ALH84001
Encyclopedia
Allan Hills 84001 is a 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), ALH 84001 is thought to be from Mars
. However, it does not fit into any of the previously discovered SNC categories. On discovery, its mass
was 1.93 kilograms (4.3 lb). It made its way into headlines worldwide in 1996 when scientists announced that it might contain evidence for microscopic fossils of Martian bacteria based on carbonate globules observed.
, proposed to have crystallized from molten rock 4.091 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.
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.
The theory holds that ALH 84001 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 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
.
It is hypothesized that ALH 84001 originated from a time period during which liquid 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.
In October 2011 it was reported that isotopic analysis indicated that the carbonates in ALH 84001 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.
, as published in an article in Science
by David McKay
of NASA
.
Under the scanning electron microscope
structures were revealed that were interpeted as remains—in the form of fossils—of bacteria
-like life
forms. The structures found on ALH 84001 are in diameter, similar in size to theoretical nanobacteria, but smaller than any known cellular life at the time of their discovery. If the structures are in fact fossilized lifeforms, as proposed by the so-called biogenic hypothesis of their formation, 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. 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 hydrocarbon
s (PAH) have been found. The debate over whether the organic molecules in the meteorite are in fact of exobiologic origin or are due to abiotic processes on Mars or contamination from the contact with Antarctic ice on Earth is still ongoing.
Early on, Ralph Harvey of Case Western Reserve University
and Harry McSween of University of Tennessee
reported evidence that the carbonate globules found in the meteorite were formed at high temperature (above 650°C) by volcanic or impact processes on Mars. At such high temperatures, it would be very unlikely that the morphology of the globules could have had any kind of biological origin. Later, however, the same authors published papers supporting a hypothesis in which the globules formed at low temperature from an aqueous solution. Most scientific papers published in the past 10 years now accept that carbonates on Mars formed this way.
Other initial skepticism towards the biogenic hypothesis focused on the idea that the nanometer-sized filaments were too small to contain RNA, but evidence continues to grow that nanobacteria do exist in nature. Furthermore, microbiologists have successfully cultured nanobacteria in the lab, with sizes within the range of at least some of the purported microfossils in ALH 84001.
Some experts argue that the biomorph
s found in the meteorite are not indicative of life on Mars, but instead are caused by contamination by earthly biofilm
s. However, scientists at NASA argue that likely microbial terrestrial contamination found in other Martian meteorites do not resemble the texture of the biomorphs in ALH 84001. In particular, the biomorphs in ALH 84001 look intergrown or embedded in the indigenous material, while likely contamination do 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 of Hernandez Engineering Inc. in Houston. 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."
In November 2009, a team of scientists at Johnson Space Center, including David McKay, reasserted that there is "strong evidence that life may have existed on ancient Mars", after having reexamined the meteorite using more advanced analytical instruments now available, in light of the objections that had been made since the biogenic hypothesis for the biomorphs first had been put forward.
Overall, the team concluded that:
In addition, they argued that since the original paper was published, the biogenic hypothesis has been "further strengthened by the presence of abundant biomorphs in other Martian meteorites."
, 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 the biomorphs in ALH 84001 to be very similar to terrestrial samples without knowing that she was describing a Martian meteorite.
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
that was found in Allan Hills
Allan Hills
The Allan Hills are group of hills, mainly ice free and about long, lying just north-west of Coombs Hills near the heads of Mawson Glacier and Mackay Glacier in Oates Land and Victoria Land, Antarctica. They were mapped by the New Zealand party of the Commonwealth Trans-Antarctic Expedition and...
, Antarctica on December 27, 1984 by a team of U.S. meteorite hunters from the ANSMET
ANSMET
ANSMET is a program funded by the Office of Polar Programs of the National Science Foundation that looks for meteorites in the Transantarctic Mountains. This geographical area serves as a collection point for meteorites that have originally fallen on the extensive high-altitude ice fields...
project. Like other members of the group of SNCs (shergottite, nakhlite, chassignite), ALH 84001 is thought to be from Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
. However, it does not fit into any of the previously discovered SNC categories. On discovery, its mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
was 1.93 kilograms (4.3 lb). It made its way into headlines worldwide in 1996 when scientists announced that it might contain evidence for microscopic fossils of Martian bacteria based on carbonate globules observed.
