidences bring life about. Thus, even if organic matter was abundant on the early Earth, it would have taken an accident, or accidents, to get organic evolution on the road to life. To illustrate the sort of disputes involved, let me consider the hypothesis that a large moon is needed for life to begin. Of course, if that hypothesis is right, we should conclude that life is probably very rare in the galaxy and not to be found in the solar system at all, except for our own kind.

According to this hypothesis, clays in shallow waters served as the templates for amino acids to combine into the first complex organic molecules by forming peptide bonds—bonds that can link carbon and nitrogen in separate organic chains. Such a bond can form when, for example, a nitrogen atom loses its bond with a hydrogen atom (H) and a carbon nearby loses its hydroxide bond (OH). Since the H and the OH combine to form water (H₂O) , the formation of the peptide bond requires a loss of water. The problem is how to lose water in the presence of all that water in which the amino acids are suspended.

The tides created by the Moon provide the solution to the problem (Dorminey 2009). When the tides go out a residue of amino acids is left on the clays, and the heavier concentration in drier surroundings allows the peptide bond to form. Once formed, the peptide bond is stable in water. After many repetitions of this process, organic molecules of an increasing complexity can be formed. The function of the clays is to provide a mechanism for replication.

Having a day–night cycle also seems important because the opportunity to move away from equilibrium gives the prebiological molecules a chance to vary, and this opportunity for variation is an essential characteristic of evolution. Cyclical events are in general favorable because they permit the molecules to reach a state of equilibrium to consolidate their gains before having to change again. Apart from the tides and the day–night cycle, we have the concomitant temperature fluctuations. And we have seasons because the Earth ’s axis is tilted in just the right way thanks, again, to the Moon . Then we must also take into account the role of the magnetosphere and many other factors whose possible relevance or even their very existence may escape the experts at this time.

Insofar as any of these are large factors in allowing life to gain a foothold on Earth, the origin of life becomes an improbable event. But we simply do not know. As plausible as hypotheses such as this may seem today, they may sound very quaint in two or three decades, let alone in a few centuries. And even if the events in question were indeed factors in bringing life into our world, on further examination, they may turn out to be just some among the many alternative mechanisms that could have provided for the evolution of evermore complex molecules in a variety of other worlds. It happens all too often that when a mechanism cannot be immediately proposed to explain a particular step on the way to life, people who ought to know better jump to the conclusion that life on Earth was an extraordinary coincidence.

Many biochemists, for example, have felt that the problem of the origin of macromolecules is insoluble. At the most basic level, life consists of nucleic acids (such as DNA and RNA ) that contain the genetic information, and of functional proteins (such as enzymes). If we imagine that DNA or RNA was the original macromolecule we have to explain how it could replicate in the absence of enzymes, which are essential in modern living systems. On the other hand, if we imagine that the proteins came first, how could they have built around themselves the nucleic acids that would carry the information necessary for future coding of the same proteins?

For years, none of the mechanisms proposed seemed satisfactory, not even co-evolutionary mechanisms, because the chemical association of nucleotides (the building blocks of nucleic acids) and amino acids (the building blocks of proteins) was just too problematic. On the face of this situation, some people thought that life was a stroke of luck. And some others even suggested that life had probably come from elsewhere to the Earth (the so-called “Panspermia Hypothesis ”), as if removing the problem of the origin of life a few light years amounted to a solution (de Duve 2002, 2005).

Nevertheless, some plausible mechanisms were proposed in the late 1960s and have been refined ever since. One of them, suggested by A.G. Cairns-Smith , is that microscopic crystals in clays can serve to replicate molecules. Such clays have a large capacity for adsorption, which causes tiny bits of proteins to stick to them, just as particles of meat do to the surface of a frying pan. The crystals in these clays would then grow and reproduce the patterns of the amino acids adsorbed in the clays (Cairns-Smith 2009).

