Compact Reconnaissance Imaging Spectrometer for Mars

Visible infrared spectrometer

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) was a visible infrared spectrometer aboard the Mars Reconnaissance Orbiter searching for mineralogic indications of past and present water on Mars. The CRISM instrument team comprised scientists from over ten universities and was led by principal investigator Scott Murchie. CRISM was designed, built, and tested by the Johns Hopkins University Applied Physics Laboratory.

CRISM was being used to identify locations on Mars that may have hosted water, a solvent considered important in the search for past or present life on Mars. In order to do this, CRISM was mapping the presence of minerals and chemicals that may indicate past interaction with water low temperature or hydrothermal. These materials include iron and oxides, which can be chemically altered by water, and phyllosilicates and carbonates, which form in the presence of water. All of these materials have characteristic patterns in their visible infrared reflections and were readily seen by CRISM. In addition, CRISM was monitoring ice and dust particulates in the Martian atmosphere to learn more about its climate and seasons.

CRISM measured visible and infrared electromagnetic radiation from 362 to 3920 nanometers in 6.55 nanometer increments. The instrument had two modes, a multispectral untargeted mode and a hyperspectral targeted mode. In the untargeted mode, CRISM reconnoiters Mars, recording approximately 50 of its 544 measurable wavelengths at a resolution of 100 to 200 meters per pixel. In this mode CRISM mapped half of Mars within a few months after aerobraking and most of the planet after one year. The objective of this mode is to identify new scientifically interesting locations that could be further investigated.

The data collecting part of CRISM was called the Optical Sensor Unit (OSU) and consisted of two spectrographs, one that detected visible light from 400 to 830 nm and one that detected infrared light from 830 to 4050 nm. The infrared detector was cooled to –173° Celsius (–280° Fahrenheit) by a radiator plate and three cryogenic coolers. While in targeted mode, the instrument gimbals in order to continue pointing at one area even though the MRO spacecraft is moving. The extra time collecting data over a targeted area increases the signal to noise ratio as well as the spatial and spectral resolution of the image. This scanning ability also allowed the instrument to perform emission phase functions, viewing the same surface through variable amounts of atmosphere, which would be used to determine atmospheric properties. The Data Processing Unit (DPU) of CRISM performs in flight data processing including compressing the data before transmission.

Understanding the composition of Mars' crust and how it changed with time tells us about many aspects of Mars' evolution as a planet, and was a major goal of CRISM. Remote and landed measurements prior to CRISM, and analysis of Martian meteorites, all suggest that the Martian crust is made mostly of basaltic igneous rock composed mostly of feldspar and pyroxene. Images from the Mars Orbiter Camera on MGS showed that in some places the upper few kilometers of the crust is composed of hundreds of thin volcanic lava flows. TES and THEMIS both found mostly basaltic igneous rock, with scattered olivine rich and even some quartz rich rocks. The first recognition of widespread sedimentary rock on Mars came from the Mars Orbiter Camera which found that several areas of the planet including Valles Marineris and Terra Arabia have horizontally layered, light toned rocks. Follow up observations of those rocks' mineralogy by OMEGA found that some are rich in sulfate minerals, and that other layered rocks around Mawrth Vallis are rich in phyllosilicates. Both class of minerals are signatures of sedimentary rocks. CRISM had used its improved spatial resolution to look for other deposits of sedimentary rock on Mars' surface, and for layers of sedimentary rock buried between layers of volcanic rock in Mars' crust.

To understand Mars' ancient climate, and whether it might have created environments habitable for life, first we need to understand Mars' climate today. Each mission to Mars has made new advances in understanding its climate. Mars has seasonal variations in the abundances of water vapor, water ice clouds and hazes, and atmospheric dust. During southern summer, when Mars is closest to the Sun (at perihelion), solar heating can raise massive dust storms. Regional dust storms ones having a 1000 kilometer scale show surprising repeatability Mars year to Mars year. Once every decade or so, they grow into global scale events. In contrast, during northern summer when Mars is furthest from the Sun (at aphelion), there is an equatorial water ice cloud belt and very little dust in the atmosphere. Atmospheric water vapor varies in abundance seasonally, with the greatest abundances in each hemisphere's summer after the seasonal polar caps have sublimated into the atmosphere. During winter, both water and carbon dioxide frost and ices form on Mars' surface. These ices form the seasonal and residual polar caps. The seasonal caps which form each autumn and sublimate each spring are dominated by carbon dioxide ice. The residual caps which persist year after year consist mostly of water ice at the north pole and water ice with a thin veneer (a few 10's of meters thick) of carbon dioxide ice at the south pole. Mars' atmosphere is so thin and wispy that solar heating of dust and ice in the atmosphere not heating of the atmospheric gases is more important in driving weather. Small, suspended particles of dust and water ice aerosols intercept 20–30% of incoming sunlight, even under relatively clear conditions. So variations in the amounts of these aerosols have a huge influence on climate. CRISM had taken three major kinds of measurements of dust and ice in the atmosphere: targeted observations whose repeated views of the surface provide a sensitive estimate of aerosol abundance; special global grids of targeted observations every couple of months designed especially to track spatial and seasonal variations; and scans across the planet's limb to show how dust and ice vary with height above the surface. The south polar seasonal cap has a bizarre variety of bright and dark streaks and spots that appear during spring, as carbon dioxide ice sublimates. Prior to MRO there were various ideas for processes that could form these strange features, a leading model being carbon dioxide geysers. CRISM had watched the dark spots grow during southern spring, and found that bright streaks forming alongside the dark spots are made of fresh, new carbon dioxide frost, pointing like arrows back to their sources the same sources as the dark spots. The bright streaks probably form by expansion, cooling, and freezing of the carbon dioxide gas, forming a "smoking gun" to support the geyser hypothesis.