X-Rays and the Mantle

Before I get into this week’s article, I would like to explain the reason why I chose it. This year has been declared the International Year of Light and Light Based Technologies, (generally abbreviated IYL) by the United Nations General Assembly. In celebration and recognition of this, I wanted to do a post about light. But here is the complication: light is surprisingly complex. It is alternating waves of magnetic and electric fields. It’s a particle and a wave. That likely doesn’t mean much to most of my readers. Don’t worry about that: after studying physics for four years at a college level, it still boggles my mind.
Most people think of light as the sunbeams that brighten your house and make you feel warm, or as something that comes in a variety of different colors. But since light is alternating magnetic and electric fields, there is a lot more to light. The electromagnetic spectrum, as it is called, contains not just the little slice of light human eyes are sensitive to as colors and light (visible light), but also includes radio waves, microwaves, infrared, ultraviolet, x-rays and gamma rays. To me as a physicist, these are all forms of light that are unseen, but still light. That’s how I came to have a paper talking about x-ray based technology for a post in celebration of IYL.

EM_spectrum

The electromagnetic spectrum. As you can see, human eyes are only sensitive to a very small portion of it. Image taken from Principles of Physics, page 823.
This paper is a bit more technical than some, so I’d like to put in some definitions in here, so they are familiar before you start to read. You likely saw many of these in high school science classes.
  • Pascals, abbreviated Pa: a measure of pressure (force per area). At sea level, there are 101325 Pascals of pressure for every square meter (about 14.7 pounds per square inch), simply due to the weight of air in the atmosphere.
  • Giga: a prefix denoting 109. Written as G[standard unit], i.e. GPa for gigapascals.
  • Nano: another prefix, denoting 10-9, written as n[standard unit], i.e. nm for nanometers
  • Fe: iron, from the symbol on the periodic table
  • S: sulfur, from the symbol on the periodic table
  • Olivine: a mineral, usually green in color, and the primary mineral in the mantle. Chemically it is a mix of silicon, oxygen, and either magnesium or iron (chemically they can subsitute for each other in this case). As a gemstone it is known as peridot.
  • Synchotron: a device that uses magnetic fields and particle acceleration to produce light, which is then often used to act like a microscope on steroids, revealing much more detail in a structure. Knowing how they work is not needed for the understanding of this post, but this IYL page describes them decently.
  • CDI: coherent diffraction imaging, also known as coherent diffraction microscopy, is a lensless technique used to take images with high detail
  • STXM: scanning transmission x-ray microscope; a type of microscope for imaging using synchrotron x-rays
It is not needed to understand the mechanics of the last three points in order to understand the results of the paper, but I am not going to stop you from doing your own research.
I also think that keeping a mental image of the Earth’s interior is helpful when reading parts of this post or the original paper, so I give you this:

earths_interior_structure

A diagram of the Earth’s internal structure. Image taken from Earth: Portrait of a Planet, page 51.
There has been a lot of introduction, but we’re finally going to move into the article. It’s titled “Three-Dimensional Coherent X-Ray Diffraction Imaging of Molten Iron in Mantle Olivine at Nanoscale Resolution”. That is a long title that might sound confusing, but it is not too overwhemling if you take it apart one piece at a time. The author list is not any shorter: the authors are Huaidong Jiang Rui Xu, Chien-Chun Chen, Wenge Yang, Jiadong Fan, Xutang Tao, Changyong Song, Yoshiki Kohmura, Tiqiao Xiao, Yong Wang, Yingwei Fei, Tetsuya Ishikawa, Wendy L. Mao, and Jianwei Miao. If you want to read it, there is a PDF here (note: it will automatically download to your computer instead of displaying). The article appeared in Physical Review Letters in May 2013.
This study was looking at properties such as density, structure, formation and composition of the cores of terrestrial planets (these are planets like Earth or Mars, as opposed to ‘gas giant’ planets like Jupiter or Saturn). The formation of the core of planets is not entirely known, so this study investigated it a little further.
A sample comprised of 80% San Carlos olivine, combined with 20% Fe and 10% S by weight was used to simulate the Earth’s upper mantle. It was subjected to 6 GPa of pressure at 1800 degrees C (3272 degrees F), for one hour to simulate the conditions in this section of the Earth. It was then imaged using both CDI and STXM techniques. A combination of 27 images at a range of angles was used to compile a three dimensional view of the sample. This revealed that the Fe was mostly at the edges of the sample, with higher densities near the center. Though not uniform entirely, the change in density was more gradual than sudden. It was also noted that the molten Fe formed isolated spheres, and more irregular shapes of different sized.
You might be wondering why I did so much introduction for just a few paragraphs on the actual paper. First of all, the properties of the earth investigated in this paper have an impact on seismology, and trust me, earthquakes are important. Secondly, think about this: the diagram above of the interior of the earth earlier in this post represents what is below you right now. The earth, the environment, surrounds you at all times, whether you are aware of it or not. This has been a very small insight as to how the structure below you became what you live on today.

 

Bibliography:
These are sources directly used to write this post. More sources are listed in the article’s references section.
Jiang, Huaidong, et al. “Three-dimensional coherent X-ray diffraction imaging of molten iron in mantle olivine at nanoscale resolution.” Physical review letters 110.20 (2013): 205501.
The main paper for today.
Serway, Raymond A., and John W. Jewett. Principles of Physics: A Calculus-Based Text, Volume 1. 4 edition. Cengage Learning, 2005. Print.
My intro physics textbook, used for the image of the electromagnetic spectrum.
Marshak, Stephen. Earth: Portrait of a Planet. 3rd ed. W.W. Norton & Compant. Print.
My geology textbook, used for the diagram of the earth’s internal structure. Also used for resource about olivine.
Olivine: A Rock-Forming Mineral. Used as the Gemstone Peridot.” N.p., n.d. Web. 2 Feb. 2015.
A website used for some information about olivine.
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