PIXI: Micro-CT for Mars exploration
NASA is funding eXaminArt LLC under an SBIR Phase 2 grant to develop an in-situ high-resolution X-ray computed tomography (microCT) instrument to analyze geological samples on the surface of a planet or planetesimal. 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.
We demonstrated the concept feasibility during our SBIR Phase 1 study (2018) using a miniature breadboard instrument and computer simulations. Our quality 3D reconstructions of core samples compared well with data from commercial laboratory instruments. We are currently developing the second generation of PIXI for enhanced performance and relevance to future space missions.
PIXI (Planetary In-situ X-ray Imager) is a microCT instrument concept intended for deployment to Mars, cometary nuclei, or other planets/planetesimals. PIXI will provide in-situ 3D-reconstructions of rock fragments or rock cores, enabling the study of microstructures of potential biogenic origin. The instrument is based on a cone-beam geometry with a simple architecture combining a microfocused X-ray tube, an X-ray image sensor and a core scanning stage. PIXI can also provide X-ray fluorescence (XRF) data of the surface of the sample.
During SBIR Phase 1, we built a breadboard prototype by combining a microfocus X-ray tube, a custom sample rotation stage and a custom X-ray camera.
As part of our SBIR Phase 2 research, we are currently developing a new X-ray CMOS camera based on the Open Source Axiom Beta Camera developer’s kit. The sensor will be modified in-house with new scintillators. The method involves bonding a fiber optic plate (FOP) to the sensor and depositing a scintillator material on the surface of the FOP. High resolution images are obtained with this method, and the detectors show good linearity with the incoming X-ray signal.
We are also developing miniature bipolar X-ray sources to allow high energy X-rays to be produced with limited high-voltage. These new upgraded components will be combined in the second generation of PIXI in 2020.
Application 1: 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 , 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).
Examples of 3D reconstruction slices of ice cores. PIXI (left) vs lab microCT (right)
Ice cores with metallic inclusions
Ice cores with mineral grains
Application 2: 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
Example of Sandstone core
Application 3: 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 . On Earth, hot spring microfacies display predictable changes in population along thermal and chemical gradients .
An example of hotspring core analysis with PIXI is given here with a sample from Excelsior Geyser (Yellowstone).
Application 4: 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.
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)
 N. T. Vo, et al. “Superior techniques for eliminating ring artifacts in X-ray micro-tomography” Opt. Express 26, 28396-28412 (2018)
 Byrne, S. (2009) The polar deposits of Mars. Annual Review of Earth and Planetary Sciences, 37 doi:10.1146/annurev.earth.031208.100101.
 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
 Walter, M.R. and Des Marais, D.J., (1993) Icarus, 101(1), pp.129-143