Different Directions

Different Directions

The Golden One (Melrose a)

Melrose is like a movie story: “Farm hand hits Hollywood -- Dad's a big star...”

It's always interesting to report the stories of meteorites, and this one is – in one respect – somewhat typical: A farmer near Melrose, NM, kept bumping into a buried rock for years, each season as he plowed his fields, so eventually he dug it up, and eventually began using it as a weight on his hay rake.

 

Horse drawn plow

Figure 1.

Harvey H. Nininger, one of the great American meteoriticists, purchased the stone for a dollar after running an ad requesting meteorites in a local paper.

That was where typical stopped and unusual began...

When Melrose was analyzed, it was more than an ordinary rock from space. Yes, it had small inclusions of metal (troilite) and a dark brown matrix tinged with green – but it also contained gold!

Melrose a, slice

Figure 2. Melrose, 30 gram slice.

 

Edge view of Melrose

Figure 3. Melrose, crusted edge.

In this image, you can see the Earth weathering along the right side.

 

Because of this finding, Nininger had it further analyzed for confirmation, and the results of two duplicate tests, done in separate locations, underscored that Melrose was an “ore grade” meteorite (called a chondrite. See Specs.). And this was something never seen before.

So, where did that lead science? Where did the gold come from? Was there an ore-grade asteroid out is space? Is there a rich reserve of gold in the solar nebula?

Well, this leads to two comments:

  1. Here's another example of meteorites educating us.
  2. While gold is rare in the crust of the Earth, it is not from the Earth.

All the gold on this planet was created before the formation of our Sun.

Once again, a meteorite pointed in a new direction, and that direction would not become clear to science or to us until 1956 and the publication of “Synthesis of the Elements in Stars.”

And, to the topic of Nucleosynthesis....

Side Trip – Nucleosynthesis

(Excerpted from Russell Kempton, his 1998 article Melrose (a), No “Ordinary” Chondrite, published in Meteorite!, Pallasite Press, February 1998, Vol. 4, No. 1.)

Somewhere around 15 billion years ago, the universe was a vast, expanding cloud of hydrogen.

Expanding cloud of hydrogen.

Figure 4.

With a single proton and electron, the hydrogen atom is the simplest atom that can exist. If you take away either of the parts, you no longer have an atom at all.
Hydrogen atom, the simplest atom.

Figure 5.

Within the cloud, gravitational “ripples” arose, clumps of hydrogen started to collapse under their own force of gravity forming spherical globes.

Clumps of hydrogen start to collapse.
Figure 6. Color composite image of RCW120 reveals how an expanding bubble of ionized gas is causing the surrounding material to collapse into dense clumps. And this is where new stars form.

The collapse caused compression, producing the enormous amounts of heat needed for nuclear fusing – producing the very first stars.

Deep in the interiors of these first generation stars, protons and electrons collided forming deuterons, the nuclei of hydrogen isotopes. They, in turn, collided forming the element helium.

Like a first generation star.

Figure 7.

As the stars aged and their cores heated up, a successive stream of heavier elements were synthesized: carbon, nitrogen, oxygen, neon, and others all the way up to iron (1 to 26).

Periodic Table - FE.

Figure 8.

 

But Gold is not synthesized. It takes a different type of star to move past Iron.

Star structures.

Figure 9.

The largest members of this first generation of stars burned so furiously, consuming their hydrogen nuclear fuel, that they burned themselves out in around only 20 million years, finally exploding as supernovae, seeding the interstellar medium with their mix of heavier elements.

Stellar mass being ejected.

Figure 10. Eta Carinae - stellar mass being ejected.

Nature recycles everything, and the second generation of stars incorporated this mixture of elements during formation. The fusion process and life cycle of these now, lightly, metal-enriched stars is a little different.

The structure of a massive star.
Figure 11. The structure of a massive star with an iron-rich core, silicon, oxygen,  and carbon burning shells surrounding the core. There is no fusion reaction involving iron that produces energy (it has the strongest binding energy in its nucleus of any element, thus there is no way to add protons and get energy out).

At the end of their life, a massive star of this generation is layered similar to an onion, beginning with an outer layer of hydrogen encasing successively thinner layers of helium, carbon, oxygen, on to silicon, along with trace proportions of other elements.

At the center is an iron core of growing mass, as silicon burning in the layer immediately outside it creates additional iron. The core's density, at this point, is somewhere around 3000 to 12,000 tons per cubic centimeter and the pull of gravity is immense.

Unfortunately, Iron does not "burn" to create even heavier elements. So, the dynamic balance between the outward-directed pressure caused by heat release and the inward-directed gravitational pressure is lost....

And, gravity wins – the inner core implodes in less than a second, collapsing to an incredibly dense solid sphere generating temperatures of over a billion degrees Kelvin. The still-collapsing portion of the outer core collides with this surface and immediately rebounds outward at high speed.

