Friday, May 1, 2015

The History Recorded in Mars’ Meteorites

Mars resembles the Earth in many ways, including its day length, its temperature, and its gravity, thus many people consider the possibility that one day we may inhabit it. However, there are many mysteries that scientists would have to solve before determining whether Mars is capable of supporting life. If they ultimately show that life once existed on Mars, it will confirm that life can exist on other planets, even if might not be plausible for life to exist now or in the future on Mars itself. Scientists approach this question from several directions. One of them is studying the SNC meteorites, which originated on Mars.

Most of the meteorites from Mars fall into one of three groups –Shergotty, Nakhla, and Chassigny –thus people use the term SNC meteorites to refer to meteorites from Mars. These meteorites contain similar compositions and have similar structures, thus when petrologists first studied them, they speculated that they might have come from the same “parent body.” Even though they come from the same planet, they also differ in many ways. Most SNC meteorites belong to the shergottite category, which is a kind of basaltic meteorite consisting mostly of pyroxenes and relic plagioclases. Some of the Martian meteorites fall into the category of nakhlites. These meteorites are clinopyroxenes, which are monoclinic pyroxenes and are either calcic or sodic, and they contain augite, olivine abundant in iron, titanium-rich magnetite and interstitial and glassy material that was formed when the meteorite was formed. “The nakhlites have preserved some of the clearest traces of aqueous alteration within the parent rocks on Mars” (Martian Meteorites). Last but not least, chassigny is a cumulative of dunite, containing more than 90 percent of olivine, with pyroxene, chromite and plagioclase making up the rest (Martian Meteorites).

Since the SNC meteorites share so many similar features, researchers were certain that they originated on the same planet. They also speculated that they came from Mars, but not until 2000 were they confident that the SNC meteorites indeed originated on the Red Planet. According to Allan Treiman and his fellow researchers, the 14 SNC meteorites that were identified at that time “contained traces of gas which is similar in elemental and isotopic compositions to the modern Martian atmosphere as measured by Viking landers on Mars and spectroscopy from Earth.” Specifically, the EETA 79001, which is in the class of shergotite, is the first SNC meteorite that found to contain the compositions that were also identified on Mars. It has glass inclusions that contain rare nitrogen isotopic compositions, which was also found in the Martian atmosphere by the Viking spacecraft in 1976. As Treiman said, since Mars has such a unique composition, having a similar composition with it is a strong indication that the SNC meteorites came from Mars. If they are not from Mars, “it (the planet that SNC meteorites actually came from) would have to be substantially identical to Mars as it now is understood”, Treiman confirmed.