These processes can be repeated millions of times, until with the development of enzymes, as J.D. Bernal notes, we would also see the appearance of co-enzymes, some of which are identical to the nucleotides of RNA . As the co-enzymes are adsorbed, their efficiency in chemical energy transfer would give clear reproductive advantages to their associated enzymes. Under these conditions, a co-evolution of functional proteins and nucleic acids becomes possible. And this result presumably paves the road to the eventual origin of the first cells. This view has been buttressed by the work of the space scientist James Lawless , who has shown that clays do select precisely the amino acids that can form biologically active proteins.

There are many other hypotheses buttressed by experimental work that fill in some of the steps deemed necessary to take us from atmospheric gases to living cells (e.g., proteins that can “make” their own RNA ). But, what is necessary and what is not depends on the approach one takes to explain the origin of life. First, there are different starting points. Some want, even demand, a reducing atmosphere (poor in oxygen, rich in hydrogen and other gases like methane). Others think that the original atmosphere was composed largely of carbon dioxide. Second, then comes a story of the evolution of organic matter, a story that may involve thousands of steps, of which only some are specified. And of course, there could be alternative plausible stories. What makes them plausible is that some of the steps that may have seemed baffling at one time can be produced in the laboratory now, while others can be explained theoretically. For example, the first generally accepted story gained its plausibility from the Miller-Urey experiment , in which a reducing atmosphere in a flask was subjected to electrical discharges. Presumably, the result of the experiment was a soup containing the building blocks of life. Apparently, however, only very few interesting organic molecules were actually produced in such an experiment, and those were of very little complexity. The steps from there to, say, a self-replicating molecule, let alone a cell, are truly gigantic.

Since there are so many ways to tell the story, most equally unconstrained by the scant evidence, it is not surprising that the intuitions of different investigators differ on what is crucial and what is not. And even if most of the apparently necessary steps of a particular story can be accounted for by experiments, there remains the difficulty that the answer to one part of the puzzle is often at odds with the proposed answer to the next part, e.g., a molecule used as a building block for a more complex molecule is produced in an alkaline solution, but the more complex molecule has to be produced in its opposite, an acidic environment; this is not a fatal setback, since in living things the product of a reaction can be transported to a different internal environment to be used to build something else, and in general we find that natural processes have co-evolved in the living world to accomplish just this transport. The problem is that it all seems just too convenient. What we want to know now is not just how it could have been, but how it was—we want the “real” story.

To go from just-so stories to compelling hypotheses, we need a better understanding of the initial conditions on the Earth , and as we develop our hypotheses accordingly, we will get ideas of what sorts of evidence about the subsequent evolution of the global environment we may want to look for. One helpful way to proceed is to examine those worlds where life might have started, or at least where we should expect some small amount of organic evolution. To the extent that organic evolution has taken place there, we learn much about our own, once we factor in the relevant differences. To the extent to which organic evolution has not taken place, we also learn much about the failure of some forms of reasoning about the origin of life and perhaps get some clues about more appropriate forms of reasoning.

Astrobiology and comparative planetology will merge in many other contexts. Take, for example, the search for the origin of the organic carbon on the Earth, surely a needed background to make a definitive determination of how life started on the Earth. To have organic compounds, we first need to trace the carbon and the other relevant elements (hydrogen is normally easy, since it is almost everywhere, with exceptions such as the Moon and Mercury ). We begin the search for carbon in the solar system and then see how it was apportioned to the Earth. If the Earth had a disproportionate amount of carbon, we must deal with a certain set of scenarios in which the Earth comes to occupy a privileged position. If carbon is very common in the solar system, as indeed it is, our scenarios are of a different sort, but we still want to know how the Earth came by the amounts that it has: Did it happen during the initial accretion of the planet, or was most of the carbon brought in by the subsequent bombardment by comets and asteroids? We might get some useful clues by examining Mar’s history.

