The question before us is this: Are we life forms alone within the Sun’s family of planets and moons, or is there any other life, however primitive, nearby?

Planets and Moons Hardly anyone expected the Apollo astronauts to discover evidence for life forms, living or fossilized, while collecting lunar soil such as that shown in Figure 8.13. Still, some researchers thought that simple organic chemicals might have formed on the Moon's surface over eons of time. If so, then we might have a means to retrace the early steps of chemical evolution that led to life as we know it. However, even though some carbon monoxide (CO), carbon dioxide (CO₂), methane (CH₄), and a few other simple carbon-rich molecules were indeed found trapped in the lunar soil, careful analysis of the lunar samples showed that the amount of heavy organic matter was so small that no definite conclusions could be drawn.

FIGURE 8.13 — U.S. astronauts collected many samples of lunar soil during several Apollo landings in the early 1970s. In this dish of loose debris from the Moon, a small scale with millimeter markers grants perspective. (Smithsonian)

The total amount of carbon-rich matter contained in lunar soil is so minute that probably none of it is native to the Moon itself. Instead, all of it could have been deposited on the lunar surface by impacting meteorites, or even by carbon particles escaping the Sun. Since the Moon lacks a protective atmosphere and magnetic field, it’s openly subjected to fierce bombardment by solar ultraviolet radiation, the solar wind, and cosmic rays. Simple molecules, even those linked by strong carbon bonds, couldn’t possibly survive in such a hostile environment.

The planet Mercury isn’t a much better candidate to harbor life, or even the basic ingredients for life. Without much of an atmosphere, heat doesn’t transfer well from one side of the planet to the other. Hence Mercury's surface is expected to be alternately very hot and then very cold every other orbital period of 88 days. To an even greater extent than on the Moon, the Sun's radiation would destroy any carbon bonding that might happen to develop on this innermost planet.

The outermost planet, Pluto, has the opposite problem. Being so far from the Sun, it’s too cold and water there couldn’t possibly remain in the liquid state. Hence the building blocks of life, even if they did exist on this frigid planet, would have no easy way to interact to form the larger organic chemicals needed to construct life.

The Jovian Planets—Jupiter through Neptune—are all quite far from the Sun and have cloud tops well below the freezing point of water. However, we must not be too quick to dismiss any possibility for life somewhere within these large gas balls. Surely, greater warmth exists deep down in their clouds. Both the greenhouse effect (which traps solar radiation) and internal sources of energy (at least within Jupiter and Saturn) provide some heat. Consequently, a few astronomers have argued that intermediate levels of the Jovian atmospheres might be warm enough for the development and maintenance of life.

Other researchers contend that, despite the heat, life is unlikely. To survive and prosper, Jovian life would have to remain at a reasonably stable atmospheric altitude. If it floated too high in the atmosphere, it would freeze; if it sank too low, it would fry. Freezing wouldn’t necessarily destroy large molecules or organisms, but it would surely impede their further development. (Refrigerators are technological devices that don’t harm bacteria as much as slow their metabolism.) Heat, on the other hand, is more dangerous than cold. Too much heat would physically destroy any carbon bonding. (Stoves are devices that can sterilize things by killing life.)

Trace amounts of life's building blocks have been detected in the atmospheres of Jupiter and Saturn. Ammonia (HN₃), methane (CH₄), hydrogen (H), and energy of many types are known to be present. Still, a basic problem plagues the idea that these chemicals might produce life: As these molecules combine to form amino acids and nucleotide bases, and in turn possibly larger polymers, the products would become progressively heavier. These products would then be pulled more strongly by gravity, making them likely to sink. The heavier they got, the deeper they would sink, and the greater the chance of their destruction by heat.

It would seem unlikely, then, that life could originate and sustain itself at a comfortable altitude within the atmosphere of either of these giant planets. Nonetheless, future studies of their atmospheres might provide important clues to the early stages of chemical evolution—that road that led to life.

