Allan Hills 84001

Allan Hills 84001 (commonly abbreviated ALH84001 ) is a Martian meteorite that was found in Allan Hills, Antarctica on December 27, 1984, by a team of U.S. meteorite hunters from the ANSMET project. Like other members of the group of SNCs (shergottite, nakhlite, chassignite), ALH84001 is thought to be from Mars. However, it does not fit into any of the previously discovered SNC groups. On discovery, its mass was 1.93 kg.

The meteorite is best known for gaining intense media attention in 1996 when a group of scientists claimed to have found evidence for microscopic fossils of Martian bacteria in it, culminating in then U.S. president Bill Clinton giving a speech about the potential discovery. These claims were controversial from the beginning, and the wider scientific community ultimately rejected the hypothesis once all the unusual features in the meteorite had been explained without requiring life to be present. Despite there being no convincing evidence of Martian life, the initial paper and scientific and public attention caused by it are considered to be turning points in the history of the developing science of astrobiology.

History and description
This rock is considered to be one of the oldest Martian meteorites, proposed to have crystallized from molten rock 4.091 billion years ago. Based on chemical analyses, it is thought to have originated on Mars from a period when liquid water existed on the now arid planet's surface.

In September 2005, Vicky Hamilton of the University of Hawaii at Manoa presented an analysis of the origin of ALH84001 using data from the Mars Global Surveyor and Mars Odyssey spacecraft orbiting Mars. According to the analysis, Eos Chasma in the Valles Marineris canyon appears to be the source of the meteorite. The analysis was not conclusive, in part because it was limited to areas of Mars not obscured by dust.

The theory holds that ALH84001 was blasted off from the surface of Mars by a meteor impact about 17 million years ago and fell on Earth roughly 13,000 years ago. These dates were established by a variety of radiometric dating techniques, including samarium–neodymium (Sm–Nd), rubidium–strontium (Rb–Sr), potassium–argon (K–Ar), and carbon-14. Other meteorites that have potential biological markings have generated less interest because they do not contain rock from a "wet" Mars. ALH84001 is the only meteorite collected from such a time period.

In October 2011 it was reported that isotopic analysis indicated that the carbonates in ALH84001 were precipitated at a temperature of 18 °C with water and carbon dioxide from the Martian atmosphere. The carbonate carbon and oxygen isotope ratios imply deposition of the carbonates from a gradually evaporating subsurface water body, probably a shallow aquifer meters or tens of meters below the surface.

Hypothetical biogenic features
On August 6, 1996, ALH84001 became newsworthy when it was claimed that the meteorite may contain evidence of traces of life from Mars, as published in an article in Science by David S. McKay of NASA.



Under the scanning electron microscope structures were revealed that some scientists interpreted as fossils of bacteria-like lifeforms. The structures found on ALH84001 are 20–100 nanometres in diameter, similar in size to theoretical nanobacteria, but smaller than any cellular life known at the time of their discovery. If the structures are fossilized lifeforms, as was proposed (the bogenic hyothesis), they are the first solid evidence of the existence of extraterrestrial life. Terrestrial contamination was unlikely.

The announcement of possible extraterrestrial life caused considerable controversy. When the discovery was announced, many conjectured that the fossils were the first true evidence of extraterrestrial life—making headlines around the world, and even prompting the President of the United States Bill Clinton to make a formal televised announcement to mark the event.

David S. McKay at NASA presented three main lines of evidence
 * Detection of polycyclic aromatic hydrocarbons, or PAHs. These however are common in comets, asteroids and the Antarctic ice
 * The shape of the blobs resembling bacteria. The problem here is that they are so small, down to 20 nm in diameter. The smallest microbes known today are less than 100 nm in diameter but not as small as this.
 * Presence of carbonate, often used for microbes to make shells
 * Extraordinarily small and pure crystals of magnetite.

[work in progress] In particular, the shapes within the ALH84001 look intergrown or embedded in the indigenous material, while likely contamination does not. While it has not yet conclusively been shown how the features in the meteorite were formed, similar features have been recreated in the lab without biological inputs by a team led by D.C. Golden. David McKay says these results were obtained using unrealistically pure raw materials as a starting point, and "will not explain many of the features described by us in ALH84001." According to McKay, a plausible inorganic model "must explain simultaneously all of the properties that we and others have suggested as possible biogenic properties of this meteorite." The rest of the scientific community disagreed with McKay.

