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August 2004
There is geochemical evidence of water on Mars. Photographer: Andy Zeigert, Creative Commons
Many people who watched the TV images coming back from the two Mars rovers this year were probably disappointed. The bleak-looking, arid desert they saw could not possibly support life. Or could it? Before explaining why scientists are more optimistic than ever about finding life on Mars, it’s worth reviewing what we know about life on Earth.
Was there life on early Earth?
If space travelers had visited Earth during the first 3.5 billion years of its 4-billion-year existence, they would have seen large expanses of water, with smaller expanses of land, but no plants, insects, or animals. Moreover, if the spaceship had arrived during the first 2 billion years of Earth’s existence, its measuring devices would have reported that there was virtually no oxygen in the atmosphere and no ozone layer to protect the surface from UV radiation.
Based on these observations, the space travelers might well have concluded that there was no life on Earth. If so, our hypothetical visitors would have been spectacularly wrong. In fact, the Earth was teeming with microscopic life—tiny creatures called bacteria and archaea. Scientists believe that these microbes were the first forms of life on Earth. After about 2 billion years, bacteria and archaea were joined by eukaryotic microbes. Only within the last few hundred million years—an eye blink in Earth’s history—would forms of life large enough to be seen with the unaided eye start to appear.
The activities of these early microbes and their descendants have direct relevance for us today.
Scientists now know that those microbes, especially the bacteria and archaea, formed the Earth’s life-support system. Their activities made it possible for the more complex forms of life we see today to evolve. Microbes also do most of the recycling of dead things, so that life can continue.
A type of photosynthetic bacteria called cyanobacteria(originally misnamed blue-green algae) put the first molecular oxygen in the atmosphere about 2 billion years ago, raising the oxygen level to about 10% of what it is today, allowing the ozone layer to form and oxygen-using creatures to evolve.
Much later, plants took over the bulk of the job of producing oxygen, but they can do this only because of organelles called chloroplasts, which evolved from cyanobacteria. Given Earth’s history, it makes sense to look for microbial life on Mars.
What is a “microbe”?
To most scientists, and for purposes of this article, the term “microbe” means any creature capable of reproducing itself that is too small to be seen with the unaided eye. This includes bacteria, archaea, fungi, protozoans, algae, and (possibly) viruses.
Archaea are microbes that look like bacteria but have now been found to form a separate domain of life, the other two domains being bacteria(single-celled microorganisms lacking a nucleus) and eukaryotes(all nucleated organisms, e.g., fungi, protozoa, plants, animals). There are two caveats to this definition:
- Some scientists place viruses in a separate category because they are parasitic on cells capable of reproducing themselves, such as bacteria, fungi, and mammalian cells. Other microbes, with a few exceptions, are capable of reproducing themselves without this type of aid. Many of these, but not all, reproduce by dividing (called “binary fission”).
- Some microbes, including bacteria and fungi, are large enough to be seen without a microscope. These exceptions are included under the term “microbe” because they are closely related to other microbes that are not visible to the unaided eye.
It is difficult to come up with an airtight definition of microbe because, as scientists now know, microbes are an incredibly diverse collection of creatures. There is far more genetic diversity in the microbial world than in the plants, insects, or animals—a fact that is not surprising when you consider that microbes had a 3 billion year evolutionary head start. To get an idea of what this genetic diversity means, consider this: If insects (which are sometimes called, incorrectly, the most diverse group of living things) were sorted into species using the criteria employed by scientists who study bacteria, archaea, or other microbes, there would be one, or at most two or three, species of insects. The diversity of microbes is also evident in the variety of environments in which they live: volcanoes, clouds, sulfuric acid mine drainage, hydrothermal vents, deep subsurface rocks, hot springs.
How likely is there to be life on Mars?
Because water is essential for life as we know it, the discovery of recent geochemical evidence of water on Mars, some of which may still be present, was an exciting finding for biologists. To learn whether life could exist in a barren landscape such as that seen on the surface of Mars, where any water present is mostly present in the frozen state, some microbiologists have journeyed to a part of Earth that resembles Mars in some respects: the polar deserts of Antarctica.
- This region of Antarctica has very little water, and most of the year what little water there is exists in the form of ice.
- The hole in the ozone layer that has developed over Antarctica allows high levels of ultraviolet radiation to reach Earth’s surface, a condition that would be experienced by any creatures located on the Martian surface.
- The level of radiation encountered in Antarctica is not nearly as high as that encountered on the surface of Mars, but it is higher than that encountered on most other parts of Earth’s surface.
Is there life in the polar desert of Antarctica? The answer is an unequivocal Yes. Bacteria and fungi have been found in the Antarctic deserts, not only in the soil of the region but also inside rocks. Scientists speculate that bacteria enter the porous matrix of rocks as a means of protecting themselves from radiation.
