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ALH84001

ALH 84001 (full name Allan Hills 84001) is a meteorite found in Allan Hills, Antarctica in December 1984 by a team of US meteorite hunters from the ANSMET project. Like other members of the group of SNCs (shergottite, nakhlite, chassignite), ALH 84001 is thought to be from Mars. On discovery, its mass was 1.93 kg. It made its way into headlines worldwide in 1996 when scientists announced that it may contain evidence for microscopic fossils of Martian bacteria.

History

This rock is theorized to be one of the oldest pieces of the solar system, proposed to have crystallized from molten rock 4.5 billion years ago. Based on hypotheses surrounding attempts to identify where extraterrestrial rocks come from, it is supposed to have originated on Mars and is related to other martian meteorites. The theory holds that it was shocked and broken by one or more meteorite impacts on the surface of Mars some 3.9 to 4.0 billion years ago, but remained on the planet. It was later blasted off from the surface in a separate impact about 15 million years ago and, following some interplanetary travel, impacted 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.

Possible lifeforms

On August 6, 1996 ALH 84001 became newsworthy when it was announced that the meteorite may contain evidence for traces of life from Mars, as published in an article in Science by Dr. David McKay of NASA.

Under the scanning electron microscope structures were revealed that may be the remains—in the form of fossils—of bacteria-like lifeforms. The structures found on ALH 84001 are 20-100 nanometres in diameter, similar in size to the theoretical nanobacteria, but smaller than any known cellular life at the time of their discovery. If the structures are really fossilized lifeforms, they would be the first solid evidence of the existence of extraterrestrial life, aside from the chance of their origin being terrestrial contamination.

The announcement of possible extraterrestrial life caused considerable controversy at the time and opened up interest in Martian exploration. When the discovery was announced, many immediately conjectured that the fossils were the first true evidence of extraterrestrial life—making headlines around the world, and even prompting U.S. President Bill Clinton to make a formal televised announcement to mark the event.

Several tests for organic material have been performed on the meteorite and amino acids and polycyclic aromatic hydrocarbons (PAH) have been found. The debate if the organic molecules were created by nonbiological processes or are due to contamination from the contact with Antarctic ice is still ongoing.

As of 2006, some experts still argue that the microfossils are not indicative of life, but instead are caused by contamination by earthly biofilms. It has not yet conclusively been shown how the features were formed, but similar features have been recreated in labs without biological inputs. Nevertheless, evidence continues to grow that nanobacteria do exist, in spite of initial skepticism (based on the idea that the particles were too small to contain RNA).

Recent studies on ALH 84001 have shown that, although chances are low, eventually, Martian rocks such as ALH 84001 could actually transfer Martian life to Earth. Around 1 in 10 million meteorites from Mars arrives in less than a year, and around 10 rocks that weigh more than 100 grams make the journey in 2-3 years. Bacterial spores, and rock dwelling organisms are speculated to survive in space for 5 years, meaning transfer of Martian life to our planet is theoretically possible.

If Mars's atmosphere at the time life started on Earth was like it is now, though, survival and propagation of any life form after arriving would be even less likely. The life form's native environment probably would be completely unlike anywhere it would land on Earth. Mars has an atmosphere many times thinner than that on top of Mount Everest, with almost no water. A life form evolved to survive in such conditions would almost invariably find dense air to be toxic, as it would the relatively high temperatures: even the Antarctic does not get as cold as much of Mars does most of the year. However, it is hypothesized that ALH 84001 originated from a time period during which water may have existed on Mars. Other meteorites that have potential biological markings have generated less interest because they do not originate from a "wet" Mars. ALH 84001 is the only meteorite collected from such a time period.

Origin on Mars

In September 2005, Vicky Hamilton of the University of Hawaii at Manoa presented an analysis of the origin of ALH 84001 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 parts of Mars not obscured by dust.

Student participation

The analysis of ALH84001 was unusual in that an undergraduate student, Anne Taunton of the University of Arkansas, performed much of the SEM work used to correlate the suspected nanobacterial fossils with known terrestrial nanobacterial fossils. NASA's David McKay hired Anne Taunton for a 10-week student internship to perform the SEM analysis, but did not inform her about the nature of what she was investigating. This technique is known as a single blind. Taunton reported the morphology of nanofossils in ALH84001 to be very similar to terrestrial samples without knowing that she was describing a Martian meteorite.