Main Content

How do you squeeze a comet or an asteroid into an electron microscope?

Rhonda Stroud

by Rhonda Stroud

Head, Nanoscale Materials Section, US Naval Research Laboratory

Hint: you don’t need a bigger microscope; you just need returned samples. Of course, having a really good microscope helps too. This this month at the Naval Research Lab (NRL) we are making the final preparations to take delivery of PRISM (Picometer Resolution Imaging and Spectroscopy Microscope). PRISM, funded jointly by the NASA LARS program and NRL, will allow RIS4E team members to get a very detailed look samples from asteroids, comets, and the Moon. And by very detailed, I mean close enough to see individual atoms!

PRISM at the Nion factory in April 2014.

PRISM (above) is a scanning transmission electron microscope. From the outside, without its cover panels, PRISM looks like most any other advanced scientific instrument— all grey metal and dangling wires. The exciting part is what comes out after the samples go in. To make images, PRISM scans a focused beam of high energy electrons (at 40 to 200 kV, or about 1000× higher voltage than the outlets in your house) over thin samples (about 100,000× thinner than a human hair). As the electrons travel through the sample, they interact with the atoms inside. We will collect the electrons transmitted through the sample, and also X-rays emitted from the sample due to atom-electron interactions. From these signals, we will be able to see the atoms in the sample, and learn what elements they are.

The atomic-scale view of diamond nanoparticle (white arrows) surrounded by amorphous carbon from the Murchison meteorite. The area shown is ~ 10 nanometers across, or 1/10,000 of the thickness of sheet of paper. The white dots are individual impurity atoms. Vac indicates a hole or vacuum region of the image.

Above is an example image from a similar microscope at Oak Ridge National Laboratory that shows diamond particles that are about 10 atoms wide, called nanodimaonds, surrounded by amorphous carbon. The nanodiamonds and amorphous carbon come from the Murchison meteorite, but where they originally formed is still something of mystery. To help figure out if the nanodiamonds formed in ancient stars or in our own solar system,, we need to look for tell-tale signs of different growth modes or growth environments in the physical structure and elemental composition of the nanodiamonds and amorphous carbon. If we could find differences in impurities (e.g., N, He, Ne, Ar or Xe) between the individual nanodiamonds or carbon, then we would have a better idea of how and where they formed. But the atoms don’t always like to sit still for the picture taking. Impurity atoms on the surface of the sample can jump around, or diffuse away (movie).

One of the main questions the RIS4E team will be trying to answer with PRISM is how space weathering changes the structure and composition of materials on the surfaces of airless bodies, such as the Moon and asteroids. When astronomers measure the light reflected from asteroids, it appears “red” compared to the light reflected from meteorites. The “reddening” is thought to come from a combination of changes to the composition and structure, caused by space weathering, a complicated process that includes surface changes by the solar wind and galactic cosmic rays, and bombardment with micrometeorites. One of the classic signatures of space weathering is the formation of nanoscopic metallic iron particles. PRISM will allow us to the formation of the iron particles in the earliest stages.

Team members Drs. Rhonda Stroud and Brad De Gregorio will use PRISM to get the atomic-scale understanding of samples processed in the lab with well-known histories, and eventually all also returned asteroid and lunar samples. To connect this very detailed look to the “big picture”, i.e., the spectra from whole rocks or asteroids, other team members will be analyzing the same samples with more macroscopic methods. Stayed tuned in the coming months for updates with the first data from PRISM, and to read more about the happenings at other RIS4E participating sites.

Content Info/Footer