Curiosity’s Mars Hand Lens Imager (MAHLI) Investigation

Abstract

The fine-scale textures of rock surfaces reflect a complex interplay of geologic processes that include those related to rock formation, exposure, weathering, and erosion. Typically, such features are studied in two dimensions using forensic geological techniques from scales from those visible to the naked eye to those in a wider-area context. Rock surface metrology is an emergent technique in which extremely fine-scale textural information is measured in 3D using laboratory instruments. Here we describe its application on the surface of Mars. With the advent of sub-millimeter resolution (14-100 µm per pixel) imaging on Mars, a relatively low-cost (i.e., camera-based) approach for computing quantitative relief models (QRM) of rock surfaces has been investigated. We present preliminary assessment of 20 different martian rock surfaces using this QRM technique; the observations have implications regarding the depositional, diagenetic, and weathering/erosional history of sedimentary rocks on Mars. The Mars Science Laboratory (MSL) Curiosity rover landed in Gale crater, Mars, to investigate a portion of the 5-km-thick stratigraphic section of largely sedimentary rocks exposed within the crater. These rocks display records of depositional and diagenetic environments thought to be ~3.6 billion years old and include fluvial conglomerates; fluvial, deltaic, and eolian sandstones; and lacustrine mudstones [1]. The Mars Hand Lens Imager (MAHLI) is a 2-megapixel Bayer pattern micro-filtered color camera with a focusable macro lens mounted on the turret at the end of Curiosity's robotic arm [2]. We used MAHLI to acquire sets of 5 overlapping frames arranged in a "+" pattern with ~31 µm/pixel resolution to compute QRMs at scales never before possible on natural planetary surfaces [3]. These QRM's allow analysis and interpretation of geologic surfaces at horizontal scales as fine as 100 µm, akin to non-destructive engineering metrology approaches. In principle, the quantitative textural information that QRMs convey for exposed rock surfaces on Mars is tied directly to the cumulative processes that formed, emplaced, and modified them. Thus, our QRM analysis of 20 different Mars rock surfaces offers a new opportunity to extract information about the depositional, diagenetic, and in situ weathering processes on Mars at spatial scales greater than 0.1 mm, extending the array of information about the role of diagenesis and other processes on Mars from mm-scale chemistry (i.e., ChemCam on Curiosity). We generated a series of QRM datasets, starting with a high-silica sandstone known as Greenstone (Sol 1130) and continuing to our latest rock surface targets named Belle Lake (Sol 1586) and Misery (Sol 1593). We analyzed the spatial and vertical distribution of rock surface 'texture elements' at multiple scales, from 0.1 mm to several mm in an effort to unravel their geologic histories. Critical to understanding the geologic phenomena captured in the QRMs are quantitative comparisons with terrestrial reference surfaces, which is an effo...

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