Extraterrestrial life forms examined
There's been a lot of excitement about extraterrestrial life found in meteorites. Can you give us a brief history about that?
The meteorite was found on 27 December 1984 by the National Science Foundation's Antarctic Meteorite Program annual field expedition on blue-ice near the Allan Hills of Antarctica. It was designated ALH84001 for the first meteorite found in the Allan Hills area in 1984. The discovery of evidence for biogenic activity (Figure 1) was made by David S. McKay of the NASA Johnson Space Center (JSC) in Houston, Texas, and his colleagues.
The meteorite was carefully collected and sealed for protection from contamination and returned to JSC, where it was maintained in a precisely controlled environment as used for moon rocks. For several years this particular meteorite was believed to be a diogenite, which is an achondritic meteorite. In October 1993, David Mittlefehldt, a JSC planetary scientist, realized that ALH84001 had been misclassified. It was not a diogenite, but instead belonged to the family of SNC (pronounced "snick") meteorites.
There is significant evidence to indicate that SNC meteorites, which are exceedingly rare, come from Mars (Figure 2). More than 20,000 meteorites have been found on earth, but only 12 of them are grouped together as SNC meteorites. The ALH84001 meteorite is considerably older than all of the other SNC meteorites that have been found and identified to date.
But how do you know this stuff came from Mars and not from the asteroid belt?
There are gases trapped within the rock. The relative abundance and isotopic compositions of the trapped gases in the SNC meteorites have a direct one-to-one correlation with the Mars atmospheric composition as measured in situ by the Viking landers. In changing from a molten state to a solid state, the rock absorbed such gases as carbon dioxide and isotopes of nitrogen and oxygen, as well as noble gases such as xenon, argon, and krypton. These gases distinguish the SNC meteorites from any earth rock, moon rock, and all other types of known meteorites.
How did ALH84001 get to Antarctica?
It was probably ejected from Mars about 16 million years ago, when a large asteroid or comet slammed into the martian surface. The impact accelerated rock material to velocities exceeding the planetary escape velocity and the ejected rocks spent some 16 million years in space orbiting the sun. One of these rocks then entered the earth's atmosphere and landed in the Antarctic, burying itself in the blue ice some 40,000 years ago. It was slowly brought to the surface by the glacial motion of the ice moving toward and being lifted up by the hard rock structures of the Allan Hills. As the uppermost layers of ice were removed by dry winds, the meteorite became uncovered and was subsequently discovered by the NSF expedition.
How are these dates arrived at and how old is this metorite?
Carbon 14 dating gave a value of 13,000 years for the time the meteorite spent in Antarctica. Its time in deep space, between 12 and 17 million years, was based on the amount of helium 3, neon 21, and argon 28 isotopes absorbed by the meteorite during its long exposure to cosmic rays. Samarium-neodymium radioisotope and rubidium-strontium dating methods indicate that ALH84001 was crystallized from a molten state some 4.5 billion years ago, about the time Mars was formed. A first shock event was indicated about 4 billion years ago. Potassium-argon dating (using laser mass spectroscopy, aka laser dating - see OE Reports, May 1996) indicates a second shock event around 3.6 to 4 billion years ago. These shock events caused fissures, cracks, and pores in the rock where carbonate globules formed.
Carbonate globules could have formed when water was present in this rock some 3.6-3.8 billion years ago. Then, Mars was much warmer and wetter than at present. It had a much thicker atmosphere. Streams, rivers, maybe even an ocean existed. Water seeped into the fractures of this rock and possibly brought along mineral salts and even microorganisms, similar to terrestrial bacteria or archaea (a newly identified life form - more later). These microorganisms quite possibly played a critical role in the formation of the carbonate globules that are found with the fractures.
What is the evidence for life in ALH84001?
Carbonate is a mineral that on earth is commonly produced by the action of microorganisms. Limestone is an example. Furthermore, the carbonate globules in ALH84001 are similar in size and texture to carbonate precipitates that are often formed by terrestrial bacteria. The McKay group demonstrated that these carbonate globules contained fine-grained secondary phases of single domain magnetites and iron sulfides. These minerals probably formed in water solutions at temperatures amenable to microbial life. This result is extremely significant.
On earth, magnetites and iron sulfides can be produced by sulfur-eating bacteria. Of course, each of these minerals can be formed inorganically by chemical processes. However, they are opposing processes! Magnetite is formed by a chemical process called oxidation, while iron sulfides are formed by the opposite chemical process known as reduction. Hence, the presence of these two minerals in close proximity is characteristic of their formation by a biological process instead of a chemical one. Also, the magnetite and iron sulfide minerals in ALH84001 were found concentrated in small grains around the rims of the small (~0.25 mm diameter) carbonate globules.
