Scientists dig inside mineral-rich meteorites for answers

Mineral-rich meteorite

According to the scientists, neutron imaging is ideal for searching for water and other hydrogen-bearing compounds because neutrons remove hydrogen so quickly. In contrast, X-ray imaging is better for detecting deposits of heavier elements such as iron and nickel, because the X-rays are primarily scattered by heavy atoms with lots of electrons.

Unlike other methods of analyzing the chemical composition of rocks, the imaging technique required to cut thin slices of meteorites does not significantly harm or alter the meteorites. Although each imaging method has been used separately in the past, the team is the first to use the two techniques simultaneously to create X-ray and neutron-beam snapshots.

in Initial study, the team examined two meteorites whose mineral and water contents were already known to assess the accuracy of the combined imaging methods. One of the rocks, called EET 87503, is a fragment from the surface of the large asteroid Vesta, but also contains material from a different, water-rich asteroid.

Movie overlay of X-ray and neutron imaging of meteorite EET 87503. Purple and orange indicate two different classes of iron-rich minerals; Green indicates minerals that contain water in their composition. (Video courtesy of NIST).

Another meteorite, GRA 06100, which is rich in iron and nickel, is classified as a chondrite — one that has not been altered by melting or other processes since the early days of the solar system. It also contains significant amounts of hydrogen-bearing silicates formed by past contact with water.

To create three-dimensional views of the meteorites, the researchers used X-rays and neutron beams to image cross sections of the rocks. Individual images of different cross sections are combined to create a 3D image, a technique known as tomography or CT scan.

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The imaging methods precisely revealed the locations of metal-rich minerals, silicate minerals, water and other hydrogen compounds in the two meteorites. Neutron imaging pinpointed chondrite grains within GRA 06100 that could be isolated for further study. 3D imaging can test theories about how water entered the rock and the path the fluid took to change the mineral composition and become bound in the sample.

Although 70 percent of the Earth’s surface is water, how this substance arrived on our planet remains a long-standing topic of debate. Some planetary scientists suggest that meteorites and comets — icy debris from the cold, outer solar system — delivered water along with the building blocks of proteins necessary for life after our planet’s core formed. Others suggest that Earth acquired water during its formation 4.5 billion years ago from gas and dust that pulled the infant Sun and coalesced together to form our planet.

Water comes in two forms: normal water, which contains hydrogen and oxygen, and heavy water, which contains deuterium (hydrogen with a neutron) and oxygen. One way to determine whether meteorites are the primary source of Earth’s water is to compare the relative abundance of these two types in rocks with the relative abundance of water on and below Earth’s surface. Planetary scientists have measured abundances in some meteorites, but many more need to be tested.

Neutron and X-ray imaging will aid these studies. Pinpointing the location of mineral, metal and water deposits locked inside meteorites, these images can guide researchers on how to better cut sections of rock so they can measure the abundance and composition of other compounds.

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Following this initial experiment, the team now plans to use its dual imaging technique to study unfamiliar meteorites so that their water and mineral content can be mapped in detail for the first time.

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