The MOLA instrument has the ability to detect atmospheric reflections, providing unique information on the dynamics of the Martian atmosphere throughout the seasons. Previous work by the MOLA Science Team has shown that atmospheric reflections obtained from MOLA consist of CO₂ that condenses from gas into the solid phase, forming dry ice snow. Reflective clouds composed of dry ice are detected from altitudes greater than 20 km (and probably extend higher) down to the surface. They are observed during the winter night, particularly over the polar caps. Atmospheric reflections sometimes exhibit dynamical forms like lee waves and gravity waves.

The observations below were obtained during the MGS mapping mission during southern hemisphere winter. In the figures, colored ellipses of different sizes represent returns from the four different MOLA receiver filters. The heights are proportional to pulse width and the width is proportional to pulse energy. Small ellipses indicate sharp returns from dense reflecting surfaces, while large ellipses indicate returns from diffuse atmospheric reflectors with low particle densities. All figures show spacecraft time (corresponding to distance over the surface) along the x-axis, and altitude above the surface in kilometers along the y-axis. The longitude and latitude of the beginning and end of each profile are shown at the upper left and right of the figure. The white lines represent ground returns (surface topography).

Publications that contain technical and scientific details about MOLA's observations of CO₂ clouds and snow include:

Neumann, G.A., D.E. Smith and M.T. Zuber, Clouds detected by the Mars Orbiter Laser Altimeter after one Mars year, submitted to J. Geophys. Res., 2001.

Ivanov, A.B. and D.O. Muhleman, Cloud reflection observations in the Mars Orbiter Laser Altimeter (MOLA) investigation, submitted to Icarus, 2001.

Smith, D.E., M.T. Zuber, H.V. Frey, J.B. Garvin, J.W. Head, D.O. Muhleman, G.H. Pettengill, R.J. Phillips, S.C. Solomon, H.J. Zwally, W.B. Banerdt, T.C. Duxbury, M.P. Golombek, F.G. Lemoine, G.A. Neumann, D.D. Rowlands, O. Aharonson, P.G. Ford, A.B. Ivanov, P.J. McGovern, J.B.Abshire, R.S. Afzal, and X. Sun, Mars Orbiter Laswr Altimeter (MOLA): Experiment summary after the first year of global mapping of Mars, J. Geophys. Res., 106, 23,689-23,722, 2001.

Pettengill, G. H., and P. G. Ford, Winter clouds over the north Martian polar cap, Geophys. Res. Lett., 27, 609-612, 2000.

Ivanov, A.B., Some Aspects of the Martian Climate in the Mars Orbiter Laser Altimeter Investigation, Part II. Polar Night Clouds, Ph.D. Thesis, Calif. Inst. Tech., Pasadena, 2000.

Zuber, M.T., Smith, D.E., S.C. Solomon, J.B. Abshire, R.S. Afzal, O. Aharonson, K. Fishbaugh, P.G. Ford, , H.V. Frey, J.B. Garvin, J.W. Head, A.B. Ivanov, C.L. Johnson, D.O. Muhleman, G.A. Neumann, G.H. Pettengill, R.J. Phillips, X. Sun, H.J. Zwally, W.B. Banerdt, T.C. Duxbury, Observations of the north polar region of Mars from the Mars Orbiter Laser Altimeter, Science, 282, 2053-2060, 1998.

Dense clouds over the south polar cap. (Credit: MOLA Science Team)

Dry ice snow condensing out of the Martian atmosphere near the south pole. Note that the CO₂ crystals extend from an altiude of 18 km all the way down to the surface. (Credit: MOLA Science Team)

Two types of clouds viewed at mid-southern latitudes. Red and black returns indicate a relatively thin layer of optically-dense clouds and green and blue indicate diffuse clouds. (Credit: MOLA Science Team)

Multiple "cloud decks" near the south pole. (Credit: MOLA Science Team)

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