One day, ALH84001 will be remembered as the Rosetta Stone of the hieroglyph of life.
When the Rosetta Stone was discovered in 1799, our information about the ancient world was limited to the Bible and Greek historiography. The triple script of King Ptolemy gave Champollion the opportunity to transliterate the hieroglyph, providing the key to decode older writings.
We have moved beyond the Bible and the Greeks, as regards human history. And yet, when it comes to the meaning of life, we find ourselves still imprisoned in the biblical-ptolemaic paradigm. You don’t have to be an advocate of intelligent design, to think that life on our planet is something unique in the Universe, regardless of whether you conceive it as a miracle, or as an accident. It comes naturally to think that we are center-stage, albeit no longer the center. We are the protagonists. The Universe is just mise en scene.
However, “somewhere, something incredible is waiting to be known”, as Sagan said. On August 6, 1996, David Stewart McKay, Chief Scientist for astrobiology at the Johnson Space Center, published an article in Science, claiming that his team has discovered microbial fossils of extraterrestrial origin in a martian meteorite indexed as ALH84001. For the first time, we looked at another script of life.
As usually happens, the media distorted the news under sensational headlines, and, accordingly, offered a convenient straw man to be criticised by both the skeptic and the true believer. The dust has settled over the headlines ever since, but the evidence was strengthened by new findings, unnoticed by the sensation machine.
Skepticism is a sound reaction, but if the history of science teaches us something, the guts of a passionate scientist is worth more than technically correct objections from philosophical skeptics. McKay’s passion was aroused in 1962, as he watched JFK’s speech about putting a man on the Moon by the end of that decade. He trained the astronauts of the Apollo missions in geology, and served as advisory during the Moon trip. He was the principal investigator of the samples brought back from the Moon. He published over 200 research papers over lunar dust. To put it plainly, when it comes to extraterrestrial life, McKay knew better than the rest of us what to look for.
So, what is the evidence carried by the martian meteorite? To answer this question, we must start with what one may call the Rosetta Stone hypothesis. In order to transliterate the hieroglyph, Champollion had to assume that the three writings on the stele had the same content. McKay and his team looked for traces of Martian life that were consistent with what we know about life on Earth.
We are searching for martian biomarkers on the basis of what we know about life on Earth. Therefore, if there is a martian biomarker, we may not be able to recognize it, unless it is similar to an earthly biomarker.
The research result was summarised by McKay in five points.
In examining the martian meteorite ALH84001 we have found that the following evidence is compatible with the existence of past life on Mars: (i) an igneous Mars rock (of unknown geologic context) that was penetrated by a fluid along fractures and pore spaces, which then became the sites of secondary mineral formation and possible biogenic activity; (ii) a formation age for the carbonate globules younger than the age of the igneous rock; (iii) SEM and TEM images of carbonate globules and features resembling terrestrial microorganisms, terrestrial biogenic carbonate structures, or microfossils; (iv) magnetite and iron sulfide particles that could have resulted from oxidation and reduction reactions known to be important in terrestrial microbial systems; and (v) the presence of PAHs associated with surfaces rich in carbonate globules
Let us review them in some detail.
Allan Hills 84001 (ALH84001) is a 1, 93 kg (4 pounds) meteorite, discovered on December 27, 1984, in Allan Hills, Antarctica.
Meteorites originate in the asteroid belt. They coalesced 4,55 billions years ago from primeval matter in the proto-planetary disk. Most of them formed through the accretion of silicate minerals floating freely in the molecular cloud.
A few other other formed as debris of differentiated crust and melted iron-nickel core, ejected into space by impacts between the first asteroids and planetoids. A narrower class have been ejected from Mars or the Moon by asteroid impacts.
During the early stage of the solar system, planets were subjected to heavy bombardments by asteroids and comets. The traces of such cosmic cataclysms have been eroded away on Earth, but we can see them plainly on the Moon, and identify them on Mars. Such impacts often project into space pieces of molten rock. Escape velocity on Mars in 5 km/h, less than half its value on Earth, therefore strong collisions may project pieces of Martian crust into space. The sun’s gravity will attract such objects toward the earth, and sometimes they end up landing.
Meteorites from Mars are indexed as the SNC (Shergottites, Nakhlites, Chassignites) group, after the name of the location where the first meteorite of each type was discovered. They are younger than other meteorites, and mass spectrography indicates isotope ratios that put earthly origin out of question, while being consistent with other SNC samples. After the Viking Landers (1976) measured the isotopic composition of the atmosphere on Mars, and analyzed the chemical composition of rock samples, it turned out that they match those of the SNC group. More recent measurements by orbiters and rovers, have demonstrated beyond doubt the Martian origin of these meteorites.
