Dusting for Fingerprints of Mars' Water

CRISM, a visible-infrared imaging spectrometer, is one of six scientific instruments on the Mars Reconnaissance Orbiter. When MRO flies over a given area, CRISM can image the surface using either of two modes:

• In a targeted observation, the scanning mechanism tracks a region on the dusty Martian surface and maps it in 544 wavelengths at scales as small as 18 meters (60 feet) across, from an altitude of 300 kilometers (186 miles).

• Between targeted observations, the instrument points at nadir and images the surface in 72 colors at a reduced resolution, showing features 200 meters (660 feet) across  This "multispectral survey" provides continuity between targeted observations and finds new sites of interest for targeting.

CRISM uses the spectrum of reflected sunlight to determine the mineralogy of the surface, a technique called reflectance spectroscopy. Someone with a little familiarity with rocks can easily distinguish a piece of basalt (black, hardened volcanic lava) from a limestone (a light-colored sedimentary rock). However some very different rocks may look alike based on color alone – at least to the human eye. For example, slate (a metamorphic rock) is also black and fine-grained like basalt, but is made up of very different minerals.

Spectroscopy extends mineral discrimination based on color to wavelengths not visible to the eye. Most  minerals – chemical compounds that make up part or all of a rock – preferentially absorb light at certain wavelengths. Dips in the spectrum of reflected light at those wavelengths, called absorptions, provide "fingerprints" that identify the minerals.

On MRO, the Sun provides light that is reflected by the Martian surface back towards space to be detected by CRISM. For every pixel of one of its images, CRISM measures a spectrum of that part of Mars. Matching the absorptions that are observed with known mineral absorptions allows us to make images, and maps, of Mars' mineralogy. For example, the chart below shows the fraction of light reflected by three common Martian minerals at different wavelengths. Each of the minerals is generally found in a different kind of rock and indicates a different kind of environment. Pyroxene indicates igneous rock, whereas hematite shows where those rocks have been weathered and altered by water and gypsum indicates a past liquid water environment. Each of the three minerals can be identified – and the past environment inferred – from the wavelengths of the absorptions.

Different Martian minerals, some of which are signposts of past liquid water, can be identified by the "fingerprint" of absorptions in their spectrum of reflected sunlight.

• how CRISM builds an image

• how different minerals look to CRISM

• the CRISM spectral library

• an overview of the geology and weather of Mars

• what questions about Mars'  geology and climate CRISM will investigate