History
This rock is theorized to be one of the oldest pieces of the solar systemSolar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, proposed to have crystallized from molten rock 4.091 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.
In September 2005, Vicky Hamilton of the University of Hawaii at Manoa
University of Hawaii at Manoa
The University of Hawaii at Mānoa is a public, co-educational university and is the flagship campus of the greater University of Hawaii system...
presented an analysis of the origin of ALH 84001 using data from the Mars Global Surveyor
Mars Global Surveyor
The Mars Global Surveyor was a US spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It began the United States's return to Mars after a 10-year absence. It completed its primary mission in January 2001 and was in its third extended mission phase when, on 2...
and Mars Odyssey spacecraft orbiting Mars. According to the analysis, Eos Chasma
Eos Chasma
Eos Chasma is a chasma in the southern part of the Valles Marineris canyon system of Mars.Eos Chasma’s western floor is mainly composed of an etched massive material composed of either volcanic or eolian deposits later eroded by the Martian wind. The eastern end of the Eos chasma has a large area...
in the Valles Marineris
Valles Marineris
Valles Marineris is a system of canyons that runs along the Martian surface east of the Tharsis region...
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.
The theory holds that ALH 84001 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 impacted Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
roughly 13,000 years ago. These dates were established by a variety of radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
techniques, including samarium-neodymium
Samarium-neodymium dating
Samarium-neodymium dating is useful for determining the age relationships of rocks and meteorites, based on decay of a long-lived samarium isotope to a radiogenic neodymium isotope. Nd isotope ratios are used to provide information on the source of igneous melts as well as to provide age data...
(Sm-Nd), rubidium-strontium
Rubidium-strontium dating
The rubidium-strontium dating method is a radiometric dating technique that geologists use to determine the age of rocks.Development of this process was aided by Fritz Strassmann, who later went on to discover nuclear fission with Otto Hahn and Lise Meitner....
(Rb-Sr), potassium-argon
Potassium-argon dating
Potassium–argon dating or K–Ar dating is a radiometric dating method used in geochronology and archeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium into argon . Potassium is a common element found in many materials, such as micas, clay minerals,...
(K-Ar), and carbon-14
Radiocarbon dating
Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...
.
It is hypothesized that ALH 84001 originated from a time period during which liquid 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.
In October 2011 it was reported that isotopic analysis indicated that the carbonates in ALH 84001 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.
Possible biogenic features
On August 6, 1996 ALH 84001 became newsworthy when it was claimed that the meteorite may contain evidence of traces of life from MarsLife on Mars
Scientists have long speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. Fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries, but it remains an open question whether life currently exists on...
, as published in an article in Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
by David McKay
David S. McKay
David S. McKay is Chief Scientist for Astrobiology at the Johnson Space Center. McKay was the first author of a scientific paper postulating past life on Mars on the basis of evidence in Martian meteorite ALH84001 found in Antarctica. This paper has become one of the most heavily cited papers in...
of NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
.
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
structures were revealed that were interpeted as remains—in the form of fossils—of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
-like life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...
forms. The structures found on ALH 84001 are in diameter, similar in size to theoretical nanobacteria, but smaller than any known cellular life at the time of their discovery. If the structures are in fact fossilized lifeforms, as proposed by the so-called biogenic hypothesis of their formation, they would be the first solid evidence of the existence of extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...
, aside from the chance of their origin being terrestrial contamination.
The announcement of possible extraterrestrial life caused considerable controversy. 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
Bill Clinton
William Jefferson "Bill" Clinton is an American politician who served as the 42nd President of the United States from 1993 to 2001. Inaugurated at age 46, he was the third-youngest president. He took office at the end of the Cold War, and was the first president of the baby boomer generation...
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 hydrocarbon
Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...
s (PAH) have been found. The debate over whether the organic molecules in the meteorite are in fact of exobiologic origin or are due to abiotic processes on Mars or contamination from the contact with Antarctic ice on Earth is still ongoing.
