NASA is digging ever more deeply into understanding the makeup of the surfaces of Mars and our Moon. A lot can be learned by sending instruments to land on these places, but vastly more can be discovered by bringing back samples for analysis here on Earth thanks to increasingly capable laboratory instruments. The only difficulty with that is, “every gram back from Mars will cost millions of dollars to transport,” said James B. Garvin, Chief Scientist at NASA's Goddard Space Flight Center. With that in mind, Dr. Garvin and colleagues have proposed “triaging” the samples in place to decide in advance which would be most valuable to transport back to Earth. Recently, Garvin turned to NASA engineers Justin S. Jones and Ryan Kent to investigate the feasibility of achieving instrumentation to do that on the surfaces of the Moon and Mars. Their solution is to use x-ray Computed Tomography (xCT) to obtain 3D images of the interiors of potential samples as a first step in “high-grading.”

Will it Work?

They first needed to demonstrate that xCT could indeed obtain x-ray images of Mars-like or lunar rocks. To investigate whether it could work, Jones used an industrial CT to evaluate volcanic rocks from a newly formed island in the South Pacific as well as other specimens, such as two lunar brecciated meteorites that fell in Northwest Africa. “Even with samples encased in protective glass and metal cases filled with nitrogen, the scanner revealed previously undetected minerals and 3-D arrangements at extremely fine scales,” said Garvin.

Figure 1. Lunar meteorite ready for xCT scan.

Industrial vs Medical Scanners

CT scanners used for imaging people and for imaging objects are conceptually the same: in both, you're utilizing x-ray beam projections from multiple angles around the sample. The main difference is, in the medical side, the CT technique is what's called a helical CT scan — you're taking a line scan of x-rays through a helical sweep around the body, which is stationary. With an industrial CT, the only moving part is the sample, which rotates; the x-ray source and the detector are stationary. Rather than a helical line scan, the industrial CT uses a planar, full screen digital detector to capture a two-dimensional radiograph from different angles as the sample rotates. The rotation stage is incremented perhaps a tenth or a quarter of a degree, another image is captured, and so on until you get all the way around. The raw data is a series of two-dimensional radiographs that are used to assemble a 3D image through a process known as filtered back-projection reconstruction.

Figure 2. 3D xCT view of the lunar sample showing the bright, highly x-ray absorbing mineral, believed to be barite (barium sulfate) or possibly Fe-based. (Sample borrowed from the Dr. Jay Piatek Meteorite Collection)

That wasn't always the case. According to Jones, CT computers have significantly evolved, partly due to the gaming community. The key development was improved graphics processing units (GPU) that can process multiple reconstruction paths simultaneously. This means it is no longer necessary to capture industrial CT data using incremental slice scans, which was very time consuming.

Extraterrestrial CT Scanning

So, getting a CT scanner to Mars, or even to the lunar surface, or the International Space Station, would be a good way to determine the most valuable samples to transport. There's just one difficulty with this plan: today's CT scanners are typically large, high-power devices that generally require human assistance for sample setup. To send one into space, it would have to be drastically down-sized — a daunting challenge. “There are some hurdles we would have to encounter, the power source is the big one,” said Jones. But Garvin and Jones think it can be done and are working with experts within and outside of NASA to find workarounds for these hurdles. “And if so, what a tool for future astronauts and robotic rovers to use to select the most compelling samples,” said Garvin.

Figure 3. Lava sample.

“Once the CT is in place, picking up the sample and rotating it is achievable,” said Garvin. He explained that “right now, the sample handling system on the Mars Curiosity Rover has all kinds of motions and spinning cells and 'shake and bake’ things to do what it does, so, going the next step: putting marble-sized samples of Mars or the moon or an asteroid for example, into one of these miniaturized X-ray computed tomography devices on another planet is not inconceivable.” All that has to be done is put a little piece of rock into a chamber on a rover spacecraft like Curiosity, align, and rotate it.

The Advantages of XCT

As you explore the surface of an extraterrestrial body, it's hard to decide just by the look of things what might be useful to examine further. The trouble is knowing, “is this a useful sample, this gray grungy thing from the Moon or Mars — most rocks kind of look gray and grungy,” said Garvin. So, it would be a great help to have an advance idea of what the sample might have to offer internally.

Figure 4. 3D/volumetric view of the lava sample. The bulk of the rock has been shaded as translucent. The bright inclusions within this volume are a highly x-ray absorbing phase of material within the rock. The plane intersecting this volume is a “cutting plane” (2D slice). It shows what the rock would look like if you were to cut it open along that plane.

There are other ways to examine the interior of a sample, so why CT? One option could be to cut open a small sample of a specimen. The trouble with that is that since the sample is just a small part of a larger object, it is impossible to use it to accurately judge the structure of the whole. In one sample that the team recently studied, “The state of the art published data, gleaned through destructive physical cross-sections, says certain features are 100 microns in scale (0.1 mm). Then, when you look at the scale of the whole rock, all of a sudden there are ones that are 100 times that — that changes the interpretation,” Garvin said. Even with samples that have been brought to earth, a CT could be used to precisely locate features you want to study so that you know in advance where to slice.

What Could We Learn from a 3D X-Ray?

The X-Ray CT is like a three-dimensional picture that can be enhanced by adding pseudo-colors and analyzed with the help of 3D rendering software. “It's not telling us: this is iron, or this is manganese, but it will tell you: this element, or this grain, or this structure in the rock is a higher density than the neighboring grain,” said Jones. It shows relative densities by virtue of the fact that x-rays are absorbed as a function of the atomic number in the periodic table — the higher the atomic number, the higher the absorption. “The chemistry of the way the rock forms, produces 3D signatures that are definitive,” added Garvin.

The XCT — Engineering Tool in Space

There are other uses for an extraterrestrial CT: inspection and maintenance of equipment. For example, there is now a 3D printer on the International Space Station and there is talk of employing one in a future human habitat on the Moon to create tools and instruments. Computed Tomography is the only practical method for inspecting parts created with 3D printers, due to their inherent complexity. If a part fails, the CT can be used to analyze the fault. It gives you a virtual three-dimensional view, including any pores, inclusions, cracks, or crack-initiators. Especially in a habitat on the moon, resources are very limited so it will be valuable to know whether it is safe to reuse a component from a failed mechanism.

What is the Future for 3D CT Scanning in Space?

“An exciting step would be to put one of these things into the next generation crew-vehicle we're hoping to send to cislunar orbit as part of the president's new space policy and have samples come to it, but not back to Earth,” Garvin said. He explained that robots and people could go to the Moon and bring the samples back to this orbiting gateway. We would then see a three-dimensional image and triage for the most interesting samples for return to Earth. “This would save the expense of bringing home samples that have mass and need care and feeding to do it right — a big step when you bring something home to earth, particularly if it's with a crew, as we did so well with Apollo,” said Garvin. “xCT and other innovations (e.g. laser confocal microscopes) that permit detailed studies of the interiors or surfaces of samples in 3D, at micrometer scales, could revolutionize planetary sciences in the upcoming decades, especially for the Moon and Mars.”

This article was written by Ed Brown, Associate Editor of Photonics & Imaging Technology. For more information, Click Here.