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Emily LakdawallaMay 1, 2008

Seeing Mars in many colors

I've enthused before about websites that make it easy to access the archives of images from Mars missions, present and past. My favorite of these, from an ease-of-use standpoint, is the Mars Express HRSCview site; and a close second is ASU's global maps of Mars website, which collects under one roof all the data from Viking's cameras, Mars Global Surveyor's cameras and laser altimeter, Mars Odyssey's THEMIS imaging spectrometer, and Mars Reconnaissance Orbiter's CTX and HiRISE cameras. Now there's another site to add to the list: the Mars Reconnaissance Orbiter's CRISM imaging spectrometer team has just launched a new website that provides a graphical interface to the publicly released data from their instrument, which currently includes approximately a thousand images.

Although I'd prefer one-stop shopping for my Mars data, it makes sense for CRISM to have their own site, because their instrument just isn't at all like a camera. CRISM does produce images, sure, but a single image from CRISM totally fails to communicate what CRISM can do, which is to tease out subtle details from the way Mars' surface reflects different wavelengths of light, information that can be used to map out the distribution of minerals across Mars' surface. The CRISM team has come up with a way to use a small number of images to display a highly informative subset of their data; I wrote about the different kinds of images they produce last October, when I was covering the Mars Science Laboratory landing site selection meeting.

I wandered around the map and grabbed a few sets of images representative of different kinds of locations across Mars. There are seven images for each location. The left two contain versions of the CRISM data that are most similar to regular photographs (the first is approximate true color, the second a black-and-white view in an infrared wavelength), while the right five contain versions of the data that map the relative abundance of interesting minerals (for those in the know about CRISM, they are in this order: FEM, MAF, PHY, HYD, ICE). Pretty, pretty colors...

CRISM view of dunes and gullies within Russell crater


CRISM view of dunes and gullies within Russell crater
Russell crater is in Mars' southern highlands. The CRISM thumbnails show little diversity, which is no surprise, given the fact that the area is covered with sand dunes.
CRISM images over the Spirit landing site


CRISM images over the Spirit landing site
CRISM observed the Spirit landing site in Gusev crater on November 22, 2006. There's not much diversity within the images here, either, and that's more or less what Spirit observed from the ground: it took grinding and trenching to expose interesting materials at this landing site.
Being neither a spectroscopist nor a mineralogist, I don't feel qualified to interpret much from these images. But I thought I'd make some broad observations. First of all, the simplest observation is this: Mars looks, well, interesting in CRISM images. In the left two columns you can see a lot of interesting features in these images: dunes and dust devil tracks, fresh craters and degraded craters, dark streaks on steep slopes, crazily eroded terrain, and the polar caps. If you look at the right five columns, one thing that you can conclude is that there is diversity. Each one of those images is designed to reveal whether or not some interesting mineral is present. If the image looks black, none of the minerals is present in that area, at least not in significant quantities. If the image is bright, several interesting minerals are present. If the image looks like one of the six primary and secondary colors -- red, green, blue, cyan, yellow, or magenta -- then only one or two minerals of interest are present, while others are missing. Of course, the most exciting places are the ones that display different colors within the same image thumbnail.

If you want to try to do some geologic interpretation on your own, here's the basic info that the CRISM team offers about the sets of images. Again, they are displayed in this order: RGB, IRA, FEM, MAF, PHY, HYD, ICE.

Two versions of the data provide an overview of surface brightness. vnir_rgb is enhanced color over the visible range, and ir_ira is brightness at a wavelength near 1330 nm.
ompositional information on the surface is concentrated in four of the browse products (vnir_fem, ir_maf, ir_phy, and ir_hyd). Not all of the sites exhibit spectral evidence for mineralogical diversity. If a location is covered in dust, it appears red in vnir_fem and bland in the other products. Sites with diversity in igneous mineralogy will appear interesting in ir_maf. Sites with minerals formed by interaction of crustal rocks with liquid water will appear interesting in ir_phy and ir_hyd.
ites that had permanent water ice on the surface or as clouds will appear pink, yellow or green in ir_ice, whereas those with carbon dioxide frost on the surface will appear bluish. Please note, icy sites also have bluish colors in ir_phy and ir_hyd due to spectral effects of water ice.

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
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