Coming Out of the Kitchen

Posted on December 31, 2012 by Richard Greenwood

(image: Smithsonian Museum)

How things have changed!

Way back when, I started this blog off with an entry that suggested you should never bring up the subject of meteorites in polite, social conversation for fear of being thought of as, well, a bit boring, or perhaps worse! But that was back in the days before the so-called “Brian Cox effect”. As a result of the popularity of the astronomy programmes presented by the charismatic Professor Brian Cox, all matters “space” are now seen as cool and trendy. So, for example, astronomy clubs, such as the Derby and District Astronomical Society, have seen at least a doubling of their membership. In addition, science subjects have become much more popular at A-level. Space enthusiasts no longer need to seek refuge in the kitchen at parties (unless they want to of course).  This positive trend is set to continue with three consecutive nights of Stargazing Live starting on Tuesday 8th January at 8pm on BBC2.

BBC2′s Stargazing Live hosts Professor Brian Cox (left) and Dara Ó Briain (right).

So people are now interested in space and want to know how meteorites fit into the bigger picture? Well, up to a point.

The problem is that even among some scientists the field of meteorite research is sometimes seen as being highly specialised, or worse still, a bit of a backwater. Nothing could be further from the truth. In reality, much of what we know about the origin and early evolution of our Solar System comes from the detailed laboratory study of meteorites.

Perhaps these misconceptions about meteorites arise in part from the random way these space rocks arrive on Earth. After all, how could such rocky debris, that plunges through the atmosphere so fast that it creates a spectacular fireball and sonic booms, be left in any sort of state to provide meaningful scientific information once it hits the ground?

Fireball of the Pasamonte meteorite as seen early in the morning on 24th March 1933. This famous image was taken by a ranch foreman who just happened to have a loaded camera beside him while he was eating his breakfast! The twisted trail is sometimes taken to indicate that the meteroid spiralled during flight. However, it is more likely that it was caused by camera shake.

In fact, despite the pyrotechnics that often accompany the arrival of a meteorite, the material that makes it to the ground is in a near-pristine state. Here’s why:

Meteoroids (the name given to space rocks before they land) enter the atmosphere at velocities in excess 11.2 km per second, the Earth’s escape velocity.  If the meteoroid has a retrograde orbit, that is opposite in direction to the motion of the Earth around the Sun, it can arrive with a relative velocity as high as 70 km per second. That means it would take less than 10 seconds to travel from Edinburgh to London, compared to 7 hours 25 minutes by car (as estimated by the AA). The enormous speeds at which meteoroids enter the atmosphere results in extremely intense frictional heating due to collisions with the surrounding air molecules. This causes their outer surface to melt and partially vaporise.

Bolide (a large meteor), seen over the Flinders Ranges, in the South Australian desert on 24th April 2011. The object broke up during flight into about a dozen fragments, each with an individual dust trail. The bolide was observed for about seven second. (image: wikipedia)

Now here’s the neat bit!

As soon as the outside of the meteoroid melts the molten liquid is swept off the back of the speeding object taking all the heat with it. It’s nature’s own heat shield! As a result, the material that reaches the Earth’s surface has a cold interior, with the last remaining melt on the outside solidifying to form a thin (usually about 1 mm thick), black, glassy layer, known as fusion crust.

Small fragment of the Tissint Martian meteorite, partially covered in black, shiny, fusion crust (field of view ~ 2cm) (image: Andy Tindle)

So, while meteoroids take a huge battering when they enter the Earth’s atmosphere and often break-up  into a shower of fragments, the material that is eventually recovered (it can now be officially called a meteorite) is close to pristine and provides science with a treasure trove that has been used to unravel the secrets of the Solar System.

TEN THINGS YOU NEED TO KNOW ABOUT METEORITES

Here are just some of the amazing discoveries that have been made as a result of the detailed study of meteorites:

1. The precise age of the Solar System

 When scientists confidentially state that our Solar System formed 4,567 million years ago (give or take a million years) this age is based exclusively on precise laboratory dating of objects known as calcium aluminium-rich inclusions (CAIs for short). CAIs were the first solid objects to form from the cloud of gas and dust from which the Solar System formed. CAIs are found in an important class of meteorites known as carbonaceous chondrites.

The large (~ 1cm long), pale, irregularly-shaped object on the left-hand side of the image is a calcium aluminium-rich inclusion (CAI) in the Allende carbonaceous chondrite. (image: Andy Tindle)

2. Meteorites contain grains that predate the Solar System

One of the most astonishing breakthroughs in the study of meteorites was the recognition that they contain mineral grains that originated in stars that predated the formation of our Solar System. Approximately 20 different types of so-called “presolar” grains are now recognised and these provide a unique record of the processes that take place in a wide range of different types of stars.

