Combining neutron and X-ray imaging, scientists study meteorites to explore how Earth got its water

Every year, hundreds of meteorites — rocky bodies left over from the formation of the solar system — bombard Earth, carrying metals, minerals and water to our planet. Analysis of the cracks and metal-rich deposits inside meteorites not only reveals the early history of planet formation, but may provide clues about how the young Earth acquired water and other ingredients necessary for life.

Now researchers at the National Institute of Standards and Technology (NIST) have combined two complementary techniques—X-ray imaging and neutron imaging—to look inside these rocky remains.

Neutron imaging is ideal for searching for water and other hydrogen-bearing compounds because neutrons easily remove hydrogen. In contrast, X-ray imaging is best for finding deposits of heavy elements, such as iron and nickel, because X-rays are scattered mainly by the large number of electrons in heavy atoms.

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 their pilot study, the scientists examined two meteorites whose mineral 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.

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 3D views of the meteorites, NIST researchers Jacob LaManna and Daniel Hussey, along with colleagues from the Lunar and Planetary Institute in Houston, Oak Ridge National Laboratory in Tennessee, and the University of Chicago, used X-ray and neutron beams to illustration of rock sections. Individual images of different cross-sections were then combined to create a three-dimensional image, a technique known as tomography. (Doctors commonly use CT scans, more commonly known as CT scans, to image the human body.)

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. The 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, Hussey said.

Although water represents 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 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 .

The NIST team used the NIST Center for Neutron Research, one of only three research neutron beam sources in the US. The researchers reported their study in the October issue of Meteorological and Planetary Science.

The team now plans to use the dual-imaging technique to study less familiar meteorites so that their water and metal content can be mapped in detail for the first time, LaManna said.