CENAP Infoline - no 043

Magazine Overview

Title: cenap-infoline
Issue: nr.43
Date: 28. August 1996

This issue of cenap-infoline, dated August 28, 1996, focuses heavily on the ALH 84001 meteorite and its potential implications for the existence of past life on Mars. The magazine delves into the scientific investigation, the evidence presented, and the ongoing debate within the scientific community.

The ALH 84001 Meteorite: A Search for Life on Mars

The lead article, "Meteorit ALH84001," details the recent "exciting weeks" surrounding the meteorite. It begins by recapping the long-standing question of whether life ever existed on Mars, noting that the Martian surface was previously considered "self-sterilizing" due to harsh conditions like unfiltered UV radiation, extreme dryness, and oxidizing nature.

The article highlights ALH 84001 as the most significant meteorite from Mars, found in Antarctica 10 years prior and confidently attributed to the Red Planet. Its age of 4.0 billion years is particularly noteworthy, placing it in a mysterious era of the early solar system when celestial bodies frequently impacted the Moon (Lunar-Cataclysm or Late Heavy Bombardement). The presence of certain carbonate minerals in ALH 84001 initially suggested a "wet" environment on Mars, hinting at a watery past. However, this interpretation is now being questioned, with the possibility that these minerals formed through "impact metasomatism" at high temperatures during a meteorite impact on a dry Mars.

The issue also touches upon the "Mars 96" mission from Russia, which was expected to launch that year, contributing to a renewed "Mars fever."

Mars Missions and Pathfinder

Page 2 discusses the upcoming Mars missions. The launch of a US company's TV satellite, intended for the international Mars orbiter with German cameras, was postponed due to the Proton rocket being needed for the Mars 96 mission. Meanwhile, tests for the Mars-Global Surveyor were underway, with its launch scheduled for November 6, 1996, to orbit Mars for a Martian year starting September 1997.

A significant challenge was selecting a landing site for the Pathfinder, which was slated for its Mars landing on July 4, 1997, the first since the Viking missions. The site needed to be scientifically interesting, not too rocky, and in a low-lying region with sufficient atmospheric pressure for parachute deployment, while also being well-illuminated for solar cells. After eliminating most of Mars based on these criteria, only four small areas remained. Ares Vallis was chosen as a compromise between scientific value and safety. Approximately 20% of its surface is expected to be covered by stones, with 1% being larger than 50 cm. These stones are considered a highlight, representing material washed down from highlands.

Allan Hills 84001: A Stone from Mars?

This section details the discovery of ALH 84001. It began in August 1984 during an Antarctic expedition organized by the National Science Foundation, NASA, and the Smithsonian Institution. Roberta Score and her colleagues found a potato-sized meteorite in the Allan Hills region. Initially cataloged as a common type, it was preserved from terrestrial influences due to its exceptional resistance to weathering in Antarctica. The meteorite is stored under extreme cleanroom conditions at the Johnson Space Center in Houston.

The possibility of meteorites originating from Mars gained traction in the 1980s, supported by the chemical similarities of "SNC" (Shergottite, Nakhlite, Chassigny) meteorites to the Martian surface and the discovery of mechanisms for transporting Martian material to Earth. These SNC classes, named after their most common types, were found to be younger than other meteorites and had distinct chemical compositions. Isotopic ratios of Argon and Xenon in these meteorites matched those measured in the Martian atmosphere by the Viking landers, and the enrichment of Nitrogen-15 further supported a Martian origin.

By the early 1990s, the term "possible" was often dropped when referring to SNCs as Mars meteorites, as plausible mechanisms for their safe transport to Earth were identified. Laboratory simulations showed that impact ejecta could be accelerated to escape velocity and travel through space, influenced by planetary gravity and resonance effects, with a high probability of landing on other planets. While lunar meteorites typically reach Earth within 50,000 years, Martian meteorites take longer, with their initial orbits not necessarily crossing Earth's. Some may take millions of years to reach Earth, while others might fall into the Sun or be flung beyond Jupiter.

Currently, 12 Mars meteorites are known, with ALH 84001 being assigned to this group in 1994. It is unique for its age (4.0+0.1 billion years) and evidence of significant shock events, possibly related to the Late Heavy Bombardement. These shock events reset its radioactive clock, with current measurements indicating 4 billion years of radioactive decay. The meteorite shows signs of further shocks, suggesting it witnessed the Great Bombardment approximately half a billion years after the solar system's formation. The impacts have caused numerous fractures, with chemical or biological processes potentially occurring within these fissures.

