It came from Mars – or did it?

Mars – Source of the SNCs?(image: NASA)

Recently, we have been running a fair few SNCSs on our oxygen isotope line here at the Open University.

SNCs? You ask. What are they?

Well, they are meteorites that come from Mars.

From Mars! How do you know that then?

Now, that is a very good question!

SNC is an acronym derived from Shergotty, Nakhla and Chassigny, which are three of the five meteorites in this particular group that have been observed to “fall” to Earth . The other two “falls” are Zagami and Tissint. So the group should now be called SNCZT, or something like that. Not surprisingly this name hasn’t caught on and most people just refer to them simply as Martian meteorites, which brings us back to the original question: how do we know they are from Mars?

Well, there are a number of lines of evidence that were nicely summarised in a paper by Professor Hap McSween in 1994. So here they are:

  1. SNC meteorites generally show young crystallization ages, ranging from about 1,300 to only 180 million years before present. This compares to meteorites from melted asteroids which give very old ages close to that of the Solar System itself, i.e about 4.567 million years before present. That suggests that the body from which the SNCs originate must be large i.e. a planet, in order to sustain long term volcanic activity. However, the SNC meteorite ALH84001, famous for the controversial claims that it contained evidence for Martian life, is an exception, with an age of approximately 4,100 million years before present.
  2. The two NASA Viking landers which arrived on Mars in the summer of 1976 made measurements of the composition of the Martian atmosphere. Then in 1983 analysis of trapped argon, krypton and xenon in the SNC meteorite EETA79001 were shown by two NASA scientists, Bogard and Johnson, to be a close match to the Viking data. Further detailed studies of trapped gas components in SNC meteorites have also shown a good match to the Martian atmosphere. It is fair to say that the trapped gas evidence remains the most solid argument in favour of a Martian origin for the SNCs.
  3. Additional evidence in favour of the SNCs being pieces of Mars relate to aspects of their chemical composition that seem to require formation within a large planet-sized object, rather than an a relatively small asteroid.

Martian meteorite EETA79001 in which trapped gases were first recognised to have a composition similar to the Martian atmosphere as measured by the Viking lander spacecraft (image: NASA)

So just how strong is the evidence in favour of SNCs coming from Mars?

Well, in a review article in 2000, Trieman, Gleason and Bogard summarized things as follows: “There seems little likelihood that the SNCs are not from Mars. If they were from another planetary body, it would have to be substantially identical to Mars as it now is understood”.

So how can you tell if you have a Martian meteorite?

Well, it turns out that all SNCs have a number of distinct features that can be used to separate them from other groups of meteorites. They have an oxidised mineralogy and so contain minerals such as magnetite, chromite and rutile, while containing no Fe metal. They contain the iron sulphide mineral pyrrhotite rather than troilite as found in more primitive meteorites. The minerals olivine and pyroxene in SNCs show a distinctive ratio of Fe to Mn, which helps to separate them from rocks that come from the Earth, Moon or the asteroid 4 Vesta. But finally, and most importantly for our work at the Open University, SNCs have a unique oxygen isotope composition.

As we discussed in a previous blog, oxygen has three stable isotopes (16-O, 17-O and 18-O), which have an absolute abundance of approximately: 16-O = 99.76%, 17-O = 0.04% and 18-O = 0.20%. It turns out that rocks from Mars show a very constant enrichment in 17-O compared with terrestrial samples. This is clearly seen on the diagram below. Rocks on Earth plot along the line labelled TFL (Terrestrial Fractionation Line), whereas rocks from Mars plot on a second line labelled MFL (Mars Fractionation Line). No other known group of meteorites plots on or near the MFL.

So it turns out that oxygen isotopes are a very useful way of identifying SNCs, which is the reason why we get sent so many samples. And you can be sure we are not complaining, because SNCs are fascinating meteorites and until we actually go there are the only samples we have of Mars.

Oxygen isotope plot showing how various types of SNC meteorites plot along a distinct line labelled MFL (Mars Fractionation Line) compared to rocks on Earth which fall along the line labelled TFL (Terrestrial Fractionation line). (All data obtained at the Open University)

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