Science-Filters Study of Martian Rock Sees Hematite

This false-color image demonstrates how use of special filters available on the Mast Camera (Mastcam) of NASA's Curiosity Mars rover can reveal the presence of certain minerals in target rocks. It is a composite of images taken through three "science" filters chosen for making hematite, an iron-oxide mineral, stand out as exaggerated purple. This target rock, called "Christmas Cove," lies in an area on Mars' "Vera Rubin Ridge" where Mastcam reconnaissance imaging (see PIA22065) with science filters suggested a patchy distribution of exposed hematite. Bright lines within the rocks are fractures filled with calcium sulfate minerals. Christmas Cove did not appear to contain much hematite until the rover team conducted an experiment on this target: Curiosity's wire-bristled brush, the Dust Removal Tool, scrubbed the rock, and a close-up with the Mars Hand Lens Imager (MAHLI) confirmed the brushing. The brushed area is about is about 2.5 inches (6 centimeters) across. The next day -- Sept. 17, 2017, on the mission's Sol 1819 -- this observation with Mastcam and others with the Chemistry and Camera (ChemCam showed a strong hematite presence that had been subdued beneath the dust. The team is continuing to explore whether the patchiness in the reconnaissance imaging may result more from variations in the amount of dust cover rather than from variations in hematite content. Curiosity's Mastcam combines two cameras: one with a telephoto lens and the other with a wider-angle lens. Each camera has a filter wheel that can be rotated in front of the lens for a choice of eight different filters. One filter for each camera is clear to all visible light, for regular full-color photos, and another is specifically for viewing the Sun. Some of the other filters were selected to admit wavelengths of light that are useful for identifying iron minerals. Each of the filters used for this image admits light from a narrow band of wavelengths, extending to only about 5 nanometers longer or shorter than the filter's central wavelength. Three observations are combined for this image, each through one of the filters centered at 751 nanometers (in the near-infrared part of the spectrum just beyond red light), 527 nanometers (green) and 445 nanometers (blue). Usual color photographs from digital cameras -- such as a Mastcam one of this same place (see PIA22067) -- also combine information from red, green and blue filtering, but the filters are in a microscopic grid in a "Bayer" filter array situated directly over the detector behind the lens, with wider bands of wavelengths. Mastcam's narrow-band filters used for this view help to increase spectral contrast, making blues bluer and reds redder, particularly with the processing used to boost contrast in each of the component images of this composite. Fine-grained hematite preferentially absorbs sunlight around in the green portion of the spectrum around 527 nanometers. That gives it the purple look from a combination of red and blue light reflected by the hematite and reaching the camera through the other two filters. https://photojournal.jpl.nasa.gov/catalog/PIA22066