Allan Hills 84001 is a martian meteorite from the Shergottites group. Radiometric datings show that the rock crystalised on Mars 4.5 bya ago, was blasted off about 4.09 bya ago, and landed in Antarctica 13,000 years ago. The meteorite has thin fractures that have been penetrated by martian water. Within these fractures, the research team found carbonate globules containing tiny structures identical to earthly bacteria.
The carbonate globules formed on Mars (as determined by isotopic content), from CO2 in the martian atmosphere, recycled in water at 18 degrees C, and are flattened parallel to the phracture, evidence of restricted growth within the fissure..
The martian origin of the globules is important because, along with the analyses of the environment where the meteorite was found, additional tests in laboratory, and strict precaution in handling the sample, excludes the possibility of contamination.
Structures that are similar to bacteria on Earth were seen through the scanning electron microscopes in the carbonate globules. They are very small (20 to 100 nanometers), and are similar in shape and size to fossil and living nanobacteria on earth. Such organisms on earth have been found to live rocks, some of them similar to ALH84001.
Three mineral by-products similar to those produced by bacteria on Earth were found in the carbonate mineral globules: magnetite (iron-oxide), pyrrhotite (iron-sulphide), and greigite (iron-sulphide).
Magnetite is magnetic iron-oxide. Bacteria produce magnetite and use it as an inner compass, to orient themselves along the lines of the magnetic field. Mars used to have a magnetic field like Earth, which protected the atmosphere against solar winds, and, accordingly, made liquid water possible. Once upon of time, Mars had a large ocean, third of its surface, lakes and rivers crossing the land. Then something happened, the geological activity ceased, the magnetic field died, and Mars along with it. The fossils inside ALH84001 come from that remote past when Mars was like Earth. There are a lot of questions about us to be answered on Mars.
Iron sulfides may appear on Earth as waste bacterial products, along with iron oxide, like within ALH84001. The iron-sulfides in ALH84001 are similar to those produced by earthly bacteria. Any of these minerals could have been generated by nonbiological processes. However, what matters as a biological marker is their combination, iron-oxide and iron-sulphide being patched together inside the globules of magnesium carbonate. This combination is very unlikely to happen without biological activity. Moreover, the carbonate globules bear the marks of acid water. Both minerals can form inorganically, but only in alkaline environment. The acid water would have dissolved the iron-oxide and iron-sulphide at least partially. The only explanation is biogenic origin.
Another biomarker is the presence of polycyclic aromatic hydrocarbon (PAH) around and in the carbonate mineral globules. PAHs are chemical combinations of carbon and hydrogen, spatially structured in cyclic rings (mostly hexagons) of carbon atoms. On Earth, PAH is generated naturally through the activity of bacteria and other living organisms, or by their decay.
The presence of PAH in ancient sedimentary rocks indicates the degradation of dead marine plankton and vegetation, and is interpreted as a sign that petroleum might be present. Nevertheless, most PAH in today’ environment comes from burning fossil fuels. PAH is also found in meteorites and this is thought to originate in the early molecular cloud.
McKay and his team proved that PAH molecules in ALH 84001 are from Mars, and are of the same class as those that on Earth come from the decomposition of bacteria. The PAHs in ALH 84001 represent only a limited variety, as it has been observed on Earth to form from the natural breakdown of simple living organisms.
From my perspective, their strongest conclusion is that ALH84001 contains polycyclic aromatic hydrocarbons (PAHs) that formed on Mars. These PAH molecules may be related to martian microorganisms, as McKay and co-workers suggest. The PAHs might also have formed without assistance from living organisms, in what might be called a prebiotic organic chemistry. Proof of a prebiotic organic chemistry system in Mars would be nearly as exciting as proof of life itself. (Allan H. Treiman, Lunar and Planetary Institute, August 21, 1996).
Each of these features can be explained by a naturally (although unlikely) occurring abiotic processes. It is their grouping that makes the inorganic origin highly improbable. As McKay said:
None of these observations is in itself conclusive for the existence of past life. Although there are alternative explanations for each of these phenomena taken individually, when they are considered collectively, particularly in view of their spatial association, we conclude that they are evidence for primitive life on Mars.
In November 2009, NASA Johnson Space Center announced that the case for martian fossils on ALH84001 “further strengthened by the presence of abundant fossil-like structures in other Martian meteorites.” So, even if the jury is still out, the evidence is compelling.
Ludwig Wittgenstein declared that the sense of life must lie outside of the world. I will go beyond his intention in reformulating this statement in astrobiological terms: the sense of life is to be found in comparative abiogenesis. We seem to have come closer to it.