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High Spatial Resolution Visible Color Units on Mars from the Mars Odyssey THEMIS/VIS Instrument.J.F. Bell III1, T. McConnochie1, D. Savransky1, B. Stiglitz1, M.J. Wolff2, P.R. Christensen3, G. Mehall3, P.B.James4, M. Malin5, M. Caplinger5, M. Ravine5, L.L. Cherednik3, K.C. Bender3, K. Murray3, and the THEMIS Sci-ence Team, 1Cornell University, Dept. of Astronomy, Ithaca NY 14853-6801; [email protected] , 2Space ScienceInstitute, Boulder CO, 3Arizona State University, Dept. of Geology, Tempe AZ, 4U. Toledo, Toledo OH, 5MalinSpace Science Systems, Inc., San Diego CA.

Introduction: The Visible Imaging Subsystem(VIS) on the NASA Mars Odyssey spacecraft'sTHEMIS instrument has been obtaining high spatialresolution 5-color visible wavelength images of Marssince mapping began in February 2002 [1,2]. VIS is a1024¥1024 interline transfer CCD camera that usesnarrowband interference filters bonded to the CCD toacquire multispectral images from Mars orbit at centralwavelengths of 425, 540, 654, 749, and 860 nm (±25nm). This abstract describes the newest data reductionand calibration methods used to process VIS data, andpresents some initial results on surface color propertiesat high spatial resolution.

Calibration and Data Processing: Raw VISimages returned from the spacecraft suffer from anumber of the "usual" calibration challenges in-herent in CCD imaging, as well as from uniquecalibration and processing issues related to theVIS design and the specific environment in whichVIS is operating. These issues include (a) Biasand dark current signals which need to be re-moved; (b) pixel-to-pixel responsivity (flatfield)variations that need to be characterized and ac-counted for; (c) frame transfer smear signal re-sulting from the finite time it takes to transfercharge to the CCD serial register and the fact thatthe frame transfer masks are not completelyopaque; (d) scattered light impinging on the VISfield of view; (e) derivation of a set of coeffi-cients to convert corrected DN values to radiance;(f) mapping and coregistration of different wave-length bands; and (g) Generation of "true color"display products.

(a) Bias and Dark Current. Bias signal ischaracterized and removed from VIS images us-ing images obtained over the nightside of theplanet. Typical bias signal levels are only a fewDNs and there is a repeatable spatial pattern to thesignals. Dark current has not been detectable inflight because of the low operating temperaturesand the extremely short exposure times (less than10 msec) used for VIS images.

(b) Flatfield. Pixel to pixel responsivityvariations arise from intrinsic variations in thedetector, spatial variations in the transmissivity ofthe color filters, or geometric effects from theTHEMIS optics. These effects were assessed us-

ing a combination of pre-flight calibration filesand in-flight averages of hundreds of images togenerate "superflat" frames. In practice, flatfieldvariations are not completely separated fromscattered light artifacts (see (d)), so the calibrationpipeline handles both effects simultaneously.

(c) Frame Transfer Smear. Charge generatedby the VIS CCD pixels is transferred to a maskedregister adjacent to each pixel for eventualclocking and transfer off the chip. However, theframe transfer time is much longer than the expo-sure time and the masked areas are not completelyopaque. The result is extra signal added to thedesired signal, in proportion to the distance ofeach pixel from the CCD horizontal register. Thisis commonly known as frame transfer smear, andcan be accurately characterized and removed us-ing a combination of knowledge of the CCD tim-ing and specially-designed zero exposure timeimages. The zero images provide measurementsof only the frame transfer smear component. Wehave obtained in-flight zero measurements over avariety of albedo terrains in order to characterizeboth the magnitude and "color" of the frametransfer smear, in order to model and remove it aspart of our processing pipeline.

(d) Scattered Light. The beam from theTHEMIS telescope impinging on the CCDslightly overfills the field of view. Some of thisout of field light is reflected off structures in boththe VIS detector and telescope assemblies, im-parting a scattered light signal onto the raw im-ages. Fortunately, the scattered light pattern isrepeatable, and in-flight observations are helpingto characterize and remove it from the desiredsignal. Accurate knowledge of the scattered lightand frame transfer smear components has alsoallowed us to develop an accurate exposure timeprediction model so that we can optimize the de-sired signal level in each wavelength even in thepresence of these "contaminating" signals.

(e) Radiance Conversion. The above stepsresult in a set of instrumentally-calibrated imagesexpressed in "corrected DN" values. Because themeasurement environment experienced by VIS inorbit is substantially different than that which wasable to be simulated during pre-flight testing, we

Sixth International Conference on Mars (2003) 3238.pdf

THEMIS/VIS COLOR ANALYSIS: J.F. Bell III et al.

