To make another astrobiological connection at the interface of astrophysics and biochemistry, the issue of life beyond Earth looms large—especially the possibility that evidence for Martian fossils might already exist. The prospects for life on alien worlds, such as the Jovian moons Europa and Titan, are equally titillating. Biologists have recently broadened their view of life, not only in extreme Earth environments such as the hydrothermal vents noted in the previous CHEMICAL EPOCH, but also in distinctly new venues elsewhere in our Solar System.

Martian Microbes? During the past generation or so, several robots have orbited and even landed on Mars, foremost among them the two spacecraft of the 1976 Viking project—perhaps NASA’s boldest scientific mission to date. Scientists strongly suspect that Mars currently has no liquid water, dimming the chances for life there now. But running water and a denser atmosphere (again, to “keep a lid on”) in the past could have conceivably fostered conditions suitable for the emergence of life. And there is some visual evidence that water did flow on Mars long ago, including the possibility of meager oceans (or at least large ponds), presumably before the planet entered its current ice age <1 billion years ago. Both Viking landers were programmed to perform some simple experiments designed to detect biological activity or at least organic matter, in the hope that some microbial life forms might have survived to the present day. None was found—as explained in greater detail in the eight, FUTURE EPOCH.

However, the Viking experiments searched only for life now living. Perhaps ancient fossils of long-dead Martian life—simple bacterial life possibly enduring prior to the arrival of the numbing cold that likely prohibited sustained life as we know it—might show paleontological evidence of rudimentary life. If a severe ice age had locked Earth into a deep freeze a billion years ago, the only evidence that life arose on our own planet would be microscopic remains of fossilized microbes—and we wouldn’t be here to ponder it.

Surprisingly, one place to look for Martian fossils is on Earth itself! Planetologists agree that a small fraction of meteorites found scattered across Earth’s surface have actually come from the Moon and from Mars. These meteorites were apparently blasted off these bodies long ago during collisional impacts with other celestial objects, thrown into space violently enough to escape their parent bodies, and eventually captured by Earth’s gravity, ultimately to fall to the ground. The most fascinating of these rocks are surely a dozen or so from the Red Planet—their trapped gases match exactly those present in Mars’ atmosphere—and one of them might harbor fossil evidence of past Martian life.

Based on estimates of the cosmic-ray exposure it received while drifting toward Earth, the meteorite catalogued as ALH84001 and shown in Figure 6.6 must have been ejected from the Martian surface ~16 million years ago. The blackened rock itself, ~4 billion years old and about the size and weight of a grapefruit, was found in 1984 in the Allan Hills of Antarctica, a place where pristine meteorites often just sit atop the icy wastes of the barren, frozen landscape. Upon breaking the rock open and examining its insides closely with a microscope, scientists can see rounded, brownish “globules” of carbonate matter no larger than the period at the end of this sentence, a little like those made during the origin-of-life experiments discussed in the previous CHEMICAL EPOCH. Because carbonates form only in the presence of water, these small globules imply CO2 gas and liquid H2O near ground level at some time in Mars’ history. This matches the inferences drawn earlier from orbital photos of valleys and tributaries apparently carved by water when the Martian climate was wetter and warmer.

FIGURE 6.6 FIGURE 6.6 – This is the meteorite ALH84001, thought to be a rock that chipped off Mars and made its way to Earth. (For scale, the cube is 1 cm3.) When broken open, its insides show circumstantial evidence of microbial life—including the carbonate globule insert at upper left—but the results are very controversial. (NASA)

Claims that the ALH84001 rock contains traces of primitive Martian life stem mainly from our knowledge of bacteria on Earth. Terrestrial bacteria do produce structures similar to the Martian globules, and they also manufacture iron-rich chemical crystals that the rock displays as tiny, teardrop-shaped crystals embedded in places where the carbonate has dissolved. Furthermore, the rock contains traces of PAHs—chem-talk for a class of messy organic molecules known as polycyclic aromatic hydrocarbons—often found among the decay products of terrestrial plants and other Earth organisms. None of these data would individually indicate life if found on Earth, but all of them collectively make the case stronger for life on Mars. Even so, as is often said in science, extraordinary claims require extraordinary evidence.