These and other investigations underscore the intimate connections between astrobiology and those aspects of space science that deal with the formation and evolution of planets . Since the role of life has been of crucial importance for the Earth, and since we need to know specifically how terrestrial life may not only survive but also prosper, this study of origins is highly justified. To put the point differently, the biota and many of the other elements of the global environment co-evolved. Thus, to understand the evolution of one of those elements we need to understand it in its relationship to the evolution of the others. Moreover, the very role of the imagination in trying to determine the range within which life can be born and the possible forms life may take provides a fruitful context in which to discuss questions of origin and evolution. For by the consideration of likely scenarios for life, and by the comparative examination of the planets in our solar system and of other planetary systems , we will be better able to understand not only how life came about but also why it took the paths that it did when it apparently had others available.

4 The Value of Searching for Martian Life Even if no Specimens Are Found

As we have seen, even if we find no life in Mars, the search may still help determine how life can be born and the possible forms it may take. But Even presumed dead ends, such as the Martian meteorite ALH84001 , have spurred fruitful biological investigations. Martian Meteorite found in Antarctica in 1984 had arrived 13,000 years earlier, after being blasted from Mars some 17 million years before. It had crystalized perhaps about 3–4 billion years ago, when there was liquid water on Mars .

In 1996, David McKay , of NASA , claimed evidence of fossil life in ALH84001 (worm-like structures in the center of the meteorite, 20–100 nm long, although some were larger). In close proximity, he also found globules of carbonate, polycycle aromatic hydrocarbons (PAHs) , clear evidence that there has been organic carbon in Mars , as well as magnetite and iron sulfides (McKay et al. 1996). Meteorite experts and many other scientists disagreed with McKay’s conclusion (Kerr 1997)² on the grounds that:

• (1) All the compounds and structures found in ALH84001 could have been produced by inorganic processes.

• (2) Therefore, by Occam’s Razor , we should eliminate the extraordinary conclusion (Martian Life ).

• (3) Moreover, the smallest worm-like structures were up to 50 times smaller than Earth bacteria (not enough space for all the materials a cell needs).

A number of replies were made to the majority arguments:

• To (1): Inorganic magnetite forms at three times the temperature that ALH84001 experienced. And the magnetite in the sample is extremely pure (on Earth only bacteria make it that pure).

• To (2): In ALH84001 McKay found three things in space a few nanometers across: (a) typical bacterial food (hydrocarbons), (b) structures that look like typical bacteria, and (c) typical excreta of bacteria (magnetite and iron sulfides). It thus seems to me that one simple hypothesis, LIFE, accounts for all these phenomena and the fact that they are closely packed together: Martian bugs ate the hydrocarbons and left the droppings behind. On the other hand, the inorganic-origins hypothesis requires at least three separate mechanisms and has little to say about why they are together in such a small space.

At any rate, the controversy spurred interest in the possible existence of undiscovered Earth bacteria that small (a subject of very little interest in the 80s and early 90s). Soon enough some biologists claimed to have found many such varieties of bacteria (nanobacteria), even smaller than the presumed Martian fossil bugs. Later investigations revealed, however, that main nanobacteria candidates are actually nonliving mineral structures, e.g., calcium carbonate crystals, which mimic bacteria in some respects and even reproduce (Martel and Young 2008). This was a very interesting discovery with practical importance: Those nanostructures are apparently involved in the formation of kidney stones.

The search for ultrasmall bacteria did not stop there. In 2015, scientists from the University of California at Berkeley and the Lawrence Berkeley National Laboratory announced in Nature their discovery of ultrasmall bacteria, about the size of the larger Martian worm-like structures (250 nm). And just to be sure, they also sequenced the ultrasmall bacteria genomes (Luef et al. 2015). These ultrasmall bacteria are four times smaller than life “could be” (according to the previous common “wisdom”). Thus, the pursuit of the possibilities suggested by a “failed” hypothesis led to significant biological discoveries. Consider now that the primitive Martian RNA bacteria (without DNA ) could be even smaller: About two or three times smaller than ultrasmall bacteria. That is, they could be the size of the worm-like structures in ALH84001 ! On the extremely dynamic Earth, such fossils would be far older than those of the DNA bacteria we have found (less than four billion years old). But on a far less dynamic Mars they might have been preserved. Finding them would be far more than exciting.

