CURIOSITY AT GALE CRATER, MARS: GEOLOGIC OBSERVATIONS, IMAGE SCALE, AND RANGE-FINDING WITH THE MARS HAND LENS IMAGER (MAHLI) ON THE ROVER’S ROBOTIC ARM

MAHLI is a color camera with a macro lens aboard the Curiosity rover that can focus at working distances of 2.1 cm to infinity. The highest resolution images permit distinction of silt from very fine sand. Here, we discuss: (1) MAHLI’s first 12 months of geoscience activities on Mars and (2) the manner in which the scale of features is determined for tactical planning and science analysis.

Operating in northwest Gale crater, MAHLI provided critical observations to characterize the size, shape, and roundness of particles as well as sedimentary structures, diagenetic features, and cementation of rocks. The images assisted in differentiating major rock types, particularly mudstones and sandstones, and helped in reconstruction of depositional setting and evaluation of the record of a past, habitable environment. MAHLI was also used to interrogate the physical properties of sand and rock in support of sample collection and analysis, assist robotic arm engineering and tool placement, and document aspects of the engineering state and condition of the rover.

Conveyance of scale in geological photography is often facilitated by placing or locating an object of known size— e.g., telephone pole, ruled scale, coin, rock hammer—in the scene. In field settings inaccessible to humans, knowledge of camera properties and observing geometry inform the user of scale. During such robotic field operations, quick assessment of scale—which can be challenging to access tactically—is vital to make informed, resource-saving decisions. MAHLI image scale is a function of the distance between the lens and a target; range is determined by autofocusing on a portion of a scene. The lens uses a stepper motor to change focus and the number of steps is reported. Testing on Earth and validation on Mars provide an empirical relationship between motor count (m) and range (r, in cm) for distances of 2.1 to 210 cm when the MAHLI dust cover is open. That relationship is: r = ((0.576786m^–1) + (–11.8479) + (2.80153×10^–3m) + (–2.266488×10^–7m²) + (6.26666×10^–12m³))^–1. Range, r, is related to the scale of an image pixel (p, in µm), as projected onto the target, by: p = (6.9001 + 3.5201r). The highest resolution MAHLI images collected thus far on Mars are 16–17 µm/pixel; more typically, the images are in the 20–35 µm/pixel range and nested within context images of ~100 µm/pixel.