A final piece of relevant Martian data provides the most dramatic—and the most controversial—evidence. On very small scales seen only through a powerful microscope, elongated and egg-shaped structures are discernable inside the carbonate globules of ALH84001. And these are what some scientists have taken to be fossils of primitive organisms. Outwardly, the photomicrographs reveal curved, wormlike structures clearly resembling bacteria on Earth. But scale is a crucial part of any interpretation. The minute structures are only ~0.5 micron across, or ~10 times smaller (hence ~1000 times less voluminous) than ancient bacterial cells found fossilized on Earth. Many biologists argue that such minute bags of chemicals are simply too small to have functioned as life as we know it. What’s more, the Martian rock contains no evidence of amino acids, cell walls, semi-permeable membranes, or any kind of internal cavities for bodily fluids—all of which properties accompany even the oldest and most primitive fossils found on Earth.

Most experts are of the opinion that life has not been found on Mars—not even fossilized life. They maintain that all the meteorite data (and all the Viking data) could be the result of chemical reactions not requiring any kind of biology. Carbonate compounds are common in all areas of chemistry; PAHs are found in many lifeless places (for example, glacial ice, interstellar clouds, even the exhaust fumes of automobiles); bacteria are not needed to produce crystals; and it remains unclear whether the tiny wormlike structures within ALH84001 are animal, vegetable, or merely mineral. Contamination is also a potentially huge problem since this rock apparently sat open to the elements on Earth for >10,000 years before being picked up by the meteorite hunters. As with most frontiers in science, early pioneering results are not as clear-cut as one might hope.

All that said, should the claim of life on Mars hold up against the weight of healthy skepticism in the scientific community, these findings will go down in history as one of the greatest discoveries of all time: We are—or at least were—not alone in the Universe! Distressingly, if life conceivably did originate on Mars and later arrived on Earth, then we might all be Martians!

Extremophilic Life When considering the presence of life under adversity, we shouldn’t be too quick to rule out environments based solely on extreme properties. The underwater hydrothermal vents noted in the previous CHEMICAL EPOCH are very hostile places as judged by life on or near Earth’s surface, yet life manages to thrive around them under conditions quite unlike anything at the surface. Undersea volcanic activity spills forth scalding-hot water rich in sulfur and poor in oxygen that manages to feed “extremeophilic” life by a process known as chemosynthesis—an analog of photosynthesis, yet one that operates in total darkness. Here, teeming colonies of organisms survive and prosper at temperatures close to, and sometimes even exceeding, the usual boiling point of water on a diet of H, CO2, and elemental sulfur (S), while exhaling toxic (to surface creatures) hydrogen sulfide (H2S). A variety of deep-sea animals resembling clams and worms form symbiotic partnerships with bacteria that get their energy from sulfides rather than light. Despite the decidedly odd conditions and even odder metabolisms, all known extremophiles inhabiting vent environments are still based on the element carbon, just like the rest of us living in the more traditional biosphere in or around Earth’s surface.

The volcanically heated springs of Yellowstone National Park (Figure 6.7) are another good example of an exotic site where a wealth of life flourishes under extreme conditions inhospitable by human standards. There, the rich microbial diversity hardly includes garden-variety types, yet, surprisingly, many of the microorganisms’ genes approximate many of ours. At the molecular level, these hot little creatures resemble eukaryotes more than bacteria, in fact they differ from conventional bacteria more than do humans from a crab. Though in the public eye microbes are often seen in the context of disease and rot, these heat-loving bugs might be telling us something important about how the earliest life forms employed inorganic nutrition (in the absence of carbon) and geothermal heat (without the Sun’s radiation). Carbon-centered metabolism and solar-driven photosynthesis arose comparatively later.

FIGURE 6.7 FIGURE 6.7 – Yellowstone’s hot springs teem with microbial life despite being scalding hot. Apparently, some life forms can adapt to extreme environments, including acid baths. The colors are caused by the microbes photosynthesizing and their chemical oxidizing in different ways.

A decidedly unorthodox category of life—indeed, a newly proposed primary lineage—is the archaeabacteria that are often found in extreme environments once thought devoid of any life. These microorganisms populate only oxygen-free ecosystems, such as on the seafloor, in sewage, or in the hot springs seeping through Earth’s crust. The archaea (as they are called for short) stay alive by converting CO2 and H into methane (CH4), which is the main chemical of natural gas (explaining why some of them are called “methanogens”). Such scalding, oxygen-less conditions resemble those thought to have existed on our planet during its first billion years or so, implying that the archaea hark back to times prior to the formation of conventional bacteria. Accordingly, today’s archaea might be relatively unchanged descendants of the most ancient class of life on Earth—and as such the best link to that ultrasimple, original, “last common ancestor” from which all forms of life have subsequently evolved.