Presolar silicon carbide (SiC) grain isolated from a carbonaceous chondrite meteorite. Most presolar SiC grains are thought to have formed in asymptotic giant branching stars. (image: Larry Nittler)

3. Meteorites tell us how the Solar System formed

Meteorites contain evidence that certain short-lived radionuclides (41Ca, 36Cl, 53Mn, 26Al and 60Fe) were “live” in the early Solar System. The abundance of these isotopes means that they must have been synthesised just before the formation of the Solar System. The most likely explanation is that they were formed when a nearby massive star exploded to produce a supernova. The shock wave from this explosion would have initiated the gravitational collapse of the surrounding gas and dust to produce new stars, including our own.

4. Meteorites tell us about conditions in the early Solar System

The study of meteorites allows us to construct a detailed picture of how the Solar System evolved. Recent dating studies have demonstrated that small planets formed much more rapidly than was previously thought and from these evolved the larger rocky planets, including Earth. The results of these studies may have implications for the evolution of planetary systems around other stars.

5. Samples from Mars

While the NASA Curiosity rover is doing some fantastic science on Mars, one thing it cannot do is return a sample from the red planet for detailed study in the laboratory. However, thanks to a group of meteorites known as SNCs we already have a large number of samples from Mars. The total number of official Martian meteorites currently stands at 113 (source: Meteoritical Bulletin), of which 5 were witnessed “falls”. The most recent arrival from the red planet was the Tissint meteorite which fell in Morocco in 2011.

The NASA Curiosity rover is doing amazing things on Mars. Unfortunately, one thing it isn’t going to do is bring back samples. Fortunately, we already have samples from Mars in the form of Martian meteorites.

6. Samples from the Moon

In addition to the 382 kg of Moon rock returned by the NASA Apollo program and  0.32 kg returned by the Soviet Luna missions, we also have 165 official lunar meteorites with a combined mass of 63.6 kg (source: Meteoritical Bulletin). The Apollo and Luna samples were collected from a relatively restricted region of the Moon’s near side, whereas lunar meteorites are believed to be derived from much more diverse areas, including the Moon’s far side. While lunar meteorites are significantly less pristine than the samples returned by the Apollo and Luna missions, they do provide a very important complimentary suite of samples.

7. Samples from asteroids, including 4 Vesta

The majority of meteorites are believed to be fragments derived from asteroids that lie between the obits of Mars and Jupiter, in what is generally termed the Asteroid Belt. These meteorites are extremely varied in both their physical characteristics and chemical composition and are thought to be samples of at least 80 asteroids.  One asteroid from which we have a particularly large number of samples is 4 Vesta, with a total of 1158 officially recognised specimens of which 61 are witnessed falls. The group of meteorites that are believed to originate on Vesta are known as the HEDs (howardites, euctites and diogenites). One of the major scientific reasons for sending the NASA Dawn spacecraft to visit 4 Vesta was the wealth of information about the asteroid that had been obtained through the study of the HED meteorites.

Composite view of asteroid 4 Vesta, as imaged by the NASA Dawn spacecraft. The wealth of knowledge gained about Vesta through the study of the HED meteorites was a major factor in the decision to send a spacecraft to study this important asteroid, the second largest, by mass, in the asteroid belt.

8. Meteorites brought water and other volatiles to the Earth

The Earth formed in the inner part of the Solar System and would have been essentially “dry” until the very final stages of its growth. Water and other volatile components were delivered to the Earth by late-stage impacts of disrupted icy bodies from beyond the “snow line”. Thus, meteorite impacts were an essential prerequisite for the evolution of life on Earth. It has also been suggested that the complex organic molecules found in some meteorites and interplanetary dust particles provided the “prebiotic” ingredients for life.

9. Meteorites brought gold to the Earth!

The Earth is a “differentiated” body, consisting of an inner core of metal, surrounded by a rocky mantle and crust. During its formation the Earth grew by absorbing smaller asteroids and mini-planets. As this new material was incorporated, almost all of the “metal” loving elements in these small bodies, including gold and platinum, would have gone into the growing core. This should have left the overlying mantle essentially devoid of these elements. In fact the present day Earth’s mantle has higher concentrations of these elements than predicted by experiments. The relative abundance of such metal-loving elements in the mantle indicates that they were added at the end of Earth formation as a “late veneer”. Primitive meteorites are the most likely source of this veneer.  So, almost all the world’s available gold was transported here by meteorites. Of course there is plenty of gold in the core, you just need a very good shovel!

Almost all of the world’s available gold was transported to Earth by meteorites as a “late veneer”.

10. Meteorites are the best models we have for the bulk composition of the Earth

Unlike the Earth, a small group of meteorites known as CI chondrites have retained a primitive composition that is close to that of the Sun. This group contains only 9 known examples, but provides the best chemical reference material with which to test models for the formation and evolution of the Earth.

I could go on! But I think you will agree that meteorites provide a wealth of important information about the origin of our Solar System and even how life evolved on Earth.

