Crystal Structures or Fossils?
The prime tool for examining rocks at high magnification is the electron microscope. There are two basic types. One is called a scanning electron microscope (SEM), which sprays a highly-focused beam of electrons in a grid pattern on the sample, which can be a little chunk of a rock or a polished piece. An SEM is shown in the photograph below.
Samples are placed
in the sample chamber, which is then evacuated by powerful vacuum pumps. The tall column
labeled "electron gun" is also evacuated. It produces electrons near the top and
accelerates them down toward the sample. Electromagnets focus the beam to a very fine
point (about 1 nanometer in diameter), while other magnets cause it to move in a grid
pattern. The monitors allow scientists to view the images produced by detectors that see
electrons that bounce off the sample or others (called secondary electrons) that are
produced in it. The monitor on the far left provides informtion about the chemical
composition of the mineral in the sample.
The other type is called a transmission electron microscope (TEM), which transmits electrons through very thin (less than one micrometer) slices of rock. TEMs can also move the electron beam in a grid pattern to produce an image like a picture.
An interesting problem in looking at rocks with electron microscopes is that the instruments are so good that you can see features you might not expect to see. Crystallographic features not visible in conventional light microscopes are evident, and some types of fractures can appear curved or worm like. Even more confusing, because the samples need to be coated with some kind of conducting material to carry off the electrical charge deposited by the electrons, scientists have to worry about introducing artifacts when they apply the coating to their samples. Small crystallographic features and artifacts from sample preparation are what Bradley and coworkers argue are the so-called nanofossils observed by McKay and his team.
Pyroxene Crystal Surface |
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Scanning electron microscope images obtained on samples of ALH84001 in John Bradley's lab. Photos on the right are higher magnification pictures of regions in photos on the left. The top pair show the surface of a pyroxene crystal; the bottom pair show a carbonate crystal. |
Carbonate Crystal Surface |
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Bradley and his team suggest that the nanofossils in ALH84001 are not fossils at all, but are instead small-scale features of crystals (which Bradley calls "cleavage lamellae") of pyroxene and carbonate in the rock. Basically, these are parallel planes in mineral crystals that leave steps on broken surfaces. Many of the elongated microscopic features in the rock are lined up on the surfaces of crystals and seem to emerge from inside the grains. Bradley says that this is evidence for a crystallographic origin, not fossils deposited on the surface. Some of these features are even curved like the nanofossils reported by McKay.
In their reply to Bradley and coworkers, McKay and company agree that there are numerous crystallographic features on crystal surfaces in ALH84001, and they illustrate this with some photographs like those above. They also argue that some features may have been caused by weathering, producing clay minerals. No matter how the linear features formed, McKay and associates agree that such features are not of biological origin.
On the other hand, the McKay group points out that there are other curved and more isolated structures besides those shown by Bradley, and it is those structures that the group claims are biological. They also argue that the structures they call nanofossils are much larger, up to 750 nanometers (0.75 micrometers) long, than those shown by Bradley (see the photographs above). John Bradley counters in a communication to PSRD that many of the elongate structures in their images are just as long, and also point out that some of the previous evidence cited by McKay and colleagues, such as the herd of nanomaggots, are the same size as most of the crystallographic features observed. McKay notes that objects like those in the nanomaggot herd are more ovoid than elongate, and thus less likely to be mineralogic features.