What we don’t know is whether this anerobic, submarine life was the origin of more familiar life that eventually made its way up to the surface, or, by contrast, whether the archaea are merely the result of primitive life once on the surface that managed to survive only by diving (and adapting to) deep underwater environs in order to avoid poisonous oxygen, to develop its own odd metabolism, and to seek alternative energy sources.

Unearthly Life? Hot springs and the extremophilic life thriving within them raise the possibility of life forms with even greater diversity amid even wilder conditions than those known to us on Earth. This is especially true if we broaden our perspective yet more to consider life at the other extreme—cold. Household refrigerators (or at least their freezers) surely retard the growth of bacteria—that’s the job of those machines—but life can sometimes still eek out a living there. In fact, the bottoms of perennially frozen lakes in Antarctica harbor entire communities of microbes, despite temperatures nearly equaling the freezing point of water. These aren’t merely bacteria, but also microscopic plants and animals. In a few places, microbial life holds on and avoids death even within the thick, hardened ice itself, surviving in a kind of suspended animation, apparently indefinitely. The frigid, dry, Antarctic climate resembles that of Mars today, bolstering the idea that frozen tundra on the red planet could support life under Spartan conditions. Other inhospitable places on Earth where simple, yet active life has been found include subterranean rock, salt deposits, and even oil fields, all >1 km below Earth’s surface.

Discoveries during the past decade have brought much wider appreciation for life on our own planet, revealing bacteria in places where biologists once thought nothing could possibly live. Adaptation is the key, as so often the case, and the simplest forms of life seem to be surprisingly adaptive to all sorts of environmental extremes. Marine microbes alone, living in the unpromising milieu of seafloor sediment, are now thought to comprise nearly 1/3 of all living organisms on Earth, yet very little is known about them. Perhaps fully half of Earth’s total biomass is made of microbes, many of them extending as much 1 km into the crust. This is a whole new “deep biosphere” that geologists are only now beginning to explore by underwater drilling. Even in the more accessible (upper) parts of the ocean, bacteria are known to be much more numerous and diverse than previously thought—roughly several billion microbes infuse every teaspoon of water. All told, the oceans are brimming with an estimated 1027 bacteria, or roughly a million times more cells in the sea than stars in the visible Universe. And if many of these microbial life forms within and under the sea are sucking up carbon, they may collectively comprise a huge sink that absorbs carbon pollution produced by today’s civilization and thus mediates climate warming on a global scale. How many more species are yet to be found in the depths of this, the largest habitat on Earth? And if life is so robust in such unlikely places on Earth, to what extent does that raise the prospects for extraterrestrial life, even if it’s only simple, creepy-crawly life?

Consider two candidates for alien life: Jupiter’s moon Europa (Figure 6.8) 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. The Galileo mission to Jupiter recently returned direct imagery implying 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 probably 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.

FIGURE 6.8 FIGURE 6.8 – One of Jupiter’s Galilean moons, Europa, has an icy surface that is only slightly cratered (yet heavily streaked), implying that some ongoing process must be smoothing over impact craters soon after they form. That process might be water upwelling from trapped seas below. (NASA)

FIGURE 6.9 FIGURE 6.9– Saturn’s moon Titan is larger than the planet Mercury and nearly half the size of Earth. This Cassini spacecraft image shows some of Titan’s upper cloud deck and some of its surface, both of which are known to be laced with organic matter. (NASA/ESA)

Likewise, Saturn’s big moon Titan (Figure 6.9) is a place where odd life forms, or at least the prebiological ingredients that comprise life, could be present. 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, such as methane, CH4). Titan’s environment must resemble a gigantic biochemical factory powered by the energy of sunlight—and where there’s energy and organic matter, well, who knows? It’s very chilly there, though; direct measurements indicate that Titan’s surface temperature is a frigid ~75 K (-200oC)—so cold that ordinary ice would be as hard as steel and even methane lakes mostly frozen. If life forms do exist, or have existed, on alien worlds, they will probably be quite unlike those populating the sea ice on Earth today.

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