As noted briefly in the BIOLOGICAL EPOCH, some of the moons of the Jovian planets are probably better candidates to house life as we know it. For example, Saturn's moon Titan (see Figure 6.9), with its methane-ammonia-nitrogen atmosphere and its possibly solid surface, is a potential site for life, though the results of the 1980 Voyager fly-by mission suggest that its surface conditions are rather inhospitable for any kind of life familiar to us. Even with some greenhouse heating possible, its surface temperature was measured to be a frigid ~75 K (or -200^oC). Still, Titan has twice the mass of our Moon and an atmosphere thicker than Earth’s. Nearly 90% of Titan’s gas is nitrogen, much like Earth’s air, laced with hydrocarbons (which are molecules made solely of H and C). Speculation runs the gamut from oceans of liquid organics to icy valleys laden with hydrocarbon sludge. At the least, Titan’s environment must resemble a gigantic chemical factory powered by the energy of sunlight—and where there’s energy and organic matter, well, who knows. If life forms do exist—or have existed, and might have adapted—on alien worlds, they will probably be quite unlike those populating the sea ice on Earth today.

Jupiter’s moon Europa (see Figure 6.8) is also intriguing. It has a metallic core, rocky mantle, and probably more water locked in and beneath ice near its surface than in all the seas on Earth. Though the evidence for water is only conjectural, its likelihood opens up many interesting avenues for speculation about life, or at least the prebiological steps that led to life. The Galileo mission to Jupiter recently returned direct imagery showing Europa totally ice-bound, yet those pictures also show a smooth yet tangled surface resembling the huge ice flows that cover Earth’s polar regions. Something, most likely the tidal effects of Jupiter, is causing this moon (which is comparable in size to our own Moon) to be active and thus to allow water to be energized independent of the Sun. But a caveat is in order: Where there’s water doesn’t necessarily mean there’s life.

The chances for life on Venus approximate those on Jupiter and Saturn. The Venusian surface temperature and pressure are much too high to support life as we know it—750 K and 90 bars, respectively. (Earth’s atmospheric pressure equals 1 bar.) However, the tops of the clouds are cooler and thinner. As for the Jovian planets, we can again reason that an intermediate level exists for which the physical conditions might be more hospitable. The chemical conditions, on the other hand, are most undesirable, as Venus' atmosphere contains large amounts of highly corrosive sulfuric acid (H₂SO₄). At any rate, organisms or even proto-organisms would be unlikely to remain at a constant altitude needed to avoid either freezing or cooking.

Search for Life on Mars For centuries, the planet Mars has been the center of speculation about extraterrestrial life. Of all the planets, its physical conditions most resemble those on Earth. Mars has a hard surface, moderate temperature, and mild pressure. It rotates in ~24.5 hours and experiences seasons during its nearly 2-year trek around the Sun. And it has an atmosphere, although one made mostly of CO₂ (~95%) which ensures some greenhouse heating; still, its surface temperature averages ~50 K less than those at corresponding latitudes on Earth.

All things considered, and as earlier noted in the BIOLOGICAL EPOCH, Mars today seems harsh by Earth standards. The scarcity of liquid water implies dehydration, its thin atmosphere (~0.01 bar) ensures a continuous freeze-thaw cycle as the planet spins, and the lack of global magnetism and an ozone layer allows the solar high-energy particles and ultraviolet radiation to reach the surface virtually unabated.

To be sure, Earth life would have a hard time surviving on Mars. But here we’re not examining Earth life; we’re evaluating the prospects for Martian life. Our views become tempered given that any Martian life would have originated, evolved, and adapted in that alien environment. Continued adaptation of native Martian life could have presumably made even advanced life possible, especially in the past when Mars' atmosphere was thicker, its surface warmer and wetter. Thus, Mars is indeed harsh by our standards, but not entirely incapable of supporting some form of life.

An early American Mariner spacecraft that traveled past Mars in the 1960s sent back disappointing pictures. The chances for life seemed bleak. Packed with numerous craters, Mars seemed to resemble the desolation of our Moon more than a comfortable abode for life. However, the last in the series of Mariner spacecraft carried improved equipment and took sharper pictures while orbiting Mars in the early 1970s, and here the results were totally unexpected. Mariner 9 showed Mars to have not only volcanoes and canyons, but also what resembled dried river beds or channels through which liquid of some sort likely flowed. Figure 8.14 shows one such notable example. The chances for life on Mars increased greatly, especially if that liquid was the water now trapped in the frozen polar caps. The possibility of finding evidence for life there suddenly brightened—if not current life, then possibly extinct life that flourished at some earlier time when the liquid flowed.

