Microbial Life in the Solar System

The search for microbial life in the solar system began in earnest with Apollo 11. Although it was clear that the Moon was not a teeming abode of life, it was thought that the Moon might provide clues to early life or at least to pre-biotic chemical conditions. The astronauts and the samples they collected underwent elaborate quarantine, lest lunar microbes attack Earthlings like the disastrous diseases carried across the Atlantic Ocean just 450 years earlier. Before Apollo, some thought that the Moon was similar in composition to primitive meteorites—that it might contain abundant carbon and water in the form of hydrated minerals. A popular theory for the Moon's origin was that it formed elsewhere and was captured during a close encounter with Earth. This theory had it that the Moon initially had a much smaller orbit and then retreated outward in response to tidal effects. Harold Urey, the Nobel Prize—winning chemist (and one of the leading pioneers in the field of planetary science), imagined that the Moon would have passed so close to Earth that the immense tidal interaction would have resulted in parts of the ocean sloshing into space and landing on the lunar surface. Although few believed that any living organisms could flourish in the harsh environment of the airless Moon, Urey thought the Moon might retain critical records of prebiotic chemistry and desiccated remains of the earliest forms of life on Earth. Urey called the Moon the Rosetta Stone of the solar system.

When the Apollo 11 samples were returned, the first tests were toxico-logical, to see whether the samples had any dire effects on terrestrial life. Some of the priceless cargo of lunar soil was fed to rats and placed in the root systems of growing plants. No negative effects were observed, and detailed analysis of the rocks and soils revealed no organic material of biological origin. There was carbon, but it all appeared to have been derived from impacting meteoritic bodies and implantation by the solar wind. As mentioned above, the lunar samples were extraordinarily "dry"; they contained no bound water. The Moon was found to be a lifeless body that did not even contain the building blocks of life or a life-supporting environment.

The Viking program was the only space mission that directly included life detection among its goals. This extraordinary program involved four spacecraft: two that landed on the surface of Mars for detailed in situ studies and two that went into orbit for global-scale mapping and to relay lander information to Earth. With the possible exception of the Hubble Space Telescope, Viking was the most expensive NASA mission launched purely for scientific exploration. (Apollo had a large scientific component, but the mission was largely motivated by national priorities—getting to the moon first.) The Viking missions cost about 4 billion 1999 dollars and required robotic spacecraft to land on another planet to conduct chemical searches for the presence of life. The first mission landed on Mars in 1976, the American Bicentennial, and many of the scientists involved referred to each other as the Vikings of '76. The large, cowboy-style brass belt buckles that many of them wore, with the mission logo engraved thereon, are still seen at various meetings of planetary scientists and engineers.

Viking was an enormously difficult and successful mission. And yet, in a sense, the Viking mission was a failure in that it did not detect life. Not only did it not detect life, but the results revealed the Martian surface to be a highly inhospitable environment for life. There was less carbon in the soil of Mars than there was on the Moon, and worse, the presence of highly oxidizing conditions indicated that organic material could not survive in the soil. If a dead mouse were buried in shallow Martian soil, its carbon would be converted to carbon dioxide, which would flow into the atmosphere. The results from Viking drove many nails into the coffin of the belief that Mars was an Earth analog that might harbor life.

The Viking missions carried three major life detection experiments. Each was a miniature, highly specialized chemical laboratory designed to detect chemical changes characteristic of biological activity. Each lander had a retractable arm with a scoop at the end. One of the joys of the mid-1970s was watching these scoops actually dig trenches and collect samples on Mars, the famous red planet of so many science fiction stories. The scoops would dig soil samples and drop them through a screen into the analysis instruments. The major life detection experiments were the gas exchange (GEX), labeled release (LR), and pyrolitic release (PR) techniques. The first data, 8 days after the Viking I landing, came from the GEX experiment, and the results were positive—or appeared to be so. A gram of soil was placed in the experimental chamber, and a small amount of water and nutrient was added. Two days later a large amount of newly generated oxygen gas was detected, an expected signal of biological activity. The LR experiment also yielded positive results only a day later. In the labeled release experiment, water and nutrient labeled with radioactive carbon-14 was added to a soil sample, and the equipment recorded whether C14-labeled carbon dioxide or methane was released.

The signal was again positive, and—startlingly more positive, in fact, than in many soils on Earth! In the pyrolitic experiment no nutrient or water was added, but the soil was exposed to C14-labeled carbon dioxide and to carbon monoxide gas and light. After an exposure, the soil was heated (pyrolized) to see whether C14-labeled material would be released from any newly formed organic compounds. There was a weak but positive signal.

