By Science Hooker, Mar 24 2016 02:16PM
Space scientists often publish research findings based on meteorites that are supposedly from Mars, but how on Earth (forgive the pun) do they know for sure they come from Mars? Why not from another planet? Or from random space debris? Besides, does it really matter where meteorites come from?
What is a meteorite? A meteorite is a relatively small solid piece of rock, often containing metal, that lands on Earth from space. Many form from disintegration of asteroids or comets. Asteroids are rocks greater than 1km3 and comets are icy rocky bodies. However, some meteorites come from planets, such as Mars, and are ejected into space when their surfaces are struck by large impacts. This ejection process is called spallation. When in space the rock is called a meteoroid, and if it happens to impact Earth and survive entry it is then called a meteorite. Scientists have currently confirmed 89 meteorites found on Earth that originate from Mars .
How do we know they are from Mars? We have good knowledge of the gas composition in the Martian atmosphere via measurements taken by landers, rovers and satellites. Within meteorites there are tiny pockets of trapped gas, often tightly sealed in melt glass, and by carefully measuring the elemental composition of such gas bubbles using a technique called spectrometry. The spectrometry results can be compared to the known values from Mars. When scientists claim a meteorite is from Mars it is because the gas proportions have matched precisely.
Is this the only test? Although a good initial test, there are other lines of evidence to confirm whether the meteorite came from Mars.
• Rocks from Mars contain iron rich metal oxide minerals such as magnetite, chromite and ilmenite, however, the rocks are poor in pure iron metal .
• They all contain the magnetic sulphide mineral called pyrrhotite .
• Within certain minerals called olivine and pyroxene there is a chemical element ratio of iron to manganese (Fe:Mn) that is highly distinctive of Mars .
• Rocks from Mars possess highly specific oxygen isotope ratios. Isotopes are when the atomic mass of an element, such as oxygen, varies due to the number of neutrons present. Oxygen can be present as O16, O17 or O18. Rocks with the same 18O / 16O ratio usually have the same 17O / 16O ratios. Mars meteorites possess a higher 17O / 16O ratio than rocks from Earth or the moon, and are distinctive from meteorites that have not come from Mars .
How can scientists be sure all this information isn’t just contamination from when the meteorite landed on Earth? When meteorites descend incredibly fast through the Earth’s atmosphere a great amount of heat is generated through friction with atmospheric particles. This heat produces a semi-melted outer layer called a fusion crust, which is characteristically dark and shiny. This layer can create an impenetrable barrier to moisture, microbes and other contamination. However, over time even the best seals break and contamination occurs. A great deal of effort is spent assessing whether findings are genuine results or contamination effects .
Are all Mars meteorites made of the same stuff? No. Meteorites from Mars divide into three main sub-types and are named after the meteorites that best represent those sub-types. These are called Shergotty, Nakhla and Chassigny, and are collectively known as SNC meteorites. There are a couple of rare Martian meteorites outside of the SNC range, but these will not be discussed here. Shergottites are the most common and are made of basaltic rock, much like the sea floor. There are seven Nakhlites and these are made of clinopyroxenite; these Nakhlites contain the best evidence of ancient water on Mars. Two Chassignites have been found and are made of dunite, a greenish rock caused by the mineral olivine, and in Earth dunites form quite deep in the crust.
Why do these Mars meteorites matter? By studying the chemistry and mineralogy of Mars meteorites we can discover what the conditions of early Mars was like and how these conditions have evolved over time. They help us unpick whether the planet once had vast oceans or were covered in icy glaciers. The ancient environments then inform us as to whether life may have been present, and help us in deciding whether life might currently be present. We are now sure Mars had surface water in its past, and this would have required a thicker atmosphere which has now gone. Some was lost to space, some stored in the poles as CO2 ice. Yet some of the mostly CO2 atmosphere became stored in the crust as the mineral carbonate, which we can study in Mars meteorites. The very same process of capturing atmospheric CO2 in rock is being tried by engineers on Earth to counter dangerous climate change caused by anthropogenic global warming. It is quite possible that the ancient environmental history of early Mars, brought to us in the form of meteorites, can teach us a great deal about managing our own planet’s carbon future.
Also, meteorites are just awesomely cool!
 Rochette P., Lorand J., Fillion G., Sautter V. (2001). ‘Pyrrhotite and the remanent magnetization of SNC meteorites : a changing perspective on Martian magnetism.’ Earth and Planetary Science Letters 190:1-12.