[Picture:Mars, from Earth]4500 million years ago – Mars Cools, Formation of Rock
The main rock body, or matrix, of ALH 84001 is igneous rock, similar to lavas on Earth that have cooled just beneath the surface. The striking difference is that this is a meteorite and that it belongs to a small group of meteorites with several features which are characteristic of Mars and completely different from anything else yet discovered in the solar system The crystallization age is determined by the samarium-neodymium (Sm-Nd) radioactive isotope decay system. This is one of the most reliable techniques for determining the date of cooling and crystallization of igneous rocks. A similar system, rubidium-strontium (Rb-Sr), gave the same age for one sample of ALH84001. None of the other "Martian" meteorites are older than about 1300 million years.
The main rock body, or matrix, of ALH 84001 is igneous rock, similar to lavas on Earth that have cooled just beneath the surface. The striking difference is that this is a meteorite and that it belongs to a small group of meteorites with several features which are characteristic of Mars and completely different from anything else yet discovered in the solar system The crystallization age is determined by the samarium-neodymium (Sm-Nd) radioactive isotope decay system. This is one of the most reliable techniques for determining the date of cooling and crystallization of igneous rocks. A similar system, rubidium-strontium (Rb-Sr), gave the same age for one sample of ALH84001. None of the other "Martian" meteorites are older than about 1300 million years.
The main rock body, or matrix, of ALH 84001 is igneous rock, similar to lavas on Earth that have cooled just beneath the surface. The striking difference is that this is a meteorite and that it belongs to a small group of meteorites with several features which are characteristic of Mars and completely different from anything else yet discovered in the solar system The crystallization age is determined by the samarium-neodymium (Sm-Nd) radioactive isotope decay system. This is one of the most reliable techniques for determining the date of cooling and crystallization of igneous rocks. A similar system, rubidium-strontium (Rb-Sr), gave the same age for one sample of ALH84001. None of the other "Martian" meteorites are older than about 1300 million years.
[Picture: a small crater (only a mile wide...), here on Earth]4000 and 3800 million years ago - Shock and Fracture
The Rb-Sr crystallization age can be "re-set" by later events, such as a shock or heating. Something (like a nearby meteor impact) shocked the rock, producing the many fissures now seen filled with other material. Such a shock would have partially re-heated the rock, allowing the gas Argon (Ar) to escape. The radioactive potassium isotope 40K decays directly to 40Ar, so K-Ar ages tell us when the rock was last cooled down, trapping the argon. Some Rb-Sr ages confirm such events at 4000 and 3800 million years ago.
[Picture: thin section from ALH 84001; credit: NASA/LPI]1390 to 3500 million years ago - Deposition of carbonates
The dates here are very uncertain and depend on the dates of the events before and after. The formation of carbonates in the rock cracks indicates the presence of liquid water. In this microscopic cross-section, the yellow globules are the carbonate mineral, surrounded by black and clear shells of Whether the water was at temperatures lower than 113C, allowing life as we know it, is still a matter of research and debate. In any case, the carbonates were formed on Mars, before the rock’s ejection into space.
[Picture: micrograph of worm-like "fossil" in ALH 84001]1996 - Evidence for Fossil life or Alien life ?
Announcement of possible evidence for fossilized remains of life from Mars in ALH84001 announced. This sparked worldwide interest, and although the debate about these results continues, this is surely the most famous rock in the solar system! The controversy has perhaps tightened the definition of what constitutes a "biomarker" - a believable sign of life, that will be useful in decades to come. And remember, the absence of signs of past life in ALH84001 does not mean the absence of life on Mars, now or at some time in the past.
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