The vigorous search for life in Mars would inevitably lead to the profound transformation of our views of the living world. And since those views are linked to our understanding of the global environment, the resulting theoretical adjustment would be of great magnitude—and so eventually would be the change in the way we may interact with the universe. Whether or not we ever find a single Martian specimen !

5 Science and the Welfare of Human Explorers in Mars

Steve Squyres , a robot expert and the lead scientist on the Martian rovers , said in 2009 that

… the most successful exploration is going to be carried out by humans, not by robots. What Spirit and Opportunity have done in 5 1/2 years on Mars , you and I could have done in a good week. Humans have a way to deal with surprises, to improvise, to change their plans on the spot (Squyres 2009).

Human scientific explorers in Mars will arrive there after a long and expensive trip and will require a very sophisticated infrastructure to survive, let alone thrive in their work. It is reasonable to suppose, then, that their tours of duty should be rather long, years perhaps. One concern that arises from any human presence in Mars is the possibility of contamination, both of Mars by terrestrial life, particularly microbial, and of the human outposts by extant Martian life . The first is, of course, a greater concern, though the planetary surface is so inhospitable as to make it unlikely that even extremophile may just barely survive in pockets. Interested readers may consult the latest in-depth discussion of planetary contamination in Review and Assessment of Planetary Protection Policy Development Processes (National Academies 2018).

The other concern that arises from the human exploration of Mars is the protection and well-being of those human explorers . The thin Martian atmosphere does not offer much protection from radiation, a situation made worse by the lack of a significant magnetosphere. Living spaces, vehicles, and special outdoor suits, however, can be built to offer the needed protection. Air, fresh food, and comfortable temperatures will also be available. The low gravity of the small planet, about 1/3 that of the Earth , may become an obstacle to very long stays. Perhaps the problems will be far less significant than they are in microgravity, e.g., in the Space Station , but it would be highly irresponsible not to investigate the potential ill effects. In the rest of this essay, I will describe some of the relevant studies of the biological effects of microgravity and make some remarks about possible extrapolations to effects on humans on Mars.

Our main interest lies in the range between 0 and 1 g, so as to study not only the perception of gravity but perhaps even the role that gravity has played in evolution. By experimenting in that range, we may be able to determine gravitational thresholds of biological importance; that is, we may determine the minimum level at which gravity can be detected and at which it becomes a significant factor in physiological or developmental functions. With some luck, the gravitation humans will experience on Mars will be above that threshold.

Gravity is all-pervasive in our planet; it is not hard to imagine that life took advantage of its presence to favor some avenues of evolution over others. As Galileo noticed as early as 1638, how much an animal weighs depends on how well its bones can support it.³ Thus its anatomical structure depends on gravity. In a planet with lower gravity, we may find much taller animals and more symmetrical trees (the symmetry is often broken because slight differences in mass in the branches weigh the tree down in different ways; the more the gravity the more pronounced those differences become in the development of the tree).

As for the perception of gravity, it greatly influences the way a plant grows. In the microgravity of space, roots grow out of the ground into the air and the shoots are generally disoriented in spite of the constant illumination from the top. Gravity is obviously the main factor in the case of the roots; shoots require both gravity and illumination. But how do plants recognize gravity? They have gravity receptors, and thanks to space research we are beginning to understand what those receptors are.⁴

So far there are indications that gravity may also play a role in the axial orientation of amphibian embryos (which is a factor in the normal development of amphibians) and perhaps also in that of birds (Kochav and Eyal-Giladi 1971; Neff and Malacinski 1982).

