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 8^th January at 8pm on BBC2.

The Hosts of Stargazing Live Professor Brian Cox (left) and Dara O Brian (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.

Blog image credit: Julia Childs Kitchen at the Smithsonian Museum

Richard Greenwood