The steps of star collapse.

Figure 12. Within a massive, evolved star here are the steps of its collapse:

a The onion-layered shells of elements undergo fusion, forming an iron core
b
The iron core eventually starts to collapse. The inner part of the core is compressed into neutrons
c This causes infalling material to bounce
d This forms an outward-propagating shock front (red). The shock starts to stall
e But it is re-invigorated by a process that may include neutrino interaction. The surrounding material is blasted away
f What's left is only a degenerate remnant.


The shock wave generated in this collision does two things:
  1. It rips the star apart not only at the physical level,
  2. But also at the atomic level – Stripping the electrons out of their orbits (which requires massive amounts of energy) and creates high energy radiation and particles called neutrinos.

As the shock wave moves up through the layers, gases fuse forming more elements, all the way up to uranium.

Periodic Table.
Figure 13.

Finally, this supernova explodes, adding its metal-rich mix of elements to the interstellar medium as seeds for the construction of new solar systems!

Pictures of Supernova remnants.
Figure 14.

And Gold is sprayed among the stars to be added to the next mix of planets, planetoids, and, of course, asteroids and meteorites.

Melrose (a)

At around 4.5 billions of age, our Sun is a metal-rich, third generation star and, like Melrose (a), it is the current link in a very long chain. Trace levels of gold have been found in other stone meteorites but at much lower abundances ( Orgeuil – 0.145 parts per million, Bayard – 0.180 ppm ) than those reported by Nininger.

While Melrose (a) is classified as an “ordinary” chondrite (See Terms below), the high abundance of gold measured by Nininger indicates an uneven mixing of this element within the solar nebula and therein may lie its contribution to meteoritics – that our nebula is not homogeneous.

 

Surprises lead to knowledge and, if nothing else, an ore-grade meteorite here on Earth means there's an ore-grade asteroid out there!

Side Trip – Nebular Hypothesis


Here's an illustration that shows the nebular hypothesis. It does not, however, show the first and second generation stars that came before ours. It starts with the early formation of our solar system, seeded with material from earlier stars. When we learn about other meteorites, this view will evolve:

Nebular Hypothesis.
Figure 15.

The Nebular hypothesis
A. solar nebula
B. contraction into rotating disk
C. Cooling causing condensing into tiny (dust sized) solid particles
D. Collisions between these form larger bodies
E. These accrete to form planets

Side Trip Hydrogen Burning


The process a star takes to create new elements may take several forms, but commonly involves the fusion of light atomic nuclei to form heavier nuclei.

As a by-product of these reactions, energy is released (the life-sustaining heat and light we receive from the Sun), and various other particles are produced (gamma rays, electrons, neutrons, and even hydrogen or helium nuclei).

Hydrogen (H) may burn to produce helium (He, with two protons) but only at temperatures that exceed 10 million K (See Figure 15).

Helium burning can produce carbon (C, atomic weight 12), which can combine with other helium nuclei to produce oxygen (O, atomic weight 16).

Similarly these burned products, sometimes called "ashes," may react during carbon burning to produce oxygen, neon (Ne), sodium (Na), and magnesium (Mg); during neon burning to produce oxygen and magnesium; during oxygen burning, to produce the element magnesium through sulfur (S); and during silicon (Si) burning to produce elements up to iron (Fe, atomic weight 56).

Notice that this is the very process discussed in second generation stars and shown above.

This figure shows how Hydrogen is burned.

It's the proton-proton chain, the important energy-producing nuclear reaction that takes place inside stars like our Sun.

In this process two hydrogen nuclei (protons in red) combine to form a deuterium nucleus (D, hydrogen with an atomic weight of 2--red and gray) and a positron (electron with a positive charge--white).

Neutrons are gray; protons are red. Reaction with an additional hydrogen nucleus (red) produces a helium nucleus with a mass of three (3He).

The fusion of two such nuclei results in the production of stable helium (4He, which has two protons and two neutrons) and the ejection of two hydrogen nuclei (red), which can be consumed in other reactions of this sort.

Energy is released by these reactions as a small amount of matter is converted into energy in accordance with Einstein's equation,

E (energy) = m (mass converted to energy) c2 (the speed of light squared).

Fusion in the sun.
Figure 16.

Just The Specs


Ordinary Chondrite (L5)
Curry County, New Mexico
Found: Sometime before 1934
Stone. Olivine-hypersthene chondrite

Here's a link to The Meteoritical Society database:

http://tin.er.usgs.gov/meteor/metbull.php?sea=melrose+%28a%29&sfor=names&ants=&falls=&valids=&stype=contains&lrec=50&map=ge&browse=&country=All&srt=name&categ=All&mblist=All&rect=&phot=&snew=0&pnt=Normal%20table&code=15475

Terms

As we have seen with other meteorites, Melrose is a beautiful chocolate-colored chondrite. Here's some of the terms used to describe it.

