Creating False Color Images of Mars

By Jim Bell and Dmitry Savransky (Arizona State University, Cornell University)

Introduction

These pages display false color representations of rocks, soils, and other surface features photographed by the Pancam instruments on the Mars Exploration Rovers Spirit and Opportunity (Bell et al., 2004a,b). Images from Spirit are shown in the left column, and images from Opportunity are shown in the right column. Next to each image is some information about the Sol when the image was acquired on (a sol is a Mars day, which is 24 hours and 39 minutes long), the Pancam sequence number, the Local True Solar Time (LTST) when the image was acquired (assuming 24 "hours" per sol), the Earth date corresponding to that sol and LTST, and the 3 filters used to create the image. You can click on the JPG or TIFF links to download full-resolution versions of each image.

It is important to remember that these images are not intended to represent what you would see if you were standing on the surface of Mars. They are False Color views created to emphasize differences in visual color, and to produce color images when there is not enough information available to calculate the true color of a scene.

The Basics

The two Panoramic Cameras (called Pancams) on each of the Mars Exploration Rovers work somewhat like a pair of human eyes. Each camera's light sensitive "cells" are called pixels, and they are part of a light detecting "eye" called a Charge Coupled Device, or CCD. However, unlike the human eye, the Pancams only measure one single wavelength or color at a time. In front of each camera is a filter wheel with eight different filters (seven colors plus one filter for looking at the Sun), each of which allows only certain wavelengths to hit the CCD. Five of the filters in the left Pancam eye of each rover are filters which span the colors that we can see, from blue to green to red. The other filters that Pancam uses can detect colors of light that we cannot see, called "ultraviolet" and "infrared." To create these false color pictures of Mars, we use three pictures taken of the same scene from the Pancam's left eye. We use computer software to combine the separate pictures into one where differences in color are stretched and highlighted.

The purpose of these images, unlike the true color images we produce is to enhance subtle differences in color. These differences are sometimes so small that a person looking at them would not be able to see them at all - something you can see by comparing a true color image with its corresponding false color version. Images like these allow scientists to quickly asses even subtle color differences, and to choose the most interesting regions for possible study with the rover arm instruments. Subtle color changes may indicate changes in the materials making up rocks and soils, or how these materials are concentrated or deposited.

The Specifics

The first step in creating false color images is to select which filters are used. When more than 3 left eye filters are available, the filters are chosen such that they span as much of the Pancam CCD's wavelength response as possible. For this reason, the most common filter combination is L2, L5 and L7, which correspond to wavelengths of 753, 535, and 432 nm, respectively. When these are not available, the filter with the shortest wavelength available is used for the blue channel, the longest available wavelength is used for the red channel, and the median wavelength is used for the green value. We use images calibrated using the absolute radiometric calibration of the cameras - radiance calibrated images produced by the calibration pipline as described in Bell et al. (2003, 2006). For each filter image, a maximum and minimum pixel value is selected by adding and subtracting 3 times the pixel values standard deviation from the image's pixel median value. If these values fall outside the bounds of values in the image, then the image actual maximum or minimum value is used. Using these minima and maxima as value bounds, each image is scaled independently between values of 0 and 255. Then the three scaled images are used as the red, green, and blue channels of a color image.

Table of Pancam Left Eye Filter Wavelengths

Filter Number	Wavelength (nm)
L2	753
L3	673
L4	601
L5	535
L6	482
L7	432

Citation and Photo Credits

If you plan to use any of the MER/Pancam "False Color" images for academic or publication purposes, please cite the following references:

Bell III, J.F., S.W. Squyres, K.E. Herkenhoff, J.N. Maki, H.M. Arneson, D. Brown, S.A. Collins, A. Dingizian, S.T. Elliot, E.C. Hagerott, A.G. Hayes, M.J. Johnson, J.R. Johnson, J. Joseph, K. Kinch, M.T. Lemmon, R.V. Morris, L Scherr, M. Schwochert, M.K. Shepard, G.H. Smith, J.N. Sohl-Dickstein, R. Sullivan, W.T. Sullivan, and M. Wadsworth, The Mars Exploration Rover Athena Panoramic Camera (Pancam) Investigation, J. Geophys. Res., vol. 108, no. E12, doi:10.1029/2003JE002070, 2003.

Bell III, J.F., J.R. Joseph, J. Sohl-Dickstein, H. Arneson, M. Johnson, M. Lemmon, and D. Savransky, In-Flight Calibration of the Mars Exploration Rover Panoramic Camera Instrument, J. Geophys. Res., 111, E02S03, doi:10.1029/2005JE002444, 2006.

If you plan to use any of the MER/Pancam "False Color" images for articles in newspapers, magazines, web sites, or in textbooks or other places of general public usage, the preferred photo credit line is:

Photo credit: D. Savransky and J. Bell / JPL / NASA / Cornell / ASU

References

Bell et al. Mars Exploration Rover Athena Panoramic Camera Investigation, Journal of Geophysical Research, Vol. 108, No. E12, p. 8063, 2003

Bell III, J.F., S.W. Squyres, R.E. Arvidson, H.M. Arneson, D. Bass, D. Blaney, N. Cabrol, W. Calvin, J. Farmer, W.H. Farrand, W. Goetz, M. Golombek, J.A. Grant, R. Greeley, E. Guinness, A.G. Hayes, M.Y.H. Hubbard, K.E. Herkenhoff, M.J. Johnson, J.R. Johnson, J. Joseph, K.M. Kinch, M.T. Lemmon, R. Li, M.B. Madsen, J.N. Maki, M. Malin, E. McCartney, S. McLennan,H.Y. McSween, Jr. , D.W. Ming, J.E. Moersch, R.V. Morris, E.Z. Parker, J. Proton, J.W. Rice, Jr., F. Seelos, J. Soderblom, L.A. Soderblom, J.N. Sohl-Dickstein, Noe Dobrea, T.J. R.J. Sullivan, M.J. Wolff, and A. Wang, Pancam multispectral imaging results from the Spirit rover at Gusev crater, Science, 305, 800-806, 2004a.

Bell III, J.F., S.W. Squyres, R.E. Arvidson, H.M. Arneson, D. Bass, W. Calvin, W.H. Farrand, W. Goetz, M. Golombek, R. Greeley, J. Grotzinger, E. Guinness, A.G. Hayes, M.Y.H. Hubbard, K.E. Herkenhoff, M.J. Johnson, J.R. Johnson, J. Joseph, K.M. Kinch, M.T. Lemmon, R. Li, M.B. Madsen, J.N. Maki, M. Malin, E. McCartney, S. McLennan, H.Y. McSween, Jr., D.W. Ming, R.V. Morris, E.Z. Noe Dobrea, T.J. Parker, J. Proton,J.W. Rice, Jr., F. Seelos, J. Soderblom, L.A. Soderblom, J.N. Sohl-Dickstein, R.J. Sullivan, C. Weitz, M.J. Wolff, Pancam multispectral imaging results from the Opportunity rover at Meridiani Planum, Science, 306, 1703-1709, 2004b.

Bell III, J.F., J.R. Joseph, J. Sohl-Dickstein, H. Arneson, M. Johnson, M. Lemmon, and D. Savransky, In-Flight Calibration of the Mars Exploration Rover Panoramic Camera Instrument, J. Geophys. Res., 111, E02S03, doi:10.1029/2005JE002444, 2006.

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