The search for life on Mars began in earnest when two American Viking spacecraft arrived at Mars in the mid-1970s. After dispatching a robot to the surface, each mother craft continued orbiting the planet. Figure 8.15 shows (prior to launch) one of these robots that soft-landed on the Martian surface. Each robot was programmed to sense life in several ways, initially using a television camera to seek fossilized remnants of large plants and animals. They also scanned the horizon for any gross motion—trees blowing in the wind, giraffes scampering across the landscape, whatever. Neither fossils nor movements of any kind were seen.

FIGURE 8.14 — The red planet, Mars, shown in true color here, displays an array of fascinating surface features, most notably in this image a vast canyon—the Mariner Valley—which extends for ~4000 km, or roughly the width of the United States. Volcanoes can be seen at left. (NASA)

FIGURE 8.15 — Many important features of the Viking robot can be seen in this photograph, taken prior to launch from Earth—its long surface-sampling arm, its cameras and meteorological gear, and its antenna to transmit information back to Earth. (Jet Propulsion Lab)

In addition, as can be seen in Figure 8.16, each robot was able to scoop up and then ingest Martian soil. Instruments onboard the robot then tested for life by conducting a variety of chemical experiments. The results of these experiments were encouraging, although they remain ambiguous to this day.

FIGURE 8.16 — Several trenches dug by the shovel-like "arm" of one of the Viking robots scar the rock-strewn plains of Mars, along with a discarded canister that measures ~20 cm long. Soil samples were scooped up and taken inside the robot, where instruments tested them for chemical composition and any signs of life. (NASA)

Figure 8.17 outlines the main features of the first chemical test for life conducted by the Viking robots. This test was designed to detect even the most primitive plant-like organisms on Mars. Plants normally require gas, water, and light for sustenance, and they grow exceptionally well if that gas is mainly CO₂. The resulting chemical reaction is photosynthesis which yields oxygen, as noted in the BIOLOGICAL EPOCH. Plants were thought to be the best candidate for life because 95% of the Martian atmosphere is in fact CO₂. To check for plants, each robot inserted a small amount of Martian soil into one of its experimental chambers containing CO₂ along with some H₂O (brought along by the robot), while a lamp bathed the interior with radiation that imitated Martian sunlight. This rich mixture of gas, water, and light should have enhanced the growth of any Martian plants. After an incubation period of a few days, this first test gave a positive result; oxygen gas was released. The soil seemed to contain plants or plant-like organisms of some sort.

FIGURE 8.17 — One test for life on Mars was designed to detect photosynthesis occurring in any Martian plants that were bathed in a rich mixture of carbon dioxide, water, and sunlight.

The second chemical test was designed to detect the by-products of any microbial animals. Sketched in Figure 8.18, this test required moistening the Martian soil with a nutrient broth rich in amino acids and other food stuffs. The nutrients, transported to Mars by the Viking spacecraft, weren’t just ordinary food. Specially prepared for the flight, some carbon atoms in the food were radioactive. After any microbes in the Martian soil ate and digested it, some of the basic ingredients of these consumed foods would be released as wastes, which could then be measured by radioactive detectors on-board each Viking robot. As for the first chemical experiment, the results of this second test were also positive; radioactive CO₂ was detected. The implication was that Martian microbes might be present in the soil.

FIGURE 8.18 — Another test for life on Mars used radioactive carbon from a rich nutrient broth. Assuming that any microbes in the Martian soil would consume and digest it, as they do food on Earth, radioactive gases in the form of released CO2 would be detected.

FIGURE 8.19 — A third test for life on Mars sought waste gases once the Martian soil was moistened a little. Neither food nor enhanced CO2 were present.