Early on, Ralph Harvey of Case Western Reserve University
Case Western Reserve University
Case Western Reserve University is a private research university located in Cleveland, Ohio, USA...
and Harry McSween of University of Tennessee
University of Tennessee
The University of Tennessee is a public land-grant university headquartered at Knoxville, Tennessee, United States...
reported evidence that the carbonate globules found in the meteorite were formed at high temperature (above 650°C) by volcanic or impact processes on Mars. At such high temperatures, it would be very unlikely that the morphology of the globules could have had any kind of biological origin. Later, however, the same authors published papers supporting a hypothesis in which the globules formed at low temperature from an aqueous solution. Most scientific papers published in the past 10 years now accept that carbonates on Mars formed this way.
Other initial skepticism towards the biogenic hypothesis focused on the idea that the nanometer-sized filaments were too small to contain RNA, but evidence continues to grow that nanobacteria do exist in nature. Furthermore, microbiologists have successfully cultured nanobacteria in the lab, with sizes within the range of at least some of the purported microfossils in ALH 84001.
Some experts argue that the biomorph
Biomorph
Biomorph may refer to:*A shape resembling that of a living organism , though not necessarily of biotic origin*One of the virtual creatures in a computer simulation described by Richard Dawkins in his book The Blind Watchmaker...
s found in the meteorite are not indicative of life on Mars, but instead are caused by contamination by earthly biofilm
Biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
s. However, scientists at NASA argue that likely microbial terrestrial contamination found in other Martian meteorites do not resemble the texture of the biomorphs in ALH 84001. In particular, the biomorphs in ALH 84001 look intergrown or embedded in the indigenous material, while likely contamination do 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 of Hernandez Engineering Inc. in Houston. 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."
In November 2009, a team of scientists at Johnson Space Center, including David McKay, reasserted that there is "strong evidence that life may have existed on ancient Mars", after having reexamined the meteorite using more advanced analytical instruments now available, in light of the objections that had been made since the biogenic hypothesis for the biomorphs first had been put forward.
Overall, the team concluded that:
None of the original features supporting our hypothesis for ALH84001 has either been discredited or has been positively ascribed to non-biologic explanations.
In addition, they argued that since the original paper was published, the biogenic hypothesis has been "further strengthened by the presence of abundant biomorphs in other Martian meteorites."
Student participation
The initial analysis of ALH 84001 was unusual in that an undergraduate student, Anne Taunton of the University of ArkansasUniversity of Arkansas
The University of Arkansas is a public, co-educational, land-grant, space-grant, research university. It is classified by the Carnegie Foundation as a research university with very high research activity. It is the flagship campus of the University of Arkansas System and is located in...
, 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 the biomorphs in ALH 84001 to be very similar to terrestrial samples without knowing that she was describing a Martian meteorite.
See also
- Deception PointDeception PointDeception Point is a 2001 techno-thriller novel by Dan Brown. The plot concerns a meteorite found within the Arctic Circle that may provide proof of extraterrestrial life, and attempts by dark forces to prevent this finding from becoming public.-Plot:...
– fiction around an ALH84001 theme - Life on MarsLife on MarsScientists have long speculated about the possibility of life on Mars owing to the planet's proximity and similarity to Earth. Fictional Martians have been a recurring feature of popular entertainment of the 20th and 21st centuries, but it remains an open question whether life currently exists on...
- Nakhla meteoriteNakhla meteoriteNakhla is a famous martian meteorite fallen in Egypt in 1911.-History:It fell to Earth on June 28, 1911, at approximately 09:00, in the Nakhla region of Abu Hommos, Alexandria, Egypt...
- Panspermia
- ExogenesisExogenesisExogenesis may refer to:* Exogenesis , the hypothesis that life originated elsewhere in the universe and was spread to Earth* Exogenesis , an episode of the science-fiction TV series Babylon 5...
- Shergotty meteoriteShergotty meteoriteThe Shergotty meteorite is the first example of the shergottite Mars meteorite family. It was a Martian meteorite which fell to Earth at Shergotty , in the Gaya district, Bihar, India on 25 August 1865, and was retrieved by witnesses almost immediately...