Gourt Home::Gourt :: Mars meteorite
A Mars meteorite is a meteorite that has landed on Earth but is guessed to have originated from Mars. This could have been the result of an impact of a celestial body on Mars, sending material from Mars into space.
Note that this does not refer to meteorites actually found on Mars, such as Heat Shield Rock.
The Martian hypothesis
A number of different hypotheses attempt to place this or that meteor's origin at some given point, like Mars. These include attempts to predict what gases would be inside such a rock once it arrives, or what the rock's composition should be, or its isotopic composition or its radioactive signature. All of these are truly little more than guesses, since actual scientific testing has not been conducted on rocks from those foreign bodies -- in most cases, we have never even had or measured (for their atmosphere, composition, or other signature) such rocks, much less measured them after they are blown into space by a huge collision, floated around for millions of years, and then plunged through the Earth's atmosphere.
Composition
Mars meteorites include three rare groups of achondritic (stony) meteorites: Shergottites, Nakhlites, and Chassignites. Together they form a class of meteorites known as the SNC group. They have isotope ratios that are said to be consistent with each other and inconsistent with the Earth. The names derive from the location where they were discovered: Sherghati, India (1865), El-Nakhla, Egypt (1911), and Chassigny, Haute-Marne, France (1815) respectively.
One of the shergottites, known as ALH84001, received a lot of attention after an electron microscope revealed structures that were considered to be the fossilized remains of bacteria-like lifeforms. As of 2005 however, most experts agree that the microfossils are not indicative of life, but of contamination by earthly biofilms. It has not yet conclusively been shown how they formed. ALH84001 is much older than the others in the SNC group — dating back to the original formation of Mars about 4.5 billion years ago. In this respect, it resembles a typical meteorite rather than the other SNC's, which all appear to be formed less than 1.3 billion years ago.- Mars Meteorite ALH 84001 – An Illustrated History. URL accessed March 18, 2006.
All the meteorites are igneous rocks. Lherzolitic shergottites (one from Antarctica, 2 from California) are identified by their Deuterium/Hydrogen ratios. The crystals appear to be 154-187 million years old and they appear, from cosmic ray analysis, to have spent 2.5 to 3.6 million years in space. There are also basaltic shergottites, some of which appear (from the presence of hydrated carbonates and sulfates) to have been exposed to liquid water prior to injection into space.
In March 2004, it was suggested that the unique Kaidun meteorite landed on March 12, 1980 in Yemen, may have originated on the Martian moon of Phobos.
Possible evidence of life
Possible evidence of life has been hypothesized in three meteorites.
- A 1.3 billion-year-old meteorite from near El-Nakhla, Egypt. Small structures that look vaguely like Earth bacteria. More like bacteria than those in the better-known Allen Hills meteorite.
- A 4.5 billion-year-old meteorite found in the Allen Hills of Antarctica (ALH84001). Ejection from Mars seems to have taken place about 16 million years ago. Arrival on Earth was about 13 000 years ago. Cracks in the rock appear to have filled with carbonate materials between 4 and 3.6 billion-years-ago. Evidence of polycyclic aromatic hydrocarbons (PAHs) have been identified with the levels increasing away from the surface. Other Antarctica meteorites do not contain PAHs. Earthly contamination should presumably be highest at the surface. Several minerals in the crack fill are deposited in phases, specifically, iron deposited as magnetite, that are claimed to be typical of biodepositation on Earth. There are also small ovoid and tubular structures that might possibly be nanobacteria fossils in carbonate material in crack fills (investigators McKay, Gibson, Thomas-Keprta, Zare). Micropaleontologist Schopf, who described several important terrestrial bacterial assemblages, examined ALH 84001 and opined that the structures are too small to be Earthly bacteria and don't look especially like lifeforms to him. The size of the objects is consistent with Earthly "nanobacteria", but the existence of nanobacteria itself is controversial.
In August 2002, a NASA team led by Thomas-Keptra published a study indicating that 25% of the magnetite in ALH 84001 occurs as small, uniform-sized crystals in a crystal form that, on Earth, is associated only with biologic activity. The remainder of the material appears to be normal inorganic magnetite. The extraction technique did not permit determination as to whether the possibly biologic magnetic was organized into chains as would be expected.
References
External links
- Mars Meteorite Home Page (JPL)
- http://www.rmki.kfki.hu/~lukacs/PADVARX.htm
"Mars meteorite"