On 7 August 1996, a team of NASA scientists working at the NASA Johnson Space Center stunned the world by announcing that they had found evidence of past life on Mars.¹ Spokesman David S. McKay reported at a press conference that the group had detected four distinct signatures of martian biology in the SNC meteorite designated ALH 84001, including:

 

• Dark specks known as 'carbonate rosettes' on the sides of the cracks and crevices that perforate the meteorite's crust. Such formations, consisting of magnesium-rich cores surrounded by a layer of iron carbonate and a rind of iron sulfide, are similar to ones produced on Earth by bacteria in ponds as they metabolize minerals.

 

• Polycyclic aromatic hydrocarbons (PAHs) in and around the rosettes.

 

• Microscopic, tear-shaped crystals of magnetite and iron sulfide embedded in places where the iron carbonate had been dissolved, presumably by some kind of acid. Certain terrestrial bacteria manufacture crystals like these and use them as internal compasses (see magnetotactic bacteria).

 

• Segmented, rod-shaped structures, a few tens of nanometers long, interpreted by the team as the actual fossil remains of martian microbial life.

 

On the day of the announcement, President Bill Clinton declared: "If this discovery is confirmed it will surely be one of the most stunning insights into our universe that science has ever uncovered. Its implications are as far-reaching and awe-inspiring as we can imagine." However, the discovery was not confirmed. Although some researchers, including David Black, Director of Houston's Lunar and Planetary Institute, initially supported the claims, others criticized them, pointing out that plausible explanations of the four main findings in terms of various inorganic chemical processes were to hand.

 

The most dramatic claim centered on the purported fossils. Under a scanning electron microscope, these appear as elliptical, rope-like, and tubular structures in the carbonate rosettes, and are extraordinarily small. Each measures a mere 20 to 100 nanometers across and has only one thousandth the volume of the smallest known terrestrial bacteria. Despite the recent claim that "nanobes" have been found, it is far from clear if anything this tiny has ever lived on Earth or even that something so small could encapsulate the minimum biochemical machinery needed to sustain and replicate a living organism. Moreover, no independent analysis carried out since the original announcement has produced evidence that the structures in question are anything other than fragments of inorganic minerals or clay.

 

Concerning the carbonate rosettes, it was the contention of the NASA group that these were (1) formed on Mars (rather than on Earth after the meteorite's arrival), and (2) were deposited by liquid water which infiltrated the rock billions of years ago. A martian origin for the rosettes now seems beyond doubt, but controversy still surrounds the conditions under which the rosettes were laid down. In particular, there is strong evidence² to suggest that the carbonate formations grew at temperatures of more than 650°C (1,200°F) which would rule out a biological explanation. On the other hand, a low-temperature, possibly bacterial origin for the structures has not yet been discounted.

 

In addition to the purported fossils, McKay and his coworkers described two other features in and near the rosettes which they believe formed biogenically in a watery environment: microscopic mineral grains and PAHs. Noting that some terrestrial bacteria manufacture iron sulfide and magnetite, they interpreted the crystals of these substances in ALH 84001 as products of martian microbial action. They conceded, however, that similar grains can result from by purely inorganic processes. An analysis conducted in 1997³ showed that the sulfides in the meteorite are too rich in sulfur-34, a heavy isotope of the element, to have come from microbes like any seen on Earth. Moreover, the long chains of magnetite which are characteristic of known bacterial activity have not been reported, although magnetite crystals have been observed growing directly out of other minerals in the meteorite-proof that at least some of them arose through simple chemical means. There is also the difficulty in understanding what use magnetite would be to a martian organism, since Mars has virtually no magnetic field. Adrian J. Brearley of the University of New Mexico has suggested that a sharp blow to ALH 84001 (it is known to have suffered at least two) could have produced the effects seen by heating the rock to more than 550°C (1,020°F). This would have been sufficiently hot to break down the iron-rich carbonate into magnetite but leave the more thermally stable magnesium-rich cores intact. When the iron subsequently crystallized, it would have released carbon dioxide and created tiny voids around the magnetite grains, as observed.⁴

 

As for the PAHs, it is true that they could have come from decomposed martian microbes, particularly since the variety in the meteorite is unusual and very limited – just what would be expected, according to McKay and his group, of a biogenic origin. However, Edward Anders of the University of Chicago has shown how, given magnetite as a catalyst, such a blend could have come about inorganically. If it did, this would explain the discovery by Thomas Stephan and his colleagues at the University of Munster of PAHs all through the meteorite.⁵ The fact is that PAHs are not good biomarkers since they are so ubiquitous. For this reason, Jeffrey L. Bada and his colleagues, of the Scripps Institution in La Jolla, California, looked instead for amino acids in ALH 84001.⁶ Although he found some, they were present in virtually the same proportions as those in the Antarctic ice in which the meteorite had lain for 13,000 years, indicating extensive terrestrial contamination. If evidence for martian life is eventually found it will almost certainly have to come from elsewhere – probably samples obtained fresh from the surface or subsurface of Mars and analyzed in situ or following their return to Earth.

 

References

1. McKay, D. S., Gibson, E. K., Jr., Thomas-Keprta, K. L., Vali, H., Romanek, C. S., Clemett, S. J., Chillier, X. D. F., Maechling, C. R., and Zare, R. N. "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH 84001," Science, 273, 924 (1996).
2. Harvey, R. P., and McSween, H. Y., Jr. "A Possible High-Temperature Origin for the Carbonates in the Martian Meteorite ALH 84001," Nature, 382, 49 (1996).
3. Greenwood, J. P., Riciputi, L. R., and McSween, H. Y., Jr. "Sulfide Isotopic Compositions in Shergottites and ALH 84001, and Possible Implications for Life on Mars," Geochimica et Cosmochimica Acta, 61, 4449 (1997).
4. Bradley, J. P, Harvey, R. P., and McSween, H. Y., Jr. "Magnetite Whiskers and Platelets in ALH 84001 Martian Meteorite: Evidence of Vapor Phase Growth," Geochimica at Cosmochimica Acta, 60, 5149 (1996).
5. Stephan, T., Rost, D., Jessberger, E. K., and Geshake, A. "Polycyclic Aromatic Hydrocarbons in ALH 84001 Analyzed with Time-of-Flight Secondary Ion Mass Spectrometry," paper presented at 29th Annual Lunar and Planetary Science Conference, Houston, Tex., March 16–20, 1998.