Meteorites:

the inside story

Natural History Museum (NHM) meteorite experts Caroline Smith (right), Sara Russell (left) and Gretchen Benedix (middle) talk to Astronomy Now about how to recognise meteorites, what's on display at the NHM, and some of the most interesting meteorite finds.

Posted: 20 April 2010

Gretchen Benedix is a researcher in the Meteorites and Cosmic Mineralogy division. Her main area of research is the evolution of the Solar System through study of altered or differentiated meteorite groups. She also studies the geology of Mars.

Caroline Smith is the Curator of Meteorites. Her role is to maintain and manage the collection and to facilitate use of the collection by researchers both in the Museum and those from institutes around the world. Her main areas of research are the identification of new meteorites and planning for future space missions designed to return samples from asteroids and the planet Mars.

Sara Russell is the Head of the Division of Meteoritics and Cosmic Mineralogy. Her main areas of research interest are in learning about the events and conditions that prevailed in the early solar system, when the planets began to form.

What are the key diagnostics for identifying a meteorite?
There are several diagnostic features that can help determine if a rock is a meteorite:


1. Does the sample have a dark fusion crust? (Image, left: naturalsciences.org)
2. Is the sample rounded?
3. Does the sample contain holes?
4. Is the rock different from other rocks in the local area?
5. Is the rock attracted to a magnet?
6. Does the rock feel heavy for its size?

These tests can be helpful in determining if a rock might be a meteorite. Meteorites have exterior surfaces that were melted as they passed through the Earth's atmosphere (the fusion crust); it is very rare that they contain holes; they are usually solid objects with irregular, rounded but not spherical, shapes; they will generally look different from other rocks in the same area; they are usually heavier than comparably sized Earth rocks; and they will usually attract a magnet. (Image, above: Dr Svend Buhl, Meteorite Recon).

If someone suspects they do have a meteorite, what should they do? Contact our colleagues in the Angela Marmont Centre at the Museum. It will pass through the enquiry service there and if it is promising, will be passed along to the Meteorite team to offer an expert opinion about it.

How often do people think they have found a meteorite, but in fact it's just a rock they've picked up in the garden? About 99 percent of samples that come to us as enquiries are "meteorwrongs". The most common meteorwrongs we get sent are industrial slags and pyrite nodules. Typically, the only times when the samples to turn out to be meteorites are the occasions when perhaps the person has purchased the rock as a meteorite or when they have visited an area known for meteorites e.g. the north African deserts and found the rock there. We answer hundreds of enquiries every year and this certainly shows that people are good at recognising rocks that are different from those in the near surroundings.

What types of meteorites can people see at the NHM?
Unfortunately we currently only have a very small number of meteorites (24) on public display. This represents a tiny proportion of the 4,809 registered in our collection. If people visit us they can see examples of the three main classes of meteorites – the stony, iron and stony-iron meteorites. Some of the meteorites are on display in our Mineral Gallery and some particularly important samples are on display in our Vault Gallery. There are others on display in our Earth Galleries. The meteorites on display in the Vault include a spectacular slice of a stony-iron meteorite (a pallasite) called Imilac, which is one of the largest slices of this type of meteorite on display anywhere in the world. Also in the Vault is a large piece of the Nakhla meteorite, one of the rare Martian meteorites. This meteorite fell in Egypt in 1911. Another important specimen, which you can see in the Earth Galleries, is the Cranbourne meteorite. This is an iron meteorite which was found in Australia in the 1850s. Weighing in at 3.5 tonnes it is the largest meteorite in the Museum’s collection and is the largest meteorite on display in a museum in Europe. It has recently been placed in a new, purpose built case in an inert atmosphere to help preserve this iconic sample for the future.

How common is it that meteorites originate from the same asteroid, or same location on the Moon or Mars?
There are approximately 920 meteorites that are thought to come from the Asteroid 4 Vesta. This is based on a link in the spectra of the rocks in the lab and of the asteroid in space. A NASA space mission called DAWN is currently on its way to 4 Vesta to go into orbit around this asteroid. This will greatly expand our knowledge about 4 Vesta and strengthen the tie between the meteorites we have on Earth with this asteroid.
It is difficult to say from where on the Moon or Mars the meteorites come. We know most of the lunar meteorites are different from the Apollo samples that the astronauts brought back and must come from different areas on the Moon than were visited and sampled. This is an area of active study for both the Moon and Mars. For Mars, there are four main groups of meteorites, but it's still unclear whether even the rocks from a single group came from the same place on the planet.

What is the biggest known meteorite that has fallen to Earth? The largest meteorite to fall on Earth is the Hoba meteorite (pictured), which is sitting in the ground where it fell in Namibia. It has an estimated weight of 60,000 kg (60 tonnes). It was not seen to fall, but terrestrial age estimates indicate it may have fallen around 80,000 years ago. This meteorite is unusual in that the total known mass occurs in one stone. Most of the bigger meteorites break up during transit through the atmosphere before hitting the Earth. These next 3 biggest meteorites (also iron meteorites) are Cape York (Greenland - 58.2 tonnes), Campo del Cielo (Argentina – 50 tonnes), and Canyon Diablo (USA - 30 tonnes). But in all these other cases, there are multiple stones associated with the fall. We would also expect large stony meteorites to break up during atmospheric flight. For example in the case of the Allende (Mexico, 1969) over 2 t of material has been recovered and it is estimated that the strewnfield (the area where the meteoritic stones fell) is more than 150km² in size.

Which meteorites do you think are most important to the study of meteorites?
Wold Cottage (Ordinary chondrite - England - 1795) and L'aigle (Ordinary chondrite - France - 1803) bracket the time when meteorites were first determined to be rocks from space. It was examination of these rocks along with a few others that helped prove that meteorites are extraterrestrial.

Nakhla (Martian – Egypt – 1911) is a beautiful rock and some of its properties were used to show that some meteorites could come from Mars, rather than all from asteroids. (Image, right: NHM).

Allende (CV3 - Mexico - 1969) and Murchison (CM2 - Australia - 1969) were two big falls, which supplied enough material to allow all kinds of primitive components (chondrules and CAIs) to be studied in much greater depth. From these two rocks we’ve learned much about the early history of the nascent solar system, from it’s age (based on the ages of CAIs) to various geologic processes. These two also fell fortuitously the same year as the first moon landing. This influx of extraterrestrial material was used to test techniques that were designed to analyse the samples brought back from the Moon.

Do you have a favourite sample in the collection?
GB: Pontlyfni meteorite - a meteorite that fell in Wales in 1931. This meteorite represents the freshest and most pristine sample of a group of meteorites that fall between primitive and evolved meteorites. I worked on this meteorite when I was a PhD student and only saw thin sections of it. It's very exciting that so many years later, I now work at the Museum where the main mass exists and I can look at the meteorite anew in a completely different context.

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