Micro computed tomography for in situ analysis of subsurface structure

We present the development of an X-ray micro computed tomography (microCT) instrument for in-situ analysis of ice or rock cores on planetary bodies. Based on X-ray attenuation, microCT provides three-dimensional visualization and characterization of internal features of multiphase materials with spatial resolution down to several microns that would otherwise only be revealed by physically sectioning the material. It has been used to analyze micron-scale porosity and particle distribution in terrestrial ice sheets. We are developing a planetary instrument concept for in situ microCT analysis for use on the North Polar Layered Deposits (NPLD) of Mars. The NPLD are a multi-kilometer thick sequence of dusty-ice layers thought to record previous climate conditions much like Earth's ice sheets. Deciphering this polar record is a major goal of Mars research. Our objective is to develop a sampling and analysis system to characterize the porosity and distribution of dust in the Mars NPLD with higher resolution and to greater depths than is possible with current techniques. Micro In Situ Tomography (MIST) is a coupled coring and microCT-analysis system. A coring drill mounted on a lander or rover produces a sample core 2.5 cm in diameter and 0.5 - 1 m in length and captures it within an X-ray transparent carbon fiber tube. As this tube is withdrawn from the surface, a miniaturized microCT rotates around it collecting X-ray attenuation images. The microCT subsystem is based on a conebeam geometry with a simple architecture combining a microfocused X-ray tube, a core scanning stage, and an X-ray image sensor. MIST will enable 3D reconstructions of ice cores to evaluate internal, i.e. subsurface, microstructure. A 1 m core of the Mars NPLD sampled by MIST would provide information about approximately 1000 Martian years of climate history. We expect to be able to detect layering and porosity related to climate cycles and deposition processes. A breadboard prototype has been built and tested on ice and ice simulants. A dedicated calibration method was developed to compute instrument geometry parameters that couldn't be measured with high precision on the breadboard. 3D reconstruction is done using a Filtered Back-Projection method. Results have compared favorably to those from a commercial instrument. We have developed software for reconstruction of a mis-aligned cone-beam CT system, likely a necessary component of a tomography system for remote in situ use.