Different Directions - Moon & Meteorites

Late in the evening on June ninth, 1952, the population in the province of Alberta, Canada, reported a fireball crossing the sky from the Northwest.

Explosions were heard followed by concussions as the fireball proceeded through the atmosphere. Some reported that it even split into pieces....

A search was mounted, and five days later, Harry Buryn, while tending his new wheat field, found a hole 2 to 3 feet round, six feet deep, and inclined at an angle. At the bottom sat a large 107 kg blackened, fusion-crusted stone.

Here's the only picture I found of this mass. Note the flow lines, cracks, and orientation (right to left).

Like all meteorites, scientists named it after the town closest to the impact site, Abee.

The Analysis

Eugene Poitevin, Chief of the Mineral Division and Collection Curator of the Geological Survey of Canada (GSC), determined Abee to be an enstatite chondrite – a rare type of stone meteorite, and the largest found at that time.

Enstatite chondrites consist of a group of minerals dominated by the magnesium silicate mineral enstatite (MgSiO3), which represents less than 2% of all known meteorites.

Enstatites are divided into two classes: high-iron (EH) and low-iron (EL) (Keil, 1968).

Being an enstatite chondrite, Abee developed in an oxygen depleted area of the solar nebula. As a result, virtually all of its iron occurs either as metal (Kamacite – Fe, Ni) or as sulfide.

Also, because of the lack of oxygen during its formation, Abee contains some rare, oxygen depleted minerals:

Because Abee was formed in such an environment, in the place of oxygen was sulfur. In fact, if you cut open a piece of Abee, you smell the sulfur.

Birth Place

These rare minerals formed 4.49 billion years ago, but it was within an unknown oxygen-poor area of the solar nebula.

Well, as our solar nebula cooled, specific minerals formed relative to the pressure and temperature of the nebula. The cooling gas would condense nickel and iron before any silicates.

Also, in our disk-shaped nebula, the gas pressure would be highest at the center and would decrease outward to the edge.

In our early solar system, the orbit of Mercury inward would have been such a high energy environment.

Comets or other objects in eccentric orbits around the Sun accelerate as they approach due to the immense pull of the Sun's gravity. If Mercury crossed their paths, the impacts could be immense.

Such a cometary object impacting Mercury, prior to its complete development (differentiation), might have blasted pieces of both the outer rubble (chondritic) layer and its core.

Some impacts could have been so devastating as to blast the pieces into orbits within the asteroid belt.

As this hypothesis goes, it is perhaps these pieces that became asteroids, parent bodies, which when impacted by other asteroids, produced the meteorites that crossed Earth's path and come to us as Abee and the other Enstatite Chondrites.

This would explain the unique minerals found in these meteorites – that they were born in Mercury's high energy environment, close to the sun.

And as we can see here, asteroids get bombarded, and pieces are blasted off of their surfaces.

This hypothesis is further strengthened when scientists analyze the light reflected from asteroids – an analysis of color – and compare it to light reflected from meteorite specimens, such as Abee, found on Earth.

This has allowed scientists to determine that the most likely source for most meteorites is the Asteroid Belt between Jupiter and Mars.

Notice in the following graph how the patterns of reflectance from groups of meteorites and some asteroids are closely similar:

So, we have one possible solution to the mystery of Abee's birthplace. But what else is yet to be discovered?

A Closer Look

When you really look at Abee, and perhaps use a magnifying glass, it has several unique features.

What we see is an impact-melt breccia (see below) that is a high metal (EH) chondrite.

Its internal structure is a myriad of granulated metal-rimmed, varying-sized clasts embedded in a dark gray, fine-grained groundmass.

Scientists say that Abee's brecciated structure is a vivid representation of a violent and complex sequence of impacts -- large angular clasts of partly melted material with dark inclusions (oldhamite), all embedded within a previously melted, but similar, groundmass.

Impact Melting

In fact, Alan E. Rubin and Wdward R.D. Scott have written that Abee experienced two episodes of impact melting with an intervening period of brecciation.

Rubin and Scott also write that previously described diamonds in Abee containing typical solar-system compositions of N (Nitrogen) and Xe (Xenon) were probably produced from graphite by shock (impact).

Chondrules

Like all chondrites, Abee contains chondrules. But in Abee, about 80–90% of the chondrules were melted, and then many of those that remained were partly reabsorbed. So, you will only see a few indistinct chondrules.

Of the Abee chondrules, they are rich in free silica – with abundant free silica in some of its clasts – and abundant niningerite (See below).

Scientists report that this composition is consistent with the enstatite sulfurization reaction.

Side Trip -- Mercury

Mercury was named by the Romans after the fleet-footed messenger of the gods because it seemed to move more quickly than any other planet.

It is the closest planet to the Sun, and second smallest planet in the solar system. Its diameter is 40% smaller than Earth and 40% larger than the Moon.

Backside of Mecury
Figure 13. This photograph of Mercury's unseen side reveal a dramatic system of globe-straddling rays.

Taken on October 6, 2008, when NASA'sMESSENGER spacecraft flew past Mercury.
If we stood on Mercury, the Sun would be two and a half times larger than on Earth; however, we would also notice that the sky is black because Mercury has virtually no atmosphere to cause scattering of light.

We also might see two bright stars. One appearing as cream colored Venus and the other as blue colored Earth.

Mercury has been now been visited by two spacecraft, Mariner 10 and MESSENGER.
Mariner 10 flew by three times in 1974 and 1975. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safely imaged by HST).

MESSENGER was launched by NASA in 2004 and will orbit Mercury starting in 2011 after several flybys. Its first flyby in Jan 2008 provided new high quality images of some of the terrain not seen by Mariner 10.

Unlike Earth, Mercury has only a relatively thin silicate mantle and crust. Inside,however, there is a large iron core. Part of the core is probably molten, but Mercury's weak magnetic field is only one percent of the strength of Earth's.

Core Comparison - Mecury & Earth

Figure 14.

Some astronomers suggest that Mercury lost much of its lighter rocks to the Sun in early catastrophic impacts.

Moreover, iron has not been detected on the surface, and so given its presumably large iron core, Mercury may be much more thoroughly differentiated than the other terrestrial planets.