Mars Orbiter Laser Altimeter Radiometry: Phase Functions and the Optical Depth of Nocturnal Water Ice Clouds

Friday, 19 December 2014
Gregory A Neumann, NASA, Baltimore, MD, United States, Michael Kenneth Barker, Sigma Space Corporation, Hyattsville, MD, United States and Xiaoli Sun, NASA Goddard Space Flight Center, Greenbelt, MD, United States
Over the course of more than 3 Mars years the MOLA instrument on board Mars Global Surveyor (from 1999 to the loss of MGS in Nov. 2006) obtained passive reflectance measurements of Mars at 1064 nm wavelength from the solar background. As an altimeter, the quantity of light removed from a laser beam by scattering or absorption during the roundtrip to the surface may be calculated knowing the energy returned, the surface geometric albedo and the instrument parameters for each laser shot. These opacity measurements indicate the combined effects of dust and condensates, particularly those seen during during the night. The measure of opacity, to optical depths exceeding unity, correlates well with daytime measurements by the Hubble Space Telescope and with the broadband Thermal Emission Spectrometer. Applying a simple phase function to passive radiometric observations obtained at emission angles varying from 0 to 80 degrees, upper and lower limits are obtained for atmospheric opacity as a function of season and time of day. The implications for the effects of nocturnal water ice clouds on radiative transfer, and for future applications to the detection of icy plumes from moons of the outer solar system will be discussed.