have relied almost entirely on in-flight observa-tions to derive a set of absolute radiometric scal-ing coefficients. Our approach is twofold: (1)We use our "superflat" observations in each filterto derive an average 5-color Mars corrected DNspectrum as observed by VIS. We then use well-calibrated Hubble Space Telescope (HST) WideField/Planetary Camera 2 clear-atmosphere ob-servations from 255 to 1042 nm [3] to generate anaverage HST spectrum of Mars using data ob-tained only at the same average incidence angle asthe VIS measurements. This average HST spec-trum is then convolved to the VIS bandpasses andthe ratio of it to the average VIS corrected DNspectrum results in a set of scaling coefficients toconvert corrected DN per msec to W/m2/nm/sr.(2) The validity of this approach is being testedby new sets of simultaneous VIS and HST obser-vations during 2003. The first set of simultaneousmeasurements was performed on March 8, 2003;29 sets of 5-color VIS sequences were acquiredwithin two hours of HST multispectral imagingfrom 220 nm to 892 nm using HST's new Ad-vanced Camera for Surveys (ACS) instrument; 10of these VIS image sets were acquired at exactlythe same time as the HST measurements. Initialanalysis of these simultaneous observationsshows that our default average-based radiancecoefficient approach appears to be accurate towithin ~5% for VIS bands 1 through 4, and within~15-20% for VIS band 5 (860 nm), which has thelargest scattered light component. These resultsare preliminary and will be augmented by addi-tional simultaneous HST/Odyssey observingcampaigns. Notably, a planned campaign duringthe August 2003 opposition will take advantageof the fact that HST will be able to obtain a spa-tial resolution of ~4 km/pixel using ACS, mean-ing that typical 36 m/pixel color VIS image stripswill correspond to about 5x15 HST pixels, a sta-tistically more significant number for refining thecalibration coefficients.

(f) Mapping and Spectral Coregistration.Calibrated radiance images are map projectedusing SPICE pointing information that predictsthe corners and center latitude and longitude ofeach VIS framelet based on knowledge of thespacecraft's orbit and attitude history. Our map-ping software warps each framelet onto a simplecylindrical grid, but does not yet take into accountthe small amount of geometric optics distortionacross the VIS field of view. Each band in a VISmultispectral sequence maps to a slightly differentpart of the surface [2]. Dead-reckoning the

pointing and mapping this way does not result ina perfectly coregistered set of multispectral im-ages, however, because of small pointing uncer-tainties and distortion errors. Therefore, aftereach band is mapped, we use an autocorrelationroutine to co-align the bands (usually using the654 nm band as the reference) for multispectralanalysis and spectrum extraction. As part of thismapping process, we also generate "backplanes"of additional information (latitude, longitude, in-cidence, emission, and phase angle for each pixel)that are needed for photometric correction and/orradiative transfer modeling of the calibrated data.

(g) True Color Images. For education andpublic outreach as well as aesthetic purposes weare also generating sets of "true color" VIS im-ages using the radiometrically calibrated data.The intention of these products is to simulate thecolor of the surface as it would be seen by a per-son with an average human photopic responsefunction, were they riding along with Odyssey atMars. Our radiance images are first converted tothe CIE's xyz color space (x and y are chromatic-ity; z is brightness [4]) using the CIE's standard"D65" color matching functions [5]. The xyz col-ors are then converted to the Web-friendly sRGBcolor system and written to TIFF files that pre-serve the sRGB standard color map and allow thecolors to be uniformly displayed by many differ-ent software packages or Web browsers.

VIS images are currently downlinked using acombination of 12 to 8 bit companding andlossless DCT compression. Possible use of lossycompression is being investigated for future en-hancement of the total VIS surface coverage.

Results: Examples of some mapped andcoregistered images are provided below, alongwith some typical 4-color VIS spectra extractedfrom these data (band 5 is still problematic). Sev-eral themes are emerging from this high resolu-tion multispectral imaging campaign: (1) Colorvariations are usually subtle–many occur but of-ten at the limits of VIS resolution; (2) Dark, ap-parently aeolian deposits exhibit different colors,suggesting particle size and/or compositionalvariations; and (3) An enigmatic class of “blue”crater floor deposits has been identified but itscomposition or origin is not yet understood.

References: [1] Christensen, P.R. et al. (1999)LPSC XXX, Abstract #1470. [2] Bell III, J.F. et al.(2003) LPSC XXXIV, Abstract #1993. [3] Bell III, J.F.et al. (2003) submitted to I c a r u s . [4] e . g . , hyperphysics.phyastr.gsu.edu/hbase/vision/cie.html [5]Estrada, P.R. & J.N. Cuzzi (1996) Icarus, 122, 251.



Figure 1 (left) Approximate true color image of an un-named 23 km diameter crater in western Arcadia Planitia,near 39°N, 179°E. The crater shows a number of inter-esting internal and external features that suggest that ithas undergone substantial modification since it formed.(top) Approximate true color image of part of the lowalbedo channel floor of Granicus Vallis, near 28°N,132°E (V03578003). North is up in all mosaics shown.

Figure 2. (left) False color RGB composite of part of VIS image V04277003showing craters in Aeolis near 13°S, 157°E. The enlarged segment (top)shows an example of an enigmatic and substantially "bluer" deposit on thefloor of one crater. The deposit is heavily cratered and appears near thedownslope mouth of a small valley incised in the crater wall. The higheralbedo deposit itself is surrounded by a diffuse and also bluer floor unit.



Figure 3. Examples of 4-color VIS spectra extracted from different units in Figure 2. (left) I/F spectra (I is detectedradiance and pF is the incident solar radiance in that bandpass); "Blue Deposit 1" and "Blue Deposit 2" are from thehigher albedo floor deposit adjacent to the small valley; "Blue Floor 2" is from the surrounding diffuse and bluer unit;"Red Floor 1" is from the surrounding crater floor region that does not show this unique spectral signature. Even the"blue" deposits can be seen to be quite red in color. The relative differences are better shown in the spectra on theright, where the average spectrum of the whole scene has first been divided out. The blue deposit is 20-30% brighterin VIS band 1 than the average. The calibration of band 5 (863 nm) is not yet adequate to allow spectral analysis.

Figure 4. (left) False color RGB composite from VIS imageV03233002 of the deposit known as "White Rock" in Pol-lack crater, near 8°S, 25°E. (top) I/F spectra from the de-posit–which is clearly not white–and the surrounding craterfloor and dunes; (bottom) Average-removed spectra.


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