Figure 8.19 outlines the third chemical test for life on Mars. This test was designed to detect any gases emitted as waste products by primitive forms of life now thriving on Mars. In the event that the Martian soil contained small plants or animals, the Viking robots were prepared to detect either CO₂ or oxygen (O₂) gas. This was a Spartan experiment; the third chamber contained no enriched CO₂ gas or any delicious food for the organisms. Only moisture was injected into the Martian soil. Even so, and surprisingly, both gases were detected. CO₂ was released in a slow and steady way, much as do animals on Earth. But the O₂ came off in a burst. This was puzzling since such a burst differs from the steady emission of O₂ by Earth plants. Furthermore, the sudden release of O₂ occurred shortly after the soil had been moistened, suggestive of a chemical reaction among soil compounds—not a biological one involving life. But what kind of chemical reaction?

Chemists subsequently discovered that reactions involving superoxide (i.e., O₂-rich) compounds can mimic some of the expected gas emissions of Earthly life forms. They didn't know this before the Viking mission arrived at Mars and only later realized this novel form of chemistry so strange that its chemical reactions never occur naturally on Earth. Such reactions require abnormally large amounts of superoxides, implying that the Martian soil differs markedly from that thought possible when the Viking experiments were designed. At any rate, this third test implied that all the Viking tests might be indicative of chemistry and not necessarily biology. The results were simply unclear at the time.

The chemical explanation is now favored by most researchers, not only because of the O₂ burst in the third test, but also because of a fourth test done by the Viking robots. This final experiment was designed to detect directly the organic molecules that form the basis for life as we know it. If life really exists on Mars, and in any way resembles that on Earth, the Martian soil should be abundant in organic molecules. However, this fourth test failed to find any organic molecules, at least down to a level of one organic molecule for every million inorganic molecules; this is roughly the relative abundance of organic matter in a sample of Antarctic soil on Earth. Test four, then, didn’t absolutely disprove the existence of organic matter on Mars, since this final test was a lot less sensitive than any of the first three tests. What it does suggest is that if life does exist on Mars, it must be spread extremely thinly throughout the soil.

The consensus among scientists today is that Mars doesn’t seem to house anything now alive and similar to life on Earth. Chemical, as opposed to biological, explanations for all the Viking tests are generally preferred by most researchers, even though peculiar chemical reactions are required to understand the gases measured in each case.

Alas, not all scientists agree. Some suspect that an odd type of biology might be operating on the Martian surface. They contend that Martian bugs capable of eating and digesting superoxides (rather than conventional organic matter) could easily explain all the results of the Viking mission. What’s more, microbial life as we know it might reside in more habitable regions on Mars, such as near the moist polar caps. After all (partly for political reasons, given that these robots were approaching Mars on July 4th, 1976), the two spacecraft landed on the safest Martian terrain, not in the most scientifically interesting regions.

The search for life on Mars took a dramatic and controversial turn in the mid-1990s when NASA announced the possible discovery of fossilized microbes from an ancient Martian era. This issue was briefly broached in the BIOLOGICAL EPOCH, while noting extreme forms of life that might have arisen on Earth. Figure 8.20 (see also Figure 6.6) shows ALH84001, a blackened, 2-kg meteorite ~17 cm across, found in Antarctica. Chemical analysis of its trapped gases indicates that it likely came from Mars—a not so surprising interpretation as planetologists have now found several dozen rocks on Earth that were likely chipped off either the Moon or Mars. This one was apparently blasted off the red planet long ago by a meteoritic impact of some sort, thrown into space, and eventually captured by Earth's gravity. Based on the cosmic-ray exposure it received before falling to Earth, it must have left Mars ~16 million years ago. The rock itself dates back ~4 billion years.

FIGURE 8.20 — This meteorite has the peculiar name ALH84001, having been found in the Alan Hills part of Earth's Antarctica in 1984. It was apparently chipped off Mars long ago, and its magnified views under a microscope (inserts) have been used by some researchers to argue for fossilized, microbial life on Mars. (NASA)

In 1996, a group of scientists argued, based on all the data accumulated from microscopic studies of ALH84001, that they had found fossilized life on Mars. Their main pieces of evidences were globular, carbonate structures similar to those produced by bacteria on Earth (top left insert in Figure 8.20), the presence of chemical compounds often associated with biology on Earth, and curved, rodlike structures (right insert) resembling Earth bacteria, which the researchers interpreted as fossils of primitive Martian organisms.