So these minerals may have been formed by bacteria living within this rock?
Thomas Gold of Cornell University suggests that a significant portion of terrestrial life may be contained in vast microbiota living deep within the earth's crust. These microorganisms may be closely related to the earliest forms of life on earth. As early as the 1920s, scientists had studied sulfate-reducing bacteria from deep oil deposits. They proposed these were anaerobic microbes living deep underground.
About a decade ago, Frank Wobber initiated a Department of Energy program to systematically study life forms deep within the earth and to examine their behavior. Scientists established conclusively that anaerobic microorganisms are actually living in rocks deep below the surface of the earth, at very high temperatures and pressures of several thousand pounds per square inch.
Back to life in ALH84001?
Within the meteorite, the McKay group found tiny structures, about 100 nm in size, that bear a strong resemblance to terrestrial bacteria. However, they are much smaller than most terrestrial bacteria, which is of great concern to some scientists. However, I don't think that is a serious concern. Ultra-microbacteria have been found living in rocks on earth. Some of the smaller terrestrial bacteria, such as the Mycoplasmatales, exhibit minute forms at the very limit of resolution of the visible light microscope.
The McKay group also established that the fresh fractures in the ALH84001 meteorite had abundant complex organic molecules called polyclic aromatic hydrocarbons (PAHs). A benzene ring (C6H6) is a simple aromatic hydrocarbon. When two or more of these benzene rings are linked together they constitute a polycyclic aromatic hydrocarbon. The simplest PAH is naphthalene(C10H8), the aromatic poison that gives mothballs their familiar smell. These polycyclic aromatic hydrocarbons are usually, but not always, produced by living things on earth. They certainly provide evidence that perhaps there was some type of microorganism living in this meteorite when the rock was on the surface of the planet Mars.
Could these PAHs have just been terrestrial contaminants?
McKay's group was careful to rule out terrestrial contamination. The concentration of PAHs increased as they probed farther into the meteorite. If they were terrestrial contaminants, one would have expected the concentration to have been higher on the surface of the meteorite and to diminish as you go into the meteorite.
What about the concept of microbes in the rock?
The biggest controversy is over whether or not the rock contains evidence of microorganisms, and therein lies the most fundamental question. There's the frequently quoted saying, "Extraordinary results require extraordinary proof." It's true that scientists must always exercise careful skepticism. However, skepticism can reach a point where valid evidence can be rejected simply because it does not fit into the conventional view of the world at that time. Sometimes scientists also oppose new ideas because they may contradict ideas that one has published in a paper years earlier.
If ancient microbial life did exist on Mars or is currently living there, alternative explanations for the observational data will not alter reality. The only way we will know the answer for sure is to conduct a thorough search and be prepared to accept either positive or negative results as the observations indicate. Even the Theory of Relativity was widely criticized when first advanced, but we now know it is correct.
Healthy skepticism must be tempered by an open mind and a willingness to objectively analyze the data even when it is contrary to the conventional wisdom. Sometimes a paradigm shift occurs and thinking is radically altered, and it is quite possible that the McKay data will lead to a paradigm shift.
In 1961, scientists reported finding "organized elements" in certain meteorites called carbonaceous chondrites. They cautiously concluded their paper by saying, "We are of the opinion that these observations suggest that the organized elements may be microfossils indigenous to the meteorite."
Subsequent studies by them and by several other groups established that the carbonaceous chondrites also contain a host of amino acids, but in relative abundances that do not support the contention that was initially advanced that they were merely terrestrial contaminants. In addition, these meteorites contain amino acid analogs that are not commonly encountered in living things on earth. An even more important indication that they were not simply terrestrial contaminants was the fact that the amino acids in the meteorites were present in both the dextro-rotary and levo-rotary stereoisomers. Since almost all amino acids in terrestrial life forms are levo-rotary, clearly the amino acids in these carbonaceous chondrites were not from this zip code.
Is there anything we know about Mars, now or as it was 3.6 billion years ago, that tells us there could not have possibly been life there?
We know absolutely nothing about Mars today, or as it was 3.6 billion years ago, that indicates it is (or was) incapable of supporting microbial life. Strong arguments were made a few years ago that, due to the lack of atmosphere, due to lack of water on the surface and so forth, there couldn't possibly be life on Mars. We now know that terrestrial microorganisms do not require the presence of water in large quantities. We know that microorganisms exist deep below the surface of the earth in these rock cores such as basalt. These are anaerobic life forms. Oxygen is deadly to them. To these kinds of microorganisms, the hostile environment is at the surface of the earth, where the temperature would seem very cold, the pressure very low, and the atmosphere highly toxic.
Then there are the exciting results of Raul Cano, of California Polytechnic in San Luis Obispo, California. He has succeeded in propagating bacteria found in the intestines of bees that have been trapped in amber for 35 or more million years. Yet these bacteria are still alive. They have been dormant for tens of millions of years but didn't bother to die.
