Neither imaging technique significantly damages or alters the meteorites, unlike other methods of analyzing the chemical composition of rocks, which require cutting thin slices of the meteorites. Although each imaging method has been used separately in the past, the team is among the first to use the two techniques simultaneously to create X-ray and neutron beam snapshots. In the pilot study, the team examined two meteorites whose metal and water content were already well known so they could assess the accuracy of the combined imaging methods. One of the rocks, named EET 87503, is a fragment from the surface of the large asteroid Vesta, but it also contains material from a different, water-rich variety of asteroid. Film of meteorite EET 87503 shows overlay of X-ray and neutron imaging. Purple and orange indicate two different classes of iron-rich minerals. Green indicates minerals that contain water in their structure. (Video courtesy of NIST). The other meteorite, GRA 06100, rich in iron and nickel, is classified as a chondrite—a rock that has not been altered by melting or other processes since the early days of the solar system. It also has a significant amount of hydrogen-bearing silicates formed from previous exposure to 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 were then combined to create a three-dimensional image, a technique known as tomography or computed tomography. Imaging methods accurately revealed the locations of metal-rich minerals, silicate minerals, water and other hydrogenated compounds in the two meteorites. Neutron imaging identified and characterized the chondrite grains within GRA 06100, which could then be extracted for further study. 3D imaging can test theories about how the water entered the rock and what path the liquid took to change the composition of the minerals and bind to the sample. Although water accounts for 70% of the earth’s surface, how exactly the substance arrived on our planet remains the subject of long-standing debate. Some planetary scientists suggest that meteorites and comets — frozen remnants from the icy, outer solar system — delivered the water, along with the protein building blocks necessary for life, after the formation of our planet’s core. Others suggest that the earth got its water during its formation 4.5 billion years ago from bits of gas and dust that wrapped around the infant sun and glowed together to form our planet. Water comes in two forms: ordinary water, which consists of hydrogen and oxygen, and heavy water, which consists of deuterium (hydrogen with a neutron added) and oxygen. One way to determine whether meteorites were the main source of terrestrial water is to compare the relative abundance of these two types in rocks with the relative abundance of water above and below the earth’s surface. Planetary scientists have measured the abundance in some meteorites, but they need to look at a larger number. Neutron and X-ray imaging can aid in these studies. By pinpointing the location of deposits of minerals, metals and water locked inside meteorites, the images could guide researchers on how best to cut sections of rock so they can measure those abundances as well as the composition of other compounds. Following this initial test, the team now plans to use the dual-imaging technique to study less familiar meteorites so that their water and mineral content can be mapped in detail for the first time.