In spite of high hopes and expectations, however, the Viking scientists had built in some backup tests. The instruments were designed so that multiple experiments and samples could be run repeatedly, as in a "real" Earth-based laboratory. Repeated tests showed that the "positive" results could be attributed to unusual chemical properties of Martian soil. With no ozone layer to block it, the harsh ultraviolet light from the sun lands directly on the soil and produces highly oxidizing and reactive compounds, such as peroxides, that can produce the reactions observed. After severe heating that would have killed any terrestrial organism, the soils still yielded "positive signals." The Viking team's interpretation of their data indicate that instead of the actions of living Martian organisms, the observed results were caused by surface chemical reactions of nonbiological origin.

The Viking landers did not convincingly detect Martian life, but they did show how difficult it is to identify microbial life, with unknown properties, on a planet that is quite different from Earth. Viking was capable of detecting organisms in most of Earth's surface materials, but our planet teems with life, and a gram of typical soil contains over a billion individual organisms. Viking found that the surface soils on Mars did not and could not support any of the forms of life found on Earth. If life does exist on Mars, we must look for it in subterranean regions beneath the frozen "cryosphere" at depths where liquid water can persist. Future missions cannot simply look for life in spoonfuls of surface soil; they must search the warm, wet regions beneath the inhospitable surface. To directly reach wet rock, future searches for living organisms will have to drill. And drilling must not be done just anywhere, because the frozen cryosphere normally extends to depths of several kilometers. Instead of attempting to drill so deeply, future missions will search out rare geothermal hot spots where liquid water might reach close to the surface. These missions will search the "Yellowstones" of Mars. It is also possible that samples of life-bearing rocks might be found on the surface as debris excavated by impact craters. Studies of terrestrial and lunar craters have shown that large rocks—some the size of houses—can be lifted from considerable depth and deposited on the crater rim. Organisms could not live in these cold dry rocks, but they might survive in a dormant state for thousands or even millions of years.

Although finding living creatures that could be observed to reproduce would be the most convincing discovery, the next searches for life on Mars will have less ambitious goals. They will search for fossils or for chemical, isotopic, or mineralogical indicators of past life. Even if Mars is currently a totally sterile planet, it may well have harbored life in its distant past. Channels and other surface features suggest that Mars was much more Earth-like three or four billion years ago. Mars occasionally had liquid water on its surface, and it probably had lakes or larger bodies of water persisting for moderate periods of time beneath thick ice crusts. If the Rare Earth Hypothesis is correct and life forms readily, then we would expect life to have evolved on Mars during the early period of its evolution when it had more Earth-like surface conditions. The search for life on Mars is a key test of this hypothesis.

Searching for microscopic fossils or other indicators of life is enormously complex and is difficult to do in a convincing way with spacecraft instruments. Space instruments must be designed to perform very specific tasks. Constraints on power, mass, cost, reliability, and remote operation in a hostile environment mean that the kitchen sink and most of the other items that scientists would like to include on the spacecraft are inevitably left behind. Space instruments are usually marvels of their time, but their capabilities rarely are competitive with those of their heavy, power-guzzling and inelegant siblings that are used for day-to-day work in Earth-bound laboratories. The most important limitation of spacecraft instruments is their inflexibility to adapt to new findings. They usually do what they are designed to do, but not more. This differs considerably from normal laboratory studies, where initial results provide new insights and lead to investigations that were not previously anticipated. For these reasons, the most detailed searches for life and fossils on Mars will require sample return. Searches for evidence of life in Martian meteorites have produced intriguing results and have shown what types of investigations could be done on returned samples. The first such mission is now planned for launch in 2005. Although they pose an enormous technical challenge, the Martian sample return missions offer the best current hope for finding evidence of life on Mars. Once returned to Earth, the samples will be examined with the most sensitive instruments we have for clues of the past existence of microbial organisms. Even if life exists on Mars, of course, its discovery may require a series of exploration and sample return missions.

In addition to Mars, there are many other bodies in our solar system that might have microbial life. These include the three outermost large moons of Jupiter (Europa, Ganymede, and Callisto), the large moon of Saturn (Titan), and possibly other moons as well. Europa presently is the most attractive prospect apart from Mars. Images of its surface show a complex landscape of shifting ice and mysterious ridges. Heated by tidal energy, liquid water lurks beneath the surface. Lesser amounts of water or brine are also thought to exist inside Callisto and Ganymede. Although twice as far away, Titan is also an exploration target of great interest. Its dense nitrogen atmosphere and hydrocarbon-rich surface is tantalizing—in spite of cryogenic temperatures at its surface. In 2004 the Cassini mission will parachute an instrumented package onto the surface of Titan. It is not designed to detect life, but this probe will measure environmental parameters that are important to life.

Most of our discussion up to this point has been couched in relatively qualitative terms. It's time now to look at some efforts that have been made to quantify the probability that life will evolve and persist, and to suggest a few numbers of our own. This is the subject of the next chapter.

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