In 1996 scientists stunned the scientific world when they reported finding evidence in a meteorite indicating that life once existed on Mars. Since then, a great deal of research has been done, attempting to prove or disprove their idea. Since most of the research involves chemistry, let's look at it.
The meteorite, named ALH84001, is a 1.9-kg rock that was found in the Allen Hills region of Antarctica. It formed on Mars about 4.5 billion years ago. Approximately 3.9 billion years ago, some kind of shock or impact produced cracks and fractures in the rock. It was catapulted into space about 16 million years ago when a comet or asteroid smashed into Mars. The rock floated around in space until it was captured by Earth's gravity about 13,000 years ago and smashed into Antarctica. Meteorite hunters found it there in 1984. For more information and links related to ALH84001, click here.
How do we know the meteorite came from Mars?
Chemists studied oxygen isotopes in ALH84001 and several other meteorites thought to come from Mars. The ratios of 16O, 17O, and 18O were different from values found in rocks from Earth or the Moon, but identical to isotope ratios on Mars. ALH84001 contains trapped gases (Xe, Kr, Ne, Ar, N2, and CO2) that are identical in chemical and isotopic composition to gases in the atmosphere of Mars that were measured by the Viking landers in 1976.
How do we know how old it is?
The other Martian meteorites contain basaltic rock that is less than 1.3 billion years old. ALH84001, however, is composed almost completely of pyroxene ((Mg,Fe)SiO3), which would have been one of the first minerals to solidify when Mars was young. Scientists estimated the length of time since the rock solidified from molten materials by using three different dating techniques. One used isotopes of rubidium and strontium. A second test used neodymium and samarium. The third employed argon. All three methods indicated that the rock is 4.5 billion years old. Analysis of other radioisotopes confirmed how long the meteorite drifted in space and how much time it spent in Antarctica.
Carbonates are the key to the mystery
The meteorite contains carbonate deposits. These appear to have been precipitated in cracks in the rock from water that was present in the past (between 1.3 and 3.6 billion years ago). The carbonate deposits exist as tiny flattened globules, 20 to 250 microns in diameter. Isotopic analysis of carbon in the carbonate globules indicates that they have too much carbon-13 to be from Earth but precisely the right amount to have come from Mars. Analysis of the amounts and types of oxygen isotopes in the carbonates suggests that the globules formed at temperatures no higher than 100 degrees C, a temperature that makes life possible.
Go To Page: 1 2
| Here's the follow-up discussion on this article: | View all related messages |
For a complete listing of article comments, questions, and other discussions related to Linda Mamassian's Chemistry topic, please visit the Discussions page.