Oued Tissint, Morocco close to where the Tissint Martian meteorite was recovered in October 2011 (image: Wikipedia).

Following my last post on the SNC meteorites, there was a bit of frenzy in the media about the latest Martian arrival, known as Tissint. Now, I am of course not claiming that this unprecedented media interest was in any way inspired by this blog. But you have to admit it was a bit of a coincidence!

So what was all the fuss about?

Well, there is no doubt about it, having another fresh meteorite sample from the red planet is a big deal. The Meteoritical Bulletin database currently lists 103 valid meteorites of Martian origin (also known as SNCs: see last blog entry).

Sounds like we have a lot already! Why should one more make a difference?

Well, for starters not all of these 103 specimens represent individual meteorites. Some are certainly related stones that fell during the same event. It is quite normal for a meteor to disintegrate during atmospheric entry, resulting in a shower of stones. Once on the ground these related stones form what is known as a strewn field.

So how many distinct Martian meteorites do we have samples of?

That’s not an easy question to answer. Dr. Anthony Irving (University of Washington), one of the team who classified the Tissint meteorite, estimates that we currently have samples of about 61 distinct Martian meteorites. That might still sound like a lot, but it is important to realise that the vast majority of these meteorites are “finds”, i.e. recovered without their atmospheric entry being witnessed. Most will have been on Earth for a considerable length of time before recovery and hence have experienced variable degrees of terrestrial weathering and contamination. In fact Tissint is only the fifth witnessed “fall” of a Martian meteorite, the others being: Chassigny (1815), Shergotty (1865), Nakhla (1911), Zagami (1962).

Even though Tissint is classified as a meteorite “fall” actual specimens were only recovered about three months after the sightings, on 18^th July 2011, of the bright fireball that formed as the object decelerated in the Earth’s atmosphere. This delay in recovering material is fairly normal for falls that take place in relatively remote areas. In a previous blog entry we looked at the fall of the important meteorite Almahata Sitta, which landed in October 2008, also accompanied by a bright fireball. Despite being tracked by satellites and ground-based telescopes throughout its atmospheric entry stage, pieces of the Almahata Sitta meteorite were only recovered two months later and required a dedicated and extensive search involving a large group of students from the University of Khartoum. In the case of Tissint, the recovery of material was not the result of a systematic search but due to the discovery of freshly fusion-crusted material by local nomads. Morocco is a region that has yielded a very large number of meteorite finds in recent years and as a result local people have a considerable expertise in spotting meteorite samples. The hot and dry conditions that prevail in the south of Morocco, where Tissint was recovered, mean that the delay in collecting pieces of the meteorite should not cause too many problems.

So what sort of meteorite is Tissint and how big is it?

The Meteoritical Bulletin indicates that a number of stones varying from 1g to 987g have been collected, with a total mass of about 7 kg. However, this is likely to be an underestimate because a lot of the material collected since the initial discovery will have been traded privately. A striking feature of Tissint is the extremely fresh, black, glassy appearance of the fusion crust found as very thin layer that encrusts the stones. Fusion crust forms due to melting along the outer surface of the meteorite as a result of frictional heating as it passes at high speed through the Earth’s atmosphere. This process helps to keep the inside of the meteorite cool, because as the melt layer forms it is swept off the back of the rapidly moving object. To understand why frictional heating takes place it is worth remembering that meteors entering the atmosphere travel at velocities of between 10 to 70 km per second i.e. 36,000 to 250,000 km per hour.

An examination of the interior of Tissint shows that it consists of relatively large crystals of the mineral olivine, up to 1.5 mm in diameter, set in a finer grained groundmass. Tissint shows many mineralogical similarities to the previous Martian fall Shergotty and so is classed as a shergottite. Shergottites are the most abundant type of Martian meteorite. While they are a relatively diverse group, most samples appear to represent igneous rocks that cooled on or close to the surface of Mars.

But how did material from a lava flow on Mars end up on Earth?

In order to reach Earth Martian meteorites must start out as debris ejected into space by a large impact on Mars. Tissint, like all Martian meteorites, bears the scars of this catastrophic launch event. It is crisscrossed by tiny veinlets of black glass formed by melting that took place when the rock was blasted into space. This process is called shock melting and the resulting glass produced by this process can trap tiny amounts of the Martian atmosphere. As discussed in the last blog entry, analysis of this glassy material provides one of the most powerful lines of evidence linking the SNC meteorites to Mars.

So as the fifth Martian fall, Tissint is clearly a very important meteorite. The detailed scientific study of Tissint has only just started. We will certainly learn a lot more about the red planet as a result of all this research activity. There will probably be a few surprises along the way.