PIXI
MICRO-CT FOR MARS EXPLORATION
eXaminArt LLC was funded under an NASA SBIR Phase II grant to demonstrate a concept of an in-situ high-resolution X-ray computed tomography (microCT) instrument for analysis of geological materials in future planetary exploration. The instrument will double as an X-ray spectrometer to map the chemical composition of the surface of the rock core when its surface is not shielded by an opaque container. The instrument will rely on a coring drill to collect and deliver the core sample. Small rock samples can also be analyzed.
Instrument concept
PIXI (Planetary In-situ X-ray Imager) is a microCT instrument concept intended for deployment to Mars, cometary nuclei, or other planets/planetesimals.
The principle of the instrument is illustrated in Figure 1. A microfocused X-ray tube illuminates a sample, and an X-ray sensitive camera collects a radiography (projection absorption image) of the sample. The sample is rotated
precisely and an image is collected at each angular position. The series of projection images is used to reconstruct a 3D map of X-ray attenuation, which allows separation of mineral phases, void, and the study internal microstructure of the sample in 3D, non destructively. The technology proposed can also be used without 3D reconstruction, for radiographic analysis of sample in a single or a few angular positions.
PIXI Applications
Ice core scanning at the Martian poles
One of the key possible applications of microCT on Mars is the analysis of the North Polar Layered Deposits (NPLD), a multi-kilometer thick sequence of dusty-ice layers thought to record previous climatic conditions much like Earth’s ice sheets record terrestrial climate fluctuations in their stratigraphy. Deciphering this polar record is a major goal of Mars research[2], and X-ray microCT is the means to unlocking the information stored in the ice. A micro-CT instrument would be coupled to a coring system collecting 0.5-1 m long sample cores (2.5 cm in diameter), captured within an X-ray transparent tube. As this tube is withdrawn from the surface, the miniaturized microCT rotate about it to fully characterize the core in three dimensions. A 1 m core will detail approximately 1000 martian years of climatic history. This deployment concept was tested in collaboration with the SETI Institute and Honeybee Robotics. The images bellow show comparisons of 3D reconstructions of ice-cores with inclusions, comparing the PIXI breadboard (left) and a laboratory microCT instrument (right).
Ice cores with metallic inclusions
Ice cores with mineral grains
Figure 2. Examples of 3D reconstruction slices of ice cores. PIXI (left) vs lab microCT (right).
Analysis of rock core samples
In-situ analysis of cores collected from rocks on the planetary surface is another key potential application of microCT. In-situ microCT will reveal the grain size and organization of mineral phases and the rock porosity distribution. To demonstrate this application, a series of 8 mm diameter rock cores were prepared by Honeybee Robotics and analyzed in the breadboard PIXI. High quality data were obtained with a resolution better than 40 μm. Mineral phases are well contrasted in the 3D reconstructions, despite the limited kV settings of the X-ray source.
The PIXI breadboard shows a resolution of <40 µm. ~25 µm is expected with the next generation.
Example of Saddleback basalt core
Figure 3. Example of Saddleback basalt core. a) 8mm diameter core. b) microCT images. Left: PIXI breadboard. Right: commercial instrument.
Example of Sandstone core
Figure 4. Example of sandstone core. a) 8mm diameter core. b) microCT images. Left: PIXI breadboard. Right: commercial instrument. c) 3D rendering.
Search for signs of ancient life in hot spring deposits
Martian hot spring deposits are prime targets for astrobiological exploration because of the likelihood that life on Earth developed in hydrothermal environments, their ease of detection from orbit, and their high habitability and preservation potential[3]. On Earth, hot spring microfacies display predictable changes in population along thermal and chemical gradients[4].
An example of hotspring core analysis with PIXI is given here with a sample from Excelsior Geyser (Yellowstone).
Figure 5. Excelsior Geyser (Yellowstone) hotspring core analysis with PIXI. a) Geyser core. b) 3D PIXI rendering showing core slides number. c) PIXI XCT image of slides.
Looking inside the Mars-2020 coring tubes
MicroCT is demanding in terms of downlink bandwidth, mechanical precision and analysis time. Simple radiographic images can be very informative on the internal structure of rock cores. We used the breadboard to image the content of titanium tubes similar in design to the Mars 2020 coring tubes. X-ray attenuation images were collected with several types of rock cores, with a steel screw for reference. A PIXI instrument dedicated to this type of core radiographic analysis would not require high mechanical precision nor high bandwidth.
Figure 6. a) Titanium canister and cores. b) X-ray attenuation images of cores with a steel screw for reference.
Primary partners in this research
- NASA Ames Research Center (USA, CA)
- SETI Institute (USA, CA)
- Honeybee Robotics Spacecraft Mechanisms Corporation (USA, CA)
- Baja Technology LLC (USA, AZ)
- Battel Engineering (USA, AZ)
- Diamond Light Source (UK)
- RÖNTGEN-TECHNIK DR. WARRIKHOFF KG (Germany)
- Scintacor (UK)
- Apertus (Austria)
References
[1] N. T. Vo, et al. “Superior techniques for eliminating ring artifacts in X-ray micro-tomography” Opt. Express 26, 28396-28412 (2018)
[2] Byrne, S. (2009) The polar deposits of Mars. Annual Review of Earth and Planetary Sciences, 37 doi:10.1146/annurev.earth.031208.100101.
[3] Cady, S.L., Skok, J.R., Gulick, V.G., Berger, J.A. and Hinman, N.W., (2018) From Habitability to Life on Mars (pp. 179-210), Elsevier
[4] Walter, M.R. and Des Marais, D.J., (1993) Icarus, 101(1), pp.129-143