Could you tell me about archaea? What is it?
Archaea represents an entirely new kingdom of life on earth. These life forms were discovered living in material brought back from the the deep sea thermal vents by Alvin, the deep sea research vessel.
Forty years ago, living things were classified as Plant and Animal. Then about 20 years ago, they were classified in one of three kingdoms: Plant, Animal, and Protists (single-cell organisms). Subsequently, living things belonged to either Eukaryotes or Prokaryotes. Eukaryotic organisms are those that have a distinct nucleus and the genetic material is within this distinct nucleus. That includes everything from fungi to palm trees to human beings and elephants as well as all plants, including even single-celled plants like the diatoms. In the more primitive prokaryotic organisms, the genetic material is scattered throughout the cell rather than in the nucleus. Examples are bacteria and blue-green algae.
In the archaea, the genetic material is distributed in a linked ring in the cell. Furthermore, much of the genetic material in the archaea is different from anything known before in either bacteria or higher forms of life.
What can space missions reveal about extraterrestrial life?
There are several space missions being planned to the Red Planet. Those missions offer tremendous promise. Properly designed experiments may make it possible to detect living or fossil microbes on Mars. Missions should provide vital data to help locate the most promising sites from which to collect martian rocks and soil for future sample-return missions.
One more question. I read your paper about the spectra of diatoms in space. Can you touch on that a little bit?
Sir Fred Hoyle and Chandra Wickramasinghe of University College, Cardiff, U.K., were working on the nature of interstellar dust. Most researchers were trying to match inorganic silicates to the interstellar dust. The problem was that inorganic silica has sharp infrared absorption peaks at 8.7 and 12.7 microns, which made it difficult to match the interstellar spectrum to inorganic silicates. They had previously found that the other interstellar dust features had infrared signatures that matched well with organic polymers at shorter wavelengths (3-4 micron regime). So, they then started to look if there were some type of organic silicates that might be analyzed to give a better match of the 8- to 40-micron spectrum.
They turned to diatoms. The exciting thing about diatoms is that they have the ability to take silicon dioxide, which is dissolved in the ocean, rivers, or streams and pull it out and, molecule by molecule, lay down a complex siliceous biopolymer that is called orthosilicic acid. A protein template governs the way diatoms lay this biopolymer down to fabricate their intricate and beautiful shells (Figure 3).
I collaborated with them, because Hoyle had seen my National Geographic paper on diatoms. Wickramasinghe obtained measurements of the spectral characteristics of diatoms and bacteria in the infrared that yielded a very good match with the observed infrared flux from the Trapezium Nebula (Figure 4) as well as the infrared source in the galactic center known as GC-IRS 7.
What about the future?
It's exciting that evidence of life on Mars may be encapsulated in the Allan Hills meteorite. But, it's also important that we take samples from comets. If microorganisms exist on comets, the eruption of gases as the comet goes around the sun would be a mechanism in which microorganisms could be seeded into space.
A number of scientists some years ago believed that you didn't have to worry about microbes in space because the hard vacuum of space (10-10 - 10-12 Torr) would be lethal to any type of microbe. And if it weren't, it would certainly be killed by the extreme cold (-100 degrees Centigrade or more).
However, we now know that is not correct. Pete Conrad and his Apollo 12 crew retrieved a camera from the Surveyor III spacecraft that had been left on the moon's surface. When the properly sealed camera was returned to earth and examined, scientists found Streptococcus mitas, a terrestrial bacteria, inside. They had been carried to the moon, accidentally, and had been sitting on the moon's surface for 31 months and didn't die. These microorganisms were still capable of replicating. The fact that these organisms remained alive, after prolonged exposure to the space environment, is one of the most exciting and least publicized results of the Apollo program.
It tells us that the future holds many surprises - that there is much to learn as we undertake a serious quest for knowledge not only about terrestrial microbial life in extreme environments but also about extraterrestrial microbial life in meteorites, comets, and other planets and their moons.
Richard B. Hoover is a NASA Astrophysicist at MSFC. He obtained (with Prof. Walker of Stanford) the first high resolution multilayer x-ray telescope images of the Sun. He is an internationally known diatomist, photomicroscopist, and x-ray optics specialist. He has written two books on diatoms, edited 12 SPIE Volumes on x-ray optics, holds 11 U.S. patents and has co-authored over 185 articles in scientific journals and encyclopedia. He was NASA Inventor of the Year in 1992 and will Chair the Conference on Instruments, Methods, and Missions for the Investigation of Extraterrestrial Microorganisms at the 1997 Annual Meeting in San Diego.
Hoover was interviewed by Frederick Su.