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ALH84001 - "The Rock"
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Allan Hills meteorite

How do scientists know that the rock came from Mars?

The rock has regions of glassy substances that have gas bubbles trapped inside them. The glassy areas were formed when meteorites impacted the surface of Mars, creating heat and pressure sufficient to form the material. The gas bubbles trapped inside are representative of the atmospheric composition of Mars at the time of impact. The isotopic composition of the gas bubbles was measured, and was found to be the same as that measured by the Viking landers. While it is true that the atmosphere of Mars changed greatly between the formation of the bubbles and the Viking measurements, the relative isotopic compositions stay the same, and so it was shown that the gas bubbles were once Martian atmosphere.

How was the age of the rock determined?

Three different tests were performed to determine the time since the rock solidified from molten materials. The methods used were the radioactive analyses of isotopes of rubidium with strontium, neodymium with samarium, and argon isotopes. All three of these tests concluded that the age of the rock is 4.5 billion years, which is approximately 25 times older than any other known Mars rock on Earth.

How was it determined how long the rock spent in space?

The length of time that the rock spent in interplanetary space was determined through the analysis of 3He, 21Ne, and 38Ar present in the rock. In space, the rock gets bombarded with high-energy cosmic rays, which react with nuclei inside the rock, producing these three isotopes. Now, the rate at which these reactions occur is known, so by measuring their abundance in the rock, scientists were able to conclude that the rock had been bombarded with cosmic rays for 16 million years, and therefore, had spent 16 million years in space.

Antarctica outline, Allan Hills location

How was it determined how long the rock spent in Antarctica?

Scientists used 14C dating to determine how long the rock has been on Earth. When the rock was in space, reactions with cosmic rays produce Carbon14 at a known rate. Eventually, the rock reaches its 'saturation point', where the production and decay rates of the Carbon14 balance out. When the rock landed on Earth, the production ceased, since the cosmic rays were no longer present, but the decay continued. Therefore, by measuring the amount of Carbon14 present in the rock, and comparing it with the known saturation point, scientists determined that the rock has been on Earth for approximately 13,000 years.

Meteorite magnification

What evidence is there that the rock contains ancient Martian life?

The rock contains carbonate globules, which are 20 to 250 microns in diameter. These globules were deposited by a liquid saturated with carbon dioxide, which leaked into the cracks in the rock. Studies have shown that the liquid had a temperature less than 100 degrees Celsius, which is suitable for life, and for the presence of liquid water. Attempts to age the carbonates have set a range between 1.3 and 3.6 billion years. The globules contain three pieces of evidence which, when considered together, could be explained by biological activity. The first is that minerals found within the globules are similar, in both identity and arrangement, to those minerals produced by biological life on Earth. The globules are rich in magnesite, and siderite, with small quantities of calcium and manganese also found. Also present in the rock are magnetite, and various sulfides. The second piece of evidence is the presence of organic molecules within the globules, and throughout the rest of the rock. When organisms die and decay, they leave behind organic molecules, such as polycyclic aromatic hydrocarbons (PAHs). The PAHs could be the decay products of organisms that were placed in the rock by the same fluid which formed the globules. Analysis of the PAHs show that there are relatively few types of them in the rock, and all are consistent with the decay of terrestrial organisms. The third piece of evidence is what appear to be microbial fossils present in the rock. These fossils are similar to those recently found on Earth, deep below the ground. The fossils are tube-like objects, which are about 380 nanometers long. Other structures in the rock are 500-700 nanometers long. Smaller objects, which appear to be fragments of larger ones, are also present in the rock.

Could the evidence be a result of Terran contamination?

The isotopic ratios of elements in the minerals do not match Earth, but are a match for Mars; therefore it is almost certain that they were formed on Mars. Also, scientists examined the location of these objects within the rock, and it appears that they are uniformly distributed throughout, including in the center, so it is unlikely that the evidence is terrestrial in origin.

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