The Carbonate Globules of ALH 84001

A distinguishing feature of ALH 84001, compared to other SNCs, are its enclosed carbonate minerals. The 1.9 kg meteorite is primarily composed of centimeter-sized orthopyroxene grains, with a fusion crust typical of meteorites. However, about 1% of its mass consists of fine-grained, roughly spherical carbonate structures (1-250 µm in diameter) with an amber color. These globules are composed of calcite, dolomite-ankerite, and magnesite-siderite. They are unique to this SNC meteorite and show shock structures, indicating they formed before landing on Earth.

The article then addresses the evidence that researchers have interpreted as potential traces of Martian life, found in a fragment of ALH 84001 over a 2.5-year study.

The Three-Argument Chain

US President Clinton, NASA Chief Goldin, and Team Chief McKay emphasized on August 7, 1996, that there was no definitive proof of life on Mars 3.6 billion years ago or today. Goldin stated, "The results are not conclusive, as there is no scientific consensus yet. We are not here in court to prove beyond all doubt that life existed on Mars. But we want to open the door a little today to present exciting scientific findings and tell a fascinating detective story."

McKay explained that their data comprised four different chains of evidence, each open to interpretation. However, the team believes a reasonable interpretation of these indicators points to biological activity on early Mars. The first chain of evidence is that the meteorite came from Mars and contains calcium carbonates, similar to those formed on Mars. The second chain involves the mineralogy and chemistry of the carbonate globules, which are compatible with a biological origin. The third is the presence of organic compounds believed to originate from Mars. The final set of evidence consists of images showing peculiar structures within the stone and carbonate, which they interpret as microfossil life forms. This last part is considered the most controversial.

At the press conference, McKay's group presented REM (Scanning Electron Microscope) images that were described as "seductive" and resembling microorganisms. However, McKay stressed that there was no independent data confirming these as fossils, nor images showing cell walls or internal structures characteristic of cells. The largest structures, even the seemingly segmented ones, were only about 500 nm, or the wavelength of visible light, but comparable in size to some terrestrial bacteria. Identifying their chemical composition and verifying cell walls remains a challenge for the future.

McKay's Evidence and Scientific Scrutiny

McKay's team presented four lines of evidence: 1) The meteorite originated from Mars and contains calcium carbonates, similar to those found on Mars. 2) The mineralogy and chemistry of the carbonate globules are consistent with a biological origin. 3) The meteorite contains organic compounds believed to be of Martian origin. 4) Images of peculiar structures within the stone and carbonate have been interpreted as microfossils.

McKay acknowledged that the "features described can be explained by abiotic processes," but these explanations require "restricted conditions" that may be unlikely. He also noted that "the formation of the described features through organic activity in the Antarctic seems possible, but such activity is currently poorly understood." He further pointed out that many features are associated with the carbonate globules, which likely formed on Mars before the meteorite arrived in Antarctica. This raises questions about how possible organic products (magnetites and iron sulfides) could have formed within these globules if the carbonates formed on Mars and the magnetites and iron sulfides formed in Antarctica. Additionally, these products would require anaerobic bacteria, whereas Antarctic ice is rich in oxygen.

Everett Gibson of the McKay team posed eight questions, based on established paleontological criteria for "credible evidence of past life in geological samples," all of which he answered affirmatively. These included the presence of microbial colonies, biomineralization markers (like magnetites), organic biomarkers (PAHs), and unusual carbon isotope patterns interpreted as "fingerprints" of life.

However, exobiologist William Schopf, invited as a skeptic, applied his seven criteria for detecting "ancient" life. While he found no issues with the simpler points, he remained reserved on the crucial questions of definitively biological nature. Schopf considered the PAHs almost worthless due to their prevalence in the universe from abiotic origins. Regarding the alleged microfossils, he demanded to see clear cell walls with electron density indicating carbon as a building material, and a size distribution clearly distinct from surrounding minerals. He also wished to see a cell that was undergoing division, to understand its life cycle, suggesting that these might be "foolers, fossil-like mineral artifacts" unless proven otherwise.

Recurring Themes and Editorial Stance

The issue strongly emphasizes the scientific investigation and debate surrounding the ALH 84001 meteorite. It highlights the cutting-edge research techniques employed and the cautious approach required in interpreting potential evidence of extraterrestrial life. The editorial stance appears to be one of excitement about new discoveries and the potential for groundbreaking findings, while also underscoring the need for rigorous scientific scrutiny and consensus before definitive conclusions can be drawn. The magazine promotes a sense of scientific detective work and the opening of doors to exciting possibilities in the search for life beyond Earth.