ALH84001 Analysis
In August, 1996, David McKay and his colleagues at the Johnson Space Center, Stanford University, and McGill University reported evidence for fossil life in a meteorite from Mars. Since then, scientists from around the world have been testing this idea, which in science generally means they are trying to prove the idea wrong. If an idea stands up to such scrutiny, it is on pretty solid ground. The latest round of testing comes from John Bradley (MVA, Inc. and Georgia Tech), Ralph Harvey (Case Western Reserve), and Harry Y. McSween (Univ. of Tennessee). They suggest in a short report in Nature that most of the tiny, tubular, segmented objects that resemble tiny fossils ("nanofossils") described by McKay and his associates are actually thin fractures parallel to atomic planes in minerals in the martian meteorite. The features have been modified, Bradley and coworkers say, by the way samples are prepared for study, producing the apparent segmentations. Dave McKay and his collaborators agree that some structures in the rock are mineral features, but say others are not, and that the sample preparation techniques they used do not produce pronounced segmentation. It appears that a nanofossil is in the eye of the beholder.
The Original Observations
The original paper by Dave McKay and his cohorts cited several lines of evidence that hinted at past life in martian meteorite ALH84001. One of them dealt with observations of fossil-like objects in the rock. Though much smaller, the wormy structures resemble ancient microfossils found on Earth. Because of their small size, they are called "nanofossils."
| in ALH84001 | in Earth rocks (from J. W. Schopf) |
in ALH84001 | |
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The photo on the left, which has appeared in numerous publications, shows a small, tubular, segmented object in ALH84001. Note the resemblance to microfossils found in ancient rocks on Earth (center), although the terrestrial fossils are much larger. In some cases, many of the fossil-like objects in ALH84001 are found in one place, as shown in the photo on the right. The curious alignment bothered some scientists, making them wonder if the structures might be related to the crystal structure of the underlying rocky material.
Crystal Structures or Fossils?
The main argument at first centered on the small sizes of the nanofossils. Some scientists wondered if an organism could actually be that small and still function. Others argued that such small organisms are found in the fossil record on Earth, though some questioned those observations. The viability of such small creatures is still debated, including by Bradley and his colleagues in their communication to Nature, though McKay and friends cite occurrences of bacteria as small as 70 nanometers.
Pesty Problems with Sample Preparation
An Unsolved Problem
It appears that one person's nanofossil is another person's artifact. In spite of this apparent complete disagreement, however, this latest debate has helped clarify some issues. For example, it is agreed that some of the features in ALH84001 that vaguely resemble nanofossils are actually structural features of the minerals, weathering products, or both. It shows that sample preparation is worth an even more thorough look, so that we can know which features might be artifacts and which are probably not.
Undoubtedly, the continued debate will lead to new tests of all the evidence for life in ALH84001. The debate over nanofossils is part of a larger debate on the temperature of formation of the carbonates in the meteorite, which is argued even more vigorously than is the reality of the nanofossils! Whatever the outcome of all the arguments and detailed study of the martian meteorites, it will show us how to search for life in samples returned from the surface of Mars, which is planned tentatively for the year 2005.