A major difference between the environment found in the high deserts of Antarctica and that encountered on the surface of Mars is the atmosphere. The Martian atmosphere is much thinner than that of Earth. It consists mostly of carbon dioxide, or CO2 (about 95%), and contains virtually no oxygen (O2). Because many bacteria, archaea, and algae can use inorganic carbon dioxide as their source of carbon (used to build proteins and other cell components), the predominance of carbon dioxide would be a plus. Also, as noted earlier, many of Earth’s microbes do not require O2, so the lack of O2 does not preclude life.
A more troubling feature of the Martian atmosphere is the very low level of nitrogen (N2). On Earth, N2 makes up 78% of atmospheric gases. On Mars it only composes 3%. Many bacteria can use N2 as a sole source of the nitrogen they need for proteins, nucleic acids, and other cell components, but the low level of N2 would certainly limit the amount of microbial growth. Thus, if there is microbial life on Mars, it is unlikely to be as abundant and as widespread as on Earth and may thus be harder to find.
Different compositions and concentrations of gases may exist in some areas under the Martian surface. Such a possibility would be difficult to prove—unless it is proved indirectly the presence of life in the subsurface regions and in greater abundance than expected.
Is there a historical record of life on Mars?
What if life once existed on Mars but has become extinct? Is it possible to find a geological record of microbial life? At one time the question of whether there was a geological record of microbial life on Earth would have been answered with ridicule. Today, scientists are rethinking this question, especially in view of the fact that microbial activities can leave a macroscopic mark on their surroundings. Two examples of cases in which microbes on Earth have left a fossil record are formations called “streamlets” and banded iron formations.
Microbes are generally too small to be seen with the unaided eye, but aggregates of microbes can be visible. Rocklike formations called stromatolites are large collections of filamentous cyanobacteria that are clearly visible to the unaided eye, even though the component cyanobacteria are not. Stromatolites, some which can be as big as a small boulder, formed when large aggregates of cyanobacteria became fossilized. There may be other cases in which aggregates of microbes were fossilized and can be seen with the unaided eye.
Another marker of past life is evidence of chemical activity. One byproduct of cyanobacterial production of O2 was the formation and precipitation of iron oxides. The result was colorful bands ranging from red to black. Geologists call these banded iron formations, or BIFs. The existence of BIFs illustrates the principle that microbial activities can cause changes that are evident long after the microbes are gone. Microbiologists and geologists are now looking for other visible signs of microbial activities that might tell the story of microbes long gone.
Another approach to finding a geological record of microbial life is to look for long-lived molecules. Current methods for detecting microorganisms and identifying them have focused on DNA. However, DNA is not nearly as long-lived as microbial lipids, which may be a better diagnostic indicator of ancient microbial life than DNA. Other structures found in bacteria survive long after the bacteria have died: Magnetotactic bacteria leave magnetite crystals in a beads-on-a string formation. Some scientists have reported finding these in Martian meteorites. Although the hypothesis that magnetite crystals in meteors from Mars are evidence of microbial life on Mars has been quite controversial, the example nonetheless illustrates ways in which scientists are looking for new ways to identify fossil traces of long dead microbes
Beyond Mars
Microbiologists are famous for thinking small, but some of them are thinking very large these days—beyond Mars to Europa, a moon of Jupiter, on which evidence of water has also been found. Europa appears to be covered by an ice layer that is miles thick, but there may be liquid water underneath it. Why? The core of Europa is emitting heat and energy, which may allow liquid water to exist despite the low temperature at the surface of the moon.
Judging from what we know about life on Earth, could life as we know it exist in the liquid water of Europa? One answer may come from a site in Antarctica, a lake named Vostok. This lake has a miles-thick ice covering, with liquid water at its base.
- Scientists are finding that ice cores taken from Lake Vostok contain bacteria that are still viable.
- The possibility exists that these microbes were simply blown into the area from other parts of the globe and trapped in the ice.
- Bacteria can remain viable for long periods if frozen, but they may not be living, that is, actively dividing, in the Vostok ice.
A problem for life existing below the ice sheet that covers Europa is finding a source of energy. On Earth, photosynthesis is the source of energy for much of its microbial life. Photosynthetic bacteria and algae are at the basis of most of Earth’s food chains. On Europa, however, it is unlikely that enough light could penetrate through an ice layer as thick as that thought to exist on Europa.
Scientists working at the bottom of Earth’s deepest oceans, where virtually no light penetrates, are finding that animal and microbial life nonetheless flourish there, near hydrothermal vents where magma from the Earth’s core pumps chemical energy sources such as sulfides into the water. The bacteria that grow there, and in turn feed the hydrothermal vent animals, use the oxidation of sulfides to produce energy.