Definition of Chondrite

  1. n. A meteoric stone characterized by the presence of chondrules.

Axtell Chondrite.

Axtel from http://www.meteorlab.com/METEORLAB2001dev/offering21o3a.htm

 

Definition of Chondrule

  1. n. A peculiar rounded granule of some mineral found embedded more or less abundantly in the mass of many meteoric stones, which are hence called chondrites.

A glass chondrule.

Photo from Wikipedia commons. http://commons.wikimedia.org/wiki/File:Glassy_chondrule.jpg


Definition of Gold

  1. n. A soft yellow malleable metallic element; occurs mainly as nuggets in rocks and alluvial deposits....

Exact synonyms: Atomic Number 79, Au


Gold Nuggets.

Photo from Wikimedia Commons

Definition of Inclusion

  1. n. Any small fragment of something found within another body.

Here's a picture of an inclusion:

Inclusion - CAIs

Photo from New England Meteoritical Services


Definition of Olivine-Hypersthene

  1. (ahl' a veen - hi' pers theen) - The name given to L class ordinary chondrites as they are composed of olivine and hypersthene, a pyroxene containing 22-30% FeSiO3.

Olivine.

Photo of Olivine from http://www.gc.maricopa.edu/earthsci/imagearchive/o,p,q,r.htm


  1. Hypersthene is a relatively common mineral and is found in igneous and some metamorphic rocks as well as in stony and iron meteorites. It forms a solid solution series with the minerals enstatite and ferrosilite.


Hypersthene.

Photo from http://www.galleries.com/minerals/silicate/hypersth/hypersth.htm

 

Definition of Troilite

  1. n. Native iron protosulphide, FeS. It is known only in meteorites, and is usually in embedded nodular masses of a bronze color.

Here's a picture of a troilite inclusion:

 

Troilite inclusion.

Photo from http://www.lexic.us

Links

Here's the link to the New England Meteoritical Services, directed by Russell Kempton. If looking for special meteorites or analog material, contact Dir. Kempton:

http://www.meteorlab.com

Harvey H. Nininger:

http://en.wikipedia.org/wiki/Harvey_H._Nininger/

Here's a key site that discusses the Fundamentals of Planetary Science in a clear and concise manner. Definitely worth reading two or three times:

http://explanet.info/Chapter02.htm

 

Here's a link to howstuffworks, and an article by Craig Freudenrich on “How Stars Work.”

http://science.howstuffworks.com/star.htm

 

Here's a great internet “rock shop.”

http://www.galleries.com/

 

Here's a good reference dictionary that also includes excellent pictures:

http://www.lexic.us/

 

Here's an article about "Differences of Terrestrial Alteration Effects in Ordinary Chondrites:"

http://adsabs.harvard.edu/abs/1992Metic..27..314S

 

Videos

The Synthesis of Molecules in the Universe

How Stars are Born and Die

http://videos.howstuffworks.com/nasa/3560-how-stars-are-born-and-die-video.htm

Meteorite Hunting

http://videos.howstuffworks.com/wgbh-nova/2348-meteorite-hunting-video.htm

Dark Matter and Bullet Clusters

http://videos.howstuffworks.com/wgbh-nova/13630-dark-matter-and-bullet-clusters-video.htm

Shape of the Universe

http://videos.howstuffworks.com/discovery/4864-shape-of-the-universe-video.htm

 

Figures & Acknowledgments

 

Figures

Figure 1. Photo from www.speedofcreativity.org

For a History of the Plow, go here:

inventors.about.com

Figure 2. Photo by CW Collinson. Melrose, 30 gram slice.

Figure 3. Photo by CW Collinson. Melrose, crusted edge.

Figure 4. Hubblesite.org

Figure 5. Image from www.kwugirl.com

Figure 6. Photo from www.sciencedaily.com Credit: ESO/APEX/DSS2/SuperCosmos)

Figure 7. Image from NASA

Figure 8. Table from www.astro.virginia.edu

Figure 9. From explanet.info

Credit: E. Chaisson and S. McMillan, Astronomy Today, Prentice Hall)

ircamera.as.arizona.edu/NatSci102/lectures/supernovae.htm

Figure 10. Image from www.physics.tcd.ie

Credit: Jon Morse (University of Colorado) & NASA

Figure 11. Fromircamera.as.arizona.edu

Figure 12. From commons.wikimedia.org

Figure 13. Table from www.astro.virginia.edu

Figure 14. From lithops.as.arizona.edu

Figure 15. Credit: oz.plymouth.edu

Figure 16. Credit: commons.wikimedia.org

Acknowledgments

Kempton, Russell. "Melrose (a), No “Ordinary” Chondrite". Meteorite! 4.1 (February 1998): 26-27.

From Russell Kempton: Sincere thanks to William Metropolis of Harvard University for discussions in gold and mineralogy.





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