In November 2009, a team of scientists at Johnson Space Center, including McKay, argued that since their original paper was published, the biogenic hypothesis has been "further strengthened by the presence of abundant fossil-like structures in other Martian meteorites." However, the scientific consensus is that "morphology alone cannot be used unambiguously as a tool for primitive life detection." Interpretation of morphology is notoriously subjective, and its use alone has led to numerous errors of interpretation.

List
These are some of the main features that have been interpreted as suggesting the presence of life. Any hypothesis about ALH84001 has to explain them.


 * The meteorite originates from a time on Mars 4.1 billion years ago when conditions for life on Mars were probably widespread, and carbonate globules in the meteorite suggest that it has been altered by water at low temperatures.
 * The structures resemble modern terrestrial bacteria and their appendages. Though some are smaller than any present day Earth microbes, others are of the order of 100 - 200 nm in size, within the size limits of nanobacteria and some are up to 1 to 2 microns in diameter..
 * The smallest ones are too small to contain all the machinery of modern life . However, a workshop on the limitations of size of microbes in 1999 found that though modern nanobacteria can't be smaller in volume than the interior of a sphere of diameter 250 ± 50 nm, primitive microorganisms based on a single-polymer system, for instance, RNA based, with ribozymes (catalytic RNA) taking the place of  ribosomes as enzymes for cell replication, could be as small as a sphere 50 nm in diameter. Cells could also shrink after death, or the fossils could be remains of fragments of larger organisms, or they could be pathogens or symbiotes which depend on a host, or they could live in consortia of smaller cells unable to survive independently on their own, or they could be based on biological systems different from the ones we understand.
 * Some of the structures resemble colonies and biofilms. . However there are many instances of morphologies that suggested life and were later shown to be due to inorganic processes.
 * The meteorite contains crystals of magnetite of the unusual rectangular prism type, and organized into domains all about the same size, indistinguishable from magnetite produced biologically on Earth and not matching any known nonbiological magnetite that forms naturally on Earth. . Bacteria use magnetite, not to find north, but to sense which way is up, as magnetic field lines are seldom parallel to the Earth's surface, and it occurs in some of the most evolutionarily ancient backteria on Earth. The magnetite is embedded in the carbonate. If found on Earth it would be a very strong biosignature. However in 2001, Golden and Gordon McKay were able to to make carbonate globules containing similar magnetite grains through an inorganic process simulating conditions ALH84001 experienced on Mars.
 * It contains polycyclic aromatic hydrocarbons (PAHs) concentrated in the regions containing the carbonate globules, and these have been shown to be indigenous. Other organics such as amino acids don't follow this pattern and are probably due to Antarctic contamination. However PAHs are also commonly found in asteroids, comets and meteorites
 * Evidence from isotopes of carbon originally favoured a biological origin. . However on further analysis the findings were less clear, and may indicate an extra terrestrial origin but if so the organics could also be of meteoritic or cometary origin as for the PAHs.

RNA world hypothesis
A workshop on the limitations of size of microbes in 1999 found that though modern nanobacteria can't be smaller in volume than the interior of a sphere of diameter 250 ± 50 nm, primitive microorganisms based on a single-polymer system could be. They cite the example of RNA based life, with ribozymes (catalytic RNA) taking the place of the much larger ribosomes as enzymes for cell replication. The cells also wouldn't have proteins and would have no need to translate DNA to mRNA. The result could be cells as small as a sphere 50 nm in diameter. These RNA World cells are one of the suggestions made to bridge the gap between abiotic chemistry and modern cells.

The main suggestions they gave that could explain the small size are that


 * Cells could shrink after death
 * The fossils could be remains of fragments of larger organisms
 * They could be pathogens or symbiotes which depend on a host
 * They could live in consortia of smaller cells unable to survive independently on their own
 * They could be based on biological systems different from the ones we understand (such as the RNA World cells).

The RNA world hypothesis is still under consideration, with Steven A. Benner (notable as the first person to synthesize a gene amongst many other accomplishments) and Paul Davies supporting it in a chapter in the book "Frontiers of Astrobiology" published in 2012 by the Cambridge University Press, in the chapter "Towards a Theory of Life"