The initial results from Curiosity’s first examination of soil samples (or more correctly, regolith samples), analysed by the Chemistry and Minerology instrument (CheMin) have been returned to Earth for evaluation.
CheMin took samples of the surface material gathered by the rover’s scoop and pre-processed (filtered for compositional size) using CHIMRA, which forms a part of the turret-mounted suite of instruments and systems on the rover’s robot arm, and subjected them to X-ray diffraction, which is regarded as the “gold standard” for understanding the mineral composition of soil and rock samples on Earth. CheMin marks the very first time such an analytical capability has been possible on the surface of another planet in the solar system.
The identification of minerals in rocks and soil is crucial for the mission’s goal to assess past environmental conditions. Each mineral records the conditions under which it formed. The chemical composition of a rock provides only ambiguous mineralogical information, as in the textbook example of the minerals diamond and graphite, which have the same chemical composition, but strikingly different structures and properties.
Because of the ambiguity in chemical analysis, which has to date revealed remarkably uniform results from regions which are geographically very diverse over the surface of the planet, understanding the actual minerology of Martian soil has always been a mixture of scientific study laced with educated inferences, so CheMin is a major game-changer.
“We had many previous inferences and discussions about the mineralogy of Martian soil,” said David Blake of NASA Ames Research Center in Moffett Field, California, the Principal Investigator (PI) for CheMin. “Our quantitative results provide refined, and in some cases, new identifications of the minerals in this first X-ray diffraction analysis on Mars.”
Prior to MSL’s arrival on Mars, it had been long theorised that much of the surface material may well be of volcanic origin, particularly given the ample evidence of the planet having had an extremely active volcanic past.
The results obtained by CheMin confirmed that much of the sample to be made of weathered “basaltic” materials of volcanic origin, mixed with coarser sand of more local provenance. The volcanic material is liable to have been carried far from its point of origin over the millennia by Mars’ seasonal, and frequently planet-encompassing storms. It is also remarkably similar to volcanic material found in Hawaii; while this hadn’t been altogether unexpected, it is nevertheless an important step forward.
Additionally, the mixture of minerals discovered at Rocknest has helped confirm initial suppositions made about the Gale Crater region which have resulted from a mixture of both observation and educated interpretation of images.
“So far, the materials Curiosity has analysed are consistent with our initial ideas of the deposits in Gale Crater, recording a transition through time from a wet to dry environment,” said David Bish, co-investigator on the CheMin experiment. “The ancient rocks, such as the conglomerates, suggest flowing water, while the minerals in the younger soil are consistent with limited interaction with water.”
CheMin’s presence on Mars is also a celebration of X-ray defraction itself – 2012 marking the centenary of the discovery of the process. As with so much in the space industry, it also marks the ability to take the Awfully Big and reduce it down to the Very Small; a traditional X-ray diffraction system is on the order of size of a large household refrigerator/freezer, while CheMin is perhaps the size of an attaché case. CheMin has already led to a commercial spin-off capability in the form of a portable X-ray diffraction system called Terra, which can be used to analyse soil and rock samples in situ here on Earth.
Despite its small size, CheMin does an incredible amount of work: further sorting the sample so that the tiniest particles (which are around the width of two human hairs) are delivered into a set of cell pairs (the instrument having 16 such pairs), where they are vibrated 2,000 times a second, which cause the samples to move in a liquid-like manner. X-rays fired at them during the vibration hit the samples while they are at random orientations due to the vibrations, allowing a wealth of data to be collected by a postage-stamp sized Charged Couple Device (itself developed by NASA for other applications, and now in widespread use around the globe in digital cameras and smartphones).
The Road Ahead
Over the last few days, Curiosity has been continuing its examination of Rocknest, where it will remain through until around the middle of next week, if not slightly longer, before resuming its journey to Glenelg. Part of this work has been imaging the area with Mastcam and Navcam, and also taking a look at various rocks using the Mars Hand Lens Imager (MAHLI) to judge their suitability as potential targets for further investigation.
Before Curiosity departs Rocknest, it will carry out at least one more sample gathering operation, this time delivering material into the Sample Analysis at Mars (SAM) instrument, which has been busy sampling the air within Gale Crater.
All images courtesy of NASA/JPL. Tharsis Ridge image via NASA’s Mars Global Surveyor mission (active mission life: 1999-2006)