At first, this discovery was viewed as evidence of a photosynthesis- independent type of growth and gave some people hope that the core of planetary bodies like Europa could also serve as an energy source. The problem with this argument is that sulfide oxidation depends on O2, which is derived from photosynthesis. Yet microbes have surprised us before with their metabolic diversity. Perhaps there are microbial strategies of non-photosynthetic growth that have yet to be discovered.
Should scientists be looking beyond an Earthcentric view of life?
The approach to looking for life described above is based on the idea that life elsewhere in the universe would be like life on Earth and would evolve in the same way. Science fiction enthusiasts, for example, have long fantasized about creatures composed of molecules based on silicon rather than carbon. Although scientists have tended to be highly skeptical of this view, for good reason, some scientists believe that it is important at least to consider the possibility that notions of life based on what we know might cause us to miss evidence of life. This concern about limiting the search for life to an Earthcentric approach has caused some scientists to ask the bigger question: What features of life are independent of assumptions about being Earthlike? Some traits of life have been suggested by scientist Ken Nealson and others:
Life forms move independently of external forces. This cannot be a central requirement for all forms of life because, even on Earth, there are many organisms that are not motile.
Living organisms have edges. There should be some kind of covering that separates a living organism’s interior from its exterior.
The composition of living things is complex. Living beings should contain a complex mixture of chemical compounds.
Life depends on chemical reactions that cannot occur spontaneously under the conditions where they are found to occur. On Earth, many microbes make a living by catalyzing reactions, such as the oxidation of minerals or the production of methane, that either do not occur abiotically or occur at a much lower rate.
What if scientists fail to find life on other planetary bodies?
If scientists fail to find evidence for current or past life on Mars, Europa, or other planets and moons in our solar system, all is not lost. The time, energy, and money spent on studies to develop life-searching strategies have already given us, and are still giving us, new information about Earth.
- Scientists are learning that only a fraction of the diversity of life on Earth is known, especially the staggering diversity that exists in the microbial world.
- New metabolic activities, which are being discovered as a result of astrobiology studies, may well yield new industrial processes.
- Astrobiology studies have reignited interest in the age-old question: What is life?
So, in a sense, we are going to Mars in order to study Earth.
Is it safe to bring Mars samples back to Earth?
In 1970, a novel entitled The Andromeda Strain became a bestseller and was subsequently made into a blockbuster movie. In the movie, a mysterious microbe was brought to Earth by a space probe. The microbe killed people by turning blood to powder. Initially, it appeared to be unstoppable. Fortunately for human life on Earth, the microbe mutated into a form that no longer feasted on blood but instead was satisfied with rubber. Although the premise of the movie was wildly improbable, it created a fear in the public mind that extraterrestrial specimens might introduce a new and unstoppable plague to the Earth.
Could a microbe that evolved in a place where there are no humans possibly cause infection in humans? From our experience here on Earth, we know that the answer to this question is Yes. There have been many examples of human infections caused by bacteria or viruses that have come out of soil or water, locations where they had not had much or any prior contact with humans, or that were previously known to infect only nonhuman animals. Examples are Legionnaire’s disease, a lung infection caused by a bacterium that normally resides in water, and AIDS, a viral infection that probably originated in monkeys and later jumped to humans who were hunting and eating monkeys.
The low temperatures found on Mars and Europa make it unlikely that an organism that evolved in either of those locations would do very well at the much higher temperature found in the human body. It is impossible to be sure of this, however, since microbes have so often surprised scientists with their metabolic diversity. At least we can say that an Andromeda-strain-type organism is highly unlikely to be a virus because viruses that infect humans require mammalian cells to carry out their life cycles.
The desire to make sure that infectious organisms are not brought to Earth from other planets has led to an international treaty that requires all of the countries that have a space program to have a planetary protection officer who leads efforts to make sure that no such importations occur. Currently, the planetary protection officer for the United States is NASA scientist John Rummel.
Dr. Rummel does more than protect the Earth from extraterrestrial invaders. He is just as concerned about damage going in the other direction: contamination of other planets by Earth organisms. In the early days of space travel, scientists did not realize that microbes from Earth could survive the conditions of vacuum, cold, and radiation that anything on the surface of a spaceship would experience. Nor did they consider seriously that astronauts might leave Earth organisms behind after a visit. Now that the danger is appreciated, NASA is taking many special precautions to make sure that there is no further contamination of the planets its space probes visit.
Author’s notes:
» The dates given here for the origin of life on Earth are a consensus of estimates from diverse sources. Different estimates can differ by as much as 200 million years.
» Information on the composition of the atmospheres of Mars and other planets can be obtained on the Internet or from general books on astronomy. Estimates from NASA or web sites of university scientists tend to be the most accurate. Some of the figures are given as percentages of the total and some as actual concentrations of the gas, so different tables may at first appear to be quite different even though they agree with each other.
© 2004, American Institute of Biological Sciences. Educators have permission to reprint articles for classroom use; other users, please contact editor@actionbioscience.org for reprint permission. See reprint policy.