Posted in General, Lunar Rocks, Meteorites | Leave a comment

Just Keeping it Fresh

Posted on November 19, 2012 by Richard Greenwood

(image: tvtropes.com)

It always makes for a great movie. The lonely scientist in his white coat, working alone in his lab late into the night. There he is, mixing crazy coloured chemicals as part of an insane quest to find some sort of secret elixir. Like Alec Guinness in The Man in the White Suit, or Spencer Tracy in Dr Jekyll and Mr Hyde success is pyrrhic at best. Yes, the lonely nutty professor remains a powerful cliché for how science works.

Of course reality is generally a little different. Splendid isolation is not usually a recipe for scientific success. Most researchers work as part of a well-organised group that meets regularly to share new results and discuss recent important developments. Our group here at the Open University is no exception. With the impressive title of: the Cosmochemistry Research Group (CRG), we meet up every Tuesday for a round-up of what everyone has been doing over the past week and to listen to a short seminar presentation by one of the group members. To be successful such meetings need to be properly organised. Over the past few years Dr Natalie Starkey has done an exceptional job of keeping things focussed and on track. She has recently handed on this role to Dr Romain Tartese. Longstanding readers of this blog will be pleased to know that cake remains an essential ingredient of CRG weekly meetings. As always, the chocolate brownies are particularly good.

Well, OK! this is not really a picture of a cake from one of our weekly CRG meetings, but one prepared by Margaret Tindle for the Moon Rocks ibook celebration. But you get the idea!

But while regular weekly meetings are a vital part of keeping research group members in touch with each other, it is also important to find out what’s going on in the wider scientific world. A vibrant programme of visiting speakers is an essential part of the mix. Thanks to the hard work and organisational skills of Dr Susanne Schwenzer we have been benefiting from a series of fascinating talks covering a wide range of planetary science topics. Take the last two weeks as an example. Dr Bill Bottke (Southwest Research Institute) presented a talk examining the Late Heavy Bombardment on the Earth and Moon and Professor Jamie Gilmour (University of Manchester) discussed aspects of the development and analytical application of the RELAX and RIMSKI noble gas mass spectrometers. We also had a talk about the potential for life on Mars given by NASA scientist Professor Everett Gibson, and Professor Alex Halliday (University of Oxford) gave a presentation on the delivery of volatiles to Earth both before and after the Giant Impact.

Good science communication involves using a wide range of new media. On his recent visit to the Open University Prof. Everett Gibson of NASA (left) takes a look at the new Moon Rocks ibook – the first ibook published by the Open University and now available as a free downlaod on the Apple Bookstore. In the picture with Everett are Prof. Peter Scott – Head of the Knowledge Media Institute (centre) and Dr. Andy Tindle (right), co-author of the new ibook. 

Then there are more organised scientific meetings. These range from large international gatherings, such as the annual Lunar and Planetary Science Conference in Houston, Texas, to smaller-scale more focussed events. A nice recent example of the latter was held in London on November 9th by the Royal Astronomical Society and highlighted how new lunar research was changing our views on the early evolution of the Solar System. An important theme of this meeting was the possibility that, as a result of impact processes, terrestrial samples may have been ejected from the early Earth and then landed on the Moon. Of course identifying terrestrial meteorites within the lunar regolith will be extremely challenging.

Apollo 12 astronaut Alan Bean holding a sample of lunar soil. The picture was taken by Commander Charles “Pete” Conrad, who can be seen in the reflection in Bean’s visor. One of the soil samples collected during the Apollo 12 mission contained a small meteorite sample called Bench Crater. It was the first meteorite to be discovered on a solar system body other than the Earth. The lunar soil may also contain meteorites from the Earth, although none have so far been identified.

So is that all there is to it? Lots of very enthusiastic scientists having lots of meetings! All a bit self-indulgent you might think. And of course it would be, if that was the end of the story. Happily it isn’t.

So let’s move on. It’s a cold damp Saturday afternoon in early November. A group of twelve enthusiastic members of the Derby and District Astronomical Society head down to the Open University for an afternoon visit. We kick things off with a talk about meteorites and early solar system evolution. It’s looking good, no one falls asleep. Then there’s a chance for people to handle some real meteorite specimens and later observe meteorites under the microscope.  We finish with a Cook’s tour of the various bits of kit we use to study the oxygen isotope composition of extraterrestrial materials (laser fluorination line, NanoSIMS). That’s all great. But the fun bit is done over coffee and biscuits. Here are a really enthusiastic group of people asking questions and showing real interest in the subject. They come at things from a different angle. They find it all fun (well most of it!) and ask challenging and thought-provoking questions. This sort of activity is given the jargon name “Outreach”, but that doesn’t really cover it. It’s all about dialogue and discussion.

One of the Derby astronomers told me that they now have more than double the number of people at their regular meetings as a result of the “Cox Effect”. Communication it seems is at the heart of keeping science fresh and relevant.

Members of the Derby and District Astronomical Society studying meteorites in thin section. (image: Graham Ensor)

 

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