X-rays on Mars: CheMin diffraction

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  • Published: Dec 15, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: X-rays on Mars: CheMin diffraction

Curious about Mars

The X-ray diffractometer CheMin onboard the Mars rover

The X-ray diffractometer CheMin onboard the Mars rover "Curiosity" has obtained fascinating data from five soil samples on the Red Planet revealing a complex mineralogy with hints of aqueous alterations.

Way back in August 2012 the Mars Science Laboratory Curiosity Rover landed in the Gale crater. The crater lies near the northwestern part of the Aeolis quadrangle and is 154 kilometres in diameter and estimated to be approximately 3.5 to 3.8 billion years old. At its heart sits a mountain 5.5 km high - Aeolis Mons - comprising layered materials that planetary scientists hope will reveal the intricate tale of Martian geology spanning those billions of years and perhaps even offer new clues to how the Red Planet came into being. The work complements recent observations from that suggest Mount Sharp was built by sediments deposited in a large lake bed over tens of millions of years.

Diffracting on the Red Planet

Once the Curiosity rover had got its bearings on Mars, it began field studies as it headed towards the central peak of Aeolis Mons (also known colloquially as Mount Sharp). Curiosity has so far travelled almost 10 km from its landing site and has already taken samples of the peak's lower slopes. Those samples have then been subject to the tireless gaze of the sophisticated instruments on board. Among them, is CheMin, an array of ten instruments on or inside Curiosity. The components were all designed to provide detailed information about the Martian rocks, soil and atmosphere. But, fundamentally, CheMin is a miniaturised X-ray diffraction/X-ray fluorescence (XRD/XRF) instrument, approximately the size of a shoebox, that uses transmission geometry with an energy-discriminating charge-coupled device (CCD) detector to gather data in what are obviously remarkably challenging non-standard X-ray laboratory conditions.

To date, CheMin has analysed five samples, specifically a sample of soil, three samples drilled from mudstones and a sample drilled from sandstone. Rietveld and full-pattern analysis of the X-ray diffraction data have so far revealed a complex mineralogy. There are components derived from parent igneous rocks, amorphous constituents and several minerals that hint at aqueous alteration, for example clay minerals and hydrated sulphates and the mudstone samples all containing one or more phyllosilicate is also consistent with changes caused by the presence of liquid water. In addition to the quantitative mineralogy, Rietveld refinements have also provides unit-cell parameters for the major phases. These parameters can then be used to infer the chemical composition of individual minerals and, based on the differences, give us the composition of the amorphous component too.

The small size of the CheMin system limits the quality of the X-ray data that can be extracted from the Martian samples but the emerging results are as good as powder diffraction patterns carried out on terrestrial samples. Enhancements in analysing the powder samples might be made by altering the instrument's reflection geometry remotely although the onboard sample sonic vibration device helps in this regard significantly.

A century on...

David Bish in the department of Geological Sciences, at Indiana University, Bloomington, USA and colleagues there and at NASA, the Planetary Science Institute, inXitu, the universities of Hawaii at Manoa and Arizona, Arizona State University, the CNRS in France, Chesapeake Energy and the Lunar and Planetary Institute have analysed the data so far.

"Humankind has studied the heavens for millennia and, until the 1960s, all observations of extraterrestrial bodies were made remotely, with either optical or spectroscopic measurements," the team writes. "Similarly, humankind has studied crystalline materials since the beginning of time, but it was not until the discovery of X-ray diffraction (XRD) [a century before Curiosity reached Mars] that we learned about the ordered atomic arrangements that characterize such solids." It is a remarkable thought that we have within that century advanced science and technology to the point where we could carry out the first XRD studies on another planet.

Related Links

IUCrJ 2014, 1, 514-522: "The first X-ray diffraction measurements on Mars"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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