Martian dust storm: its depth on 25 november 1971.

@article{Parkinson1972MartianDS,
  title={Martian dust storm: its depth on 25 november 1971.},
  author={T D Parkinson and Donald M. Hunten},
  journal={Science},
  year={1972},
  volume={175 4019},
  pages={323}
}
Carbon dioxide absorption on Mars was observed by Earth-based spectroscopy at 10,500 angstroms. Its abundance was equivalent to a partial pressure of 2.0 millibars instead of the normal 5.5 millibars. The dust layer was therefore opaque to a height of about 11 kilometers. On 10 December, a pressure of 5.8 millibars was found.