[Previous] | [Session 5] | [Next]
J.F. Bell III, M.Y.H. Hubbard (Cornell), M.J. Wolff (SSI)
Past UV/VIS studies of Martian dust aerosol properties have typically been handicapped by uncertain surface reflectance properties and/or viewing geometry, radiometric calibration, or wavelength coverage. Despite recent efforts from MPF and other missions, there remain large uncertainties in even the very basic dust parameters such as single scattering albedo. Fortunately, a nearly-ideal dataset now exists to address this deficiency. HST/WFPC2 observations of Mars during the 2001 global dust storm provide data that possess broad wavelength coverage (12 filters from 255 to 1042 nm), high precision radiometric calibration, well-characterized viewing geometry, and large regions with a negligible surface scattering component.
WFPC2 images were calibrated to I/F and photometrically (Minnaert) corrected. We derived spectra of the airborne dust from the resulting data cubes (spatial x wavelength) by comparing spectra of low albedo surface regions measured during low dust opacity conditions in April and May 1999 (Ls=130o; tauvis < 0.3) to spectra of the same regions measured through the same filters and under similar viewing conditions during the planet-encircling dust storm in August and September 2001 (Ls=220o; tauvis > 2.0). Pre-storm HST imaging in 2001 confirmed that the planet's major albedo markings were essentially unchanged between oppositions, so the ratio of high to low opacity observations provides the isolated signature of atmospheric dust alone. We extracted spectra from fairly homogeneous, roughly 60x60 km regions co-located to less than 1o in latitude and longitude, and avoided regions where water ice clouds could also be present.
The average dust spectrum is similar to the spectra of high albedo regions on Mars. However, the average dust spectrum exhibits no significant evidence for the weak crystalline Fe3+ absorptions seen in surface spectra, nor does it show evidence for an Fe2+ absorption typical of that seen in low albedo regions. The two most diagnostic features of the average dust spectrum are its relatively smooth drop-off in the visible/near-UV, and its positive spectral slope in the visible/near-IR. These results are consistent with the spectra of magnetic dust derived by the Mars Pathfinder team, and these characteristics are consistent with the dust being composed of fine-grained poorly crystalline ferric oxides.
If the author provided an email address or URL for general inquiries,
it is as follows:
Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.