However, many experts strongly disagree that evidence of Martian life has been found—not even fossilized life. They maintain that, as in the case of the Viking experiments, the "evidence" could all be due to chemical reactions not requiring any kind of biology. Furthermore, the scale of the supposed fossil structures is minute, as they span only ~0.5 micron, or ¹/₁₀ the size (or one thousandth the volume) of ancient bacterial cells found fossilized on Earth. Furthermore, several key experiments haven’t yet been done, such as probing the suspected fossils for evidence of cell walls, or of any internal cavities where bodily fluids would have resided. Nor has anyone yet found in ALH84001 any amino acids, the basic building blocks of life as we know it—though in a rock so old they might have all broken down by now.

At the frontiers of science—as we’ve seen many times on this Web site—issues aren’t always as clear-cut as we might hope. When it comes to life, Nature is indeed complex. A solid verdict regarding life on Mars may not be reached until we’ve thoroughly explored our intriguing neighbor, work that is only now beginning at the start of the 21^st century. Recent robotic landers, Spirit and Opportunity, do seem to have found evidence that Mars was warmer and wetter, but how widespread the water was is yet undetermined. Without a doubt, many secrets lurk on the dusty plains of this ruddy, mysterious planet.

Life Among the Debris Besides planets and their moons, several other objects in our Solar System might house some form of life. Comets, for example, are known to contain many basic ingredients for life, including NH₃, CH₄, and H₂O. And although comets are frozen, their icy matter warms while nearing the Sun. Such conditions would seem to favor the assembly of heavy molecules, but few heavier than these have yet been detected. Thus comets are probably unlikely candidates for life itself, although they might be convenient "laboratories" where we can study the early phases of chemical evolution at close range.

Meteoroids also cruise about our Solar System. A small fraction of those that survive the plunge to Earth's surface contains organic compounds. Shown in Figure 8.21, these carbon-bearing meteorites, called carbonaceous chondrites, display many embedded "chondrules" or pebble-sized granules having a few percent of their mass made of organic compounds. Often it's hard to determine if these large carbon-based molecules are really extraterrestrial remnants. In some cases, they may be mere terrestrial contaminants picked up while plowing through Earth's atmosphere, or accumulated while sitting for years on Earth's surface. Even perched on museum shelves, meteorites can become contaminated with small deposits of terrestrial carbon.

FIGURE 8.21 — A portion of a wafer-thin slice of a carbonaceous chondrite. By passing different types of radiation through it, even ordinary light like that used here, researchers can study the meteorite's material composition. The arrow denotes a carbon-rich chondrule, which is approximately the size of a dime. (Smithsonian)

Fortunately, we know of a few meteorites containing organic matter that couldn’t possibly be due to terrestrial contamination. The so-called Murchison meteorite, which fell near Murchison, Australia, in 1969, is an example of a carbonaceous chondrite that has been studied extensively. Located soon after crashing to the ground, this meteorite contains many of the well-known amino acids normally found in living cells. What's more, the meteorite's amino acids were found to be equally left- and right-handed, implying that they originated outside of Earth. (Life as we know it on Earth is exclusively left-handed, as noted earlier in the CHEMICAL EPOCH.) Other meteorites display parts of nucleotide bases as well. These acids and bases are similar in kind and in relative abundance to those produced in the laboratory simulations described in the CHEMICAL EPOCH. And the abundance of amino acids in this meteorite is impressive; it exceeds that of many desert sands on Earth. However, nothing resembling the complexity of life as we know it has ever been found in any meteorite. Nor do meteorites contain any known fossils of ancient life forms.

The moderately large molecules found in meteorites and in interstellar clouds comprise our only evidence that chemical evolution is occurring or has occurred elsewhere in the Universe. In each case, most researchers regard the organic matter to be prebiotic—that is, matter which could eventually lead to life, but which hasn’t yet done so. Few researchers think that the molecules in meteorites and interstellar clouds could be biotic—that is, the decayed remains of already established life that exists somewhere else in the Galaxy.