Astronomy is a science of the very large and the very distant. Thus, for centuries, its primary instrument has been the telescope. But during the last few decades, the availability of extraterrestrial materials has led investigators in the areas of planetary science, astrogeology, and astrobiology to utilize instrumentation developed to study matter on a tiny scale, namely the microscope. Beginning in 1969, scientists began using microscopes to examine lunar rocks and dust carried to Earth by Apollo astronauts. Similarly, they employ microscopy in the analysis of cometary material and meteorites.
Invented more than 400 years ago in Holland by Zaccharias Janssen and his son Hans, microscopy has since developed to include a wide range of sophisticated instruments and techniques. While planetary scientists use optical microscopes in various incarnations to examine astromaterials, techniques such as electron microscopy and atomic force microscopy (AFM) permit detailed chemical analysis. One type of electron microscopy, known as scanning electron microscopy (SEM), allows not only for complete molecular and isotopic analysis of tiny samples, but also for striking three-dimensional detail.
Out of thousands of meteorites discovered on Earth, to date roughly 40 or so have been identified as having originated on Mars. We are certain of their Martian origin, because scientists employing "wet lab" chemistry, as well as microscopic techniques such as SEM on samples from these rocks, have elucidated the ratios and isotope mixtures of gases contained within. When such gas characteristics match precisely with atmospheric measurements taken by science probes that have landed on Mars, the case for the rock's Martian origin is proven. Thus, while David McKay's assertion (made initially in 1996 with several colleagues) that four features demonstrated in the Allan Hills meteorite (ALH84001) are biogenic has been contested, the scientific community does not contest the Martian origin of this meteorite.
While much of the interested lay public will recognize the worm-like structure in the sample of ALH84001 shown in the image, this is but one of four features upon which McKay and his colleagues have built the case that the meteorite once contained Martian microorganisms. Beginning with the 1996 paper in "Science," and in the course of several publications that have followed, SEM data have figured prominently in debates between McKay and opponents of the biogenic hypothesis. While McKay's opponents point out, correctly, that nonbiological mechanisms could account for each of the four features, McKay, and another colleague, Kathie Thomas-Keprta, point out that the combination of such features in a single sample would be extremely unlikely in the absence of biology.
One feature of the meteorite ALH84001, the presence of tiny crystals of magnetite, can be explained only by life, according to McKay and Thomas-Keprta. Why? The geometry and other aspects of the ALH crystals mimic magnetite crystals that are made by a type of terrestrial microorganism called magnetotactic bacteria. Indeed, out of six characteristics specific to the magnetotactic bacteria crystals (which the bacteria use to sense the Earth's magnetic field and thus know the direction in which they are swimming), the crystals in the ALH sample match five. While in theory magnetite crystals with all of these features might be produced in a laboratory, thus far, no laboratory exists that is capable of generating the conditions necessary for manufacturing them. On the other hand, in nature there exists an environment where such crystals can be--and are--produced--inside magnetotactic bacteria.
While the debate goes on between those who explain the features of ALH84001 biogenically and those who attribute them to a nonbiological process, analysis of Mars meteorites with advanced microscopy has enriched the case that Mars may be a living planet. A meteorite called Nakhla, named for the town in Egypt where it fell in the 19th century, has been a source of many useful data. In Nakhla, ALH84001, and some other Martian rocks, the presence of various ions such as chloride suggest a salty environment in the Martian past. This in turn would allow for the presence of water on the Marian surface, briny, but in liquid form, despite the low atmospheric pressure, and therefore the presence of life.
While the cosmos has granted terrestrial scientists a collection of ready-delivered Mars samples, it may be that closure on the matter of Martian life will not come until samples are brought to Earth as part of a Mars Sample Return (MRS) mission. In such a case, advanced microscopic technologies, like SEM and AFM, will play a central role in the analysis, as they will in the analysis of soil from the Martian moon, Phobos, which a Russian probe is slated to carry to Earth in 2012.