Exploring Mars with the Curiosity rover means identifying rocks and minerals that can tell scientists more about the Red Planet and its distant past.
Since no human can yet walk up to a martian outcrop and examine it, scientists turn to the next best thing: studying rocks around the rover using its cameras. One of the instruments that can take images using special filters to isolate rocks and minerals of interest is Curiosity's Mast Camera, or Mastcam for short.
"Mastcam has both ordinary red-green-blue filters for taking images like what you would see by eye on Mars," said Danika Wellington, a doctoral student in Arizona State University's School of Earth and Space Exploration (SESE), "but it also has filters that let through a narrower distribution of wavelengths that isolate specific spectral features in minerals."
Wellington started her SESE graduate work in August 2012, just as the Curiosity rover was landing on Mars. But she didn't begin working with Mastcam until the following February. At that time, Curiosity was exploring an area named Yellowknife Bay, not too far from its landing site. Today, Curiosity is more than 10 miles away from there.
"I've seen all the ground Curiosity has driven over since," she said.
Each of Mastcam's two "eyes" — one telephoto and one wide angle — has several science filters that can be changed from one image to the next to assess how brightly a rock reflects light of specific colors. By design, some of the filters are for diagnostic wavelengths that certain minerals absorb, rather than reflect.
Wellington's adviser, SESE Professor Jim Bell, is the deputy principal investigator for the Mast Camera.
"Mastcam's special color filters provide the ability to 'see' the surface in color beyond the normal human red-green-blue vision range, into bluer colors near the ultraviolet and redder colors near the infrared," he said.
In addition to Mastcam, the rover also carries the Chemistry and Camera instrument (ChemCam). This instrument is best known for zapping rocks with a laser to identify chemical elements in them, but it also can examine targets near and far without use of the laser. It does this by measuring sunlight reflected by the targets in thousands of wavelengths, extending beyond visible-light colors into infrared and ultraviolet. Some patterns in this spectral data can identify hematite and other minerals.
NASA landed the Curiosity rover in Gale Crater, a 96-mile-wide impact crater that is about 3.8 billion years old. The rover's mission is to look for clues about wet environments in Mars' ancient past. During the first year after landing, Curiosity found evidence that some ancient martian environments offered conditions favorable for life. As the mission continues, it is studying how those conditions varied and changed.
Currently, the rover has driven up on top of Vera Rubin Ridge, a meandering feature that lies partway up Mount Sharp, a giant stack of sediments filling the center of Gale Crater. In fact, the ridge became a planned destination for Curiosity even before the rover landed.
Spectrometer observations from orbit revealed the ridge contains hematite. This iron-oxide mineral is detectable with Mastcam's science filters, and it is of prime interest for scientists as the rover examines the ridge because most hematite forms in the presence of water.
"We chose Mastcam's specific special color filters so that they would be as sensitive as possible to minerals like the iron oxides that we're studying now on the ridge," Bell said.
Hematite occurs at sufficiently small grain sizes in rocks found in this part of Mars to preferentially absorb some wavelengths of green light. This gives it a purplish tint in standard color images from Curiosity, due to more reflection of redder and bluer light than reflection of the green wavelengths. The additional color-discerning capabilities of Mastcam and ChemCam show hematite even more clearly.
"The colors of the rocks on the ridge are more interesting and more variable than what we saw earlier in Curiosity's traverse," said science team member Jeffrey Johnson of Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. He uses both Mastcam and ChemCam data for analyzing rocks.
Johnson added, "We're using these multispectral and hyperspectral capabilities for examining rocks right in front of the rover and also for reconnaissance — looking ahead to help with choosing where to drive for closer inspection."
For example, a false-color panorama taken Sept. 12 combined Mastcam images taken through three special filters. This provided a map of where stronger spectral signatures of hematite could be seen in a region a few days' drive away.
These features are most apparent in zones around fractured bedrock. The team drove Curiosity to a site in that scene to check the possible link between fracture zones and hematite.
Investigation with Mastcam, ChemCam and other tools — including a camera and brush on the rover's arm — revealed that hematite is also in bedrock farther from the fractures once an obscuring layer of tan dust is brushed away. The dust doesn't coat the fractured rock as thoroughly.
That finding suggests that dust and fractures cause the hematite to appear patchier than it actually is. If the hematite is broadly distributed, its origin likely was early in the ridge's history, rather than from a later period of fluids moving through fractures in the rock.
"As we approached the ridge and now as we're climbing it, we've been trying to tie what was detected from orbit to what we can learn on the ground," said SESE's Wellington. "It's still very much a work in progress. The extent to which iron-bearing minerals here are oxidized relates to the history of interactions between water and rock."
Said Bell, "Danika has been one of the leaders on the team for planning, processing and interpreting the multispectral images taken using Mastcam's special color filters. Her work, which is the basis of her PhD thesis research, is helping to guide the day-to-day decisions about where the rover is driving and sampling the surface."
Wellington added, "It has been an exciting adventure and a great privilege to work with the rover science team on this mission."