It is not difficult to imagine that what is true of anatomy may also be true of physiology. Indeed, microgravity leads to a shifting of body fluids, and such shifts affect the cardiovascular system in humans. As a result, we may have an opportunity to study how the functioning of the cardiovascular system—or rather its malfunctioning—is connected with the deterioration of muscles. This of course is a matter of potential significance for the general population, especially for the elderly. Moreover, in microgravity , we no longer need many of our big muscles to support us. As a consequence, the body begins to reduce its levels of calcium and other minerals needed to strengthen them. This presents a big problem for astronauts , whose bones become weak and brittle. On the other hand, their problem may give us a chance to study the connections between bone and mineral metabolism and endocrine action. Here, the adverse reactions of astronauts to weightlessness resemble the symptoms of some diseases on Earth . Space physiology thus offers a chance to investigate the underlying mechanisms (Rambout 1981). Vestibular research, for example, may yield some insights about Meniere’s disease , an affliction of the middle ear characterized by deafness and vertigo. Equivalent investigations should be carried out on Mars .

This fine-tuning of physiology to the Earth’s gravity should provide a fruitful theoretical perspective to study the relationships between a variety of internal systems and cycles in the human body. Are physiological functions maximized at 1 g, or can 0.34 g suffice? This leads to questions about why the body works as it does, questions that would not occur that easily otherwise. One possible answer is that gravity is used to harmonize a variety of physiological systems—gravity is like the glue that holds such systems together. Once the glue is gone, they do not quite work together. And from their failure, we learn what makes them work correctly under terrestrial conditions. Another answer is that those systems change their responses in order to adapt to the new conditions. This may also give us significant clues about their normal modes of interaction with other systems or mechanisms.

The fine-tuning to 1 g may become acute in issues of development. In microgravity, a human male excretes from 1.5 to 2 L of body fluids, with pronounced reductions in the levels of sodium and potassium. By contrast, a pregnant human female is expected, in 1 g, to show an increase of 1.5–4 L over her pregnancy, with a marked retention of sodium. Since the development of the fetus follows a strict sequence in which each event must take place within a critical period, and since the availability and composition of the body fluids are essential to the proper environment in the placenta, we can readily see that disruptions at the system level affect physiological processes at lower levels. At the present time, it would be morally impermissible to have pregnant women in microgravity .

The human body is resourceful; it may be able to compensate for the effects of microgravity in a systematic fashion even during a pregnancy. But to determine whether it can, we must resort to experiments on animals (Halstead and Pleasant 1982). Equivalent experiments on Mars are essential before giving the green light to human birth, or even human pregnancy, there.

The mere fact that many systems function optimally at 1 g provides warrant for designing experiments to determine how the timing and feedback controls of development operate. This, it seems to me, is not a matter of small importance.

“Mere” systemic effects can have profound repercussions that may extend to the cellular level. Changes in the environment of the cell lead to cellular changes in shape, in ability to move, and in internal metabolism (i.e., polarity, secretion, hormone regulation, membrane flow, and energy balance). Much has been written in this and related topics (Gravitational and Space Biology 2013). Changes that take place at the systems level in the organism, such as body fluid shifts, are then bound to affect several cellular systems, change the cellular environment, and thus affect the cells themselves.

The experiments that indicated that gravity was irrelevant at the cellular level were performed in cell cultures; they did not examine cells that formed part of the complex wholes that are the cells’ normal environments.⁵ It is not surprising, then, that such experiments could not expose the indirect action of gravity that starts at the systems level of the organism and works its way down into the realm of the small.

6 Conclusion

As we have seen, the scientific exploration of Mars offers extraordinary opportunities to challenge and improve geology, climatology, and specially biology. In biology, in particular, the new knowledge could have profound implications. As a result, I have argued, we have an ethical obligation to carry out the exploration of Mars in a manner that minimally disturbs the red planet, so as to preserve such scientific treasures. Any possible large-scale colonization should wait for that scientific exploration to be completed, while greatly benefiting from its findings. We have also seen how science must become an integral part of complying with our ethical obligations to those future explorers.