Observations of Phobos, Deimos, and bright stars with the Imager for Mars Pathfinder

N. Thomas, D. T. Britt, K. E. Herkenhoff, S. L. Murchie, B. Semenov, H. U. Keller, P. H. Smith

Research output: Contribution to journalArticlepeer-review

26 Scopus citations


The Imager for Mars Pathfinder (IMP) was used to observe several objects during the Martian night. The satellites, Phobos and Deimos, were observed on two occasions each, through the IMP geological filters covering the wavelength range 440 nm to μm. The observations were converted to geometric albedo using triaxial ellipsoid models of the satellites and phase functions derived from Viking Orbiter images. The spectral slopes over the full wavelength range were 7.9(±0.5)% (100 nm)-1 and 9.6(±0.6)% (100 nm)-1, respectively, referenced to 600 nm. In the Deimos spectra, some evidence for decreased reddening toward the trailing hemisphere was found. The geometric albedoes of Phobos and Deimos were found to be 0.065 (±0.010) and 0.068 (±0.009), respectively, averaged over 440 and 600 nm, in good agreement with previous measurements. The nighttime optical depth was investigated using observations of stars. A mean value of 0.56 (±0.09) was determined from measurements at different airmass. A possible maximum in the optical depth near 0200 local time was found, which may result from condensation during the night. A measurement of the egress of Phobos from eclipse was made. Modeling of the light curve gave a scale height for the scatterers of 10-15 km. The exact time of the eclipse reappearance over the limb could be reconstructed from the measurements and was in reasonable accord with predictions, although a small modification to the predicted position of Phobos of 6.8 (±6.0) km would have produced better agreement.

Original languageEnglish (US)
Article number98JE02555
Pages (from-to)9055-9068
Number of pages14
JournalJournal of Geophysical Research: Planets
Issue numberE4
StatePublished - Apr 25 1999
Externally publishedYes

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Atmospheric Science
  • Astronomy and Astrophysics
  • Oceanography


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